Help w/data file to understand surging idle/die

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Chadryan44

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First of all, mostly a lurker that has heavily benefitted from the group's wisdom, thank you so much for your time!

I have a 91 C1500 with a 5.7 TBI. I'm having issues with heavy idle fluctuation, especially after the engine is warm. Sometimes the fluctuation is so hard, engine dies. Also dies when shifting from park.

Thanks to y'alls expertise, I've been able to pull a few data logs from my truck. I have no idea what to look for. Could someone help me understand what to look for? I've attached a zip file with 2 CSV files showing the data. May as well be a foreign language to me! I do see MAP reading fluctuation quite a bit, but I'm clueless.

Any help y'all could give would be wildly helpful!
 

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  • 91 Chev Data Logs.zip
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PlayingWithTBI

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I'm having issues with heavy idle fluctuation, especially after the engine is warm. Sometimes the fluctuation is so hard, engine dies. Also dies when shifting from park.
I'm looking at your logs, a couple things I noticed;

You're not getting reference pulses every sample. I'm wondering if your ICM, Ignition Coil, and or Pickup Coil are going bad. Maybe poor connections/wires inside, going to the distributor, or the Ignition Coil. While idling, you can spray water on the coil and around the distributor to see if it stumbles worse which may give you an indication where the fault lies. Always buy quality parts, stay away from auto parts store brands and other cheap stuff.

Your TPS voltage is fluctuating, are you messing with the throttle during these logs?

You're not getting an "Idle Flag" all the time. Maybe your TPS is bad or not adjusted properly? .65V is a little high but shouldn't cause this.

The MAP sensor will vary when RPMs change so, it's probably good.

Your log is pulling a sample around every 1.2 seconds so, it may be missing some anomalies in between.
 

Chadryan44

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Peter, thanks for looking at the logs and replying! Lot of great info you've shared.

I did not touch the throttle while taking the logs, so I'll check the TPS. Maybe it has worked itself loose.

I'm sure there's a way to change the data log interval, I'll pull a log with shorter timing windows if that would be helpful. Should I shoot for 500ms?
 

PlayingWithTBI

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I did not touch the throttle while taking the logs, so I'll check the TPS. Maybe it has worked itself loose.
This and your ignition parts. What brand ICM, cap, rotor, etc do you have?

I'm sure there's a way to change the data log interval, I'll pull a log with shorter timing windows if that would be helpful. Should I shoot for 500ms?
Let's wait until you check your other stuff. What are you using to log with, TunerPro RT?
 

DerekTheGreat

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I did not look at the log, but if the TPS voltage is fluctuating without actual throttle input, that will cause an idle surge. Idle strategy seems to be that if you touch the gas, it will subsequently increase IAC counts.

If stabbing new ignitions parts in it, I recommend the complete dizzy from O'Reilly's. It was about $150 and comes complete with a cap utilizing brass terminals. I recently went through the gauntlet of aftermarket parts on my truck. Long story short, the complete dizzy with it's lifetime guarantee from the Irish parts store has been solid the past two or so weeks. Verdict isn't settled, but I'm happy with it so far. My idle isn't as choppy as it was before and it even seems to run smoother.
 

Chadryan44

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Will do my best to post the list of parts:
(Edited to add that these are all replaced in the last 2 years)

Coil - Standard Ignition Blue Streak Ignition Coil Line: STD | Part # DR37
ICM - MasterPro Ignition 6 Terminal Ignition Control Module Part # 2-7031 Line: MPI
Rotor, Cap, Plug Wires - AC Delco
TPS - MasterPro Ignition 3 Terminal Throttle Position Sensor


I am using TunerPro RT to pull the data.
 
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VIKING_MECHANIC

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Check your ICM and see if the thermal compound has dried up - if it's the original, most likely it is.

If the ICM gets too hot, it will cause issues with the engine and it being a solid state device, there's no "fixing" that other than replacing it.

My 89 K1500 kept blowing the ignition fuse everytime I put my foot into it. I thought the fuel pump died since I didn't hear it with the key on and it would only crank. And if it wasn't for my brother checking fuses, I would've jumped the gun and dropped the tank.

Based on what other similar issues people had with their TBI running, I replaced the ICM and it runs like a champ now!!
 

Sabinoerc

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I did not touch the throttle while taking the logs, so I'll check the TPS. Maybe it has worked itself loose

perhaps clean/check all your grounds. There are 3 of 4. I think the one on CTS is PCM ground, not sure but someone here will know. maybe not the issue but a good thing to check off.

It can also help to plot the data sometimes. Easier to visualize what's going on. My problem with this is I get impaled upon the chicken or the egg problem - you can see stuff happening but I'm never sure if it's the cause or just another effect of whatever is happening.

In your case. looking at TPS shows it changes at time of when the RPM first changes but is not changing during the rest of the event. Made me think noise/electrical.
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Looking at BLM/INT - looks like it's running lean prior to event, system is trying to make it richer (I think) and at event something changed and it's trying to lean it out. I'm never sure about this- people here who know much more about tuning and this will correct as needed.
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All the while, it appears from O2 switching properly it looks like system is controlling to whatever target it's trying to control? This confuses me.
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I would agree with playtbi that the intermittent references are interesting. He has much more experience on this kind of thing that me.

I would also look for flags in Tunerpro - something else is going on it seems? Look for a flag with name something like "idle enrichment" or the like. I had a problem once where marginal fuel pressure caused idle speed to drop, this kicked in a stall saver thing called something like "idle enrichment" which dumps fuel to save the idle - runs up idle, repeat.
Memory is it would peg BLM before almost stalling though so perhaps not same here.
It might also help to put what cell it's running during this - I think "cell" is correct name, that is, it is staying in the idle mapping or off idle, etc?
 
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