TD in DC

and I am not talking about the aftermath of a Taco Bell Visit . . .

My '84 944 with a freshly rebuilt motor is running pretty well. However, on Sunday morning at the track, it was stumbling a fair bit. Specifically, it seemed like it was "burping" or not getting fuel for a split second occassionally. Most of the time it is really no big deal, but if you are anywhere near the limit in a corner and it stumbles while you are using full throttle to keep the car planted, the burp can "unsettle" the car, which is not too pleasant (although not nearly as bad as when the hose to the overflow tank came unhooked and dumped antifreeze all over my front tire ). I am planning on changing the fuel filter. The car has new fuel injectors. How should I systematically investigate this issue, particularly when I am not even sure it is fuel related.

Also, the car often backfires and shoots flames out the tailpipe when I upshift or when I let off the gas to brake.

Ideas?

Thanks,

TD

TD in DC

Bueller?

Geo

My first guess is a leaky injector. That would explain some stumbling and would certainly explain the custom flamethrower exhaust.

Techno Duck

Burping and backfire i believe means you are running rich. Possible causes are a bad FPR (fuel pressure regulator) or even a bad fuel pressure dampener. I would start with checking your fuel pressure at the rail.

M758

Few things to check,
1) Check all lines for vacuum leaks.
2) Fuel Pressure Regulator
4) DME/Fuel Pump relay - Replace it as you these fail can could be a problem (if not you have a spare)
5) New fuel filter
6) AFM - pull the cover and check the wiper tracks. FR Wilk's website has a nice write-up on how to test and fix it.
7) replace throttle position switch
8) Swap DME units and test

TD in DC

Thanks Guys! I will start the process of troubleshooting . . .

David '96 993

TD,
Did you ever find the solution to your problem? Your symtoms sound very similar to my issues in my SPEC 944. It was a huge problem this weekend at Laguna Seca. I went 4 wheels off at T6 going full tilt boogy. Not fun.

My issue occurs under load only under specific contiditions: 5500-6000rpm, under load, but not all turns at all tracks. LS brings out the worst.

So far I have replaced or tried: brain,DME,coil,wires,plugs,AFM,flywheel sensor, head temp sensor. I thought it was electrical, but now I noticed a distintive pop when I shift which leads me to think it is fuel. I did test pressure, but obviously not under these conditions.

M758

David,
You may want to replace the fuel pressure regulator and damper. Those could be source of problems.

David '96 993

Joe,
I just ordered one from Vertex. I was reading up on fuel (electrical doesn't seem to be the problem at this point unless it has something to do with the harness) and some of the simptoms I have were mentioned on Clarks Garage.

I am also going to replace the filter. Can't hurt. I may also check the screen in the tank, but that was checked some time ago as one of the first things I looked at.

When this thing kicks in, it adds about 3-6 seconds on my lap times

Thanks for the suggestion.

Bill.e1

TD,
I'd look at high-V side of ignition. Cracked dizzy cap, bad wires, etc. If your car burped antifreeze, it can kill the plug wires.

My 944 had similar issues one eventing. I was sure it had jumped a timing belt tooth or two. Luckily problem was simple- distributor rotor screw had backed out and rotor would wander. Since actual timing was not effected, It fired the correct cylinder one out of four.

Good luck,
Bill

Charlotte944

There is no way that these cars will shoot flames on a down shift unless the TPS is bad. I say this because when the throttle is closed and the DME sees engine RPM above a set point, the DME turns the injectors off. This condition is called Trailing Throttle, or Over Run. The DME is programmed to turn off the injectors to save fuel.

M758

Well my 944 spec car occasional pops on upshift. Happens mostly exiting turn 9 at PIR. Usally 2-3 times per weekend out there it will pop as I am upshifting. Never a concern as the car runs fine, but does pop. Only seems to happen at the that corner and the pop has no impact on speed for tha lap.

That said I do not thing it is related to TD's problem which he solved or David Hirsch's issues.

David '96 993

Joe,
TD's problem turned out to be his alternator. I will have that checked as well.

I did order a fuel pressure regulator. What does the fuel dampner do? I did not order one of those yet. I have added to my checklist though.

M758

Damper controlls the fuel pressure spikes. It is less critical than the regulator.

Z-man

I'm curious - did you recently run the AC or defroster? I'm having similar problems (stumbling) and suspect my evaporator hose is clogged and is spilling H2O on some wires.

- Z