[bongos444]

Looking for some new ideas as I have run out….
I have a 1995 Mustang 5.0 (it might actually be a 331 but I am unwilling to admit this to our race director haha) track car that is running a stock T0M4 manual computer that has been giving me issues for some time.

The problem:
On free rev you can hold your foot to the floor and it revs up great and then just stops somewhere between 5800-6200rpm depending on its mood. It doesn’t bounce it just stops, almost like a modern soft limiter. Then when you release the throttle it acts like the computer is stuck on half fuel for anywhere from 10-45 seconds. It will die if you try to let it idle and sounds like it’s only running on 4 cyl. After the aforementioned 10-45 seconds it comes out of it and runs fine. This I could live with since it’s just on free rev but what I can’t live with is it will do the same thing on track with hard shifts. Going down the back straight, let it run up to 5500-5700 rpm and grab the next gear hard and bam, same thing. It acts like it is running on 4 cyl for somewhere between 10-45 seconds.

Here is what I have tried:
- new distributor twice, first a parts store one then a nice performance distributors brand billet one
-multiple TFI modules, I currently have a new motorcraft one in it now
-new maf (I think it’s an ngk)
- ohmed out the CTS and IAT and they are in spec
- double,triple checked the TPS setting and then put a new one one and double check that for the .9v.
- new NGK coil
-new wires
-New motorcraft plugs
- new 19lb injectors
Now here is what stumped me… I got a different supposedly stock 1995 T0M4 computer and swapped it and it does the same exact thing.

Otherwise the car runs fine, though there is a noticeable differential on the break point when you go from closed loop fuel <80% throttle to open loop. Going from 70% throttle to 85% throttle it’s like the car picks up 100hp. When I unplug the O2’s it doesn’t do this so I know it has something to do with the closed loop fueling. I usually just run with the O2’s unplugged to avoid this.

At this point I’m out of new ideas to try so any suggestions would be appreciated. I run with a couple other guys that have the T0M4 ECM and none of them seem to have these issues so I’m not sure where to go from here.

[JuiceSC]

I am not familiar with this strategy at all.

Pretty sure it's not the computer since you swapped it.
You have an unknown MAF, and you are trying to feed a 331 with 19s.
O2s unplugged and it runs better. Fueling is way off or you have 2 bad O2s.

Really not enough info provided.
Have you scanned for codes? Checked fuel pressure? Did this setup ever work right?

[bongos444]

Sorry, I thought about the codes and the maf right after I posted.

I have codes for the emissions component I deleted (egr, evap) but no functional codes other than the O2 heater circuits when they are unplugged.

I am running the stock 95 air box and tube and I had an oem 95 ford maf in there and have swapped it out with a NGK brand parts store replacement with no change.

I have an adjustable fuel pressure regulator that is set to 39# without vacuum hooked up. I have tried bumping it up 1-2 psi and nothing really changed.

Here is a little more background info, originally the car had a stock explorer 5.0 with an E cam 3 years ago and I was having the same issue where the engine drops 4 cyl if you rev it to 6000ish or shift it to hard and it wouldn’t come back for the same 10-45 seconds. The only thing this original setup didn’t do was the O2 sensor issue. That is probably related to the larger than stock new engine. At the end of my first season with the car the 5.0 lost a freeze plug on track and nuked the motor. Then it briefly had a 2v then a 4v and after a string of unfortunate incidents I decided to drop the 331 short block with the gt40 top end I had on the shelf. That’s probably too much info but the long and short is the issue still occurred with the original 5.0 setup.

[JuiceSC]

Pure guess, but how is fuel pressure when this happens? Good pressure at idle does not mean a whole lot, if fuel pressure tanks under high load/high flow demand.
Could also be a voltage issue. How is system voltage when this happens?

I had a turbo SVO that would fall on its face, and surge like crazy under load. IT was a clogged fuel filter causing it.

[bongos444]

I thought I had checked fuel pressure under load, but I should probably check it again. I did put a new fuel tank, pump, and filter when I got the car 4 or so years ago so it shouldn’t be clogged or anything but you never know.

I’ll definitely check system voltage under load. That is something I have not checked under load. The rpm at which it happens does seems get lower the hotter everything gets so maybe a voltage issue with heat? I’ll definitely look into that.

[bongos444]

Well no dice on voltage or fuel pressure.

When it starts to misbehave it has a solid 40psi of fuel. And once it has dropped half the cylinders it still has good fuel pressure at idle of 35-36psi w/ vacuum hooked up.

I check voltage at the pcm when it starts misbehaving and with the alternator hooked up it has a solid 14.2v and I tried disconnecting the alternator and it holds a steady 12.1-12.2v and will still get it to its cut half the cylinders mode.