Low RPM surging and stalling (1 Viewer)

[87warrior]

Tonight I hooked up TechStream to see if anything stood out to me when the RPMs drop and it tries or does stall. My wife describes the engine sound as "weak" when the RPM starts to drop.

Here is a screenshot of the Fuel parameters when it is running good.

In this shot the RPM dropped and was kind of sporadic, but did not die.

*note: the coolant temperature is higher than most, but that is because I received a 195 t-stat when I did the tbelt. The cruiser warms up to 195 and stays there.

It only died once tonight while in the garage when TechStream was running, but I wasn't fast enough to see anything obvious in the data. Are there any recommended parameters I should watch that might help pinpoint this issue?

I borrowed a fuel pressure gauge but am unable to test until I get the adapter banjo bolt for the fuel rail. Out of curiosity, I did experiment with the fuel pressure regulator. I removed the vacuum line for the FPR at the intake, hooked it to a handheld vacuum pump and plugged the nipple on the intake with a vacuum cap. I applied vacuum to the FPR and started the Cruiser. The engine smoothness and fuel trims did not change whether the FPR had a vacuum applied or not. The FPR also held the vacuum without leaking down. I do not know if this means anything or not, but it is what I noticed.

 

[white_lx]

The Techstream screenshots do not seem to indicate a low fuel pressure issue. Low fuel pressure would result in positive fuel trim numbers to compensate for the lower injector flow rate. I would also expect it to get worse at higher RPM's. They do show that both banks are equally affected, which eliminates a few candidates.

It looks as if there could be a missfire as the fuel numbers turn negative. But it would need to occur on both banks to have the symmetrical outcome. Possibly a ground issue of the ignition system affecting both sides equally. Can you look at the misfire counters when the problem occurs?

The other variable is the mass air flow sensor. MAF = 5.31 g/s at 700 rpm and 5.75 g/s at 641 rpm. That is a bit suspicious as the mass flow should increase with RPM. Is the MAF signal stable? Is there maybe any debris stuck in it, creating erroneous signals? Air filter clean?

Is your Throttle POS signal constant at 14% for both situtations?

 

[87warrior]

The Techstream screenshots do not seem to indicate a low fuel pressure issue. Low fuel pressure would result in positive fuel trim numbers to compensate for the lower injector flow rate. I would also expect it to get worse at higher RPM's. They do show that both banks are equally affected, which eliminates a few candidates.

My thought on fuel pressure is that the FPR might be stuck or malfunctioning causing excessive fuel pressure at idle. If the rail is at max PSI at idle, the injectors could be over fueling for the given injector pulse. It is just a theory I have.

It looks as if there could be a missfire as the fuel numbers turn negative. But it would need to occur on both banks to have the symmetrical outcome. Possibly a ground issue of the ignition system affecting both sides equally. Can you look at the misfire counters when the problem occurs?

I will add the misfire counters to the data view for further testing. Thanks for the idea.

The other variable is the mass air flow sensor. MAF = 5.31 g/s at 700 rpm and 5.75 g/s at 641 rpm. That is a bit suspicious as the mass flow should increase with RPM. Is the MAF signal stable? Is there maybe any debris stuck in it, creating erroneous signals? Air filter clean?

I thought the MAF readings were off as well. I am most suspicious because the issue has occurred with two different MAF sensors. I am going to pick up a can of MAF cleaner and clean both sensors and see if anything changes. Air filter is a stock paper style filter, it isn't new but it isn't plugged up either.

Is your Throttle POS signal constant at 14% for both situations?

TPS % and APPS voltage remain consistent in both situations.

I am going to comb through the EWD and attempt to trace the MAF wiring for any abrasions and locate any grounds related to the sensors and ignition system.

 

[white_lx]

My thought on fuel pressure is that the FPR might be stuck or malfunctioning causing excessive fuel pressure at idle. If the rail is at max PSI at idle, the injectors could be over fueling for the given injector pulse. It is just a theory I have.

Good theory. You can drill and tap the existing banjo bolt if you are in a pinch. Then you can replace it with a new one later. I actually have one in my Partsouq cart for that reason.

Fuel flow is a function of the pressure difference across the injector. That is the pressure delta between fuel pressure and intake pressure (vacuum). Varying intake vacuum levels because of an intermittent vacuum leak or throttle body issues would have the same effect on fuel flow. Just a thought.

 

[87warrior]

I tested the MAF. It was new from Toyota 3-4 years ago.

Ambient air temperature in the shop was 40°F today. MAF registered 16.93 kΩ. Since this temperature is between the spec in the FSM, I stuck the MAF in my deep freeze (-5°F) for an hour. The MAF read 19.1 kΩ.

I then turned my attention to the fuel pressure and fuel pressure regulator.

The fuel pressure did not change when the vacuum sensing hose was connected or disconnected. It stayed at 42psi.

I did notice the fuel pressure jumped to 48-50 psi when the engine stumbled and died at idle.

I added the cylinder miss-fire counters to TechStream and it did not register any miss-fires during the 7 stalls.

It looks like I need to get another MAF sensor and a FPR.

 

[ramangain]

Are your fuel trims ok?

 

[ramangain]

Also,if you unplug all O2 sensors, will it die as before?

 

[jerryb]

if you trust that gauge - and I saw a jump in pressure above 40 +or- what the book says I'd replace it. There shouldn't be a spike, BUT perhaps there is a momentary spike between the pump off and last injector pulse. I personally have never seen one. I have never witnessed a stumbling stall while watching a gauge either. But I do have a permanent digital gauge so I see it all the time.

did you check the leakdown time? anything funny about the return line past the regulator?

 

[jerryb]

one other thing, the pressure remains the same at all RPMs it's a constant. Reading the rest of your thread I'd replace the fpr just because.
And, the pressure will remain the same with or without a reference signal.
IF a quick leakdown without rail leaks, replace it.
If above spec running at any RPM, replace it.
That's what I would do.

 

[87warrior]

Are your fuel trims ok?

Fuel trims go way down when the stumble/surge/due happens. As shown above they bottom out at -20.

Also,if you unplug all O2 sensors, will it die as before?

I have not done this, but I am curious to hear what you think doing this might indicate. I am genuinely curious.

 

[87warrior]

if you trust that gauge - and I saw a jump in pressure above 40 +or- what the book says I'd replace it. There shouldn't be a spike, BUT perhaps there is a momentary spike between the pump off and last injector pulse. I personally have never seen one. I have never witnessed a stumbling stall while watching a gauge either. But I do have a permanent digital gauge so I see it all the time.

did you check the leakdown time? anything funny about the return line past the regulator?

The fuel pressure spike only occurs when the stumble/surge/die happens.

FSM indicates fuel pressure should be above 21psi 5 minutes after shutdown. It was around 32psi at 5 minutes.

one other thing, the pressure remains the same at all RPMs it's a constant. Reading the rest of your thread I'd replace the fpr just because.
And, the pressure will remain the same with or without a reference signal.
IF a quick leakdown without rail leaks, replace it.
If above spec running at any RPM, replace it.
That's what I would do.

Interesting. The FSM step (i) says fuel pressure should be 38-44psi at idle with the FPR sensing line disconnected. In step (n) the FSM says th fuel pressure should be 28-34psi with the FPR sensing line hooked up.

The FPR is stock and has 340k miles on it.

 

[jerryb]

32 @5 is good.

The FSM step (i) says fuel pressure should be 38-44psi at idle with the FPR sensing line disconnected. In step (n) the FSM says th fuel pressure should be 28-34psi with the FPR sensing line hooked up.

if that is true it's way different than the vvti. So running PSI as in normal operating conditions is 28-34 all the time? not 38-42-4?
what am I reading wrong?
I don't mean to sidetrack this, and it's also why I try not to post regarding early efi 100.

edit, I see the picture of the fsm you posted. The later ones don't even mention the disconnect. But reading that picture above it reads a little contradictory. it's 38-44 with connected normal, then disconnect check for same 38-44 PSI, then reconnect and it should be lower, and then leakdown. How does that compute?

 

[87warrior]

The first fuel pressure test in the FSM is with engine off using a Toyota hand held tester (38-44psi). I don't have this piece of equipment. The Second test is with the engine idling with the FPR sensing hose disconnected (38-44psi). The last test is with the FPR sensing line hooked up (28-34psi).

Here is the full fuel pressure test procedure for my '99:

 

[ramangain]

Fuel trims go way down when the stumble/surge/due happens. As shown above they bottom out at -20.

I have not done this, but I am curious to hear what you think doing this might indicate. I am genuinely curious.

If the rig doesn't stall when the O2 sensors are not being used, then .... I dunno. A new piece of data? ECM gurus can chime in and teach both of us. It may have something to do with open vs closed loop control?

 

[jerryb]

it still contradicts itself.
it says that normal is 38-44 with vac line attached.
it also says it's normal with vac line attached at 28-34
safe to say two numbers are wrong. Or there is one sentence missing between the J-N instructions. running in both instances zero load. Pump running through techstream also will get you that 38-44 number. Zero signal.

if you trust the gauge, something is restricting your flow. That should be 100% solid, believed info.

aside from that, and I'm done beating what a return to tank, non adjustable FPR does.

you can have serious random misfires without throwing a code. I want to say 2- 300 I've seen without a code. With enough time it will trigger but you could run all day without seeing it unless you were watching. these would be the type that randomly occur in all cylinders. But never accumulate to a big enough number in a single one. I think that's the p0300?
just an observation. again, maybe not in early 100s but seems like it.
bad gas best example. bad gas left in tank, old gas, degraded ethanol. water saturated.

 

[87warrior]

I do not believe this is a case of bad fuel. The issue has occured 300 miles from home, 1100 miles from home and at home.

it still contradicts itself.
it says that normal is 38-44 with vac line attached.
it also says it's normal with vac line attached at 28-34
safe to say two numbers are wrong. Or there is one sentence missing between the J-N instructions. running in both instances zero load. Pump running through techstream also will get you that 38-44 number. Zero signal.

if you trust the gauge, something is restricting your flow. That should be 100% solid, believed info.

I trust the gauge. I've used it many times to test Jeep fuel pressure within spec.

I do not see a discrepancy for the 99 LC fuel pressure test numbers in the FSM.

• 2(a-i) is tested without the engine running. So no vacuum, or the same as disconnecting the FPR sensing line. 38-44psi
• 2(j-l) is tested with engine running and FPR sensing line disconnected. No vacuum at FPR, same as 2(a-i). 38-44psi
• 2(m-o) is tested with engine running and FPR sensing line connected. This is the only fuel pressure test where VACUUM IS APPLIED to the FPR. 28-34psi

 

[jerryb]

ok

so, we go buy a new 100 in 1999. We're going to go across the country right off the lot. If we were watching a fuel pressure gauge, what would it read all the time?

the reason I posted in the first place is that no where in the 2004 and 2007 is there any mention of readings with or without the vac signal line. The signal line isn't even mentioned at all other than it's there.
In all instances the operating fuel pressure is 38-44 psi. If you watch a gauge while driving at all loads It will read 38-44 psi and always be constant.
That constant is relative to load. Yes there is a pressure change with the vac signal connected or not connected, but you can't see it.

if in vol 1 of the fsm in the spec pages at the beginning of the book says 38-44 psi, then that's what it should read on a gauge all the time.

If the 1998-02 100s are different, I didn't know that. That was partially the reason for asking. Going by the book and what you're saying the normal fuel pressure spec for 1998-2002 is 28-34, how could it be anything else? the engine is running, the signal line is attached, all is well, and the gauge reads 28-34, so it's fine. I'm not disagreeing with you, just the book and how I could actually get a gauge to read that low with the engine running. I'd have to create a leak in the return line, or a diaphragm cut leaking to open atmosphere.

bad gas was just an example of how it's fairly easy to have a somewhat high random misfire count without a code, that's much more plausible than a broke fpr.

the only wrench in that is that the fpr part number is the same for all 100 4.7s

 

[ramangain]

How about grabbing a cheap(er) Denso fuel pump from Amazon and installing it? ~$120 i think, returnable w/o Qs if it doesn't solve your issue.

 

[87warrior]

How about grabbing a cheap(er) Denso fuel pump from Amazon and installing it? ~$120 i think, returnable w/o Qs if it doesn't solve your issue.

I installed one of those earlier this year when I installed a LRA 40g tank. It seems to be working well, 42psi is within the specs in the FSM

 

[ramangain]

Did the stock one die? If not, how about swapping it back in for testing to see if your issue stops? The cost would be your time and perhaps a new external tank seal?