INURGRL951

well let me put another angle on things here it could b the reference sensors also they could be out of adjment .....have u check them and also there ohm readin also (like a small break in wire) only being bothered at heavy loads causing your problem to happen ??

968gene

I'm still having the same problem with my 944S. It was not the AFM.

951and944S

A valve spring or bad cam lobe will pop back through the intake tract.

There are a few points specific to 944S/S2/968 that you might want to learn more about.

Because of the ignition rotor cap difference where the wires plug in and face up, these cars are prone to arc the voltage destined for the plugs onto the bottom of the hood right above the cap, there's a bulletin on it.

Secondly, again, a bulletin in the 944S/S2 Bosch manual makes mention of high speed misfire due to the alternator overcharging and suggests a trial run with the alt/ac belt removed to verify cause.

T

LI Steve

Thanks for the advice - I will add these to my list of items to check into. Is there a simple way to test them, or would I have to swap them out to see if they are the problem?

LI Steve

Thanks T. There is definately no pop back through the intake. Would that still be the case if the broken valve spring were on the exhaust side?

I have replaced the cap, rotor and wires, so I don't think they are arcing at this point - how could I tell other than to have a video cam under the hood?

I'll try to check into the potential alternator issue too - thanks a lot for the info!

Makis

I had valve springs broken on my S2 and there was no specific problem on WOT. The car was just not running right and was misfiring slightly at most rpm and throttle position. Redlining was possible but not a pretty site and the power seemed to be well short by at least 30bhp. Suspect them if you think the car is not running correctly in general and not just WOT. Also as the S2 has decent power and if you never drive one before you may thing that the power delivery is OK even when springs are broken. So I will remove the cover first (be very careful how you remove the 13 screws as they can snap) and check the springs. You can not see them but you can push the tappets in and if any provides little resistance then you have broken spring. Replacing the springs in situ is possible but you need the right tools. At this stage do a compression test in case the the broken spring has damaged the valve (can happen easily on a exhaust valve as it will burn the valve if the car is driven with a broken spring) In this case you will have to remove the head to replace the spring and valve. A burned valve will give you misfire at idle so you probably OK.

In your case as it happens on WOT I believe the ECU does use fixed maps for fueling ignition. If you have a misfire and the car is mechanical sound then the problem will be the mixture at WOT is wrong. This is either a fueling problem: Injectors, FPR or fuel pump. Or air feed problem: Air filter, AFM or throttle body blocking air flow by some reason, or collapsed CAT and exhaust.

968gene

Steve, can you "sneak " up on high RPMs? In other words, can you feather the throttle to reach high RPMs without the problem? This is how my car reacts.

LI Steve

Thanks for the info on the valve spring - I will try to check this, but am not certain that I would know for sure if one of the springs are broken or not. How much pressure should I put on the tappets to see if they move or not?

As for the Fuel/Air suggestions, I am not convinced that it would be the throttle body / FPR / pump or collapsed CAT because the problem is only at 5400 RPM and only under full throttle - is that a good assumption? I have tried to 'create' the problem under a bunch of other scenarios, but can't. The car runs great in every other instance.



Yes, I sure can. As long as the pedal is not to the floor, the car runs fine, all the way to redline. Even at 95% throttle, the car revs up perfectly to redline.

Have you eliminated any of the above possibilities on your car yet?

968gene

Steve, here's what I've ruled out: the car is a track car and doesn't have a cat, the AFM was changed out, no change. The plugs, wires, cap, rotor are all new. The head was just rebuilt. Alternator reads 13.5 on a new Autometer gauge. The chip was replaced with no change. I have not checked the reference sensors.

white924s

So the TPS checks out? Have you checked the TPS wiring harness as well? This problem sounds a lot like an elecrical problem because it only happens at a certain throttle position and in a certain rev range. I would check those two things and maybe even try putting in a different (known good) DME to see if maybe there's a problem with how the DME is processing the WOT signal

LI Steve

I checked the TPS at the DME connector inside the car, so I think that would also check the harness. Maybe it is only acting up when hot???

I am also thinking along the lines of electrical for the same reasons you mention (problem only under very specific circumstances). Where can I find a known good DME without buying a new one? In the 928 world, I work with one of the "Big Three" parts suppliers that would let me try a replacement one and return it if that were not the problem...

Thanks.

Makis

My understanding is that at WOT the ECU runs with fixed maps and ignores AFM, O2. This means if problem with fueling or air flow you may get misfire. When not WOT ECU monitors AFM O2 and can correct small fueling or air flow issues.

Its easy to tell a valve spring is broken as when compared to a good one the resistance will be very small. Hope this helps.

INURGRL951

ok steve they should check out at about 1.030k ohms or abouts that but not below 1k and the fact that ur car is runnin means there are both ok but the in the area of adj they need to b 8mm for the keyways on the flywheel they way it soundin is that u have too much of a gap so u need to bring it in ..but if u dont have the tool to alin the reference sensors housin u need to have sumone turn the motor over till u c the allen keyway in there(needed even with tool) threw the sensor hole(the closes one to the block) and then put about 8mm of tape on the end of the closes to block sensor and c if it tags the keyway (u put it in all the way in place and remove again) and c if it has marked the tape if not u have to loosen the housin and adj down to where it circles the tape ......(sorry bout the typin im not a fan of it ...lol)

INURGRL951

oh yeah and are u runnin a a/f gauge on ur car??

LI Steve

OK, I have a little more info that might help narrow this down. I haven't had a chance to test the AFM, ECU or sensors yet, but:

The problem does happen every time in 1st, 2nd, 3rd gear while accelerating, but only at full throttle and only at 5400RPM. It does NOT happen above 5400 RPM. So, if I accelerate at full throttle to about 5200, then let off about 1/2 an inch and accelerate through 5400, then put the pedal back to the floor, the car runs perfect, right through 5400 and right up to redline, every time. So it seems to be only at 5400RPM, not above and not below, and only at full throttle. Full throttle above 5400 seems fine.

Does this narrow it down at all? I would think it it were a valve spring, arcing, fuel pressure, injectors or AFM, there would be some other circumstance that would cause the misfire (especially above 5400 RPM). I drove about 100 miles today, under varying conditions and could not reproduce the problem, except as described above.

Thanks again for all of your help!