Here goes. The car always starts fine and runs well, I drive it for 5 mins or so, stop, fill up with petrol, then start the engine and drive off - but sometimes the car runs rough!

It's not petrol because it sometimes happens even if I just pop into a shop etc it just feels like it is missfiring, hesitating.

This also mainly seems to happen when the weather is warmer as it has been recently. I have replaced the coils, it's had a new air flow meter so count them out! Do I need a new o2 sensor?

Any ideas?

 

bad gas cap

 

Not sure what that means!

 

I had problems that were similar that were fixed by replacing the oxygen sensor. Given your sensor is pretty old and a new one is about $85, it's not a bad start.

 

Dirty fuel filter?

Marc

 

A dirty fuel filter can slow/stop the flow of gas/petrol and can cause performance problems; IE stalling, hesitation, poor throttle reponse, no start, etc.

I thought, "Dirty fuel filter?" explained it all... Just me though I guess...

 

Also, there is a nice little temperature sensor at the front of the engine that is notorious for failing. It is a bear to get at and is generally good practice to replace it whenever the engine is dropped.

That sensor may cause the engine to run erratically if the computer sees an incorrect temperature.

Marv

 

Yeah, I kinda thought it was clear what I was suggesting.

 

I thought Indy was just having you on, referencing the earlier post about the gas cap.

 

Brookewebb is a new member... I don't think he was privy to the gas cap threads that have been floating around here... He didn't know about the gas cap sealing being an issue.

 

i thought that it might be the gas cap as some systems depend on pressure to run right. it could be anything. how about having the codes pulled on the car as we are just guessing. could be broken belt and all sorts of things, too expensive to throw parts at it. i was not joking about the gas cap i just jumped to this thought as you were mentioning getting gas.

 

Fuel filters can be a bit like having a ping-pong ball floating around in your gas tank. They can work fine sometimes and then the flow of gas dislodges something or otherwise blocks the flow and then you have problems. Just like having something rolling around in the gas tank. Sometimes the pickup will suck it up and the line gets clogged... Stop the car... release the pressure and then everything goes back to the way it was... Only to appear again in the future.

 

It could be that it takes 5 minutes for all the stuff to work loose.. Then, once free, it clogs after restart. After sitting overnight... It takes another 5 monutes to work loose again. Stranger things have happened. I don't know it that is the culprit here... But it's worth checking out... Especially if the fuel filter hasn't been changed for a while. Just my .02

I have a rough idle now too. I reset the DME, let it idle for a while then took it on a "spirited drive"... It won't idle now. I've had caps & rotors, fuel filter and my 02 sensor just came in... When I have the time (In a few weeks) I'm going to have "Porsche Weekend" and change all that plus change the brake fluid.

 

brookeweb,
Have you tried replacing (or better yet jumpering) the DME relay? Before you start replacing anything else try to find someone (a shop) with a Bosch Hammer. They can check out your O2 and CHT sensors (or you can with a multimeter). What model year is your 964?

 

The car is a 1989 C4.

I've had the car on a hammer which returned no fault codes - this was what led to me replacing the coils, which actually did help. The DME was replaced at the same time. I have done a full reset since, following the instructions on here. It really does seem to be the warmer weather as it was a dream throught the winter. Given my other post about organising the rally to monte-carlo and the heat involved I need to get this sorted! I guess that because the problem is only very intermittent the Hammer won't show it unless it experiences it 'live'. Just had the annual service - do they replace the fuel filter as part of that? I've also, god forgive me, put some of that expensive fuel injector treatment through it about 3000 miles ago. didn't do it any harm, but not sure about the good either....


Don't worry IndyCam - I wasn't offended in the slightest. I am a total Noob who gets his car serviced by the pros - in fact I am only really qualified to polish the car at weekends!