TBI woes

[66gmcguy]

Hello folks. I normally post over in the 73-87 section, but since I have a TBI setup in my 1982 GMC, I figured I'd get more help over here, since 1987 was the first year for TBI trucks.

A few months ago I bought a 1987 Chevy R20 with a TBI 350 and manual transmission to use as a parts truck for my 1982 GMC. It seemed to run okay but the throttle was extremely sticky due to the flappers in the throttle body sticking. I tore down the TBI, bushed the throttle shafts, replaced the injectors and IAC valve (both with Standard Motor Products), and did a thorough cleaning, making sure to clean all vacuum ports and the IAC passage. I replaced every gasket, seal, and the FPR diaphragm. After doing so, I've never really been able to get this thing to run right since then. Here are a few problems I can't get ironed out. I have WinALDL and no codes are present.

At idle, the RPM fluctuates up and down. If I depress the throttle a little bit, the RPMs still fluctuate, just in a higher RPM range than in idle. I know this is either due to low fuel pressure, a vacuum leak, an IAC problem, or TPS problem. Thing is, I have a new fuel pump + hose, all new fuel lines, fuel filter, all new intake gaskets, and new vacuum lines. I've checked fuel pressure (11.5 PSI), and checked for vacuum leaks by spraying starter fluid around every possible source for a leak and found nothing. I've tried plugging every vacuum port with no luck. I have the timing set at 0 degrees (with the bypass unplugged). I must have done the IAC reset procedure 50 times, I've put the old injectors and IAC back on, replaced the TPS and set it to .5 volts at idle, all with no change. The thing is, if I turn the idle screw up so it revs around 900 at idle, it seems to stabilize (only when in closed loop), but the IAC counts are zero. Another thing I can do that seems to help is unplug the CTS which richens the A/F mixture. I can't tell if it's the richer mixture or the slight raise in RPM that's fixing the problem though. When the engine is cold it idles fine until the coolant temp reaches maybe 100ish degrees, then I get the fluctuating idle (until it goes into closed loop mode.)

Another thing is when accelerating, I have a very distinct, but momentary misfire at a certain RPM. It only happens under light to moderate acceleration though, right at around 2200-2300 RPM, and it's only for a split second. The best way to describe it is it feels like the engine shuts off for a split second then turns back on almost instantly. It doesn't feel like a typical lean-sputter, or backfire. I thought maybe this was a TPS problem, but it does it on all three TPS sensors that I've tried, all at the same RPM under same conditions.

The last thing. When the engine is cold, it drives like crap and has no power it feels like, and every now and then I get a CEL but with no codes. If I shut the engine off and shut it back on right away, the CEL turns off, and it seems to be fine after that. None of this happens after it's warmed up beyond maybe 120-150 degrees. As I mentioned above, it idles fine when cold.

Since having these problems, I've replaced the TPS, MAP, CTS, knock sensor, EGR valve, PCV valve, O2 sensor (3 wire unit), wire harness (Painful Performance Swap harness), distributor (and all internal components), spark plugs, wires, every single intake/exhaust gasket, etc. Pretty much everything short of the EGR solenoid, the ESC, and the PROM. All components used apart from the IAC and injectors are all ACdelco. I've even switched out the computer from another truck that I have with no luck.

I know this is long winded, but I am genuinely stumped, and would love some input. As mentioned, I have WinALDL and can datalog and post it on here if needed. I'm also open to the possibility that I did something wrong by either not doing a test/reset procedure properly or didn't install something correctly, etc, so if you think I should recheck something that I've already messed with, just let me know.

[special-K]

I wish I could help but you already know more about the TBI than I do. I've never had to service that on any vehicle I've had with it. I'm sure some insight will be coming along

[Second Series]

Originally Posted by special-K

I wish I could help but you already know more about the TBI than I do.

Same here...
How about the temp sender? My temp sender worked, but the range was off. Also did you try to induce a vacuum leak by pulling a line to see how it affected things? This sounds like a TBI swap, how is the wiring, maybe a loose ground? Just grasping at straws here...

[66gmcguy]

Originally Posted by Second Series

Same here...
How about the temp sender? My temp sender worked, but the range was off. Also did you try to induce a vacuum leak by pulling a line to see how it affected things? This sounds like a TBI swap, how is the wiring, maybe a loose ground? Just grasping at straws here...

All grounds are good. All ground connections are bare metal and tested with a multi meter.

As far as the rest of the wiring is concerned, I don't believe that to be the issue, since all of the same problems occurred when the engine was still in the donor vehicle with the old harness. I have the same problems even with my new harness.

I will check the CTS tonight and report back.

[ChevyTech]

Make sure the PROM (chip) in the ECM matches your engine size.

[66gmcguy]

Originally Posted by ChevyTech

Make sure the PROM (chip) in the ECM matches your engine size.

It does. The code is ACTC

[Just call me Sean]

If the light comes on there should be a code. Maybe the computer isn't getting constant power for the memory.

[66gmcguy]

Originally Posted by Just call me Sean

If the light comes on there should be a code. Maybe the computer isn't getting constant power for the memory.

I'm not sure if that's the case. I only think this because the computer retains fault codes even after shutting the truck off for a while and the codes only go away after disconnecting the battery for a minute or so.

[Just call me Sean]

Originally Posted by 66gmcguy

I'm not sure if that's the case. I only think this because the computer retains fault codes even after shutting the truck off for a while and the codes only go away after disconnecting the battery for a minute or so.

The computer should retain the codes but it won't if you don't have constant power to it. So it gets power to run but shutting it off removes the only power it gets, pretty much like disconnecting the battery. You said the light comes on but no codes stored.

[66gmcguy]

Originally Posted by Just call me Sean

The computer should retain the codes but it won't if you don't have constant power to it. So it gets power to run but shutting it off removes the only power it gets, pretty much like disconnecting the battery. You said the light comes on but no codes stored.

My apologies, I was wrong in my original post. The CEL came on because of a lean code (code 44.) Long story why I originally thought that no codes were present when the CEL came on.

I just drove it around again after disconnecting the battery to wipe the memory and clear any codes, and sure enough, the CEL came on again for code 44. If I shut the truck off and shut it back on right away, the CEL turns off. Like I say, this situation only happens if I drive the truck before it fully warms up. If I let it warm up before driving it, this doesn't happen.

So to circle back, there IS constant power to the computer even with the key off, as it does store codes. so that doesn't seem to be the problem.

I should emphasize that I don't get that code at all if I let the truck warm up before driving it, AND after I've driven it around under various types of loads (I have city/mountain/highway driving everyday during my commute.)

So the lean condition probably explains the poor drivability with a cold engine, but I don't think it explains the other two problems I'm having, because even when the idle fluctuates around, I don't get any codes.

[ChevyTech]

It set a lean code more then once and a lean condition fits some symptoms.

Is the three wire oxygen sensor a Bosch unit? If so I suspect it is the problem.

I would try unplugging the oxygen sensor and seeing how it runs as it warms up.

If the system is seeing oxygen sensor readings that show the sensor is warm enough to use, it will be reaching closed loop operation when the engine is not as warm as it would be if it had a one wire oxygen sensor.

Let us know how it runs with the oxygen sensor unplugged.

[Warrens69GMC]

Post here, they will help you figure it out

https://www.thirdgen.org/forums/tbi/

[Just call me Sean]

My oxygen sensor would go into negative voltage, making the computer think it was running lean so it would dump the maximum amount of fuel it could into the engine. Within what voltages does it operate?

[Mr_Rich]

You got all complicated at first and I didn't know how to reply to your post. Now that there are comments that are weaned out I'll tell you what I think it is since it sounds like the same symptoms I experienced when I swapped out my original 305 engine in my truck for a 350. It will run but it will definitely give you lean codes and stumble. You need the correct PROM/ Programmed PROM for the engine you are running. It sounds like you've done everything but this. That's one of the things you have to consider in this day and age of computer controlled vehicles.

[66gmcguy]

Originally Posted by Mr_Rich

You got all complicated at first and I didn't know how to reply to your post. Now that there are comments that are weaned out I'll tell you what I think it is since it sounds like the same symptoms I experienced when I swapped out my original 305 engine in my truck for a 350. It will run but it will definitely give you lean codes and stumble. You need the correct PROM/ Programmed PROM for the engine you are running. It sounds like you've done everything but this. That's one of the things you have to consider in this day and age of computer controlled vehicles.

As stated above, the PROM and the engine match.

It's worth it for me to note that I didn't just cobble the system together from junkyard parts from various different trucks. Everything (including the engine) came off the same truck. The donor truck was an 87 R20 (2wd 3/4 ton) with a manual transmission. The truck it's in now is an 82 K20 (4wd 3/4 ton) with a manual transmission, so that means that the PROM is a close match for my truck.

[66gmcguy]

Originally Posted by Just call me Sean

My oxygen sensor would go into negative voltage, making the computer think it was running lean so it would dump the maximum amount of fuel it could into the engine. Within what voltages does it operate?

Just glancing down a couple times at WinALDL while driving to my shop this morning, I saw anywhere from .004 volts (when coasting) and as high as .894 when under load. I will try and get a data log today that will hopefully give me some better data.

[Accelo]

A good friend of mine had issues with his TBI. Turned out their are two systems and they require different fuel pressures and as a result different pumps. The auto parts store sold him an incorrect pump. He changed the pump and everything went back to normal.

Would be interesting to see the results of a smoke machine test.
Cheers.

[66gmcguy]

Originally Posted by Accelo

A good friend of mine had issues with his TBI. Turned out their are two systems and they require different fuel pressures and as a result different pumps. The auto parts store sold him an incorrect pump. He changed the pump and everything went back to normal.

Would be interesting to see the results of a smoke machine test.
Cheers.

Interesting. How did he find out what system he had, and which pump he needs?

And yes, I concur. I would have done that already but since installing this system in my truck, I haven't had time to bring it to a shop to have them test it. If I can't find the problem by the time work slows down a little bit for me, I will get a smoke test.

[ChevyTech]

Originally Posted by Accelo
A good friend of mine had issues with his TBI. Turned out their are two systems and they require different fuel pressures and as a result different pumps. The auto parts store sold him an incorrect pump. He changed the pump and everything went back to normal...

The 94 & 95 big block (7.4) had a 26-32 PSI TBI system.

NOTE:
Earlier I suggested to try running with the vacuum hose plugged to the EGR because there are different types of EGR systems depending on the gross vehicle weight and there could be a mis-match of parts from two different types of EGR systems causing problems. I have run into this problem in the past.

[66gmcguy]

Originally Posted by ChevyTech

The 94 & 95 big block (7.4) had a 26-32 PSI TBI system.

NOTE:
Earlier I suggested to try running with the vacuum hose plugged to the EGR because there are different types of EGR systems depending on the gross vehicle weight and there could be a mis-match of parts from two different types of EGR systems causing problems. I have run into this problem in the past.

I was going to try that this morning but I realized I left all my vacuum plugs at my shop. I'm going to try it on my way home this evening.

The EGR valve I bought is ACdelco 214-5073. I picked it out based on the listed GVWR, and because it said this particular one is for trucks with manual transmissions. But of course, it's still possible I picked out the wrong one. Also, this is the EGR solenoid I have. I guess these aren't as common as the other type that has two wires coming out of the back. This one also connects to ported vacuum, whereas the other one connects to full manifold vacuum.

[ChevyTech]

That photo is of a "Port" EGR Electronic Vacuum Regulator Valve.

An AC-Delco 214-5073 is a negative back-pressure EGR valve.

These parts are NOT compatible.

That is the wrong EGR valve for a very different system.

A negative back pressure solenoid is opened when EGR operation may be needed and passages inside the EGR valve modulate the opening.

On a port EGR system the Electronic Vacuum Regulator Valve pulses like a fuel injector to modulate the opening on the EGR valve.

Port EGR systems are used above 8500 GVW.

My notes show AC - Delco 214-5364 as possible EGR valve that might fit your truck, GM number 17087185.

There should not be any EGR operation at idle, so we found something wrong, but I think there are other problems.

I believe all the trucks above 8500 GVW used a distributor module with a less aggressive advance curve.

[Just call me Sean]

From what I've heard removing the smog pump will throw off the oxygen sensor readings. Maybe you need a PROM from a truck without a smog pump.

[66gmcguy]

Originally Posted by Just call me Sean

From what I've heard removing the smog pump will throw off the oxygen sensor readings. Maybe you need a PROM from a truck without a smog pump.

I will look into that. I have a 1989 R3500 with the same engine as my truck, same EGR system but with no smog pump (federal truck.) I think I will swap computers/PROMs and see. The only thing is, the R3500 has an automatic (TH400), whereas my truck has a manual transmission. I'm assuming the fuel maps are different. This won't hurt my engine, will it?

[Just call me Sean]

Originally Posted by 66gmcguy

I will look into that. I have a 1989 R3500 with the same engine as my truck, same EGR system but with no smog pump (federal truck.) I think I will swap computers/PROMs and see. The only thing is, the R3500 has an automatic (TH400), whereas my truck has a manual transmission. I'm assuming the fuel maps are different. This won't hurt my engine, will it?

I don't think it would matter much since the computer can't see either transmission.

[Just call me Sean]

Pretty sure ChevyTech has said before that the computer will learn what it needs to run right, the programming just starts it closer from no memory.