'00 Engine stumble @ low OR high RPM, not between

PFarkas · June 24, 2002

Sorry for the long post. I hope someone can help.

I've got a 2000 GMC 2500 (6.0 liter, 5sp manual trans, 58000 miles) that recently began "stumbling" or "missing" at engine speeds less than 1200 RPM and greater than 4500 RPM (at wide open throttle). If I keep the engine between about 1200 and 4500, it seems fine.

I wish I could describe the problem better than this. At low RPM it's not quite lugging, but it does feel similar. At high RPM, there is an occasional and unusual noise (for the truck) from the exhaust that is coincident with the feeling of miss/hesitation/stumble. The noise at high RPM sounds like the engine is spitting, but not like I would expect a backfire to sound. The truck is an extended cab long bed, and as the exhaust exits behind the rear passenger wheel, I'm not able to reproduce the sound and position myself to better observe it.

This behavior only seems to manifest itself when the engine has been at operating temperature for a few minutes and is being operated under load (gunning it while in neutral doesn't make it happen).

I've been through two complete tanks of gas (34+ gallon fillups each time) and started through a third since I first noticed this. The fuel was purchased in places over 100 miles apart, which leads me to believe that it's not a "bad gas" thing. On the last fill up I dumped some injector cleaner in with the fuel, and two hundred miles into the tank I'm still noticing the problem.

I do have headers and an aftermarket exhaust on the truck. They have been on for about 10,000 miles. I didn't notice any problems when I put them on. Otherwise, the engine is stock. Prior to this, I have not noticed any problems with the engine

The first service interval is at 100,000 miles (per the owner's manual). Prior to that, the only recommended service is to change oil, which has been done every 2,000 miles for the first 40,000 miles, and every 4,000 miles since then (when I switched to Mobil-1 Synthetic). The truck has original plugs, wires, and belt.

I'm not getting a "check engine" or "Service Engine Soon" light, so this must not be enough of a problem to get the on-board computer worked up.

I don't think this condition crept up on me - I just started noticing it a couple of weeks ago, and it hasn't gotten worse so far, although I was hoping it would get better by itself after I ran through a couple tanks of gas.

Any advice or suggestions as to where to look or what to check?

I'd really like to know what could cause this without producing an SES light.

July 9, 2002

ONLY THING I WOULD KNOW TO DO IS WHAT YOU ALREADY HAVE DONE... INJECTOR CLEANER... IS YOUR AIR FILTER DIRTY??? OF COURSE YOURE PROBABLY SMART EBNOUGH TO HAVE ALREADY LOOKED AT THAT.... WHEN IS THE SCHEDULED SERVICE FOR PLUGS??? THATD BE THE ONLY OTHER THING I COULD GUESS UNLESS YOUVE GOT A DIRTY O2 SENSOR THATS CAUSING IT TO RUN A LITTLE LEAN AND MISFIRE SOMEHOW... BUT THATD POP A CODE AND YOUD BE SEEING YOUR SES LIGHT... I DUNNO... GOOD LUCK