UK Car Repair Questions? Ask a Mechanic for Answers ASAP
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Did you have this misfire before you replaced spark plugs? What brand spark plugs did you install?
Does it run ruff all the time, or only after it warms up?
Two things I have seen cause this. First is the Catts break apart and cause a exhaust restriction. But it looks like you checked for that.
The other is the Fuel pump. It does not put out the proper pressure which can cause running issues and lack of power. To verify it's the fuel pump a fuel pressure gauge would be installed to monitor fuel pressure. It should be around 56 Psi and not drop lower than 35 to 40 psi when accelerating. I did one 2 weeks ago that fuel pressure dropped to 15 psi under acceleration.
The fuel pump I worked on, had good pressure at idle. But lost it under acceleration. You could have it low at idle causing the misfire. That is one of the basic checks you do with misfire codes.
Do you happen to have a scanner? The only thing I can think of that really may be causing this is maybe a restriction in the exhaust?
You didn't do both catts?
Do you happen to have a scanner to graph 02 sensors? That will tell you for sure if you have a restriction or Catt issue
There is only so much you can do with out a scanner. You could have a A/F sensor not operating properly which could be causing a imbalance of fuel trim which can cause a misfire. You also want to monitor intake timing to make sure the sprocket is working properly as well.