Thread: 930 misfire
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MikeSchNail MikeSchNail is offline
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Join Date: May 2012
Posts: 30
YES! More info...

Very good description of your problem. Very good points brought up by many. Forget the ignition at this point, provided the timing is correct, (likely so)

Fred has a real good point bringing up the overboost function, although usually when it kicks it, the interuption in power to is sudden for a lean condition long enough to cause a backfire. Also, I have never seen one change pressure settings from wear. 1.3 or so is the right where they kick in.

Checking CIS pressure is a great idea, provided you have the gauge. However, that is only half the story in testing your fuel system. The gentleman that mentioned one of his pumps failing was experiencing a fuel Volume problem.

The CIS system has an enormous amount of fuel volume flow compared to an EFI system due to the fact that is uses fuel pressure to control mixture via the fuel distributor.

It is common to test the pressures in the garage and all seem well and good, because you are not out under a load creating the same conditions at which you experience trouble.

The best way to test fuel volume is per the factory manual. It states to measure the fuel flow AFTER the entire system while it under pressure. This is done by disconnecting the fuel return line at the hard line on the way back to the tank. You probably need to connect o piece of hose to the metal pipe coming from the engine somehow to route it to a suitable container for measurement.

If you don't have the volume, you won't have the pressure, so it's a half@ss test to only check volume, but if this is likely you problem and you simply want to confirm it without procurring a CIS test gauge, there is no harm in checking volume alone. Again, if the vloume/flow tests pass, don't assume the pressures are good, but if they fail, start looking into fuel supply before bothering with a gauge. The most common easy problem is a bad fuel pump relay, next there could be an issue at the fuse block, there could be a bad pump, or there could be a restriction in the supply to the front pump from the fuel tank. There is a screen in the bottom of the tank, as well as some passages that can get stopped up.

If both pumps are running, one under middle of front axle centerline, one under car in front of LR wheel, but making ALOT of noise, check the supply to the front pump from the tank. They are noisy from cavitation from trying to pull fuel from the tank through the screen, passages.

If only one pump is running and making ALOT of noise, figure out which pump isn't running and why. Power? Ground? Dead pump?

I hope this is your problem as it is much easier to solve a volume issue typically than diagnosing a pressure problem, IMO

Good Luck,

Mike
Old 05-21-2012, 11:36 AM
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