98 silverado z71... Runs great most of the time - very smooth idle, but after some stop and go kind of driving, the truck will start shaking at idle almost to the point that it feels like it will die (hasn't yet though). Also will sometimes start this while at a constant speed. Feels very "off balance". Ideas as to what it could be?

 

It could be related to a fuel delivery or ignition issue.

Check the fuel pressure. It should initially be 50-60 psi when you turn the ignition on (engine off) after two or so seconds. You should be able to hear the pump prime the system. After a few seconds it should only drop about 5 psi and hold there. If it continues to drop more than that, it's either the check valve in the pump or the fuel pressure relief valve is failing to hold line pressure.

Unplug the vaccum line from the fuel pressure relief valve. If there's fuel in the vaccum line, the relief valve diaphram is torn. It will need to be replaced.

When was the last time you changed the fuel filter? If it's been a while, you may need to replace that.

If all that's checked and the problem persists, you may need to have an injector balance check done. Unless you have a bidirectional scan tool, you will need to take it to a auto technician to have this done. This will check each injector for their ability to hold fuel pressure. It may identify a problem injector.

You may have an ignition coil problem also. I don't have much experience with this so can't give you much info on how to check this. Pull the plugs and see how much carbon build up they have. You may want to replace those or clean/regap them as well.

Lastly, and probably the most expensive action, the fuel pump may need to be replaced if the above doesn't find or fix the problem.

I hope this helps some. There may may be other things to check as well. I'll let someone smarter than me chime in on what that might be.

Good luck, and let us know what you find.

 

sounds like a fuel delivery issue...........fuel pressure is the first step...also some of these trucks have a Hot Fuel Handling Module which when it starts to fail that will cause the same kind of issues.....The standard fuel pressure I find when looking for these trucks is 60-65 psi on prime....also an issue I have found recently was the fuel pump relay...pull it and look at the power distro box.........is it melted any where around the fuel pump relay??????...mine was....I just went and bought a standard 5 post relay and some ends and rewired it outside the distro box ( standard 5 posts are cheaper than the "gm relay".........what causes this is still a mystery to me but I am looking into it....I assume it is a failing fuel pump drawing to many amps.

 

This one turned out to be an o2 sensor that was bad. All better now - Thanks guys!

 

I fixed the same problem by running mine for 45 seconds at 5000rpms down the road Cleaned out all the gunk that'd stacked up in there