'88 GT Cali model w/ mass air - hesitation/stumble at high RPM WOT?

[Globule]

I just picked up my next Fox body 5.0, an '88 GT 5-speed with factory mass air (the California Mustangs in '88 got mass air instead of speed density)

The car has been running great overall, except for one big problem - under WOT, the car stumbles and hesitates when the revs go past 4000. It does it under load, with car in gear and accelerating, or without load, with a strong WOT rev past 4k RPMs.

The car has 180,000 miles and has been extremely well taken care of (same owner since '89)

Any ideas?

 

[HISSIN50]

sorry, you lost me. it bogs at WOT at 4K RPM while under load, but revs fine with no load?

have you pulled codes?

how is FP at the times it gives trouble. FP and TFI are what come to mind at first blush - need more info/clarification.

good luck.

 

[nito88stang]

Sounds fuel related. Get a fuel pressure gauge on it and see what it does as the revs climb with load, prob the stock fuel pump quiting on you or the fuel filter clogged. Since you just got the car i'd change teh fuel filter anyway.

 

[bmorrisj]

check the pickup under the rotor, thats been the cause of a lot of >3k stumbles...

 

[autoXr1]

Under load, the fuel and timing curves can be significantly different than when not under load. Typically, when reving the engine in the garage or even during part-throttle cruise, the timing may be advanced some 40* or more. Under load and WOT, however, timing is retarded to the mid-high 20's with a more rich A/F ratio.

High RPM stumble under load may be an error in the timing and/or fuel tables...or may be something more mechanical such as a distributor, TFI module, injectors, fuel pump, etc. It is difficult to tell what it is without looking at the A/F ratio, voltages off your sensors, your timing advance curves, fuel delivery, etc.

 

[Globule]

Under load, the fuel and timing curves can be significantly different than when not under load. Typically, when reving the engine in the garage or even during part-throttle cruise, the timing may be advanced some 40* or more. Under load and WOT, however, timing is retarded to the mid-high 20's with a more rich A/F ratio.

High RPM stumble under load may be an error in the timing and/or fuel tables...or may be something more mechanical such as a distributor, TFI module, injectors, fuel pump, etc. It is difficult to tell what it is without looking at the A/F ratio, voltages off your sensors, your timing advance curves, fuel delivery, etc.

The car also does it when not under load (just revving in neutral)

 

[autoXr1]

Well,...
1. Check your TPS voltage throughout it's range from closed to WOT.
2. Throw a timing light on it and check the timing at different points...1000, 2000, 3000, 4000, and so on. See how the advance curve is, and also check to see how stable the timing is...if you have quite a bit of wander...the distributor bearings may be worn. TFI modules have been known to get "irritable" with age, as well.
3. Check the fuel pressure. Set it at 39-41psi with the vacuum off, then check the fuel pressure at different RPM points like you checked the timing. If it is taking a nose dive at high RPM, your fuel pump may be failing at high RPM.
4. Pull the codes on the beast and see if anything pops up.
5. Plugged cats?

Probably some others, but you can start from this list.