944 Back Fire at Startup
 

Does anyone know what could cause this? It is a stock 944 as far as I know. I do know the engine was rebuilt but I dont know how long ago that was. The engine runs perfectly except for when I start it, it usually back fires and grinds the starter badly...

I put 93 gas in it everytime which helps reduce this but it still does it. I tried putting a booster in it and that reduced it even more. A stock 944 NA is supposed to be able to run on normal gas right? What could be causing this?

BTW, I checked the computer with the ignition timing and it is set to stock. I tried slowing it down and it just made it worse.


Thanks for any help!

i'm only guessing here, but it sounds like something to do with the fuel, too lean or too rich, at startup. i think there is something that alters the fuel ratio for starting, either the DME or a valve.

have you replaced the AFM at all? That would adjust your mixture wrong if you have a dead one.

This is a known problem, and Porsche put out a tech bulletin on it around 1989 or so.

It seems that the smaller (gear reduction) starter generates a stronger magnetic field when it is cranking, and stray magnetism fools the speed sensor into firing the ignition pulse on the compression stroke. This results in a "thud" and a halt in cranking for a second. The next few times that you attempt to start it, it may be just fine.

The fix is to retrofit the later flywheel sensor bracket and a shield for the speed sensor to stop that.

And, that's the way it is....

gb

that is almost the wierdest problem i've ever heard of. But i know that jeep had a recal one time for late 90's on the routing of the spark plug wires.

I wonder who found out the porsche problem. A magnetic field causing problems with another sensor would be a hard to find nail head. Did someone go through with some sort of eddy-current machine and find the odd magnetic field properties.

Wow thats interesting and I agree that it must have been difficult to find that!!!

How difficult is that to change out? It doesnt sound too bad.


Another thing I was wondering is this... When I am crusiign along the highway at about 2900RPM or so and then I give it just a little bit extra throttle to accelerate slowly I notice that the tach twitches a little bit. It twitches about 100-200RPM up...is the engine trying to advance the timing too much and then backing off?


Thanks guys for all of the help!!!!