Landtank MAF surprising scangauge results (3 Viewers)

[Brian894X4]

that would also explain the differing results above from hand blocking the two mafs at idle. on the stock maf at idle the ecu may be getting no maf signal and simply flowing a predetermined baseline fuel flow calibrated for a 700 rpm idle. for the landtank maf, it is still getting a maf signal and so trying to respond to the differing air flows caused by hand blocking.

I know the stock MAF is providing some useful signal to the ECU at idle, because if you block the sensor hole, the engine will die. Also, to be fair, the sensor hole in the stock MAF is right out in front of the housing, while the sensor hole for the LT MAF is inset, but there should be a difference in how the two MAFs behave when partly blocking the intake at the filter canister lid.

plugging or unplugging the Landtank MAF seemed to have no affect on its sensativity, but once the engine started acting up with the filter removed, I stopped messing around, so more testing would probably have been valuable.

Again, assuming the Landtank sensor was working correctly and baring any unforseen problem like a vacuum leak I missed, the LT mod sensor acted like it was causing an extreme lean or rich condition right on the edge of the ECU parameters and the disruption of the airflow as just more than the ECU could handle and it shut down the engine. I don't know if the sensor is just too sensative at idle, or if the ECU is confused by a signal its not expecting to see, or if the sensor is not calibrated to the ECU and that's causing the ECU to run right on the edge of it's limits at idle.

Again, with the filter attached, it seemed to run just like Landtank and others said it would with the FPR mod, so there's no dispute there. The question seems to be why does lifting the filter canister lid have an affect on the sensor, why does only partly blocking the intake have such a dramaticly affect on the sensor and why does removing the filter canister lid from the sensor have such a dramaticly affect on the sensor. It seems to be that there could be real calibration issue between this sensor and this engine and this ECU at least at idle.

To be very fair, someone else might want to test my results to make sure this wasn't because of something silly like a loose intake hose or crack that came open and then later closed up and sealed up with the stock MAF.

 

[Ebag333]

i wish i could find my 1994 FSM i am 95% sure it tells me that the FPR on the 93-94 compensates for the lack of sensitivity of the VAF at low rpm.

if that is so, then the generic discussion of toyota the purpose of FPR's that christo has attached is not the whole story. if it does the same thing for the stock MAF, this would also potentially explain why the landtank FPR is "conflicting" with the FPR. basically, the ecu may be getting a reading from the newer MAF when it is not expecting one because the stock MAF will normally have dropped out. the FPR "baseline" fuel pressure may be calibrated to compensate for an expected zero "baseline" maf signal to the ecu at or near idle, but the landtank MAF may still be generating a calibrated signal.

Well, Christo's point on the FPR is really one of two (as I understand it), though they are related.

1. Why do you have to remove the FPR to get it to run right?
2. How do you know that the sensor is reporting the correct amount of air to the ECU?

I didn't find anything on the FPR, but I did find some interesting things on the MAF.

One thing is that the ECU expects 3.8 +- 1.2 gm/sec for airflow. (If I'm reading that right, thats at 2500 RPM, warm, and no load.)

So since Landtank's mod is pushing more air, the voltage that the new sensor reports to the ECU needs to match what the old one did, compensating for the extra air that's being added (Christo's second point).


I also found this:





(Sorry if any of this has been posted before.)

 

[Beowulf]

I also found this:

Is that from your 1997 FSM?

-B-

 

[Ebag333]

Is that from your 1997 FSM?

-B-

Yup. Sorry, should have prefaced that.

 

[semlin]

Well, Christo's point on the FPR is really one of two (as I understand it), though they are related.

1. Why do you have to remove the FPR to get it to run right?
2. How do you know that the sensor is reporting the correct amount of air to the ECU?

i agree. i understand that christo is saying "what if disconnecting the fpr is compensating for the fact that this maf is sending a signal that understates the airflow at all rpms?". he fears that that would be bad since the ecu would constantly be playing ketchup based on o2 sensors to an excessive degree.

however, the problem with christo's theory is that there is quite a bit of stochiomatic testing and according to that testing, the FPR vacuum only causes an issue with fuel trim at low air flow. with the landtank maf you have an 18% discrepancy at idle, but it disappears as soon as you get any rpm.

that causes me to question a base assumption of christo's, which is that the FPR on an 80 will vary according to manifold pressure to maintain a constant fuel pressure at all rpms. the testing results suggest that it does not.

so, i am suggesting that in the generic toyota discussion of the FPR that christo has linked, Toyota did not mention that at least some FPRs have a base fuel pressure at low rpms. think of it as a stop preventing the diaphragm from moving any further beyond a certain vacuum pressure. i am suggesting this is done because the stock vaf or maf is not expected to accurately meter air at low rpms so toyota has basically deemed a stock minimum fuel flow rate that is calibrated to be correct at 700 rpms for a certain deemed manifold pressure. if they have done this, then they would not want the fuel pressure to vary below the correct predetermined pressure. they would want the engine to automatically recover back to and hold at 700 rpm based on the fuel supply / fuel pressure.

this could explain why the landtank maf conflicts with the fpr only at low rpm.

if it is correct, then it does not actually answer christo's concerns, just explain them. you would still have the problem that landtank maf trucks do not have this failsafe mechanism so that if the landtank maf gets dirty and no longer reads low rpms

another possibility is that the deemed minimum fuel and fuel pressure is overriding the landtank maf signal, but since the landtank maf is flowing more air than the stock maf would flow, this is upsetting the deemed air/fuel/pressure calculation for 700 rpm.

either way, the solution is to try and adjust the idle to find an idle rpm that matches the interaction here.

I didn't find anything on the FPR, but I did find some interesting things on the MAF.

One thing is that the ECU expects 3.8 +- 1.2 gm/sec for airflow. (If I'm reading that right, thats at 2500 RPM, warm, and no load.)

So since Landtank's mod is pushing more air, the voltage that the new sensor reports to the ECU needs to match what the old one did, compensating for the extra air that's being added (Christo's second point).


I also found this:




(Sorry if any of this has been posted before.)

i do not understand how this line of analysis could be a concern. a supercharger pushes far more air into the engine compared to stock than the landtank maf, yet toyota had no hesitation in selling one and trusting the ecu to compensate. why would the modestly larger air flow area of the landtank maf chamber in any way tax the ecu as long as it is constant? the ecu is going to adapt to that using o2 feedback exactly the same way it will adapt to using larger injectors.

 

[Ebag333]

i do not understand how this line of analysis could be a concern. a supercharger pushes far more air into the engine compared to stock than the landtank maf, yet toyota had no hesitation in selling one and trusting the ecu to compensate. why would the modestly larger air flow area of the landtank maf chamber in any way tax the ecu as long as it is constant? the ecu is going to adapt to that using o2 feedback exactly the same way it will adapt to using larger injectors.

It's not about a SC really.

Remember the stock sensor is going to show the correct amount of air flowing up to a point when it becomes saturated, then the ECU goes off programmed numbers. But from idle to that point we know that what the ECU is seeing is good valid data.

If with LT's mod the fuel trim monitor is active (which is what I understand) at idle when the stock setup is not, then the MAF is reporting that it's pushing more than 4 gm/sec. How much more is the question.

Now if the FPR is causing it to run rich, then that's because it IS pushing more air through at idle than stock. As I understand it, that's part of the FPR's job.

By disabling the FPR you are causing a lean condition. My guess (and this is purely a guess based on my limited knowledge) is that the MAF sensor is not reporting as much air flowing through at idle as the FPR is seeing. Basically at idle there's more air entering the system than what's reported.

Now it may only be at idle and be a non-issue throughout the rest of the RPM range....but is there any way to show that?

 

[semlin]

It's not about a SC really.

Remember the stock sensor is going to show the correct amount of air flowing up to a point when it becomes saturated, then the ECU goes off programmed numbers. But from idle to that point we know that what the ECU is seeing is good valid data.

If with LT's mod the fuel trim monitor is active (which is what I understand) at idle when the stock setup is not, then the MAF is reporting that it's pushing more than 4 gm/sec. How much more is the question.

Now if the FPR is causing it to run rich, then that's because it IS pushing more air through at idle than stock. As I understand it, that's part of the FPR's job.

By disabling the FPR you are causing a lean condition. My guess (and this is purely a guess based on my limited knowledge) is that the MAF sensor is not reporting as much air flowing through at idle as the FPR is seeing. Basically at idle there's more air entering the system than what's reported.

first off, unless i have it backwards, disabling the fpr is not "causing a lean condition". doing that will increase the fuel pressure and send more fuel to the engine per pulse, which is the opposite of causing a lean condition. also, the fpr's "job" is to ensure there is always more fuel pressure as manifold pressure increases so that fuel can enter the engine, but i am saying it may also have the job of setting a minimum fuel pressure at low idle that works with a similar minimum fuel flow rate to keep the motor idling properly at 700 rpm.

secondly, what the FPR "sees" is manifold pressure, not air volume. if more air is flowing into the manifold then this increases manifold pressure which would cause the fpr to increase fuel pressure and thus send more gas so, again, it is not going to cause the engine to run lean.

thirdly, the maf is always going to be reading the correct rate of air flow and density. what you are suggesting is that since the air passage around the sensor is larger than stock, the ecu is using a multiplier based on the stock maf chamber dimensions and therefore miscalculating the total volume of air entering the engine at a given rate of air flow. i think that christo and you are right: that is obviously the case as soon as you change the chamber dimensions since the ecu cannot know that has happened.

but my point is -- so what? the ecu has a feedback loop to address this so it does not depend on having exact air metering. if increasing air volume to this small extent was a problem for the ecu then it should be a problem at all rpms, not just at idle. the calculation of volume by the ecu would always be out by the same amount so the fuel trim problems should occur at all rpms with the FPR connected. also, the problem should resurface at higher rpms even with the FPR disconnected.

but it isn't a problem except at idle. the very same tests that show the "problem" with the fuel trim caused by the fpr at idle show no issue any other time. in other words, this air volume fluctuation is well within the ecu's power to adjust for except at idle. in fact, the test show that, except at idle, the ECU can adjust for this increased air volume whether or not the fpr is connected which shows that it is well within the ECU's comfort zone.

so you are missing something. you lack a theory to explain the observed results.

i have made two possible suggestions as to what you are missing. my suggestions could explain why the problem only occurs at idle. i would be interested in your explanation for why the problem only appears at idle.

 

[Cattledog]

By disabling the FPR you are causing a lean condition.

This is incorrect.

On the US OBD II 80 At higher (i.e. -29" H20) vacuum the FRP lowers the rail pressure from about 44 psi to 33 or 38 psi, this occurs at idle. -29" H2O is just an example. At WOT the FPR is partially closed as compared to idle, this raises the rail pressure.

I and Rick have also done the experiment that if you artificially raise the pressure on the FRP to above atomspheric you can further richen the fuel mixture until the ECU will adjust STFT (under1 second) and after a bit of time LTFT will correct the air fuel mixture. Under boost the rail pressure is above 44 PSI. the ecu has no problem correcting the increased fuel from the rail by shorting the injector pulse.

If you really are wonder about the rail pressure vs intake vacuum run a fuel pressure gauge. You will see on the US OBD II 80 that the rail pressure is raise very quickly to about 43.5 psi of idle. Eveyone who runs Rick MAF wonders why the throttle respones is better. Well the fact is that the rail is already at about 43.5 psi and the air is correctly metered by the O2 system feedback. Thus better throttle tip in. IMO this also part of the reason the shifts smooth out with Rick's setup.

Sorry, Christo your piggyback was the link if my memory serves me correct not the unichip if I am wrong please correct me on what system is used and every detail on how you "tune" the system I am interested in here how your shop does it. Unichip was safari. IMO, piggyback systems are a bandaid for proper fuel control. In most cases the piggyback ends up either modifying the orginal sensors signal or running extra injectors that are poorly placed in the intake system causing single or multiple lean cylinder issues.

I still think you are blowing this incorrectly metered air bit out of proportion.

 

[Brian894X4]

When you guys did all your tests, did you ever run the new MAF without the air filter attached? I'm wondering if you had the same issues I did, if you did? Or if it ran perfectly with the LT MAF and no filter canister attached like the stock MAF does?

 

[sleeoffroad]

i wish i could find my 1994 FSM i am 95% sure it tells me that the FPR on the 93-94 compensates for the lack of sensitivity of the VAF at low rpm.

93 94 uses a totally different fuel metering system as well as ECU, so it would be irrelevant to this discussion.

 

[jamisobe]

93 94 uses a totally different fuel metering system as well as ECU, so it would be irrelevant to this discussion.

Totally irrelevant?

They are both on the same vehicle and they do both use the same FPR. It is obvious that if the 93 94 models use the FPR to overcome flow inefficiencies at idle that the Toyota engineers have at least used that strategy before.

Irrelevant maybe. But nobody has shown anything that proves any risk in disconnecting the FPR on this application. Only unfounded precaution. With the vacuum disconnected, the pressure in the fuel rail is much higher than normal at idle which means the engine would run richer if anything. Then it would contiune to be richer as RPMs build.

The FPR has nothing to do with anything at higher RPM ranges since it saturates at about 80% flow.

The only risk of running lean would be at higher RPMs when there would be a potential of increased airflow from a better MAF outpacing static fuel settings. But the fuel settings are not static, have nothing to do with the FPR at WOT, and are adjusted by the ECU throughout the RPM range.

Lean schplean.

 

[jamisobe]

After running back on the road with the stock MAF, the performance gains of the Landtank MAF were pretty apparent. Testing under the exact same conditions, including weather and temp, I couldn't get the scangauge to indicate anything more than 260 gross h.p. Remember that with the Landtank MAF it was grossing 305 h.p. The stock MAF intake just feels restricted compared to the Landtank MAF and I'm sure that's a significant reason for the power increase.

That would indicate a 17% increase in power. Even if the scan guage figures are too high they are apples to apples if the scan gauge is the finely calibrated instrument that people seem to think it is. I would say that the increase definitely feels like about 17% more power.

This mod rocks. Nervous nellies wearing pants that are too tight and socks that are too loose have no business enjoying such gains.

 

[sleeoffroad]

Totally irrelevant?

They are both on the same vehicle and they do both use the same FPR. It is obvious that if the 93 94 models use the FPR to overcome flow inefficiencies at idle that the Toyota engineers have at least used that strategy before.

Can you please send me the reference that this is the case? The flapper style meter as used in the 93 to 94 is a totally different thing. The only thing common to 93/94 to the later models is the motor. The OBDII system that is used on 95 and later can not be compared to the earlier one.

Irrelevant maybe. But nobody has shown anything that proves any risk in disconnecting the FPR on this application. Only unfounded precaution. With the vacuum disconnected, the pressure in the fuel rail is much higher than normal at idle which means the engine would run richer if anything. Then it would contiune to be richer as RPMs build.

if the MAF was original and no boost was supplied. In that case the truck would run leaner.

The FPR has nothing to do with anything at higher RPM ranges since it saturates at about 80% flow.

What saturates?

 

[jamisobe]

Can you please send me the reference that this is the case? The flapper style meter as used in the 93 to 94 is a totally different thing. The only thing common to 93/94 to the later models is the motor. The OBDII system that is used on 95 and later can not be compared to the earlier one.

The premise is that the 93 and the 94 use the FPR to lean for proper idle.

What can make this relevant is that it proves that Toyota has used the same exact FPR to do what you say it is not doing on an OBDII (without proof). Not saying either way, just that it is not neccessarily irrelavent since it proves that Toyota did not use the FPR to prevent a lean condition in the 93-94 models, but rather to induce a lean condition at idle.

if the MAF was original and no boost was supplied. In that case the truck would run leaner.

Not if the pressure increase causes more velocity of fuel through the injector at idle or if the ECU tells the injector to stay open longer during operation.

What saturates?

The FPR is fully open when it only allows 20% of rail pressure to the tank. Vacuum connected or disconnected, at WOT the FPR holds 80% of the pressure in the fuel rail.

The fuel return saturates at WOT regardless of whether the FPR sees manifold pressure.

The more I study this the more evident it is that the only reason Toyota links to manifold pressure is to prevent rich idle or to prevent poor throttle response due to poor airflow. But then again, I am not wearing a tight belt or loose socks.

 

[sleeoffroad]

i agree. i understand that christo is saying "what if disconnecting the fpr is compensating for the fact that this maf is sending a signal that understates the airflow at all rpms?". he fears that that would be bad since the ecu would constantly be playing ketchup based on o2 sensors to an excessive degree.

however, the problem with christo's theory is that there is quite a bit of stochiomatic testing and according to that testing, the FPR vacuum only causes an issue with fuel trim at low air flow. with the landtank maf you have an 18% discrepancy at idle, but it disappears as soon as you get any rpm.

OK, I thought I would summarize my concerns/questions for whoever is still in this thread. This is from the viewpoint of people installing this mod and expecting it as a factory replacement that works better.

Now playing with MAF signals and modifying them is commonly done. From the time of opening the flapper style of the 22re and then messing with that, to piggybacks that intercept the curves and modify them. It doesn't mean it is a bad thing, it just means when you start experimenting, you have to be aware of the consequences.

In this case, I would say that 10% of the people know what they are installing, the rest if following the crowd. This is an assumption.

For normally aspirated trucks.

1. The new MAF caused excessive LTFT at idle. The FPR was disconnected and this was corrected. The purpose of the FPR line was explained as compensating for a original sensor that did not read the low air flow accurately. I asked for where is the proof of this. This has not been shown to be true or not.

2. I asked if any work was done to calibrate the sensor to provide accurate information airflow numbers to the ECU in terms of actual airflow. None was done, just observations and testing based on checking fuel trim values.

Boosted trucks,

1. Assumptions was made regarding open loop vs closed loop and the amount of time that is spent in each. These assumptions were wrong since there is no evidence that the air flow numbers as calculated my the ECU are correct. The ECU calculates the air flow from the voltage it receives from the MAF sensor as well as the information it has hard coded regarding the size (flow) of the original housing. The ECU does not know the housing size changed, so there is no way to know how that any air flow number reported by the ECU is an correct value or can be compared.

Open loop vs closed loop calculations is based among other things, on engine load (being the most important). Engine load is derived from airflow numbers. So it is easy to see that if airflow numbers are off, then load calculations is off. If that is off, then the decision to go from open to closed loop is incorrect. If that is done, you have removed the failsafe that Toyota engineered into the system to say, hey dump a bunch of fuel into this thing to ensure it doesn't run lean.

I will put the other side of the argument here. If Toyota was that confident in the feedback systems, why did they not design the vehicle to run in closed loop all the time? I am sure they could if they wanted to.

2. Comments were made about the saturation of the stock MAF and the fact that the new MAF read higher numbers. This was incorrect as well (or never proven). There is no proof that the stock MAF is saturated. The only way to prove this is to flow it and then map the voltage readings vs actual airflow.

--------------------------------------------------

that causes me to question a base assumption of christo's, which is that the FPR on an 80 will vary according to manifold pressure to maintain a constant fuel pressure at all rpms. the testing results suggest that it does not.

It maintains constant fuel pressure differential. This is not an assumption.

so, i am suggesting that in the generic toyota discussion of the FPR that christo has linked, Toyota did not mention that at least some FPRs have a base fuel pressure at low rpms. think of it as a stop preventing the diaphragm from moving any further beyond a certain vacuum pressure. i am suggesting this is done because the stock vaf or maf is not expected to accurately meter air at low rpms so toyota has basically deemed a stock minimum fuel flow rate that is calibrated to be correct at 700 rpms for a certain deemed manifold pressure. if they have done this, then they would not want the fuel pressure to vary below the correct predetermined pressure. they would want the engine to automatically recover back to and hold at 700 rpm based on the fuel supply / fuel pressure.

this could explain why the landtank maf conflicts with the fpr only at low rpm.

I have never seen a comment or anything in Toyota literature that suggest the above for a fuel system that uses a FPR and a return line.

another possibility is that the deemed minimum fuel and fuel pressure is overriding the landtank maf signal, but since the landtank maf is flowing more air than the stock maf would flow, this is upsetting the deemed air/fuel/pressure calculation for 700 rpm.

either way, the solution is to try and adjust the idle to find an idle rpm that matches the interaction here.

Idle is not adjustable.

i do not understand how this line of analysis could be a concern. a supercharger pushes far more air into the engine compared to stock than the landtank maf, yet toyota had no hesitation in selling one and trusting the ecu to compensate.

Because they left the stock MAF in place and assumed increased airflow will be handled by the stock MAF. I am not necessarily agreeing that this is the correct way as well. That is why I said repeatedly Ricks aim of using a different MAF and larger injectors is a good idea, but without the initial calibration and incorrect assumptions it would be a very difficult thing to get right.

why would the modestly larger air flow area of the landtank maf chamber in any way tax the ecu as long as it is constant? the ecu is going to adapt to that using o2 feedback exactly the same way it will adapt to using larger injectors.

It does not tax the ECU, but you are relaying on feedback systems that are not very reliable. You can blow a motor up without a check engine light coming on due to lean conditions.

 

[Cattledog]

When you guys did all your tests, did you ever run the new MAF without the air filter attached? I'm wondering if you had the same issues I did, if you did? Or if it ran perfectly with the LT MAF and no filter canister attached like the stock MAF does?

No, IMO, it would be dumb to run the engine without the air cleaner attached.

 

[sleeoffroad]

thirdly, the maf is always going to be reading the correct rate of air flow and density.

This is not correct. It produces a voltage based on a air sample it takes in a air stream. The conversion of that voltage measurement to actual airflow is done in the ECU.

what you are suggesting is that since the air passage around the sensor is larger than stock, the ecu is using a multiplier based on the stock maf chamber dimensions and therefore miscalculating the total volume of air entering the engine at a given rate of air flow.
i think that christo and you are right: that is obviously the case as soon as you change the chamber dimensions since the ecu cannot know that has happened.

Correct.

but my point is -- so what? the ecu has a feedback loop to address this so it does not depend on having exact air metering.

So what? The feedback is 100% dependant on accurate air flow readings. Here is why. Closed and open loop decision is based on AIR FLOW readings (I posted a link to a document explaining this) . Open loop is when Toyota said, hey, we can't use feedback to ensure that the truck does not blow up, use these fixed readings.

With incorrect air flow numbers, this whole decision point has been altered. You are changing both sides of the equation.

if increasing air volume to this small extent was a problem for the ecu then it should be a problem at all rpms, not just at idle. the calculation of volume by the ecu would always be out by the same amount so the fuel trim problems should occur at all rpms with the FPR connected. also, the problem should resurface at higher rpms even with the FPR disconnected.

You do not know how much it is off. You don't know the response curve or the actual flow of the housing. It does surface at higher RPM. Although might not be a problem yet. Boosted trucks are running leaner, without an explanation if it is drop in fuel pressure differential, or airflow readings.

but it isn't a problem except at idle. the very same tests that show the "problem" with the fuel trim caused by the fpr at idle show no issue any other time. in other words, this air volume fluctuation is well within the ecu's power to adjust for except at idle. in fact, the test show that, except at idle, the ECU can adjust for this increased air volume whether or not the fpr is connected which shows that it is well within the ECU's comfort zone.

Again, you are relaying on the fact that the ECU feedback decisions are not changed, but it is. If the sensor and housing was calibrated, you could assume that.

i have made two possible suggestions as to what you are missing. my suggestions could explain why the problem only occurs at idle. i would be interested in your explanation for why the problem only appears at idle.

You don't know it only occurs at idle. Without measuring NOX, you do not know what is going on. The problem might be less, but it might still be there.

 

[sleeoffroad]

Sorry, Christo your piggyback was the link if my memory serves me correct not the unichip if I am wrong please correct me on what system is used and every detail on how you "tune" the system I am interested in here how your shop does it. Unichip was safari. IMO, piggyback systems are a bandaid for proper fuel control. In most cases the piggyback ends up either modifying the orginal sensors signal or running extra injectors that are poorly placed in the intake system causing single or multiple lean cylinder issues.

I would rather not bring this into this thread as well. It is already as confusing to most people. I will agree, that piggyback is not always the ideal solutions, but without calibrating this MAF, it is a much more accurate system and predictable.

I still think you are blowing this incorrectly metered air bit out of proportion.

I don't think I am. I am trying to point out incorrect assumptions so that people that do this to the trucks, understand what they are doing.

 

[jamisobe]

a original sensor that did not read the low air flow accurately. I asked for where is the proof of this. This has not been shown to be true or not.

Uhhhh, proof? Sounds like a double edged sword to me.

How about 3 years of trouble free (exceptional) operation on boosted and non-boosted trucks.


Then there is that common sense thing...

If the concern is disconnecting the FPR the risk would be too rich, not too lean. The FPR being disconnected will not be the cause of lean operation.

If the concern is more airflow then the ECU can compensate for then an O2 sensor would send a lean signal at WOT at least once on the hundred or so trucks running this upgrade. Anyone???

Sometimes it helps to be a simpleton when you really want to understand things.

 

[semlin]

christo, do you have a 93-94 FSM? I cannot find "volume 2" of mine right now anywhere (it tore in half), but I think that if you look at the FSM discussions on the VAF and the FPR or in the introduction to the EFI section you will find a comment about the FPR compensating for the fact the VAF does not work at low rpms.

if it's not there then i guess i hallucinated it, but i recall reading it and being concerned about it when i was looking at doing a vaf-maf conversion. if it is there, then i do think it is relevant. it offers a possible explanation here since we seem to be talking about the same parts on the same motor.

and the obd1 and obd2 toyota ecu's are very similar (obd1 and obd2 ecus on karman vortex supras are plug and play interchangeable, and on the vaf supras the stock obd1 ecu will accept a maf or map conversion easily with a maf translator to reverse the voltage signal).