Another SBC TBI Problem Thread

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.

BrianSanDiego said:
If that 10.0 doesn't change at all then it's not giving you the correct readout. The mixture is going to vary quite a bit depending on the throttle position and load. Yes 10:1 would be very rich but it would run. You'd want around 12:1 for max power and 15:1 for max MPG, and beyond those extremes it won't run very well. I'm not familiar with Tuner Pro, but if it could somehow get a real-time display or at least a data log from your O2 sensor, it would go a long way towards sorting out the injection.
B.
Click to expand...
That is what it seemed like. Thanks. I am still trying to make sure that I have the correct .adx datastream file so it interprets the data correctly. When I have it set up correctly, there is a cell to monitor real-time AFR from the o2 sensor. I am switching out my radiator fan today, I have to modify my shroud because the fan is 19.5in vs the old 18in, ugh. After that, I am working on the o2 sensor data. Thanks for the recommendation.
 
jcj_78FJ40 said:
Kinda sounds like a fuel delivery issue. I had very similar issues from a clogged fuel filter. They can clog up quick when your messing with the fuel system. Are you using high pressure hose (ethanol resistant)between the tank and the pump? I have read that low pressure hose may collapse with the higher pressure pump.

As someone mentioned, see if you can rig up your fuel pressure gauge while you are driving.
Click to expand...
I think it sounds like fuel too. When the problem started I replaced my filters and the problem didn't change. I have also replumbed my fuel lines several times to prevent kinks and stay away from heat. I am using high pressure ethanol resistant hose on all the feed line sections. I have rigged up a fuel pressure gauge, but have only used it in the garage (because it was under the hood), but I was able to make the engine backfire through the intake with no perceptible change to the fuel pressure, 13-15psi. My next step, after the fan change and O2 AFR data, is to pull the stock tank and check it out. I would like to retrofit it with a larger line inside it, or fabricate an in-tank fuel pump for it. Time and money...
 
You could gopro the fuel gauge under the hood.

Popping in the exhaust if it's rich, and I think could backfire out the tbi if lean.

When it's rich you can really smell it too. Very distinct raw fuel smell. Also possibly black smoke out the exhaust. Black spark plugs too.

Good luck, B.
 
Did you check/set the minimum air? Have you checked if it's running closed loop at idle? (I'm guessing it isn't).
Also, what intake manifold are you using? Is it from the '95? Does it have EGR and did you block it off or keep it?
 
BrianSanDiego said:
You could gopro the fuel gauge under the hood.

Popping in the exhaust if it's rich, and I think could backfire out the tbi if lean.

When it's rich you can really smell it too. Very distinct raw fuel smell. Also possibly black smoke out the exhaust. Black spark plugs too.

Good luck, B.
Click to expand...
The gopro idea is a great one. I will give that a chance. Right now the cruiser is out of commission. I took it on a drive and stopped to get gas and the starter crapped out. I have had to run the starter for a long time to get it to start when the engine is warm, and it gave up the ghost. The exhaust is dark and there is a fuel smell, but I am not sure if it is from the exhaust or from all the fuel I have spilled in the truck trying to change my fuel system around.
When I have pulled my plugs I have not noticed that they are black. I will pull one and check.
 
Last edited:
terrx said:
Or cut the 19.5" fan down to 18".
Click to expand...
The 19.5 fan was all bent up, so I got an 18" reverse rotation fan from Napa and put it in. Overheating issue appears to be solved as it didn't go over 180* in 30 minutes of driving. I hope that means that I can check one problem off the list. Now it is just the backfiring through the intake (and the not wanting to start when warm, cold starts it fires up easy). On my drive yesterday if I am easy on the throttle it will accelerate easily, but if I give it quick throttle, without letting it build smoothly, it will backfire through the intake, even when stationary and manipulating the throttle lever by hand.
 
terrx said:
Did you check/set the minimum air? Have you checked if it's running closed loop at idle? (I'm guessing it isn't).
Also, what intake manifold are you using? Is it from the '95? Does it have EGR and did you block it off or keep it?
Click to expand...
Hmm, how do you check/set the minimum air? Is it this sequence?
"2. IAC reset/min idle speed- This sets the throttle plate on the throttle body to a specific rpm. I recommend about 500 rpm for most engines. Procedures- Go to sheet 7 of the pinout diagrams. At the top is the ALDL connector. You need to put a jumper between pins A and B of the connector. These are pins A9(wht/blk) and A12 (blk/wht) on the ECM. This puts the ECM into aldl mode. Turn the key on (not start) and the IAC should drive completely closed. You will normally hear a buzzing from the IAC. If you have the winaldl up and running on your laptop, you should see the IAC went from 145 to 0. Now unplug the IAC connector. turn the key off. Remove jumper. Start motor. If the idle is too low you may have to keep it running. On the front drivers side of the throttle body is the adjustment screw. It may be behind a silver plug. If so just use a nail or punch to poke it and remove it. Then use a #20 torx bit to adjust the idle speed to at least 100 rpm less then your warm idle speed. if 600 then set to 500 rpm. You are looking for the lowest consistant idle your motor will do. When set, turn off motor and reconnect the IAC. I try to keep the IAC count to at least 40 with the motor warm and idling.
This is a good time to verify the operation of the IAC. When the jumper is in and the key is on the IAC is driven closed. So while it is idling to set min rpm put a finger over the IAC port just above the IAC. There should be no air being sucked in and you should be able to see the pintle on the IAC.
When the IAC is connected and operating there should be a small amount of air being sucked in the port at idle and should increase as you apply some throttle."

My TBI makes a distinct sucking noise, I don't know if this matters, but it sounds totally different than my carb did.
It does go between closed and open loop while driving, I need to check at idle.
I am using the stock TBI intake from the truck the TBI unit, sensors, heads, cam, and wiring came from. It does have an EGR and it is functional (I have not blocked it off, I should check to see if it is functional).
 
Last edited:
Yes, that's the procedure. The IAC is a controlled vacuum leak the ECM uses to adjust idle. Ideally you want as much air as possible passing the injectors and mixing with fuel than bypassing it. The idea is to adjust for proper idle with the vacuum leak closed. It's probably OK but makes sense to check/verify it now.

The problem sounds to me like an issue with acceleration enrichment (AE). It's a specific mode with TBI used during acceleration much like a carb pumps extra fuel when you step on it. The ECM does this by reading MAP and TPS and based on tables it calculates the appropriate amount of fuel to deliver. It's not operating in closed loop in that mode, the mixture is table driven. If it's off and too lean the engine can backfire. I think that's why slowly increasing speed works without backfiring.

If that's the problem, then look for things that would cause the ECM to miscalculate AE. As I mentioned anything causing bad MAP or TPS readings could do it. Sounds like you replaced some of these already. That's also why I asked about EGR since that might affect mixture. Perhaps temporarily disable it - verify the valve is closed and disconnect the vacuum line to it; also plug the vacuum line to throttle body. Check the solenoid controlling it etc. EGR is probably not the problem but like I said try to rule things out. Look for what changed between the original vehicle and yours. A major difference like heads, intake, injectors, etc would need to retune AE in the calibration/EPROM. Also if you're trying to do something tricky like adding a vacuum reference fuel pressure regulator that would need to be tuned and I doubt is possible with your ECM.

Not related to AE but will mention it anyway, I'd change to a 3 wire heated O2 sensor. In my experience the stock non-heated 2 wire sensor will not stay hot enough to work below 1000 RPM or so when using rams horn manifolds.
 
terrx said:
Yes, that's the procedure. The IAC is a controlled vacuum leak the ECM uses to adjust idle. Ideally you want as much air as possible passing the injectors and mixing with fuel than bypassing it. The idea is to adjust for proper idle with the vacuum leak closed. It's probably OK but makes sense to check/verify it now.

The problem sounds to me like an issue with acceleration enrichment (AE). It's a specific mode with TBI used during acceleration much like a carb pumps extra fuel when you step on it. The ECM does this by reading MAP and TPS and based on tables it calculates the appropriate amount of fuel to deliver. It's not operating in closed loop in that mode, the mixture is table driven. If it's off and too lean the engine can backfire. I think that's why slowly increasing speed works without backfiring.

If that's the problem, then look for things that would cause the ECM to miscalculate AE. As I mentioned anything causing bad MAP or TPS readings could do it. Sounds like you replaced some of these already. That's also why I asked about EGR since that might affect mixture. Perhaps temporarily disable it - verify the valve is closed and disconnect the vacuum line to it; also plug the vacuum line to throttle body. Check the solenoid controlling it etc. EGR is probably not the problem but like I said try to rule things out. Look for what changed between the original vehicle and yours. A major difference like heads, intake, injectors, etc would need to retune AE in the calibration/EPROM. Also if you're trying to do something tricky like adding a vacuum reference fuel pressure regulator that would need to be tuned and I doubt is possible with your ECM.

Not related to AE but will mention it anyway, I'd change to a 3 wire heated O2 sensor. In my experience the stock non-heated 2 wire sensor will not stay hot enough to work below 1000 RPM or so when using rams horn manifolds.
Click to expand...
Thank you so much for explaining how the system works. That is super helpful. I will work on the EGR, I haven't checked it out yet. In terms of major differences that are on the motor, the heads, intake, injectors, and all sensors came together, my ramshorns do have a smaller outlet than the 95 motor came with, could it be that I am backing up exhaust?
I keep wanting to identify parts (perhaps this is misguided) that were consistent across the two TBI iterations because both had the same problem of backfire under load, the exhaust is one of those things. Also, so is the fuel tank, which I will be pulling soon to check the tank for issues. It is a 68 FJ40 tank and has a small outlet that might be constricting things. Thank you again for your help. I really appreciate it.
 
Update. Took the Cruiser out last weekend and it ran fine, as long as I allowed the revs to build slowly. No more overheating because now the fan is blowing towards the engine (that really might be one of my dumbest mistakes ever). I decided to stop for gas and it would not restart. This has been a problem for a while. When warm (cold starts are quick) the engine will turn over and over without starting. I have observed spray from the injectors and using a timing light spark in the plug wires. After an hour and a half I got it to start. The starter is done now, so I am going to pick up a new one. I hope that it is just a weak starter that caused it, but it seemed to turn over well. We shall see.
 
Having to build revs slowly would have me look at fuel delivery again. Tried to see what type of pump you are using or where it is mounted but I didn't see it. Are you using an inline pump or in tank ? What does the spray pattern look like ? Is it aspirating or squirting raw fuel?
 
I'm not going to take the time to read all 52 previous post to this thread because bottom line is with your 1995 K2500 TBI fuel injection system you have to use all 1995 matching parts to that system for dependable drivability. We fought drivability problems with our F,2F TBI conversions until we went "all Chevy matching parts" to the prom inside your ECU, and they can not be defective.
 
too tall said:
Having to build revs slowly would have me look at fuel delivery again. Tried to see what type of pump you are using or where it is mounted but I didn't see it. Are you using an inline pump or in tank ? What does the spray pattern look like ? Is it aspirating or squirting raw fuel?
Click to expand...
I am looking at fuel delivery now. It has a Walboro GSL395 inline fuel pump mounted next to the tank between the seats. It seems to have a good conical spray pattern from both injectors (using timing gun to see it), but I am not sure what it looks like under full throttle. It could be aspirating fuel, I can try to check that out. I keep coming back to fuel too. Thanks so much.
 
I personally have had the best luck with in tank pumps. If it is an inline pump it is important to locate the pump below the fuel supply. I've seen many different systems cavitate or fluctuate fuel pressure because the pump was mounted in the wrong location. Always easier to push rather than pull in regards to liquid unless you can guarantee an uninterrupted siphon. Most inline pumps have a hard time self priming from below.
 
too tall said:
I personally have had the best luck with in tank pumps. If it is an inline pump it is important to locate the pump below the fuel supply. I've seen many different systems cavitate or fluctuate fuel pressure because the pump was mounted in the wrong location. Always easier to push rather than pull in regards to liquid unless you can guarantee an uninterrupted siphon. Most inline pumps have a hard time self priming from below.
Click to expand...

X2! You need to mount the pump under the tub so it's below the tank. Inline pumps are designed to push fuel, not pull.

I'm also not a fan of inline pumps but it should work OK if it's in the right location. I'm on my third one in 3 years. Eventually I'm going install a sub tank or maybe an in-tank one from here:
In-Tank Fuel Pump Module - PA-Series
 
too tall said:
I personally have had the best luck with in tank pumps. If it is an inline pump it is important to locate the pump below the fuel supply. I've seen many different systems cavitate or fluctuate fuel pressure because the pump was mounted in the wrong location. Always easier to push rather than pull in regards to liquid unless you can guarantee an uninterrupted siphon. Most inline pumps have a hard time self priming from below.
Click to expand...
Electric fuel pumps are pushers, not suckers, always mount as close to tank as possible.
 
Thank you guys so much for your help. I am still working on this but my daughter just got swindled on her first car, a 1991 Suzuki Sidekick 4wd, that she bought with money she had raised babysitting. Basically, the guy lied about the smog history when in reality he had just failed a smog test in CA. He has since left for Chicago. So, I am currently pulling the head to check it out because it has less than 5 psi in cylinders #1 and #2. I am praying it is a headgasket. I will be back on the cruiser soon and let you know what I find out. Thank you so much!
 
Did you guys vent your stock FJ40 tank? I was wondering if this might be an issue with the hard to start condition when the Cruiser is warm. I have my stock FJ40 fuel cap (which I am sure does not seal well) and the tiny factory pin-prick vent hole in my fuel filler neck.
 
Update: Problem solved (fingers crossed).
Problem 1: Backfire through intake on acceleration under load.
Solution: Pulled the fuel tank drain plug (on the 1968 FJ40 it is an M12-1.50 Pitch bolt) went to NAPA and they had a oil drain plug that is the same pitch and perfect length and, best of all, has a hole already drilled part way through it. I bored out the rest of the new drain bolt and made the existing hole in it a little wider, I also added two cross holes in the shaft of the drain bolt because the drain threads in the Cruiser tank have two holes in them to drain the gas in the bottom of the tank (I don't know if this was necessary, but if you do this, make sure you properly clock your bolt with the gaskets installed so the holes end up in the right place). Then I took an 90* elbow section of hard fuel line from a TBI chevy and welded it inside a nut of the same diameter as the drain plug bolt (17mm). Then welded the two together. Plan two for this was to weld an AN-6 bung onto the drain plug, but I got impatient. The result is ugly, and I had to pull the tank to put it in (because the frame got in the way of spinning the plug in) but it is functional and stout. I still need to armor the lines with some frame sliders, but no more backfire (at least as of the last 2 test drives).
1395392-15a9861daa473b41c398398566f36c7d.jpg

Problem 2: Overheating.
Solution: Don't be an idiot like me, use a fan that matches the rotation of your belt. When I switched to the 95 K2500 350 one of the reasons was because I wanted a serpentine belt. I didn't realize that it spins the opposite way from my old v-belt. You might realize this is your problem when hot air is being blown at you when you set your timing standing in front of the engine.

Problem 3: Hard start when engine warm.
Solution: This one was pretty confusing. After driving, if I stopped the cruiser (gas station etc) when I tried to start again it would crank and crank, but not start. Needless to say this was totally lame. I would run my battery dead trying to start. In the end, I swapped out my starter motor, which didn't make sense to me, but it is starting fine now. If you are wondering, I don't have headers and my rams horn exhaust on the starter side doesn't go near the starter. I don't get it, but it worked.

Thank you all for your help. I really appreciate it as you got me through a pretty frustrating time where I actually considered just selling the Cruiser (gasp). Thanks for walking me back from the edge.
 
You must log in or register to reply here.

Share this page

Back
Top Bottom