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Join Date: Dec 2003
Posts: 120
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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! |
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11-12-2004, 10:50 AM
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winter-hater club member
Join Date: Oct 2003
Location: salt lake city, utah
Posts: 24,705
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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.
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2000 Corvette - ????, 2007 Buell XB9R - Astrid, 1996 Discovery - Piglet, 2000 Forester "COOL PRIUS!" - Nobody Ever |
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11-12-2004, 11:29 AM
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Join Date: Oct 2004
Location: Memphis
Posts: 340
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have you replaced the AFM at all? That would adjust your mixture wrong if you have a dead one.
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Ryan '84 944 moneysucker edition (SFR, Corbeaus, etc.) '02 Jetta TDI 280Lb/Ft 145Hp @ wheels. RC2 PP357's |
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11-12-2004, 11:46 AM
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Join Date: Jul 2004
Location: Upstate New York
Posts: 3,278
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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
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Good luck, George Beuselinck |
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11-12-2004, 04:12 PM
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Join Date: Apr 2004
Location: Circlevlle, OH
Posts: 70
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Quote:
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.
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Parting out an 83 with black interior and two red racing seats |
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11-12-2004, 05:53 PM
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Join Date: Dec 2003
Posts: 120
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Quote:
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!!!! |
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11-15-2004, 06:04 AM
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