I have a 2001 4.3L that is missfiring when I am on the highway and get over 60 mph. I noticed that it happens more when I am on an incline or needing to accelerate more than normal. It started out very random but now it is every day. I can rev the engine with no problems in park. I changed the spark plugs. The tech at Auto Zone said that I have a cylinder 5 missfire. Any advice?

 

hows the spark plug wires on it? did you replace them with some new AC Delco's?

 

It could possibly be a sticking valve, try some top engine cleaner and then an oil change.

 

Ive got the same problem but i just replaced the intake gaskets and cannot figure it out 88,000 miles

 

is your engine light on? have you hooked up a scanner under your dash to look at how your enigne is running? if so, look at the timing on the scanner and see if it jumping around...