Hesitating at high RPM

BKOMAR · BKOMAR's Garage

I just got back from a 1200 mile round trip in the 911. The car ran flawlessly throughout the trip and I got ~24 MPG which I was quite happy with. I ran at highway speeds (70-90) the entire way and I had no problems. At the end of the trip, I noticed that the car started hesitating at high RPM's and in some cases would not even get up to redline. I would have the gas floored and the car just didnt seem to have any ooomph. Driving normally at lower RPM's I could see no difference and it idled and ran fine. It almost seemed as though I had something blocking my air intake (which I checked and it was OK). Before I left, I inadvertantly overfilled the oil tank (~1/2- 3/4 qt too much) but I didnt think that this would pose a problem and I saw no blowby anywhere. The only other thing that I can think of that would be causing this, and this is my query... I let the tank run waaaay down, because I wanted to see how far I could get on a tank (for future useful reference) and I am pretty sure that this had not been done for a very long time in this car. Could I have clogged up the fuel filter? Is what I have described indicative of a clogged fuel filter? The car starts, idles and runs normally without a problem its just in the higher RPM's (when it needs more fuel?) that it stumbles and hesitates. Any input? Also, I only saw 4 other Porsches the entire trip all of which were in St. Louis. Thanks, Brian

wckrause · wckrause's Garage

Sounds like a clogged filter. It's cheap and easy to replace.

BKOMAR · BKOMAR's Garage

Thanks guys, I was pretty sure that that was my problem, but in the back of my head (always) is the worry of something more "expensive" I will order a new filter today! THanks, Brian

JackT · 04-10-2002, 12:44 PM

Check your warm up regulator. I had the same problem about a month ago Cost about $200., including labor to fix.

Superman · 04-10-2002, 12:56 PM

Do the simple and inexpensive stuff first. CIS cars don't tolerate contaminated gs very well. Rust particles can clog all sorts of CIS components. Clean the screen and the gas tank, replace the filter, and see what happens.

BKOMAR · BKOMAR's Garage

What would the warm up regulator be doing to mimic what I have described? Isn't that the same as an Auxiliary Air Regulator on D-jet (914) cars? Meaning...doesn't that just let more air in for warmup? Please correct me if I am wrong...I am trying to understand this system...and I assumed they did the same thing. Thank, Brian

Superman · 04-10-2002, 01:58 PM

Yes, but the suggestion above deals with the Warm Up Regulator, not the AAR. If the WUR were bad, your fuel mixture could be messed up enough to reduce high-rpm performance, maybe. I still think it's a fuel starvation problem. If you don't find crud in the fuel, I might do a volume test of the pump. I think it should move about a liter of gas per minute, but you should find out the actual spec. Chances are excellent that your FP is bad as JW has suggested, but I like to be sure before I go replacing hundred-dollar parts.

wckrause · wckrause's Garage

If your warm control pressure was real low (bad WUR), and you had compensated for it by just adjusting the mixture, then it may be possible for the sensor plate to rise up to its limit. This could cause a fuel starvation symptom as described.

I'd rule out the fuel filter screen and the fuel filter first, seeing how your symptoms occured after letting the gas level get low.

Brian Cameron · 07-20-2002, 11:36 PM

Brian - any update on how you made out with this? Did the fuel filter / screen clean etc work? Did you do something else that solved the problem?

My 79SC has recently started missing at 5000 rpm so I searched the archives - found your problem which sounds identical, so thought I'd see if you had any luck solving.

thx/bc