A year ago I purchased an 01 GMC Sonoma, 2.2L engine with 30,000 miles on it. The truck is immaculate both in and out and runs like new EXCEPT for a somewhat faint knocking noise that occurs at lower RPMs. The noise seems more pronounced when the engine is cold but it’s always in the background. It does not degredate the engine performance in any way. My mechanic told me that it’s likely carbon buildup but he thinks that there is not much that can be done to fix it. Anyone have any other suggestions?
I can’t accept he carbon buildup suggestion. First, modern engines are not prone to carbon buildup like the old carburated engines were. They simply burn too clean. Second, the symptoms of carbon buildup are much more pronounced when the engine is warm and under high load, like acceleration.
More likely scenerios IMHO include a sticky lifter. At higher RPMs you’re creating higher oil pressures and that may be why it’s not a problem except at low RPMs, and sticky lifters are always more pronounced when cold because the oil is more viscous, it doesn;t flow as readily into the small orafices that fill the hydraulic lifters.
Some GM engines have also been prone to “piston slap”. That’s a condition wherein the piston skirts are too short for the stroke and the angular forces created during the strokes by the crank pulling the wristpin sideways one way then the other (at the stoke’s top and bottom) cause the skirt of the piston to slap the cylinder walls. I know the V8 engines had this problem, but I don’t know if the 2.2L had it. It’s worth a search.
Bearing knock occurred to me as a possibilty. But there’s really no way to check this without disassembly.
Many thanks for your insighful response. As in many previous instances I have been advised to “turn up the radio”. The knock, according to many, is nothing to worry about. I find it hard to believe (as you said) that a modern engine and particularly one that has been maintained by the book would have any knocks or odd noises with such low mileage on it. I’ve rented cars from Avis with 50K miles on them that run quietly and smoothly.
Should I just turn up the radio?
I second the piston slap the theory, and is most likely nothing to worry about. But you may want to have a second opinion just to make sure.
Thank you as well. I’ll have to find another credible mechanic to take a look at it. In addition to my regular mechanic, I’ve also taken the truck to the GMC dealer. To be quite honest, they were less than helpful in evaluating the problem.