[RunningonMT]

I have a '93 964 C2 with stock engine, stock ECU (no performance chip) and stock ignition.

It runs well under partial throttle, but when at wide open throttle, it hesitates at ~4800rpm, then continues at a much lower power level than I'm used to. This started happening on the last day of a HPDE at VIR. At several marks on the back stretch, I was 5-7 mph slower than the previous day. At home, I've been testing on freeway on-ramps and the issue occurs in both second and third gears. I've tried a number of things: drained the fuel and re-filled with fresh Shell gas, replaced the fuel filter, swapped out the DME/ECU to my spare, checked the secondary distributor belt (its there) and checked the function of the resonance flapper valve (rotates on startup).


Carrera51 had a similar problem caused by a bad fuel pump with low output pressure at full throttle. Will probably order a new pump and new fuel pressure regulator. Considering they are over 20 years old, it won't hurt to have spares.

What steps would you recommend to trouble shoot this problem? Any specific areas that you would suspect?

How do you determine that the phasing of the two distributor rotors is correct? If the fuel pressure regulator is the problem, can it be replace without removing the intake manifold?

Just in case I can't find the issue, which shops are best suited to fix this?

Thanks.

[MatP]

Full throttle microswitch activating? Signal getting to DME?
Did you check fuel pressure into and out of the engine?
Are you getting spark on the secondary plugs? / Both coils firing?

[RunningonMT]

Originally Posted by MatP

Full throttle microswitch activating? Signal getting to DME?
Did you check fuel pressure into and out of the engine?
Are you getting spark on the secondary plugs? / Both coils firing?

Thanks for the great suggestions.

Checked the full throttle and idle switches at the engine connector. They show open and closed at the appropriate positions of the throttle for each switch. I will have to research the pin out on the DME connector to determine if the signals are getting all the way to the DME. Will post the results later.

Checked for spark by disconnecting one of the coil packs at a time. Smooth running idle speed at the same level using all combinations: both packs together, secondary pack alone and primary pack alone. The car was cold, so idle speed was ~1050 rpm.

Have not yet checked inlet fuel pressures (test port on the left side fuel rail located between the pump and the pressure regulator) or pump delivery rates. Would like to check these at idle with the car stationary. Would also like to check fuel pressure at various operating conditions; i.e., partial throttle and full throttle through the rpm range. Will do these test this weekend since I will have some semi-skilled labor available then.

[Intubator]

I had similar symptoms on my 87 and it turned out to be a dying fuel pump...

[MatP]

See if you can get a new pump by the weekend.

[Baywatch]

I had similar problems once and it turned out to be a clogged air filter. Replaced the filter and the car was back to normal. If it does not help at least you've replaced the filter which is often overlooked.

[Intubator]

Might try removing the air filter and going for a brief 5 minute drive to see if there is any effect... Then of course replace it. On the flip side it could make matters worse if you get more air flow than before with a less than optimal fuel delivery... I think if the filter was dirty enough to restrict the flow that much then it would be REALLY dirty and evident by just looking at it though.

[Trak Ratt]

Originally Posted by Intubator

Might try removing the air filter and going for a brief 5 minute drive to see if there is any effect... Then of course replace it. On the flip side it could make matters worse if you get more air flow than before with a less than optimal fuel delivery... I think if the filter was dirty enough to restrict the flow that much then it would be REALLY dirty and evident by just looking at it though.

I agree on testing but not about seeing. Fuel filter can also be culprit but that usually causes engine to mostly shut down till sediment drops to bottom again. Don't worry about the extra air as the engine can't go faster without fuel anyway. What you are describing is different than running lean at speed.

[RunningonMT]

Originally Posted by MatP

See if you can get a new pump by the weekend.

Ordered pump early yesterday, but has not yet left Arizona. Not likely to get here by the weekend. But we can hope!

[RunningonMT]

Originally Posted by GaryGanoo

I had similar problems once and it turned out to be a clogged air filter. Replaced the filter and the car was back to normal. If it does not help at least you've replaced the filter which is often overlooked.

Thanks for the suggestion, Gary. I accidentally ran this test. I had taken the air filter out for another reason and took the car out for a test without putting it back in. No change without the air filter.

Originally Posted by Trak Ratt

I agree on testing but not about seeing. Fuel filter can also be culprit but that usually causes engine to mostly shut down till sediment drops to bottom again. Don't worry about the extra air as the engine can't go faster without fuel anyway. What you are describing is different than running lean at speed.

Thanks for your comments, Dave. When this problem started, the fuel filter had only a few hours of operation on it (<15 hrs). Thought I might have gotten a bad batch of gas at the track, so changed it anyway. But no improvement with a brand new one.

Guys, I think there's a real hint in the fact that the problem starts at 4800 rpm consistently. I've tested numerous times and it always stumbles at this same engine speed. It happens in both second and third gears. MatP helped me in researching the inter webs and, IIRC, we found the ECU switches to a different fuel / timing map. Not sure how to use this information, but maybe someone out there can help!