Ok I'm stumped. I've got a 98 Chevy cheyenne C1500 4.3, replaced plugs, new cap and rotor, wires, new spider, fuel filter. Cleaned the idle air and checked the composite intake while I had it off... Still missing, throwing a P0300 random missfire, shows four flashing lights showing cat, egr, o2, eva. Only misses after 2000. Acts like its loading up and will back fire after a little bit. But will accelerate fine, just won't cruise at 2000....5 speed standard transmission. No broken wires. Cat was checked at exhaust shop when cherrybomb was installed....any help greatly appreciated...thanks in advance....

 

how did they check the cats and did they check both? Did they measure backpressure?

I'd remove the pre cat 02 fromthe exhaust and do a short test drive and see what it does.


If you have access to a scan tool that displays live misfire data, you can pinpoint which cylinders are actively missfiring. if the misfires are all on one bank the cat may be clogged on that side of the ypipe.

 

Only one cat, not sure how he checked it out. Wasn't there when he checked it. Will try the o2, ill check my friends scan tool tomorrow to see if has a live data feed...

 

I do know the cat was replaced before I got the truck but I don't know when....

 

This guy "used" to be a mechanic be careful what he tells you as it may lead you to more trouble.

 

I had a similar problem with my 97 Cheyenne. Could not pass emission testing because the check engine light was illuminated. Plugs, wires, cap, distributor, fuel injection spider, multiple flushes with sea foam in the tank and fed through air lines. Still the damned light would not go off, it was mocking me! Finally I replaced the timing chain and gears and the light lit no more. Runs great!!!

 

I've replaced all of the above except the spider on mine. Still having the same problem. Looks like a new spider in my future.