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4.2 Engine backfires/sluggish/unstable when accelerating

4.2 Engine backfires/sluggish/unstable when accelerating

 
Old 01-09-2013, 07:24 PM
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Default 4.2 Engine backfires/sluggish/unstable when accelerating

I have a 2004 Audi A8L and recently started having this weird issue that no one seems to know whats up.

the engine starts good, idles good.

but when accelerating, and especially if accelerating hard, the engine starts to backire and wants to stall, becomes really rough. if you accelerate slowly past 2600 rpm, everything from there on is perfectly fine, its a beast like nothings wrong with it. it only does this when accelerating from idle to about 2600rpm.

changed fuel filter. air filter. checked for vacuum leaks. spark plugs good. coils were replaces a few k miles ago.

heres whats interesting, i did some monitoring today with Vag-Com and noticed a few things that might be causing the problems.

at idle - its steady @ 680rpm with throttle valve angle open between 1.6 and 2.0%, occasionally jumping up to 2.4%. when accelerating slowly, the angle % slowly creeps up like its supposed to and never really goes past 30-40%.

When you hit the accelerater pedal, just fast, not far, the throttle valve opens to 100% and the engine starts to stall and backfire.

so i figured it was the throttle valve issue.

than i watched the accelerater pedal position sensor. when you hit it fast but not far, it jumps to about 20-30% when in reality your only pressing it about 10%. hit the gas and hold it at the same position, it jumps to 30% than falls back down to about 10%.

does this sound like the pedal sensor sending the engine wrong commands? or is there another issue that has something to do with the throttle body?

any ideas? suggestions that i could test out?

this car isnt driveable cuz when you need to accelerate it wants to stall.

and our local audi dealership has nothing but dumbasses. i spent so much money there already with 0 results..
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Old 07-01-2013, 10:05 AM
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Clean your MAF sensor!
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Old 07-01-2013, 01:58 PM
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can you disconnect the MAF sensor like on other VWAG cars and if it smooths out, your MAF is bad?
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Old 10-18-2013, 12:19 PM
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i am having the same issues on my 2.8 v6 30v. did you find the problem
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Old 01-23-2015, 05:44 AM
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Default Did you fix your problem ?

Hey guys Iam having the same problem 2005 A8, can someone please help
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Old 03-12-2015, 06:39 PM
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Default similar situation and more....

I have a 2007 a6 4.2 auto. Love my car but it's been acting strange. I'm even afraid to accelerate fast. When I start to accelerate slowly, no problem. When I accelerate fast from complete stop it jerks and only rpm goes up and my car still goes slow. The weirdest thing is sometimes on my dash. All the 123456 gears light up. Feels like driving both auto/manual together. When I stop the car, turn off and on. Back to normal again. I don't know much about cars so please excuse my terms. Anyone experience this before?
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Old 03-16-2015, 06:35 AM
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Originally Posted by Wing07a6 View Post
I have a 2007 a6 4.2 auto. Love my car but it's been acting strange. I'm even afraid to accelerate fast. When I start to accelerate slowly, no problem. When I accelerate fast from complete stop it jerks and only rpm goes up and my car still goes slow. The weirdest thing is sometimes on my dash. All the 123456 gears light up. Feels like driving both auto/manual together. When I stop the car, turn off and on. Back to normal again. I don't know much about cars so please excuse my terms. Anyone experience this before?
Check for a vacuum leak, check all the hoses for cracks. It's hard to say without my eyes on the car.
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Old 03-23-2015, 06:24 AM
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Originally Posted by C-ked View Post
Check for a vacuum leak, check all the hoses for cracks. It's hard to say without my eyes on the car.
Hey c-ked. I'm in Queens area. If you have some free time and not that far away. I was wondering if I can drop by sometime and you help me check out my car.
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Old 03-30-2015, 05:28 AM
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Originally Posted by Wing07a6 View Post
Hey c-ked. I'm in Queens area. If you have some free time and not that far away. I was wondering if I can drop by sometime and you help me check out my car.
I'd have no problem with you coming down but I've moved to Alabama. You're still welcme to come down but it's a monster of a drive.
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Old 05-20-2015, 11:30 AM
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Default did you reslove this issue ?

I have same issue.

slip-slip-slip, like a shudder when starting up incline
and a needle that bounces around in the 1000-2000rpm zone.
I had no misfires when VCDS while back.

discounnected MAF and things smoothed out. The slip was gone and pedal had much more response. Plugged MAF back in and old issues returned.
So ordered a MAF sensor.

I still suspect a vacuum leak and will search for that soon.
A good cleaning of air intake system is in order too.

Hoping the MAF fixes my Bank 1/2 lean codes.
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