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Engine surges at low rpm

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Old 10-24-2008, 05:10 PM   #1
brit2001
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Default Engine surges at low rpm

I've had this problem off and on for a couple of months and can't get a fix on it. I spend alot of time driving in the 30-40 mph range. My car is a A4 and has BBK intake and throttle body LG long tubes with high flow cats, X pipe and Borla exhaust. I also have 3:42 gears with a 3200 stall.

Here is the problem: the car will start to shutter and shake when the I'm in the 30-40 mph range. It is intermitten and will some time make the check engine light flash but will go out once Igive it more rpm or the torque converter unlocks. It will generally set a to rich bank 1 and 2 and intermitten misfire codes even though the check engine light is not on. The car has a good tune on, done by charlie at RPM. I'm lost. Any help would be apprectiated. Thanks, Laurence
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Old 10-24-2008, 09:14 PM   #2
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Sounds like a fuel problem. Have you changed where you get your gas over the past few weeks?
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Old 10-24-2008, 10:06 PM   #3
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Quote:
Originally Posted by mellowyellow01vert View Post
Sounds like a fuel problem. Have you changed where you get your gas over the past few weeks?
Had the same exact issue when I filled up with a bad batch of premium from Mobil in my L98 powered C4. Went through the whole "tune up" process, new plugs (only 15K on the old ones), checked wires cap and rotor - all good, checked for EGR function (checked fine), verified ignition timing - right on, ran a batch of fuel injector cleaner with pressurized can directly into the fuel rail - no change, (new injectors had been replaced less than 20K miles ago), no codes thrown, check function on the Diacom - all appeared correct, replaced O2 sensor, no change in 30-40 mph operation, except mileage did increase by about 2 mpg, and replaced the MAF sensor with a known good unit - again no change.

Ran tank almost dry, and filled with Shell Premium, and within a mile the engine was running like it should.

I have a theory (and it is only that at this point) that with the usage of 10% ethanol, some stations are salting in water as a "filler" and pocketing some extra change. Ethanol absorbs water (and then mixes with gas), and would be easy to add, and most people would not notice the difference. Back in Illinois, when Ethanol first came out we found several stations increasing the percentage of ethanol significantly. The law said no more than 10%, but we found some stations always ran 16%-18%, and others in between. The Shell and Amoco (BP now) always ran on the money at 10%.

In other words, try a different gas.

Plasticman
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Old 10-25-2008, 10:29 AM   #4
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I only use Chevron and not from the same station. This problem has been going on for about 3 months now. It seemed to start when I put a trans cooler on and bypassed the radiator. I've been wondering if the trans is running to cold and when it locks up it won't unlock when the engine starts to bog down. Thanks for the inputs, anyone else because it has me frustrated. The car runs great under brisk acelleration
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Old 10-25-2008, 10:41 AM   #5
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pull your codes - even if the check engine light doesn't come on a code may be set. see what you get and post 'em.
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Old 10-25-2008, 02:57 PM   #6
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I get every time fuel trim toorich bank 1 and bank 2, also intermitten misfire.
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