TBI woes

[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.

[66gmcguy]

Originally Posted by 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.

I plugged EGR ports on my way back from work. It didn't seem to make much of a difference. Maybe if anything, it ran a little worse when cold, but after it was hot, there was no noticeable change.

The SPID sheet on the donor truck says "NA4" which means "EMISSION SYSTEM,ABOVE 8500 LB GVW." So you are correct. I ordered the wrong EGR valve. I will get the correct one, but I'm not entirely sure that's causing any of the problems I'm having. I took a vacuum gauge and checked vacuum at the port on the throttle body (no vacuum until you crack the throttle blades open), and I also checked the vacuum at the port going from the solenoid to the EGR valve and had no vacuum, even after cracking the throttle blades open. This was of course done with the vehicle parked.

As far as the ICM goes, I installed an ACdelco D1984A. The only other one that was listed as a possible fit was D1943A, and there was virtually no info as to what the difference between the two was.

I just realized I failed to mention that I deleted the smog pump from this thing since I'm not required to smog test my truck. I didn't think that would cause any problems but I thought it might be worth mentioning in case it does. No noticeable change in the way the truck ran with it vs without it though.

[ChevyTech]

California Emissions required smog pumps where most federal emissions TBI trucks had no smog (A.I.R.) pump. The only TBI trucks I saw with pumps were very early TBI trucks. I don't think this is an issue.

As for the wrong EGR valve I am sure it would have eventually cause a code 32.

If the problem was there with the engine still in the donor truck I think there is a problem you have not found yet. Did you replaced the intake gaskets or head gasket on the donor engine? Check to see how much vacuum is getting to the MAP sensor at idle.

As for the Ignition Control Module, the D1943A is the one most people would suggest using. My bet is the D1984A has an even more aggressive timing curve. I think both of these modules have more aggressive timing then the one that came with that engine from the factory. My guess is the AC Delco D1906C module is what was used origonally in that donor engine. For performance that is terrible, but for safely of the engine it is safer.

Here is module info:

https://www.gmt400.com/threads/which...-d1984a.55395/

Every thing below is from my saved notes:
Check your ignition module and if it still has an AC Delco, see if it has 048 or 369 marked into it. The 048 modules have a lot of built-in retard. Above 3,800 rpm they will retard the timing 4* and rise to 6* retard by 5,000 rpm. The 369 is a much better module in the fact that it advances the timing 2* over 3,200 rpm and maintains that 2*. There is an 8* difference in advance at the crankshaft between the 048 and 369 modules at 5,000 rpm. That makes an 048 equipped vehicle fall over at a low rpm and the 369 equipped vehicle pull strong up to 5K and beyond.

1983G20Van from Chevytalk

The D1934A is an OE 369 module, the AC Delco D1906C is a Professional 048 module
https://www.gmt400.com/threads/acdel...s.39367/page-2

For those of you noticing a fall off of power after 3,500 rpm on even a stock TBI engine in good shape, pop off the distributer cap and look at the ignition control module. If its a stock GM module, look at the numbers inscribed in it. You will either have 048 or 369. The 048 is a TERRIBLE module to have as it will pull a ton of timing over 3,500 rpm. The 369 will actually add a 2* of timing advance over 4,000. The 048 was like a built in rev-limiter, the 369 was used in performance applications and applications that needed upper rpm performance. TPI Camaros/Firebirds and TBI Fullsize Vans and 9C1 cop cars came with the 369 module. TBI Trucks and Civilian B-cars as well as TBI F-cars came with the 048 modules.

https://www.gmt400.com/threads/acdel...choices.39367/

You might look up the fsc forums. The 369 and the D.u.i. icms are discussed. I think the later tbi trucks had the 369 one stock.

[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?

[66gmcguy]

Originally Posted by ChevyTech

If the problem was there with the engine still in the donor truck I think there is a problem you have not found yet. Did you replaced the intake gaskets or head gasket on the donor engine? Check to see how much vacuum is getting to the MAP sensor at idle.

None of these problems were there before I rebuilt the throttle body. I only rebuilt it to fix the sticky throttle, and this was when the engine was still in the donor truck. After I rebuilt it is when the problems started. I just assumed it was a low fuel pressure issue, and likely just the fuel pump, so I decided not to change it since I was going to pull the engine out soon anyways. Before I rebuilt the throttle body, the throttle shafts had maybe an 1/8" of slop in them.

With the engine out and ready to go into my truck, I changed intake gaskets, not head gaskets though. If I can't make any more progress, I will get it smoke tested when work settles down for me, which is hopefully within the next two months.

Originally Posted by ChevyTech

As for the Ignition Control Module, the D1943A is the one most people would suggest using. My bet is the D1984A has an even more aggressive timing curve. I think both of these modules have more aggressive timing then the one that came with that engine from the factory. My guess is the AC Delco D1906C module is what was used origonally in that donor engine. For performance that is terrible, but for safely of the engine it is safer.

I just checked my ICM and other than a GM logo on the top and the ACdelco part number on the bottom, there was nothing on it. I stupidly threw out the old distributor which had the old module on it, which I could have used to check the number.

I got to thinking, what are the chances a bad alternator can cause issues? Last night, I decided to disconnect the alternator belt and drive around. I still had RPM fluctuations at idle, but it kind of seems like those misfires when accelerating were gone. I put the belt back on and they came back. I'm about to do the same thing again and see if I get the same results.

[ChevyTech]

The correct running voltage makes a difference. The ECM has a correction mode for low voltage and it will try to speed up the engine and increase the fuel delivered.

Did you test the fuel pressure with a fitting so you can watch the pressure with the engine running? This is a must.

[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.

[66gmcguy]

Originally Posted by ChevyTech

The correct running voltage makes a difference. The ECM has a correction mode for low voltage and it will try to speed up the engine and increase the fuel delivered.

Did you test the fuel pressure with a fitting so you can watch the pressure with the engine running? This is a must.

Yes, I tested it when it was running and got 11.5 psi. I didn’t run it for very long but the fuel pressure seemed to be creeping up the longer it ran.

Edit: I tested it right at the throttle body.

[ChevyTech]

When you start the truck does it start without you touching the gas pedal?

[66gmcguy]

Originally Posted by ChevyTech

When you start the truck does it start without you touching the gas pedal?

Yes, it starts easily, and actually seems to run really well for the first 30 seconds or so.

Also, I just measured 20 inches of manifold vacuum at the port for the MAP sensor.

[66gmcguy]

Here is a datalog I took with the RPM fluctuations at idle. About 5-7 seconds in, I gave it a little bit of throttle input to show that it still fluctuates, just in a higher RPM range.

[ChevyTech]

That is a small data sample with an engine that is not at operating temperature, but it shows it is to lean.

Did you reused the old injector screens?

I know you tried putting the old injectors back in.

For some reason it is starving for fuel.

Could something have happened when you were in the TBI that could be blocking fuel flow. Did you used any sealant on anything?

[66gmcguy]

Originally Posted by ChevyTech

That is a small data sample with an engine that is not at operating temperature, but it shows it is to lean.

Did you reused the old injector screens?

I know you tried putting the old injectors back in.

For some reason it is starving for fuel.

Could something have happened when you were in the TBI that could be blocking fuel flow. Did you used any sealant on anything?

I will take a longer datalog tomorrow at operating temperature. I wanted to capture the fluctuating idle which seems to only happen before the engine is fully warmed up.

I did not reuse screens or use sealant. If there were a blockage of some sort, is it safe to say it’s somewhere between the inlet on the throttle body and the injectors? Because that’s where I took a fuel pressure reading.

[66gmcguy]

Well, yesterday I pulled the computer and PROM out of my 1989 R3500 and put it in my truck. I drove around a while and it seemed to make a difference. Drove it to work this morning and had no surging while accelerating or fluctuating RPM. I did let the truck warm up for about 2 minutes before driving, but I didn't have any cold drivability problems like I've been having. I would like to drive it on a completely cold engine to see if it did in fact fix that particular problem (I sort of doubt it, because I think the 3 wire O2 sensor is the problem.)

I'm not counting my chickens yet, as I would like to do some more driving before I determine that the old PROM and/or the computer were the problem, but I'm feeling more optimistic finally. It ran almost perfect last night and this morning.

My old computer is a 1227747 unit, and the PROM BCC is ACTC. This is for a 1987 R20 with a L05 and manual transmission, port EGR, and a smog pump.

The new computer is a 1228747 unit, and the PROM BCC is AKWA. This is for a 1989 R3500 with a L05 and a 3 speed auto transmission, port EGR and no smog pump.

Unless anyone tells me not to, I'm going to keep running the different computer for a bit. My only concern is if the fuel map or spark tables or something are different and it causes detonation or some other problem. I live on a mountain and have to drive up it on my way home so I put load on the engine when I do.

[Boog]

Sure helps to have another running vehicle to borrow from to test against your original parts. Curiosity has me wanting to know if the prom is failing or the pcm. On these older rigs I couldn't trust a junk yard pcm and I have no faith in remand units either. Are there new ones still available by chance?

[66gmcguy]

Originally Posted by Boog

Sure helps to have another running vehicle to borrow from to test against your original parts. Curiosity has me wanting to know if the prom is failing or the pcm. On these older rigs I couldn't trust a junk yard pcm and I have no faith in remand units either. Are there new ones still available by chance?

I suspect the PROM, because a few weeks ago, I did switch computers (but kept my old PROM) with another 1227747 unit from another truck and had exactly the same problems as before. I don't think there are new ones. I believe they are all reman. The computer I just put in yesterday is a reman unit. I don't know how old it is, but it's a Echlin unit from Napa Auto Parts and so far, it seems to be okay.

[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.

[66gmcguy]

Figured I'd update this incase anyone in the future has similar problems and reads this thread.

After switching the computer and the PROM, this thing runs great now. No cold drivability issues, and no more acceleration problems. The idle isn't 100% perfect, but at this point, it doesn't really bother me. Thank you to everyone for their suggestions, and thank you Chevytech for finding the right EGR valve for me.

[ChevyTech]

You're welcome!

Thank you for the update.