Rough idle


Asked by May 12, 2015 at 09:28 PM about the 1999 GMC Yukon SLE 4WD

Question type: Maintenance & Repair

I have a 99 Yukon with a 5.7L vortec engine.
It had a slight knock since we bought it but
it never was a problem. We had some out of
town trips planned for this summer so we
figured we should fix the knock. The pistons
for some reason were not where they
should have been. We got used pistons to
replace them. Once it was together it had a
rough idle and eventually wouldn't start
anymore. The timing was set dead center.
The spark plugs are showing signs of a
misfire. Valves were adjusted but it still
won't idle right. The ignition control module
got so hot it burned out while trying to find
the problem. Any suggestions we're running
out of ideas and our race is this weekend!?!

9 Answers

Andrew Olsen

Pistons not where they should be??? That makes no sense. Used pistons make even less sense. You mean to tell me you went through the effort to take apart the motor to use used parts, and the cylinder wasn't even bored to match?


Pistons were not in there correct cylinder #5 in #2 spot with a #5 in #5 spot. 3 of the pistons and rods were doubled 2 #5 2#7 and 2 #1 were in the engine. I hope that makes more sense. Now we have the correct # of pistons and rods in the correct order. It is not an issue with the pistons/rods because it's turning over the idle is just rough after all our adjustments. It seems like it's not firing on all cylinders but we can't figure out why.

Andrew Olsen

The number order doesnt matter as long as the rod and cap match. Do a compression test. Someone was inside that motor before to do that. They may have bored a cylinder to correct damage, if a stock piston is in a bored cylinder you get low compression = miss


Ok we'll check that, thanks for clarifying! It also has this whistling noise like it's sucking too much air.


Do you know what the compression should be?

Andrew Olsen

Compression maybe around 120ish. Your really looking for even numbers across the board. Vacuum leaks can cause problems but you should still get spark


We checked the compression cylinders #1,3,2,8 compression was 130. Cylinders #5, 7,4, and 6 compression is 120 does the 10 psi make a difference?

Andrew Olsen

No, that won't do much. there has to be a wire pinched somewhere. You got all the grounds back on? Even the elusive one behind the head?


So we got the engine out, looked it over. A couple wires needed mending. Got the motor back in, it started for a while. It seems like it's not getting enough fuel to the cylinders. Got the injectors cleaned and back in. Started up it was the same sputtering. While trying to figure out the problem it got the control module so hot it burned out. New wiring harness you think?

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