EEC-wise... what changed from 89-92 to 93-95 on the 3.0?

[morebhp]

Here's why I ask: My first SHO was a 90. I never had any drivability or power loss problems with the engine. It started and ran strong instantly even cold.

I've also had two 95 MTXs, one of which I still own. Both of them have always stumbled for about 5 minutes after a cold start between 2K and 5K at part throttle. The blue one (which I sold) would warm up and be fine and run really strong at all RPMs. My white one (which I still have) never seems to fully recover. It always has this jerky little cough between 2-4K. Some days its worse than others but always there.

Does anyone elses '93-'95 MTX have this same behavior after a cold start?? What the heck is it??

I seem to recall reading that in '92 or '93 there was a change to a two speed fuel pump. Could that be it? Or perhaps some additional emissions crap?? I truly believe something they changed weakened realiability/drivability a little. Even my dad's old beater '91 SHO (155K) doesn't do this. Grrr!!

It's not the plugs or wires. They've been replaced twice with OEM stuff. Didn't help.
I've also replaced...
-Coilpack
-HEGOs
-MAF
-TPS
-CPS
-IAC
-ECT (coolant temp sensor)
-Tested ACT (air charge temp sensor)
-The manifold very is clean.
-I've checked vacuum hoses.
-I have no codes.

I checked my butterflies tonight. They close nice and tight and won't open even with the engine off until I take out the key. Then they release.

I'm open to any suggestions or wild guesses. I'm stumped. ...and frustrated.

 

[1993MTXSHO]

being at 5k rpms on a cold start isnt a good idea to begin with, does it go away when warm?

 

[morebhp]

Right. I always keep it under 4k for the first 10 minutes.

After 10 minutes, the symptoms subside... mostly... but not completely. There's always this little surge-like thing until the butterflies open, then it pulls hard and smooth. If I hadn't owned two other SHOs that didn'tdo this... I may not even bother... but... it bothers me.

 

[Axianator]

Here's why I ask: My first SHO was a 90. I never had any drivability or power loss problems with the engine. It started and ran strong instantly even cold.

Although there were several "small" changes between the programming itself and the pinouts on the various computer models, all V6 MTX computers are essentially the same and may be freely interchanged between model years without problem.

I've also had two 95 MTXs, one of which I still own. Both of them have always stumbled for about 5 minutes after a cold start between 2K and 5K at part throttle. The blue one (which I sold) would warm up and be fine and run really strong at all RPMs. My white one (which I still have) never seems to fully recover. It always has this jerky little cough between 2-4K. Some days its worse than others but always there.

Stumbling or hesitation after a cold start is often caused by a lean air/fuel mixture, which can stem from bad emission control sensor feedback (ECT, ACT, HEGO or MAF sensor) to an unmetered air leak somewhere along the intake or exhaust path. Are you completely sure that you do not have any intake or exhaust system leaks?

Also, have you tried replacing the PCM in your '95 MTX? While most Ford EEC units are not known for outright failure, it is not uncommon for some aging PCMs to suffer from bad solder connections, burned-out drivers and other electrical problems that can adversely effect the performance and behavior of your engine.

I seem to recall reading that in '92 or '93 there was a change to a two speed fuel pump. Could that be it? Or perhaps some additional emissions crap?? I truly believe something they changed weakened realiability/drivability a little. Even my dad's old beater '91 SHO (155K) doesn't do this. Grrr!!

The '89-'90 V6 MTX cars were the only SHOs equipped with the factory two-stage, two-speed fuel pump system. Unfortunately, while the system helps to alleviate some of the problems caused by using the stock FPR with a higher-flow fuel pump (190+ LPH), it would do little to cause any driveability problems on a factory SHO with a factory-sized fuel pump.

 

[morebhp]

Stumbling or hesitation after a cold start is often caused by a lean air/fuel mixture, which can stem from bad emission control sensor feedback (ECT, ACT, HEGO or MAF sensor) to an unmetered air leak somewhere along the intake or exhaust path. Are you completely sure that you do not have any intake or exhaust system leaks?

Well, that's really hard to say for certain. This car has done this since nearly new so I've never suspected an intake leak. I've never had the main intake plenums off, I've only had the manifold and the endcap off to look inside for buildup. It's still very clean in there. Is there a semi-scientific way to search for intake leaks?

About the exhaust... are you saying an exhaust leak would cause the car to run lean and stumble? I'm not refuting that but it surprises me if that's plausible. What factors are at work in that scenario? (just trying to learn here...)

Also, have you tried replacing the PCM in your '95 MTX? While most Ford EEC units are not known for outright failure, it is not uncommon for some aging PCMs to suffer from bad solder connections, burned-out drivers and other electrical problems that can adversely effect the performance and behavior of your engine.

Again, since I've had two '95s exibit the same behavior I did not suspect the main computer. It just seems like standard operating proceedure. Maybe if I tried a computer from a '90...

I really think if I could figure out what's causing the cold start stumbling, I'll find what's causing my throttle tip-in and sputtering under load between 2K-3K.

Thanks...