96 Chevy C1500 random misfire problems
 

I have the 305 v8 5L engine, and it seems to be misfiring. It only begins to misfire when i am going above 50mph or right before it changes gears. After i fixed the check engine light i scanned it and the random misfire code came up, and a code for a bad O2 sensor after the cat converter did also. If anyone knows what could be causing the misfires i would like to know. I have already changed all the spark plugs, and the wires, and the problem is not fixed.

Re: 96 Chevy C1500 random misfire problems
 

Welcome to the forums.
First thing you need to do is check the fuel pressure or have it done for you. Low fuel pressure can cause cruise speed missfires. Low catalyst efficiency can also be caused by low fuel pressure misfires. O2 sensor behind the cat is there only to monitor the cat activity. The ecm uses the upstream cat to make fuel adjustments.

Re: 96 Chevy C1500 random misfire problems
 

I will get that done. Thanks for explaining what the O2 behind the cat does. Did not know that... Saved me some money for replacing them

Re: 96 Chevy C1500 random misfire problems
 

Jim gave some good advice.

For me, I had something similar that ended up being a dirty MAF sensor. If you have a K&N filter, or other oil-impregnated one, there's always a chance that there was too much oil used at re-charge time and the oil can leave a thin film on the sensor wire. You can use alcohol and a q-tip to clean that wire.

Re: 96 Chevy C1500 random misfire problems
 

The oil thing about the cotton filters is a farce that K&N's own website debunks.

http://www.knfilters.com/maf/massair.htm

When was the last time you replaced the fuel filter??????