2000 Yukon Denali Misfiring at 2000 to 2500 RPMS

bry21317 · September 22, 2005

My 2000 Yukon Denali with the 5.7 engine is misfiring at around 2000 to 3000 rpms. The car has 82k miles.

This has just started, about a week ago. I changed the Spark Plugs last night, and put the AC Delco Plugs in that are stated as the Plugs in the manual. They were not horrible, but not to good, and only about 9 months old. I then cleaned the Throttle body also. I took out the K+N air filter and cleaned this really good also. There was a new Fuel filter put on the car about 6 months ago.

I added 89 (instead of my normal 87) octane gas and a fuel system cleaner to the tank yesterday to see if maybe the 1/2 tank of gas I had was the problem, and this has made no difference.

I have not changed the Spark Plug wires at all, so I don't know when these were done or if they have been done.

It idles perfect, it just purs. If I put the car in 1st gear and then start to go, it will make the Misfire happen alot worse, due to the load it is under. Also, I put the break down to the floor with it in Gear and then revved the engine up, so there was a load on it, it then really started to miss when I got to about 2000 rpms.

It will do it at highway speeds at about 70mph but it is not as bad, since it is harder to feel on the Highway.

Thanks for any info you can give me,

Bryan

GM-Tech · September 23, 2005

Sounds like possibly secondary ignition. Maybe a bad wire/plug, etc.

bry21317 · September 23, 2005

Now that I have been running it with better gas for 2 days now, it seems a little better. Now I really am only getting it when I put it in 1st gear instead of drive, it will do it very bad then, almost like maybe a tranny issue to. It will really buck and sputter.

I have also noticed this a few times, but not near as bad when driving the Highway at 70+mph. It is getting really annoying.

Brand new plugs AC DELCO Platinum as Recommended- 2 days old.

Fuel Filter is about 6 months old, 5k miles on it about.

Cleaned the Throttle body, 2 days ago

Cleaned K+N Air Filter 2 days ago

Distributor cap is only about 4-5 months old and rotor

Tranny Fluid Changed and new filter 2-3 months ago. Maybe 3k.

I now tried revving the engine with it in 1st gear while my foot is on the break again, as I did before, and now it doesn't seem to misfire. It does do it very badly when driving in First gear and going any speed, as long as the RPM's are up to about 1500 or more it will do it in First bad.

Is this a Miss or could the Tranny having an Issue? I might take it by a tranny shop that I trust and have them see if that could be it, but I don't think so. It seems to have been perfect and then all the sudden started doing this.

Thanks,

Bryan