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Passat B5.5 02 model - no CEL, still misfiring

963 Views 1 Reply 2 Participants Last post by  Iowegian
Hello all,

I have a 02 passat b5.5 1.8T ABT spec. It has an AWT engine and has clocked 121k kms. The car has been running smoothly since I bought her at 98k kms. Here is my issue:
A few weeks back the car just started misfiring and running extremely rich (I hadn't used it for about 2 weeks prior). I went to a local garage where they diagnosed two coil packs as not working. They were changed together with the spark plugs. As I was about to leave the misfire was back and same garage said there's a problem in chamber 1; surprising right? Yea. Strangely a few kms from the garage the misfire just disappeared after hitting a pothole. Ok. A few days later driving on the h/way the misfire was back. Went back to the garage and this time they said its the oxygen sensors. Strangely there was no CEL light all through. I went back home and cleaned the oxygen sensors and the MAF sensor for good measure. Still no CEL but the misfire still there. The rev counter is steady at 800 and not erratic. The fuel consumption at idle varies between (1.0 - 1.4) l/h and normally its steady at (0.8 - 0.9) l/h . When driving on h/way gears change smoothly and at high speed the misfire is barely noticeable; consumption on h/way is normally 6.6 l/100 km, but now I'm getting roughly 7.5 l/ 100km.
Can anybody give me any pointers as to where else I should look or advise on what the problem might be please. :banghead:

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I'd check the temperature sensor at the back of the head. It's not easy to get at but it can cause this type of issue. It has 2 outputs, 1 goes to the cluster to display the coolant temperature, the other goes to the ECM to tell the engine computer how warm the engine is. It's not unusual for one sensor to work and the other to be bad. If the engine thinks the is cooler than it actually is, it will run rich. Does the misfire happen on a cold start or only when the engine is warm? If it runs rich immediately after a cold start it is less likely to be the temp sensor.

Having the behavior change after hitting the pothole suggests it may be a faulty electrical connection as well. Not sure where to start there but the air and temperature sensors could cause a rich condition.

You might want to try to get a VCDS setup or some other type of scan tool. If you put a lot of miles on while running rich you could ruin the 02 sensors or the catalytic converters and those are $$$$ compared to a scan tool.
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