89 Mustang 2.3 Occasional Misfire/Stall

Nov 19, 2018
5
1
13
27
Layton, UT
#1
So, I have a bit of a mystery for you guys. My car likes to stall every now and then, and I'm stumped as to why. From a cold start, it starts out fine. Once it's fully warmed up, it's also fine. However, in between then, it occasionally bucks and stalls like it has a misfire. It idles and runs just fine until it gets into these fits, it's the strangest thing. Here's the list of things I've learned so far:
-Fuses are all good.
-Battery cables/terminals are clean and free of cracks/corrosion.
-IAC is clean and functioning normally. I can hold the throttle open, and the problem persists.
-I'm reasonably sure the injectors are working normally, but I don't possess a noid light.
-TFI module is new, as is the coil, spark plugs, and wires. These were replaced since the car had been sitting and wouldn't start when I bought it. They are off brand; however, the car didn't run at all until i replaced the TFI module specifically, so I'm fairly sure it's doing what it needs to do.
-I could not find a vacuum leak, though that doesn't necessarily mean there isn't one :/
-There are no outstanding power draws, but i did notice the battery voltage hangs out around 13.2 at idle with the alternator loaded (lights on, etc.). Alternator tested good at my local parts store, battery is 3 years old. I also noticed loading the alternator tends to exacerbate the problem.
-I pulled the KOEO codes. I got 11 at first, and an 18 that was stored. This is my only real lead. That being said, the battery has been disconnected, and hasn't been driven much since then, so it really doesn't have a chance to store any other codes right now.
-Fuel pressure checks out, have 40 psi with the regulator disconnected and vacuum line plugged. It also isn't leaking fuel anywhere I can find.

I'm stumped. Like I said, it starts just fine, and runs smooth cold and after fully warmed up. Just that in between stage... I suppose my first question is how long can the computer take to calibrate? Could this be a thing I need to drive out of it? Otherwise, what should I check? My mind says PIP sensor, but it isn't really exhibiting the typical problems that would cause. Any help would be greatly appreciated.
 
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
9,777
3,082
193
polk county florida
#2
First thing I would do is get a fuel pressure gauge and mount it to a wiper arm temporarily with zip ties and monitor the pressure when the stumbling occurs. Did you get the koer codes?
 
Nov 19, 2018
5
1
13
27
Layton, UT
#3
KOER didn't show anything unordinary. I ran it on the road for a good while, ran the codes again, and got 31 added to KOEO though. I pulled the EVP sensor and gave it a good clean, along with the connector.
I'll try your suggestion with the fuel pressure gauge, should I leave the regulator disconnected as I do this? I also noticed that I oopsed and added too much transmission fluid, could that cause a problem like this? It leaks anyways, so that would fix itself i presume :D
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
9,777
3,082
193
polk county florida
#4
Trans fluid level has no effect on how the engine runs. Leave the vacuum line hooked up when driving around monitoring the fpg, pay attention when the car acts up.
 
Nov 19, 2018
5
1
13
27
Layton, UT
#5
Okay, bit of an update: I ended up testing the fuel pressure, but it seems to check out. It fluxuates with engine vacuum as it's supposed to, and never got below 26-30 psi. So, the code 18 got me looking into the timing and ignition system. When it started having a fit, I disconnected the MAP and the SPOUT at separate times. The MAP raised RPM a bit, but didn't otherwise affect the problem. However, the SPOUT made it stop. Applied throttle, turned on lights and A/C, and it runs just fine, if a little bit faster. Plug it back in, acts up again. Timing shows 10 BTDC. This leads me to believe the PIP is acting up as the engine heats up, but it doesn't explain why it evens out completely after being fully warmed up. Could the computer be on the fritz, not able to adjust timing like it's supposed to? Or, could it be that the new TFI isn't jiving with the PIP sensor?
Also, I searched again for vacuum leaks, as I thought of more places to check. Still nothing.
 
Nov 28, 2018
21
6
13
66
Hewitt,Texas
#6
I bought a brand new TFI last week and it made mine run worse than the one I took out>>> I took it back to AutoZone>>>
they tested it and said it was Fupped right out of the box and gave me a replacement
 
Nov 19, 2018
5
1
13
27
Layton, UT
#7
Final update! The car is fixed! On a hunch, i went ahead and swapped out the MAP sensor for a newer one that was in a buddy's 88 Ranger. It ran a little rough after that, but we couldn't get it to stall anymore. I checked the timing, however, and it was 16 degrees too advanced. Yikes! Base timing was still fine though, so we just reset the computer and let it relearn the timing curve. It's running like a dream. So, combination of a bad MAP, and the computer overcompensating to keep the car alive. Thanks for the help!
 

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