Tuning help needed - '91 TBI 383 w/ Vortec heads

**1project2many** · 02-22-2019

If Decel surge continues, try reducing spark advance by 2-3 degrees in the cells affected by decel.

**Daveo91** · 02-22-2019

Originally Posted by 1project2many

If Decel surge continues, try reducing spark advance by 2-3 degrees in the cells affected by decel.

Thanks for this. I guess I was wrong - the decel surge is still there although it seems to be a little softer. Now I'm wondering if there is an IAC problem that could be related. Most of the time it will idle at or near target idle speed (650 when warm), but it will occasionally idle higher. I drove it 45 miles at highway speed a few days ago, at the end it was idling over 1000. Shut it off and restarted a minute or two later and was back to normal. IAC sticking? I have a spare IAC that I'm going to swap in to see if anything changes.

If IAC does not solve problem, I will try the timing reduction in the affected cells - thanks!

**donf** · 02-25-2019

Originally Posted by Daveo91

Thanks for this. I guess I was wrong - the decel surge is still there although it seems to be a little softer. Now I'm wondering if there is an IAC problem that could be related. Most of the time it will idle at or near target idle speed (650 when warm), but it will occasionally idle higher. I drove it 45 miles at highway speed a few days ago, at the end it was idling over 1000. Shut it off and restarted a minute or two later and was back to normal. IAC sticking? I have a spare IAC that I'm going to swap in to see if anything changes.

If IAC does not solve problem, I will try the timing reduction in the affected cells - thanks!

I am not sure if it helps but there is idle overspeed spark retard and idle underspeed spark advance tables at least with the 95 tbi computer. My cam is very mild but even with it, the timing was bouncing all over the place like a yo yo with the factory settings and the idle had a little lope/instability. I neutered the tables for advance or retard up until a 50 rpm difference then after that decreased the adjustment range slightly. The idle even with the almost stock ramjet cam smoothed out and the timing is very stable at idle now. Even the definitions list the problem " Idle overspeed/ underspeed corrections .... This is why timing is constantly moving around at idle." Not sure if it will help your problem or not but since your IAC adjustment seem to help. I figured I would mention it. I did not remove all the correction, just toned it down a bit.

**Kitch** · 02-25-2019

I've been wondering for ages what was causing my spark timing to bounce around at idle so much!
It would be much appreciated if you could give me an example of how your tables vary from my stock ones (shown in the attachment).
I'm keen to try this out :-)

**donf** · 02-26-2019

Idle Overspeed Spark Retard vs. RPM Error 0.0 0.0 0.0 0.0 1.1 2.1 3.2
Idle Underspeed Spark Advance vs. RPM Error 0.0 0.0 0.0 0.0 2.1 2.1 3.2
It's not my tip, someone that is very familiar with the later LS computers told me to check that when I was wondering why the timing was so unstable at idle. My $0D computer has it, not sure about the earlier ones.

**Daveo91** · 02-26-2019

Originally Posted by donf

I am not sure if it helps but there is idle overspeed spark retard and idle underspeed spark advance tables at least with the 95 tbi computer. My cam is very mild but even with it, the timing was bouncing all over the place like a yo yo with the factory settings and the idle had a little lope/instability. I neutered the tables for advance or retard up until a 50 rpm difference then after that decreased the adjustment range slightly. The idle even with the almost stock ramjet cam smoothed out and the timing is very stable at idle now. Even the definitions list the problem " Idle overspeed/ underspeed corrections .... This is why timing is constantly moving around at idle." Not sure if it will help your problem or not but since your IAC adjustment seem to help. I figured I would mention it. I did not remove all the correction, just toned it down a bit.

Thanks for the tip, but, yeah, I don't think my older ECM has those tables. Searched for overspeed and underspeed with the parameter finder and nothing. Searching for "decel" gives me two: Decel Enlean Multiplier Map Diff (Scalar) and Decel Enleanment Coolant Factor vs Coolant temp. I've been looking at what DFCO is set at - looks like it's a pretty narrow RPM range that is a little outside where I'm seeing the surge, so not sure that would be the problem?

I just found two more exhaust leaks upstream of the O2 sensor tonight. I had a custom y-pipe made but have found three pinhole leaks in two different welds so far. A little bummed. Going to get those fixed before I do anything more with tuning or chasing out the surge. If surge still there after exhaust tightened up, I'll swap IACs and see if that changes anything. Then I'll dial back timing just a little in cells affected by decel. Thanks for the help!

**donf** · 02-26-2019

I am not very familiar with the older gm computers. I don't know if you saw this in my thread but this has a lot of info on tuning the pre 94 tbi computers. The later ones are actually just a blurb in the back pages. http://www.gmcmi.com/wp-content/uplo...EFI-System.pdf

**PlayingWithTBI** · 02-27-2019

I am not very familiar with the older gm computers. I don't know if you saw this in my thread but this has a lot of info on tuning the pre 94 tbi computers. The later ones are actually just a blurb in the back pages. http://www.gmcmi.com/wp-content/uplo...EFI-System.pdf

Great read!

**donf** · 02-27-2019

Originally Posted by PlayingWithTBI

Great read!

It's one of the better sources of info I have found. Its missing the last part, the tuned examples according to the index. Even so it's worth saving a copy.

**Daveo91** · 02-27-2019

Originally Posted by donf

I am not very familiar with the older gm computers. I don't know if you saw this in my thread but this has a lot of info on tuning the pre 94 tbi computers. The later ones are actually just a blurb in the back pages. http://www.gmcmi.com/wp-content/uplo...EFI-System.pdf

Thanks for the link, ton of valuable info in there. Saved it to my iPad and am part way through reading it. It doesn't seem to cover my specific ECM (PCM actually since I have 4L80e) but still pretty applicable on the engine side.

**Redtires019** · 03-07-2019

Originally Posted by donf

I am not very familiar with the older gm computers. I don't know if you saw this in my thread but this has a lot of info on tuning the pre 94 tbi computers. The later ones are actually just a blurb in the back pages. http://www.gmcmi.com/wp-content/uplo...EFI-System.pdf

THank you for posting this. This has answered alot of questions and is now saved. I too am looking to build a 383 (for a 1988 pickup) and am wanting to keep the TBI. I have been cruising this board for awhile trying to learn what I can.

**donf** · 03-08-2019

Originally Posted by Redtires019

THank you for posting this. This has answered alot of questions and is now saved. I too am looking to build a 383 (for a 1988 pickup) and am wanting to keep the TBI. I have been cruising this board for awhile trying to learn what I can.

Glad to share, even with the 95 computer it's helpful. I actually tried to track the author down. I wanted to see if I could get all the pages. From the index its missing the tuned examples, I think that would be educational. The website mentioned in the pamphlet is still registered to someone since early 2000's but it's got the info blocked for privacy.
Edit: I did some searching and actually found the complete pamphlet here zipped in docs, under CustomEfis Setup. The tuning examples were never in the info, probably why the others cut it off. It's just a link to a long gone website. It is a cleaner copy though.