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Old 09-25-2018, 12:41 PM   #1
5Tractorguy
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1988 TBI to 1991 TBI Conversion Q's

Swapping a '91 Cheyenne 2500 throttle body over to my '88 V30 for a last ditch effort to resolve a power issue. If you want to read more about that, see this thread here--- http://67-72chevytrucks.com/vboard/s...d.php?t=743034

Anyways, had a couple questions before I proceed.

The TPS on my '88 is a bit different than the '91. If I change the pig-tail will it still work?

I'll be installing the computer from the '91 too, I assume that will work okay with the harness and components on the '88?

'Bout all I can think of for now. I have to pull the injectors, fuel pressure reg and run everything through the glass bead blaster so it can be rebuilt.

Another quick question since I'm thinking of it, what are the symptoms of a bad distributor pickup coil? I think mine is original so I was considering replacement or a new distributor.

Any help, direction or thoughts are greatly appreciated!

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Old 09-25-2018, 04:34 PM   #2
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Re: 1988 TBI to 1991 TBI Conversion Q's

Have you done any data logging at all?

Here are a couple great videos that can help with diagnostics. While these are based on later systems, the basics still apply with the STFT and LTFT and using them to determine where your issues are.

https://youtu.be/5WnM_NsOtd8

https://youtu.be/cARQ0jZZ4Qc

What do you hope to accomplish with the TB swap? You are throwing unknown quality used parts into a system that already has issues.
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Old 09-25-2018, 05:05 PM   #3
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Re: 1988 TBI to 1991 TBI Conversion Q's

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Originally Posted by DieselSJ View Post
Have you done any data logging at all?

Here are a couple great videos that can help with diagnostics. While these are based on later systems, the basics still apply with the STFT and LTFT and using them to determine where your issues are.

https://youtu.be/5WnM_NsOtd8

https://youtu.be/cARQ0jZZ4Qc

What do you hope to accomplish with the TB swap? You are throwing unknown quality used parts into a system that already has issues.


I haven't. For some reason the Code Reader doesn't work on this truck. Last time I plugged it in it wouldn't read the ECM. If it did, I'm sure I'd have a better idea of what's going on. Check engine light also doesn't come on.

Thanks, pretty informative videos. I still have to get to the 2nd one. Looks like the possibility of a vacuum leak can be causing this too.

Looking to get it to run "normal". Of course they have been sitting a while but the TBI came out of a known running truck that ran strong but the frame/body rotted out.

What would you suggest? I'm assuming you saw my other thread so you know what has been replaced on this engine/system so far....
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Old 09-25-2018, 05:28 PM   #4
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Re: 1988 TBI to 1991 TBI Conversion Q's

Comparing the other truck to yours - how different were they? If the engine came from a 2wd standard cab 1/2 ton with 4.11 gears it is going to feel way different in your truck.

Engine specs? Mileage? Transmission? Gear ratio? There are a ton of variables on why an engine would feel "better" in one chassis vs another.

I'm chasing issues on my TBI 454 right now where I'm way down on power. I think I have traced it to weak fuel pumps based on my fuel trims and the fact that the fuel trims are different for each tank. I did the same thing with chasing parts until I got a cable and software to do some logging and saw that the fuel trims were going crazy at anything over about half throttle. Fuel pumps were "new" as of 3 years ago, but they were crap Spectra quality (installed by previous owner). So installing 1 new Delco pump and will do some logging and see where it goes.

Personally I would not throw any other parts, especially used parts, at it until you get it logging.

Get a cable from here - http://aldlcable.com/

Comes with a CD that has a couple dozen different scanning/logging programs.
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Old 09-25-2018, 05:59 PM   #5
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Re: 1988 TBI to 1991 TBI Conversion Q's

That truck was a 3/4 ton, 4WD, long bed with a 350ci and 700R4 transmission. Got around 9-10mpg. As for the rear end gears I don't know.

Thanks for the site! Remember though, even the code reader didn't show anything when I plugged it in... zip... so I'm not sure if that cable will.

As a side note, the '88 was built with about 12 other 73-87's, has a wiring harness from one, computer from another, (3rd ECM I've had in it... seemed to run the best with this one). Harness for a system with the air pump, we deleted that not long after restoration. I'm sure the mismatched components have something to do with it too.
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Old 09-25-2018, 07:01 PM   #6
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Re: 1988 TBI to 1991 TBI Conversion Q's

This will sound crazy. What year is the block. I know you say 88 engine. GM had an issue with moving timing marks around on engines. Sometimes when doing a TBI swap the timing cover from the TBI engine gets put on a block that has timing set different. It may not be the issue. But always worth a look, takes 2 minutes. Picture does not cover every timing cover, but you get the idea.
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Old 09-25-2018, 08:09 PM   #7
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Re: 1988 TBI to 1991 TBI Conversion Q's

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This will sound crazy. What year is the block. I know you say 88 engine. GM had an issue with moving timing marks around on engines. Sometimes when doing a TBI swap the timing cover from the TBI engine gets put on a block that has timing set different. It may not be the issue. But always worth a look, takes 2 minutes. Picture does not cover every timing cover, but you get the idea.
That I will have to check! Timing has always been in the back of my mind on this...

Original engine had a crack so another 350 block got rebuilt. It was buy a very reputable engine guy from the area, but maybe the cover was swapped... mistakes happen. If I remember I'll check the truck Thursday when I'm out, have it parked across town.
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Old 10-01-2018, 10:29 PM   #8
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Re: 1988 TBI to 1991 TBI Conversion Q's

Having just done a tbi swap, I doubt the body itself is the problem. the computer should work fine. chk fuel pressure and timing.
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Old 10-02-2018, 08:15 PM   #9
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Re: 1988 TBI to 1991 TBI Conversion Q's

Will do! After mikebte mentioned the marks, that is something that has to be looked at again. When this first hit the road, timing was an issue for a while. I've got it around 2 or so vs. the stock 0.
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Old 10-04-2018, 11:48 AM   #10
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Re: 1988 TBI to 1991 TBI Conversion Q's

One good test is to disconnect the timing bypass wire (tan connector right under the brake vacuum booster). Set the crank timing mark to 0, stab the distributor to TDC (generally that's the rotor pointing to 5 PM on a clock, towards cylinder #1). It should run OK, and then set timing with a timing light.

IF This doesn't work, then check the timing marks, harmonic balacer and sheer pin.

THEN try again.

IF That doesn't work, then focus on the next round of testing.

I had a problem with my setup where both the Harmonic balancer had shifted AND the computer timing tables were corrupted. Doing what I said above would have saved a lot of time.
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Old 10-14-2018, 07:31 PM   #11
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Re: 1988 TBI to 1991 TBI Conversion Q's

Alrighty, finally got around to working on the 1-Ton today.

I got the TBI glass-bead blasted and rebuilt from the '91. Went well and cleaned up nice. Also replaced the pickup coil in the distributor since I think it still had the original in it.

Reassembled everything, fired it up and it ran great! Drive it around the block quick, more power than it's had in a long time. Engine sounded smooth and strong.

Now, here's where my question comes... I checked the timing after driving (did unplug the advance wire) and realized it was at 12 BTDC on the indicator, but it wasn't knocking from what I could hear. Brought it back to 2, ran real rough/choppy like it did before. Up to 4, a bit better, 8 more so, then to 10 and I left it. Runs smooth and there is no knock going uphill or when I mat it. I'm inclined to go further but don't know if I should.

Below are pictures of the timing indicator on the timing chain cover. Starting to wonder if that's been the problem all along and the engine doesn't have the right indicator (been rebuilt)... Referencing the picture that mikebte posted, the indicator is more in the 1-2 'oclock position vs. 12 noon.


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Old 10-14-2018, 08:38 PM   #12
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Re: 1988 TBI to 1991 TBI Conversion Q's

Check to make sure the balancer hasnt slipped. Over time the rubber gets old an allows the outer shell to move, messing up the location of the timing marks.
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Old 10-14-2018, 09:42 PM   #13
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Re: 1988 TBI to 1991 TBI Conversion Q's

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Check to make sure the balancer hasnt slipped. Over time the rubber gets old an allows the outer shell to move, messing up the location of the timing marks.
That I did not know! I'll have a look tomorrow...
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Old 10-15-2018, 10:50 AM   #14
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Re: 1988 TBI to 1991 TBI Conversion Q's

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That I did not know! I'll have a look tomorrow...
These engines like a healthy amount of advance...I think you're fine at 12', mine, computer controlled, is close to 20'. Runs like a top.

Glad you've got it working.
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Old 10-15-2018, 12:12 PM   #15
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Re: 1988 TBI to 1991 TBI Conversion Q's

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These engines like a healthy amount of advance...I think you're fine at 12', mine, computer controlled, is close to 20'. Runs like a top.

Glad you've got it working.
Excellent, thanks Wwes... that's what I was looking for. Very excited to get this on the road again, once the trans gets rebuilt it'll be like a whole different truck.
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Old 10-15-2018, 02:51 PM   #16
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Re: 1988 TBI to 1991 TBI Conversion Q's

You can keep advancing the timing until it runs bad, bring it back and see how far out of the timing is. If your able to run more than 45 degrees advance, your distributor could be moved 1 tooth and check timing again.
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