Backfire problem

[Gordon Rankin]

I havent adjusted any engine settings but my car will have a small backfire upon starting, occasionally. It does not happen all the time and does not coincide with any other circumstance. It started doing this about 4 months ago and I dont understand why.

Car runs fine, does not overheat. Plugs, dist cap, oil were all changed within the last 30 days.

Any ideas?

[rsrmike]

Where does the backfire come from? Induction? Exhaust?
I assume the intake as exhaust backfires are rare when starting...

I would first check for any intake leaks that could cause a minor lean condition exaggerated when cold (not likely given your description but possible and worth the effort anyways.)

Consider getting a new blue coolant temp sensor as it is an easy cheap repair, and difficult to diagnose intermittent troubles. I've had them give me fits at times, either have an open circuit, which usually cause a rich condition, or be shorted internally giving low resistance, which will cause a lean condition (usually after the car has been overheated at some point but sometimes, just cause it felt like it.)

Either fault can rectify itself just from the simple vibration of unplugging the connector. The last one that gave me trouble was the latter example, It checked fine at the sensor w/ an old pigtail plugged in and hooked to my DVOM measuring resistance. I finally found it reading the resistance at the DME. It read really low ohms, unplugged the sensor, checked it there again, it checked fine, plugged it back in, measured at the DME, it was fine there too, and the car started and ran fine till it warmed up and cooled all the way back down again, measured again and it was shorted internally. New blue temp sensor fixed the problem after much troubleshooting. Another shop had previously made a "machine gun" attempt at fixing the problem to the tune of over $1,000. New AFM, speed and reference sensors, plugs, wires, etc. That was what prompted me to find the problem before changing ANYTHING.

In your case I would just try the sensor.

Also, the AFM could cause it, but not likely if it runs fine after starting. THe above mentioned car ran fine after a few seconds, I assume the vibration of the engine caused it to "unshort" the thermistor.

You could also be getting interference from the starter in the reference sensor. Porsche redesigned the speed and reference mounting bracket to include a shield for the reference sensor after they changed the bellhousing design which left the reference sensor exposed the the electromagnetic interference generated by the starter. (according to their service bulletin) It would usually cause preignition or rough starting, sort of like an old lead sled w/ the timing advanced too high...

But your reference sensor could be aging to the point that it still works but does not generate as strong of a signal and is more susceptible to interference. The starter could also be getting tired generating more interference, or the battery is slightly weaker or the connections are getting dirty, either of which would turn the motor over just a little bit slower happening gradually over time to where you don't notice a change, but reducing the reference sensor strengh when starting.

What does it sound like when it "backfires" does it sound like it's trying to blow the AFM or the airbox off? If so, you may wind up calling "fair catch" on your AFM in a parking lot one day, Not good

If it's more of a pop w/ a slight hesitation in turning over, I would look to the reference sensor.

If it comes out the exhaust, well, I have no idea as I haven't hunted that one on a 944...

Good luck!!

Mike