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Re: fuel system or ignition? Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Sat, 24 May 2008 21:45:03
In Reply to: fuel system or ignition?, arcticsaab, Sat, 24 May 2008 13:29:00 Members do not see ads below this line. - Help Keep This Site Online - Signup |
It being a turbo, hard acceleration usually translates to turbo boost. And stumble under boost usually points to ignition. Specifically, spark plug gap that is too big. Pull the plugs and check the gap. Make sure they are spot-on per the decal under the hood, and replace the plugs if they look old. Check the timing - too advanced can be a problem. If the cap or rotor are old, or they show signs of arcing (inside), replace them. If the spark plug wires are over 3 years old, replace them.
Under boost, it is harder for a spark to jump. The bigger the gap, the harder for the spark to jump. If it doesn't jump, you get misfire and stumble. The gap is made up of the gap at the plug, but any problems in the spark plug wires will also add to the gap, as will any defect in the cap and rotor.
At idle, the internal cylinder pressures are low, and it's easier for the spark to jump.
The other place to look is the Air Mass Meter (AMM). It measure the air going into the engine, and schedules fuel flow. If the AMM isn't measuring right, you're going to get some seriously wrong air-fuel mixtures, which can cause stalling. Fuel only burns when the air-fuel mixture is in a narrow band - way too rich or way too lean, and the engine shuts down. Now, the LH fuel injection system is pretty clever, and can adapt to a bad sensor. Up to a point. At idle, it knows the throttle plate is closed because of the throttle position switch, so it can 'guess' at the right amount of fuel to idle. A little rich, actually, which means a good idle. Driving along, it can guess at the necessary fuel from engine speed. But it's the transients - accelerating or decelerating, when it can't guess very well, and it all goes badly.
A common symptom of a bad AMM is if you try and accelerate away from a light and at first the engine seems to ignore you, then it bogs down. If you're really, really light on the gas and accelerate slowly, it's somewhat OK.
posted by 76.202.162...
, Sat, 24 May 2008 21:45:03 <-- Viewing This Message
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