Multipass

I am trying to diagnose a problem with a high RPM miss (keeps it from reving past 5K), for details see: https://rennlist.com/forums/924-931-...r-warm-up.html

I took the cam shaft off and after close inspection the valves look fine. I pressed on each one with my body weight and they all feel like they have the same stiffness.

Is there anything else I can do to check the valves?

I searched and most comments say it should be obvious when a valve is broken, a few said that they can look OK, but the springs could still be damaged, anybody have this experience?

Thank you!

931guru

It's pretty obvious if one spring is broken... either it's the outside one, and you can see it, or it's the inside one and you can press it down without too much pressure (maybe 10 pounds).

Eastep

im having miss/hesitation issues at WOT too

Makis

The valves are probably OK then.
I would check now fueling (fuel pump, injectors), inlet airflow restriction (blocked air filter) exhaust restriction (blocked cat).

Weston

AFM AFM AFM

When my car cut out at 5200+ RPM at WOT, it was a combination of two things... #1: the DME's WOT map normally runs a little bit leaner there. #2: the AFM track was worn out in that area, causing it to reduce fuel quite a bit. The combination of the two caused it to run so lean that there was no combustion, and the engine cut out like a rev-limiter. I just re-tracked the AFM, and everything was good again.

Multipass

Thank you everybody. Good news: I don't have to replace the valves, bad news: back to square One (

I have opened up the AFM, checked the voltages and lightly sanded the contacts per Clark's Garage instructions. There were groves, but they are normal.

I have also checked:
- fuel pressures at different conditions (all within spec).
- fuel flow (well above min)
- flywheel ref sensors

Replaced:
- all vacuum lines
- spark plugs, cap, rotor, wires
- ignition coil
- air and fuel filters
- injectors: cleaned and balanced

I also ran the car without the O2 sensor (to reduce back pressure) and the car behaved the same way, just louder.

The car runs hot, but the fans keep it from overheating. The missfire happens before the car warms up above normal. I was going to deal with this after resolving the first issue.

The car is a '88 NA.

I am pretty close to giving up and tacking it to a mechanic. Can anybody recommend a 944 expert in the Portland Oregon area?

Thanks again!

Makis

Removing O2 sensor will not reduce back pressure. Removing the CATwill though.
From what you saying it appears your engine is overheating. This has to be fixed for sure. As you seem to have tried a few things already it is likely the overheating is related to the miss at high rpm.

A lean mixture will help overheating and also cause misfire at high rpm. As your fuel pressure is OK it may be that your injectors are not working as they should causing lean mixture. Other causes could be a faulty ECU or even a head gasket issue (which can cause overheating).

xsboost90

could have a clogged converter...

marky522

Have you checked the WOT switch in the tps??

Mark

Multipass

Mark, good question, I checked both TB switches, they were fine.

Multipass

Update: Issue resolved. I took the car to A&P Specialties shop here in Portland and Alan diagnosed and replaced both flywheel sensors, the ones that are on top left of the engine. The lead wires were bad and were sending a bad signal to the ECU.

Hope my experience helps others in similar situations.

Now if I can just get the overheating figured out I'd have my car back