# Misfire Mystery



## eriksalo (Apr 2, 2012)

I'm getting consistent P0305 and P0307 misfire codes when my 4.2L engine is revved past 5,000 RPM. 

Have a done a bunch of diagnostics including a new timing belt to no avail. Any help would be appreciated! 

Specifics:

I have 2001 Audi A6 with a 4.2L 40V engine. Timing belt replaced about 1.5 years ago and it runs great. However, I'm getting some codes. 1) At high RPM (over 5,000 rpm) under load, I consistently get a P0300, P0305 and a P0307. Happens like clockwork. As soon as I hit 5,000 rpm, those three codes post. 2) After driving around for a bit, I get a P0340 (Cam position Sensor). Things I have done, all with no change: 1) Checked for Vacuum Leaks 2) Replaced all the spark plugs 3) Moved all the coil packs from one side to the other 4) Replaced the Cam position Sensors 5) Moved all the injectors from one side to the other 6) Inspected (as best I could) the variable cam timing system under the valve covers. Brown chain guides looked OK. 

Finally, I changed the timing belt to no avail. When I disassembled, I found the motor was perfectly in time. My marks from the first change ~18 months ago were still in place and everything lined up. I was able to install the crankshaft lock and cam locking bar with the timing belt in place (it wasn't even off a fraction, the locking bar slid right in). I installed a new belt and all the rollers + tensioner. Didn't put in a new water pump but changed all the other pieces. The new install went great and I was able to get almost exactly the 5mm gap on the tensioner after spinning the motor a few times that Bentley talks about. Put it back together and took it for a spin. [email protected] The same codes post at exactly 5,000 RPM: P0300, P0305, P0307. 

What do you think?


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## fresh1 (Aug 29, 2004)

I had an issue with misfire on bank 1. Traced it to catalytic converters, catalyst broken apart on one side and the other was restricted. Replaced the cats, and misfire instantly went away. Plugs and coils had no affect.


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## fresh1 (Aug 29, 2004)

*misfire*

I had an issue with misfire on bank 1. Traced it to catalytic converters, catalyst broken apart on one side and the other was restricted. Replaced the cats, and misfire instantly went away. Plugs and coils had no affect.


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## eriksalo (Apr 2, 2012)

*Problem Solved*

FIXED - Problem was the battery.

After trying a million things, it turns out a new battery completely fixed this problem. Either the fuel pump wasn't getting enough voltage at load, or the coil packs weren't getting enough current to fire??

Car started fine with old battery even with lights on, etc. Battery was on my list so I charged it fully with an external charger and the problem did not change. The battery in this Audi is narrower than a normal unit so when I attempted to install a standard battery from another car to check, it wouldn't fit. Finally, determined to check the battery I removed the aluminum bar forward of the battery and was able to shoehorn a standard unit in place. Problem completely went away.

In retrospect, the only clue was that the voltage shown on the instrument panel guage showed just above 12 volts. The new battery is 13-14.

All that work......


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