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
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So I’ve been having a bizarre issue with my 1991. Basically it won’t idle for more than 15 seconds once the engine has some heat in it, but doesn’t have symptoms any other time. It idles fine until it shuts off as if someone turned the key off in the ignition. The issue is explained in more detail below.

To help in the diagnosis, below is a list of modifications done by the previous owner (according to him), a list of stuff I’ve done since I’ve had the car, and a brief overview of the issues the car is having now. Sorry in advance for the wall of text……...


1991 Mustang LX 5.0 5-sp. hatchback.


Modifications per previous owner - all done within 10k miles ago (I have omitted suspension and interior mods for obvious reasons):

“Rebuilt” original 302 w/approx. 105k miles on car, 5k miles since rebuild; ARP bolts/hardware used
Ford Racing B303 Cam
SVE cylinder heads
Ford Racing hardened pushrods
Ford Racing hydraulic lifters
Trick Flow RR 1.6
Ported Ford GT40 upper and lower intakes with 1/2 inch phenolic intake plenum spacer
AEM adjustable FPR with liquid-filled pressure gauge
DUI Billet aluminum HD distributor with MSD coil
Summit Ceramic shorty headers
Cat deletes
Smog system deleted
H-pipe with Flowmaster 40 series mufflers
New brake master cylinder with brake booster
New aluminum radiator with overflow tank
New motor mounts (OEM)
Polished aluminum UDP’s (BBK)
Upgraded 3G alternator with wiring
New water pump
New timing gear kit with timing cover
New hoses for heater core and radiator
New heater core and AC evap
BBK fender well cold air intake
BBK 65MM TB with new IAC valve and Ford Racing adjustable IAC plate
New TPS sensor with adjustable plate
New IAT sensor
New oil pump with oil pressure sensor
New fuel pump (OEM) with fuel filter and fuel level sending unit
“Salt and pepper shakers reworked”
OEM computer checked and capacitors replaced
Chrome valve covers (not stock style)
New OEM harmonic balancer
New engine seals, including front and rear main seals
New OEM clutch fan



Since I have had the car, I have done the following:

New Duralast distributor with new TFI - timing set at 14 degrees
New Accel coil
New AC compressor
New pushrods, guide plates, inspected lifters, and adjusted rockers - apparently there was an issue with the lash/preload adjustment and the
guide plates were gouged out a bit and there was some valve train noise. The entire valve train was inspected professionally, pushrods replaced in an abundance of caution, and rockers adjusted properly.

Now for the symptoms and what I’ve done to try to diagnose it so far:

One day as I was driving home, the car started to stall out randomly at idle. No sputtering or rough idle, it would just shut off. I hadn’t changed anything within 500 miles before this, it just kind of started doing this randomly. No unusual gauge readings or CEL’s. Fuel pressure is about 35 PSI with the vacuum line on, and 39 with the vacuum line off.

The car will start fine, run fine and idle fine without exhibiting any symptoms. Once it has a little bit of heat in the motor, it will not idle for more than 15 seconds. Sometimes when I start it up when it’s got a little heat in it, it dies right away. Each time after it stalls, it fires up on the first crank without any issues, although one time it did crank over for a couple seconds before firing. Usually, it idles on a cold start for approximately 3-5 minutes before it starts having this issue. When it stalls, it’s as if someone just cut the ignition switch. No unusually rough idle, no surging, nothing.

I’ve tried disconnecting the IAC with the car running. Idle drops down to 500 RPM, barely stays alive. So I believe that piece is functioning correctly.

I ran codes on the car - key on, engine off:

81- air diverter solenoid fault, intake air control circuit fault/air injection diverter
82 - integrated relay control module
84- EGR vacuum regulator circuit failure

My guess is that it’s getting a bad signal once it goes into closed loop, but otherwise this is a bit beyond my skill set at this point.
 
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LILCBRA

I wish I didn't have all of these balls in the air
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Ok, I read this thread not long after you originally posted, and now that Rick has posted a link to the Corral post I've gone through that. I think someone over there is a little condescending, but we won't get into that.....

Now I will admit that I'm no expert on EFI systems or anything. Hell, I'm no expert on anything! But going from your description of the symptoms you've experienced, I think I'd look at the TPS. The reason I say this is because you stated:

One day as I was driving home, the car started to stall out randomly at idle. No sputtering or rough idle, it would just shut off.

This tells me that it's an electrical issue. And when you said that you're able to keep it running while your foot is on the accelerator, it points to the TPS in my mind. Googling "Symptoms of a bad TPS" comes back with this site at the top. You're experiencing symptom number 3 on their list:


You should be able to pull the TPS and check it with a meter to see if it's acting right. I'd suspect that there is either a write-up or a YouTube video on how to check it with a meter, I think I'd go that route before just buying a new one. But that's where I'd start.
 
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General karthief

wonder how much it would cost to ship you a pair
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Look dude, let me shine a little light here,
Yeah, I wasted my time reading the BS being tossed around on that other site, I didn't read anything remotely helpful so here is a link to the procedure you need to follow, read the instructions, hell, read the whole thing, gather up the tools and get to testing, it's all there just do it step by step. If you get stuck, ask us to explain just do in this thread so the members following now know what's going on.
 
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7991LXnSHO

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If you get that far and it tests bad, a Motorcraft TPS is about the same price at Rock Auto as the Echlin TPS at Napa. The Motorcraft TPS at Advance Auto is almost 2x the cost and probably still needs ordered. I have one on the way while it is still available. I will be testing for odd but only occasional gremlins more on a much cooler day. Heat indexes over 105 = nope.
 

LILCBRA

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Just checking in both here and there, it seems he's more active over there. Probably because there's more replies and more activity. But I'm curious as to what the conclusion will be..... :chin
 
Mar 5, 2020
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Hey everyone, sorry I wasn't getting notifications for replies for this thread and I thought it went dead.

I seem to be getting a lot of flack on the other thread. People not wanting to just "feed me the answers", people accusing me of posting "BS" about the circumstances under which I've been without a car, etc.

I'm very much a beginner to this and I promise I'm not trying to waste anyone's time. I don't necessarily expect to be spoon-fed the answers, but please don't underestimate how new I am to this and in a lot of ways I do kind of need my hand held as how to do basis tests and stuff. I only want to learn.
 

Wayne Waldrep

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Just checking in both here and there, it seems he's more active over there. Probably because there's more replies and more activity. But I'm curious as to what the conclusion will be..... :chin
I looked over there against my better judgement.....I know what you are saying and agree. But I'd rather have a few good replies than a bunch of :poo: replies.... Lol.
 
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Mar 5, 2020
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Ok, I read this thread not long after you originally posted, and now that Rick has posted a link to the Corral post I've gone through that. I think someone over there is a little condescending, but we won't get into that.....

Now I will admit that I'm no expert on EFI systems or anything. Hell, I'm no expert on anything! But going from your description of the symptoms you've experienced, I think I'd look at the TPS. The reason I say this is because you stated:



This tells me that it's an electrical issue. And when you said that you're able to keep it running while your foot is on the accelerator, it points to the TPS in my mind. Googling "Symptoms of a bad TPS" comes back with this site at the top. You're experiencing symptom number 3 on their list:


You should be able to pull the TPS and check it with a meter to see if it's acting right. I'd suspect that there is either a write-up or a YouTube video on how to check it with a meter, I think I'd go that route before just buying a new one. But that's where I'd start.

This makes sense to me; that symptom aligns perfectly. The only thing I've tried is disconnecting the TPS with the car running to see what happens. It didn't change anything; it kept running for a bit and stalled just like before.

I'm going to see if I can perform the voltmeter tests myself. I'm going to try and find a write-up.
 
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Mar 5, 2020
10
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Look dude, let me shine a little light here,
Yeah, I wasted my time reading the BS being tossed around on that other site, I didn't read anything remotely helpful so here is a link to the procedure you need to follow, read the instructions, hell, read the whole thing, gather up the tools and get to testing, it's all there just do it step by step. If you get stuck, ask us to explain just do in this thread so the members following now know what's going on.
I really appreciate it, this looks very methodical. Some of this may be beyond my skill level (for example I don't know how to set the voltage on the TPS). But it looks like it explains how to do that so I'm going to look at this in more detail and I'll come back with questions about specific things I don't know how to do.
 

LILCBRA

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I second Karthief's suggestion of the surging idle checklist thread. The TPS is the second item in his list as well, and he did a damned fine job with his writeup - as he always does!