1991 5.0 - fires up every time, idles perfectly, but once it’s warm it won’t idle for more than 15 seconds

Mar 5, 2020
12
2
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32
Southeast Florida
Hey all,

So this is a part two to this thread:

https://www.stangnet.com/mustang-forums/threads/1991-5-0-fires-up-every-time-idles-perfectly-but-once-it’s-warm-it-won’t-idle-for-more-than-15-seconds.921537/

I ended up taking the car to the shop. It had a bad TFI, computer was wired incorrectly, they had to hook up the car to a "breaker box" (not even sure what that is) to diagnose it and it took them a while to figure it out. There were some other issues as well (can't remember off the top of my head) that they sorted through as well. Also had the rear end rebuilt (previous owner lied about that, clutch packs were toast and the driver's side axle had signs of galling). They said that when they had the car on the lift, they could spin one rear wheel freely and the other wouldn't do anything. Finally, They replaced the leaking valve covers and some front suspension pieces that were bent/loose.

So anyway, they got the car running, and it was running and driving great, except that the shop mentioned to me when I picked it up that it would pop and backfire badly out of the exhaust when it was cold, but ran fine otherwise. They said to drive it for a little bit and if the car didn't stop having that issue, to bring it back to them because it likely needed a tune.

So, the problem persisted, and I brought the car back to them for a full dyno tune. They put an SCT chip on it and did two hours of dyno tuning. Baseline run was 261, they ended up squeezing 285 out of it by the end (on a Dynojet).

So now the car runs even better. Idles perfectly, runs smoothly, etc.

EXCEPT: It still doesn't run great when it's cold, and backfires/pops out the exhaust and has significant hesitation until it warms up. Once it's warm, it runs HARD, pulls smoothly throughout the RPM range, etc.


I feel like this is an issue they should have resolved. The car shouldn't be popping and sputtering when it's cold like this.

I did some quick looking on the forums and people said they had this issue and it was either the ECT, ACT, or the BAP, or dirty injectors that needed cleaning

The car does not have a check engine light on. I am getting intermittent "low coolant" lights on the dash (it's illuminated about 75% of the time), despite the coolant level being adequate. Also checked the coolant system for leaks via pressurizing the radiator, and it seems file. The car does not have an EGR or any other emissions equipment.

This is clearly an issue with a sensor or something when the car is in open loop before it warms up. The car runs quite well when it's warm, and pulls hard.

What do you guys think?
 
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General karthief

wonder how much it would cost to ship you a pair
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I merged the threads together so members know what is happening without going back 4 or 5 pages that honestly, they are not going to do, which means they will A: ignore your post B: ask a lot of questions that have already been asked.
Now without going back through the thread to see what was/is going on I will say I don't hold dyno work in high regard when it comes to fixing our era mustangs, I think, and this is my opinion, they program a chip to mask a problem that likely could have been fixed by the owner with some diagnostic work like the 'surging idle checklist'. I could be wrong though. I hang out in left field.
 
Mar 5, 2020
12
2
13
32
Southeast Florida
I merged the threads together so members know what is happening without going back 4 or 5 pages that honestly, they are not going to do, which means they will A: ignore your post B: ask a lot of questions that have already been asked.
Now without going back through the thread to see what was/is going on I will say I don't hold dyno work in high regard when it comes to fixing our era mustangs, I think, and this is my opinion, they program a chip to mask a problem that likely could have been fixed by the owner with some diagnostic work like the 'surging idle checklist'. I could be wrong though. I hang out in left field.

Thanks, I appreciate it. Wasn't sure if there was a prohibition on bumping super old threads.

Yea I'm kind of in a weird position because the shop that I took it to is legendary around here and it's the same one everyone has already been recommending elsewhere.

But I guess I trusted them that it needed a tune to fix this, so I am kind of losing faith in their expertise if the've now had the car in their shop twice and haven't been able to fix this.

So now I’m in a situation where the car has been tuned so I don’t know if I should start messing with stuff. I’m going to call them on Monday and see what’s up. This is becoming frustrating.
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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I sympathize, they may not be, wait, let me rephrase this, experienced with eec lv computers, if you cruise around here looking at some build threads there are a few that have a lot of bolt-ons that run just fine without a chip, I'll go back and read through this again and prolly give you some really bad advice, or at least make you laugh.
 
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Bullitt347

man bewbs please...
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, they had to hook up the car to a "breaker box" (not even sure what that is) to diagnose it
It is called a "BreakOut Box" It basically allows access to all of the pins off of the PCM without having to back-probe or otherwise actually disrupt or open any of the wires from the PCM. It makes it much easier to diagnose an issue.

?u=https%3A%2F%2Ftse1.mm.bing.net%2Fth%3Fid%3DOIP.jpg
 
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
20,718
7,128
203
polk county florida
It is called a "BreakOut Box" It basically allows access to all of the pins off of the PCM without having to back-probe or otherwise actually disrupt or open any of the wires from the PCM. It makes it much easier to diagnose an issue.

?u=https%3A%2F%2Ftse1.mm.bing.net%2Fth%3Fid%3DOIP.jpg
I want one, it goes with all the other old junk errr, stuff I have.
 
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Mustang5L5

Put lubricant all over the balls
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The car does not have a check engine light on. I am getting intermittent "low coolant" lights on the dash (it's illuminated about 75% of the time), despite the coolant level being adequate. Also checked the coolant system for leaks via pressurizing the radiator, and it seems file. The car does not have an EGR or any other emissions equipment.

This is clearly an issue with a sensor or something when the car is in open loop before it warms up. The car runs quite well when it's warm, and pulls hard.

What do you guys think?


Not all codes stored in the EEC will generate a CEL. When in doubt, pull the engine off, engine running codes and run the cylinder balance test. Usually the CEL will light up if a primary sensor is failed, but does not light up if one is out of range. I used to have a list of the codes that lit the light but cannot find it right this second.

During warmup (open loop) the O2 sensors are ignored and the fuel/timing curves are generated based on lookup tables based on parameters input from the TPS, MAF, coolant and intake air sensors. WHen the car transitions to closed loop, the O2's are the primary.