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Backfire Help.


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<_< I just purchased a 1999 Boxster last week with 50K miles on it. Twice in the last week upon startup in the morning (about 40 degrees F) the car starts to feel like it is stalling and it backfires repeatedly (2 per second??) as the RPMs bounce between 250 and 1000. If I shut it off and immediately restart it is fine. No CEL or any other lights. I park in a garage. It seems to hesitate a lot as I back up and as soon as I throw it into gear it starts this backfiring or whatever it is doing. It is a popping sound coming from the rear of the car (I am guessing the exhaust). Any ideas??? I am guessing that I have started the car at least 50 times in the past week and it has only happened on two occasions about a week apart.

On another note I am very elated to have just purchased my first of many Porsche's. I love the car, and other than this issue it is in pristine condition.

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Congrats, have fun with the car.

To your questions: Has the car been modified, e.g. Evo intake, sports exhaust?

It may be seasonal, something about fuel additives/mixtures differences between summer and winter fuel to which the engine is reacting, this will go away by itself. Try switching to lower octane and higher grade (name brands) for a while and see if this helps.

It may be related to dirty spark plugs or possible some dirt buildup in the fuel or oxygen lines, or possible leaks, loose fittings somewhere in the system. You're at 50k miles, it is a little bit early to do a 60k miles, but you could consider parts (e.g. change fuel filter).

It may also be related to the computer setting the fuel mixture to rich. This usual is linked to a dirty MAF sensor, more common if the intakes are changed. Try cleaning the MAF (very carefully, don't use oily substances), and consider replacing the MAF as a last resort.

I vaguely remember that the oxygen sensors are activated after a few minutes of driving, not immediately. Maybe their activation is related to your observation.

Any pattern that you can detect? E.g. only on a cold engine?

Does it only happen after you have been on a short drive (not properly warmed up the engine)?

Have you been on a long drive, when the engine gets warmed up properly?

Does it go away/happen afterwards?

Do you engage the clutch at very low RPM (lugging, i.e. below 1500RPM) with a cold engine?

Does it happen when your fuel is close to empty (causing sediment swirls in the gas tank)?

Just a couple of thoughts

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Thanks for the ideas. Since I've only had it for a week and it has only done it twice it is hard to say that there is a pattern. I hope to be able to drive it for about another month (please don't snow before December). Maybe I will see a pattern. If so I'll post. To the other post, not CEL at all. There are not aftermarket parts on it that I have found so I doubt it.

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Last week I had the same exact thing happen, I have a 98 with 33000 and It did it for about 5 min and stoped, a couple of days later again. The first time the car was parked overnight and the second for about 8 hours. I thought it might have been Shell fuel? Or spark plug or fuel filter. I am still not for certain.

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