Ford Mustang Forums banner

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

14K views 213 replies 20 participants last post by  Enjoy_the_Ride 
#1 ·
1991 5.0. HCI, all kinds of bolt ons.

I need to preface this by saying I do NOT think the issue is related to a burnt TFI or anything in the distributor. The car fires up IMMEDIATELY on the first crank, every time.

The car has recently developed an issue. I have not changed anything with the car prior to this issue developing.

The issue im having is as follows:

From cold, the car will start and idle just fine for approximately 3-5 minutes, and then it will just shut off. It will NOT stumble, search or sputter. It just shuts off. Once it’s got a little heat in the motor, it won’t run for more than 10-20 seconds before it will shut off on its own. But it will fire back up immediately every time.

Fuel pressure is 35ish, 40 without the vacuum line. The car will rev up just fine and doesn’t make any noises that suggest a vacuum leak or anything. No other signs of a problem.

If I keep my foot in the gas, I can keep it running indefinitely.

I’m guessing it’s a sensor or something that once the car goes into closed loop, it triggers and tells the car to shut off. But I can’t figure out what it is.

MAF?
IAC?
TPS?
O2 Sensors?
Stock computer?
 
See less See more
#6 ·
1991 5.0. HCI, all kinds of bolt ons.

I need to preface this by saying I do NOT think the issue is related to a burnt TFI or anything in the distributor. The car fires up IMMEDIATELY on the first crank, every time.

The car has recently developed an issue. I have not changed anything with the car prior to this issue developing.

The issue im having is as follows:

From cold, the car will start and idle just fine for approximately 3-5 minutes, and then it will just shut off. It will NOT stumble, search or sputter. It just shuts off. Once it’s got a little heat in the motor, it won’t run for more than 10-20 seconds before it will shut off on its own. But it will fire back up immediately every time.

Fuel pressure is 35ish, 40 without the vacuum line. The car will rev up just fine and doesn’t make any noises that suggest a vacuum leak or anything. No other signs of a problem.

If I keep my foot in the gas, I can keep it running indefinitely.

I’m guessing it’s a sensor or something that once the car goes into closed loop, it triggers and tells the car to shut off. But I can’t figure out what it is.

MAF?
IAC?
TPS?
O2 Sensors?
Stock computer?
Do you have a MSD box? If so I would unhook it and try without it. Mine did the same thing and was very annoying!


Sent from my iPhone using Tapatalk
 
  • Like
Reactions: bobbywayne
#9 ·
you can keep it running by keeping the AIR pedal open, correct?

kinda rules out spark or fuel or compression,,,,,,dont you think?????

by keeping the AIR pedal open, what do you think the engine is missing?
 
#12 ·
if it doesn’t run

ITS missing something, that is not up for debate

AND NO, i am not going to spoon feed you, you are going to put your diagnostic hat on.

so, again, if it requires you to ADD AIR to keep it running

what is it missing?
 
#14 ·
Come on man I’m just trying to learn. This isn’t a class at university lol I’m just asking for help fixing an issue. My other car is also undriveable because the alternator died and I’m also working on fixing that. So I am in a tough spot and if I can get this fixed it would make my life a lot easier, and the lives of those who depend on me.

Based on the fact that it only exhibits symptoms at idle, I THINK it’s not getting enough air at idle, which leads me to believe it’s the idle air control valve. Based on the similarity in how it cuts out is much like the way it cuts out when the AC is on and I’m coming to a stop, it would make sense that this is the issue. So I’m guessing I need to clean or replace the IAC. Beyond that, based on the fact that it only does this once the car has some temperature in it, my guess would be bad feedback from a sensor once it goes into closed loop.

I am still unsure because it doesn’t idle low, idles perfectly (for a cammed engine) until it just dies. If it was just not getting enough air at idle, I’d expect it to stumble and sputter before dying. Maybe with the cam it doesn’t even have enough vacuum to do that?

Am I close?
 
#15 ·
you will learn, dont worry!

no amount of begging or sob story, is gonna get ya spoon fed.

so dont waste valuable time on that.

if the IAC was the issue, how could it start and run cold? it needs more air and fuel cold.

what test can we do do prove it, when cold?
 
#16 ·
It would start and run cold because of the fuel trims in the open loop cycle. Either that, or it’s on the verge of failing completely, and somewhere in the system there’s a part that expands just enough to exacerbate the issue to where it starts stalling.

I plan on starting it up when cold and unplugging the IAC to see if the car dies. If it keeps running and potentially doesn’t stall (despite probably exhibiting some surging) I’ll know it’s the IAC
 
#21 ·
Ok here’s where I’m at.

I went outside and tried to crank it. It made crunching/clicking sounds almost like a dead battery but not quite, and wouldn’t crank. I adjusted the battery cable terminal and was eventually able to get it to start. (I have video of the noise it made).

I tried disconnecting the IAC. CEL came on, but the car would idle and did the same thing it’s been doing (dying after a few minutes).

I tried disconnecting the MAF. Same exact thing.

I tried disconnecting the TPS. Same exact thing.

I tried disconnecting the other sensor that’s in the cold air intake tube between the MAF and the TB. Same exact thing.

When I first reconnected the unknown sensor and started it with everything in place, it cranked for about 4-5 seconds before it would fire.

Observing the idle closely, it seems to sputter for just one rotation before it dies. It doesn’t struggle at a low idle, it just drops extremely quickly and dies. But prior to that, it idles at a normal RPM.

I also tried a few more times to start it and let it run with everything intact. A couple times it would fire and then IMMEDIATELY die.

My theory is that the IAC is physically stuck or clogged, and needs to be replaced or cleaned.
 
#25 ·
when it was running and you unplugged it, the idle did NOT drop or stall engine?

this is very important to be very accurate, unless you have a scope, internet diag requires you to be extremely accurate.

i am ignoring all the other stuff you are doing, i dont diag like a chicken with its head cut off.
 
#26 ·
Honestly, it doesn’t keep running for long so I had to run around and disconnect it. It eventually stalled soon thereafter (maybe a few seconds), but not immediately.
I didn’t notice much of a change in the idle at all, it’s hard to tell with the cam it has. I did also restart it with it disconnected and no difference to how it acted otherwise.
 
#28 ·
What are you even talking about? Can you show me where I said it doesn’t run cold? Where are you getting this from?

I’m not the one who installed my ****ing HCI. Honestly man what is your deal? This is what happens when people treat newcomers like **** and then wonder why the hobby is dying
 
#33 ·
The issue im having is as follows:

From cold, the car will start and idle just fine for approximately 3-5 minutes, and then it will just shut off. It will NOT stumble, search or sputter. It just shuts off. Once it’s got a little heat in the motor, it won’t run for more than 10-20 seconds before it will shut off on its own. But it will fire back up immediately every time.
poster gots 3-5 minutes........................

what is the freaking issue, or is the OP lying?
 
#37 ·
HOLY mother of pearl

what am i dealing with here......................

I never said to get it to temp, there is a reason

ONE MORE TIME

stop trying to jump ahead, like you know something, follow the direction to a f&^&ing T

allow the engine to cool, SO IT CAN RUN FOR 3-5 MINUTES LIKE IN YOUR FREAKING FIRST POST!!

ONCE ITS RUNNING, WALK TO THE IAC, UNPLUG IT,

WHAT HAPPENS TO THE rpm?

IS THIS CLEAR?
 
#40 ·
Y'all stop bickering - we as an audience want to know what the solution is.
 
#48 ·
So I went out just now and started the car up, and immediately disconnected the IAC. The idle dropped to about 500 RPM. The car barely stayed alive but did continue to idle for a bit. I did happen to notice that the fuel pressure, which was at 33-35 PSI before, was now hovering closer to 37 PSI. Not sure if that’s related or not.



My best guess as to what’s happening is that the idle is set too low via the set screw, and the IAC is held or stuck wide open. This allows it to idle just fine while in open loop, but as soon as it goes into closed loop, the IAC has no further to open and the car shuts off.



My other theory is that the IAC is just clogged or stuck in a partially closed position where it can’t open any further, with the same result. This would make more sense since the problem appeared after the car had been driving fine for a while. The IAC could have deteriorated or have had time to get gummed up.



So maybe if I adjust the idle via the set screw with the IAC disconnected, or if I perform a base idle reset and replace the IAC with a fresh one, that might solve it. But I’m really not sure.



Any thoughts?
 
#49 ·
Seems the IAC is working. What else could have changed to provide less air? Do you have a PCV on the car?
 
#51 ·
Do you have the hose from the valve cover to the throttle body connected?
 
#53 ·
Also check that the PCV isn't clogged or otherwise not pulling air - it's in back of the intake.
 
#54 ·
Does it have a tune? If so do you know who it is tuned by? If the ecu thinks it's getting hot and has a decipha tune (even his free base tune has it) it may be shutting off due to over heat protection. Overheat protection allows the car to run for about 15 seconds before dying again.
 
#56 ·
Does it have a tune? If so do you know who it is tuned by? If the ecu thinks it's getting hot and has a decipha tune it may be shutting off due to over heat protection. Overheat protection allows the car to run for about 15 seconds before dying again.
I do not believe it has a tune, and no aftermarket chip or anything. That would make sense, but how would this develop suddenly and how would I fix it? Would the ECT be involved in sending a bad signal that the car is too hot?
 
#61 ·
Something changed. I suppose you can do your re-set of the idle procedure and see it if sticks. It may help to diagnose the issue, it will also satisfy our curiosity.
 
#64 ·
I did try disconnecting the battery and letting it drain down and I also verified that the timing is set properly (14 degrees base timing) but I don’t know if it would make a difference as far as the battery disconnect.

I really would like to diagnose the issue but it seems as though I’m at a dead end. I also had my mechanic friend do a once-over and make sure everything looks good visually, and he couldn’t find anything. He’s been a Ford tech for 30 years and he’s done quite a few foxbody builds over the years so I dunno. He didn’t do any in-depth stuff though.

I may just have it towed to a shop honestly
 
#65 ·
I was thinking you could crank the throttle stop screw a set amount (so you can undo it if doesn't do what you like) to get it to idle without IAC, then plug IAC in (obviously cycle key to re-learn TPS zero position.)
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top