stumbling acceleration redux

[mytrplseven]

Before I started a serious campaign to repair practically everything on the engine and from the flywheel aft, I posted a note about stumbling acceleration on full throttle. I had lots of inputs and I'm sure I've covered all the possibilities offered. I still have the problem, however. Here's what I've done so far:

Full vacuum line check and repair
New ignition system
All new rubber seals throughout (including AOS and throttle body)
Rebuilt injectors
New fuel filter
Rebuilt AFM
New belts, rollers, seals, clutch, tranny seals and Redline MT-90,
New brake system, SS lines and fluid flush
New boots and tie rod ends in front suspension (including steering rack)

Car runs smooth, idles great, proper temps and oil pressure. Just can't figure why it stumbles when I romp on it.

Any help appreciated.

[mytrplseven]

Anybody?

[rick13]

Tps, afm

[VirginiaF1]

Originally Posted by mytrplseven

Anybody?

Considering all you tried to do to fix it, I guess I'll forget about fixing mine ....

[John_AZ]

Originally Posted by mytrplseven

Anybody?

mytrplseven,

I know you have done a lot of work on the car from your posts and it is frustrating when it is not running correct.

I read a few of your posts here and on RL. You added on RL that the car stumbles a little on warm up. This may be important.

The Bosch motronic system runs rich when cold. This is programed to speed up warm up. The DME engine temperature switch sends a signal to the DME to add fuel when cold and when warmed up the DME uses the signal from the O2 sensor.

What does this mean? You could have a bad engine temperature switch (not coolant temp switch). This could cause the car to stumble until it warms up.

Per Clarks:
Engine Starting and Running Troubleshooter

This is my first suggestion to check.

Next------"Car bogs down and stumbles at higher RPM"

I do not know what you mean by "new ignition system".
Does this mean new wires, coil, ++++?
Have you replaced the old reference sensors with new ?
Are the sensors gapped correctly?

The next question I have is what type and brand of spark plugs are you using?
Stick with cheap copper plugs. No quad tips, no platinum.

Pull the plugs and check the tips visually. Use a common chart of plug pictures as reference. Is one different than the others?

Like others have said "clean, clean, clean the grounds and all electrical contacts on top of the engine. Clean the 35 pin connector at the DME.

Have you ever replaced the in tank fuel screen?

If you do all suggested checks from me and others, you may have a bad DME and the only way to check it is to try another.

Just ideas. Good luck.

John

[mytrplseven]

Actually, John I replaced the rotor, cap, wires and plugs (Bosch per recommded number and properly gapped). It came with platinums which I removed.The engine stumbles a bit more when it's cold but after it warms up runs fine until I romp on it and then it stumbles. It idles fine and runs normal under normal conditions. I've been thinking about the in-tank screen and fuel pump. If I put a fuel guage on the fuel rail, that'll check the normal pressures but is there a test to see if I'm getting full fuel flow under WOT?

Now that I've cured all the leaks and replaced all the stuff that I'd initially worried about, i.e., belts, wp and clutch, rubber seals (including the RMS) I'm focusing on the higher performance issue. I tested and adjusted the TPS and rebuilt the AFM and took care of the vacuum leaks. I'm planning on driving it for a while so the plugs will give a good color reading and then recheck the tension on the belts (after 1500 or so miles).

I assumed that the reference sensors were gapped properly since the car runs great until putting the pedal down. All the grounds around the engine were cleaned and re-tightened. AOS seals, and FPR and FPD both show no evidence of leakage.

I guess at some point I could pull the DME and give the solder joints a going over and checking the connector for good connections. Right now the next priority is the rear hatch reseal before the rainy season arrives. It's delaminated, of course, and you know what that means.

It's all a process and I love puzzles. This one, though, is a little frustrating. Thanks for your inputs and support. I'll continue to check other resources, i.e., Clark's, factory repair manual and mechanics for inputs as this challenge progresses.

[hoover]

Might be worth checking the fuel pump wires, I had a similar issue, when calling for more power the load was too much for one of the wires which had almost parted inside the plastic covering, ok when not much demand on it.

[John_AZ]

A couple of late ideas...

When the car is warmed up, unplug the O2 sensor and see if you get the stumble at full throttle. Could be bad.

Have you checked to see if the FQS has been played with ---out of spec
FQS - Fuel Quality Switch

Do you have a stock chip in the DME?

GL
John

[mytrplseven]

FQS? Honestly, John, never heard of it on this car. I doubt seriously any mods were made to the DME (chip). It was an older couple I bought the car from and they were the original owners.

[John_AZ]

Ralph,

I saw the pictures of your '87 944 you posted, very nice:

2012 CENTRAL FLORIDA Porsche 944 Rally & Swap Meet!

The interior looks almost new.
Sport steering wheel.

The previous older couple took very good care of the car---only a couple of weekly trips to church in FL. .

John

[mytrplseven]

Actually, I bought it from a Dr. in Santa Fe and drove it home to Florida with a huge mouse nest in the air filter box. Still got good mileage. Loved the 3 day road trip.

[mytrplseven]

Hey John, it was the O2 sensor. I disconnected it and it took off like a shot. Now I'm wondering if I should just leave it disconnected or is it necessary for other stuff? Right now the car pulls like a John Deere and it's finally fun to drive. Thanks to all for their inputs.

[John_AZ]

Good news!!

Now you need to find out why by pulling the O2 sensor out.

Both my cars have the OEM O2 and not the interchange that is crimped on the old wires. AZ does the emission inspection every 2 years.

The catalytic converter could be damaged by rich fuel mixture. Somewhat expensive to replace with the price of metals now.

GL
John

[mytrplseven]

This unit appears to be the OEM since it has the 3 wire round connector and when I had the exhaust system out it appeared to be original, but then I wasn't focused on it. With all these years and miles on it, is it unrealistic to assume a general failure of the unit? I'll check to see if there's any test or appearance that I should be aware of.

Even with it being what caused my engine problems, I got great gas mileage.

[Rasta Monsta]

Factory installed = dead.

[mytrplseven]

I'd expect that it is FUBAR and worried that the cat is lousy too. Gotta look into that later. Right now just revelling in the fact that the car runs so good.