Ken7258

When I drive around with less then a 1/4 tank of gas and it's warm out, sometimes my engine cuts out for 3-4 seconds. The hotter it is outside, the more it happens. This has been going on a couple years. I took it to an indy mechanic 2 years ago and they replaced a purge valve. The problem has never gone away and it happened again this weekend. Any suggestions to what the problem may be. It sounds like pressure is building up in the gas tank and stopping flow to the fuel pump. I have a 2001 996 c2.



Thanks a lot.

Macster

Pressure in the tank could only help the fuel pump. Besides the filler tube the only other way for the gas to leave the tank is via the fuel pump and the fuel line out of the tank.

My WAG is some sensor (crankshaft position sensor located on rear of engine comes to mind) is failing in high heat and delivers an implausible signal to the Ecu. The Ecu stops the engine -- shuts off fuel -- when it gets an implausible signal from this sensor cause it can't know when to trigger the injectors or spark. The absence of an error code is explained by the fact there is no affect on emissions and apparently this cut out doesn't qualify as a misfire event so the Ecu doesn't recognize this as worthy of an CEL.

However, the Ecu may log an error somewhere that may be obtainable by proper Porsche diagnostics equipment.

Are you turning a corner when the cut out happens? Covering rough road? More details as to what you and the car are doing when the symptom appears might be of some help.

Sincerely,

Macster.

Ken7258

It happened mostly when I was tracking the car and it was hot out. I could be going straight or in a turn or cool down lap. It would never happen if I had more then a 1/4 tank of gas. A year ago when I brought this up, someone said it could be the fuel pump going. I wonder if when the gas tank is mostly empty and it's hot out and the sun is beating down on the car, the gas expands in the empty tank and causes the fuel pump or some type of venting a problem.

Ahsai

I agree with Macster about pressure can only help fuel delivery. Have you scanned the car? Any hidden codes? The fact that it correlates to the fuel level suggests a physical problem (e.g., for some reason your fuel pump is not picking up the fuel when the level is low). The correlation with temperature suggests a potential electrical problem.

I would suggest scanning the car first (preferably with Durametric) to check for any error codes, then changing the fuel relay, then open up the gas tank and take a look to see if there's anything obvious physically. Maybe even changing the fuel pump. If you really want to make sure it's NOT the fuel pump (so that you don't change out a good nuit) but rather the power supply to the fuel pump, you can probably tap into the fuel pump power and wire an LED in parallel with it so when it happens again you can see if the LED is flickering or not. If it is, the problem lies in the power to the fuel pump.

Pac996

I wouldn't cruise it with a 1/4 tank. High pressure fuel pumps burn out quick if air keeps getting into them. The fuel is required to cool the pump from what I was told. You are going to get air in the pump running the tank that low and thrashing around a track or street. Air also doesn't burn near as good as fuel

Macster

Some type of fuel starvation problem or the fuel pump's overheating.

My 996 Turbo owners manual has a caution about hard cornering on a low fuel tank. (Warning light on low.)

But under extreme track driving even if the light is not on severe G-forces can keep the fuel pump perhaps uncovered for quite a time. If the fuel is hot cause of a return type fuel system and ambient temperature coupled with a fuel pump that needs more cooling not less...

My advice would be to run the gas tank at least half full and refill when it gets to near quarter tank of gas. The few extra pounds of weight is not going to turn you from a michael schumacher into a Eddie Griffin.

In fact if the engine's cutting out on you with you running the car with a low fuel tank level you might be faster with more fuel and the engine on all the time.

Sincerely,

Macster.

Ken7258

Are there any symptoms or tests for a fuel pump going bad?

Macster

Symptoms varying from worst case, no start, to engine cut out under some conditions and this can include high speed/hard braking/cornering or hard acceleration. At higher engine speeds and loads a going bad or marginal fuel system may not be able to deliver sufficient fuel. The engine may feel flat, stumble, hesitate even cut out. The engine controller may register one or more misfires too. At idle and around town with low loads and engine speeds the symptoms may not appear.

If the fuel system is delivering less fuel or delivering fuel at a lower than expected pressure the engine controller can detect this indirectly by reaching the end of its enrichenment adjustment threshold. The engine controller will compensate for lower fuel pressure by adding more fuel, increasing the time the injector is open. (The injector pulse widths are based on an assumed fuel pressure.) At some point the engine controller will set an error code and the CEL will be turned on. What can happen is under some conditions the fuel pressure is ok, adequate. Then heavier engine loads and higher engine speeds require more fuel and the fuel system can't deliver. The engine controller compensates. Then the loads decrease the engine rpms come back down and the engine controller spends the next few moments unrolling the adjustment it made to the fuel to compensate for the inability of the fuel system to deliver the proper amount of fuel at the proper pressure.

If fueling problems suspected in the fuel system, and this of course includes the fuel pump, pressure and flow tests are generally used to verify pump can deliver the proper pressure and flow. The pressure and flow checks are done as far from the pump as possible to ensure as much of the fuel delivery system is tested and if the tests passed is working properly. The problem may not be a bad fuel pump, but a crimped/pinched fuel line, the fuel pressure regulator maybe going bad, or it could even be one more fuel injectors. The entire fuel delivery system runs from the tank to the injector.

Sincerely,

Macster.

Pac996

Pressure test the fuel outflow. Even a high pressure pump will still be putting out what you would think is a good enough gas flow. Just it isn't high enough for the system. Just don't run the car with only a 1/4 tank. Even with baffles the fuel is going to ride to one side in a hard extended turn.

Loren

I would start with simple/obvious.
Have you replaced the fuel filter?

Ken7258

No, I actually was on your site yesterday looking at the instructions to replace the fuel filter. I went to my indy mechanic yesterday and he pulled the codes, there were zero. Absolutely nothing showed up.

Over the years, I usually don't fill up before I'm below a 1/4 tank. I guess I'll replace the fuel filter and always keep it above a 1/4 tank, and at least a 1/2 tank at the track.

RPMulli

Have you figured out the problem ? i have the same issue but when i fill may tank after driving with 1/4 or less my car wont start for 4-6 hours after i fill it up.

EastBay

Just a point of general automotive, mid 90's Ford Tauruses were famous for pseudo vapor locking on less than 1/4 tank of fuel. Low fuel tank level would cause the fuel to become super heated when returned to the tank and repumped. Keep 3/8 tank of fuel in the tank as a heat sink, no problem.

Never a problem on my C2, but maybe your pump is too high, to weak, screen clogged or some other in tank issue. I'd take a look.

RPMulli

its a brand new pump....