GM Pickup Engine Miss

[pmt]

My 2003 GMC Sierra 4.3L 6 cyl (97,000 miles) has developed a serious engine miss. It runs OK when started up cold, but once it fully warms up the stumbling and missing begin; sometimes so bad that the truck shakes and slows down. But the Check Engine light never comes on.
Work done so far: new spark plugs, new distributor cap and rotor. Checked plug wire resistance - OK. Checked fuel system and pressure - OK. I agree with my mechanic, that it feels like an ignition miss.
Any GM pickup experts, or anyone - have any ideas what to look for next?

Thank you.

 

[heyu]

Coil pack maybe

 

[05GMC]

here`s what my haynes says to check , 1) fuel filter clogged or bad fuel 2) faulty or incorrect gapped spark plugs 3) fault in the fuel or ignition system 4) defective spark plug wires 5) faulty emissions system component 6) low or uneven cylinder compression 7) vacuum leaks at throttle body or hoses.

 

[Zaedock]

It could be an intermitent coil issue. I'd look at the spider injector assembly too. They can get dirty and cause a misfire when in closed loop.

 

[05GMC]

pretty good site for testing components...http://easyautodiagnostics.com/

 

[Boomer]

This still has a gear driven distributor?? If so, pull it and check the gear teeth on the shaft. This is a common problem and can give the symptoms you are experiencing.

 

[vssjim]

did you check fuel pressure when it was acting up

 

[wirelessF]

Clean the MAF and the throttle body as well.

 

[Scott_Tucker]

Injector assemblies are a huge issue on these engines. I would check that first. Replace them all, not just the cylinder that is misfiring. Which cylinders are misfiring?

 

[pmt]

Thanks for all the input. I'll be supplying these comments to my mechanic when I take the truck back to him next week.

pmt

 

[morepwr]

Check for a bad EGR valve as well. I had one go bad and couldnt hardly believe how lousy the engine ran. The was a 4cyl S-10 for reference though.

 

[mechtech2]

Vacuum leaks are masked by rich fuel mixtures and more advance when cold.
But when hot, they are more noticeable.
Also, electrical components can malfunction when hot, and return to normal when cooled down.
A dirty MAF or scratchy TPS can be a problem , too.

If I could pinpoint it on the internet for you, I would. But this is why techs and testing are in the world.

 

[mattd]

lots of things that will cause misfires on these trucks...

check coil/plugs/wires/cap/rotor as well as distributor.
injector spyder (this will usually cause a long crank cold)
also valve guides stick on these trucks that will cause misfires.
check to make sure nothing is stuck in EGR valve.

If you can have codes pulled, that may help you narrow it down.

 

[Zaedock]

Originally Posted By: mattd
If you can have codes pulled, that may help you narrow it down.


X2. Even if the check engine light is not on, you could have a code pending. Your tech should be able to see it.