Hey guys, I've got a 2000 GMC Jimmy Diamond Edition with 154,xxx miles on it.. The truck misfires when under load at about 1500 to 2k rpm, any ideas on what could be causing this problem? Occasionally (once every three or so days) it wont misfire and it will drive 100%. It throws the multiple engine misfire code. Ive replaced spark plugs/wires and dist cap/rotor to no avail.. Any ideas are greatly appreciated.

 

What brand did you use for the tune up? Do you have acess to a scan tool that you can check the timing? Sometimes, a misfire is due to the fuel system so a pressure and leakdown test wouldn't hurt. However, most of the time a random misfire is do to a fault in the secondary ignition system.

 

I used Duralast wires, Duralast gold cap and rotor, and ACDelco rapidfire plugs. The truck was misfiring before I changed those. Ill replace the fuel filter within the next day or so.. And I'll see if i cant get my hands on a scan tool to check the timing, and perform a leakdown test. Could a bad cat, or sticking valves be part of the blame?

 

Just curious, as I have a scan tool, how do you check the timing? Mine shows spark advance, but I think that's it.

 

When you say "under load" does that mean going uphill or towing something? Is the misfire code you are experiencing P0300? Are the cylinders affected 3,4,5 or 6?

 

Under load meaning just in drive, not towing or going uphill. If the trucks in neutral or park the engine wont misfire at those rpm's. and cylinders 1 and 6 also came up with the P0300 as separate codes

 

There's a chevy bulletin 1539013 (it also had your model on the list). Probably not the case here, but it mentions similar symptoms and refers to valves binding in their guides.

 

These trucks are very picky about electronics. Changing the parts to ac delco or delphi may take care of the issue. Did you replace the coil?
Here is a link that describes how to set the timing:
https://blazerforum.com/forum/showthread.php?t=24373