Engine misfires when hot, loses compression

ANONYMOUS

MEMBER
1998 BMW Z3
160,000 MILES

And it is almost impossible to drive after about 15 minutes of driving. I did some research and found out that the catalytic converter may be clogged, so I cut it, and remove it from the exhaust. This action fixed the problem temporally, but after I was convinced that I found out what was the problem, and that putting a new catalytic would definitely solve my issue, the car started to misfire and lose power again (now it drives for about 30 minutes). Now I have the car waiting to be installed a new catalytic but still don't know what is causing the misfire. Please help!

1 Reply

KASEKENNY

MECHANIC
18,907 POSTS

How do you know that we are losing compression? Have you checked the engine when it is hot?

Here is a guide that will help with this:

https://www.2carpros.com/articles/how-to-test-engine-compression

However, if you are then we need to check the head gasket. Here is a guide that will help with this as well:

https://www.2carpros.com/articles/head-gasket-blown-test

If there is no loss of compression, then we need to check the fuel pressure when this is hot and see if we are losing pressure and go from there.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Lastly if all this checks out then we need to run through this guide to help with additional issues of misfires:

https://www.2carpros.com/articles/engine-misfires-or-runs-rough

I would then suggest we find out if this is a rich or lean misfire by checking the o2 sensor data and find out what type of issue we are dealing with.

Please see the attached document that will help with determining if this is rich or lean.

Thanks

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