Quote:
Originally Posted by glatt
There were a couple other tests I could have done if I had a vacuum pump, a vacuum gauge, and a scanner tool that showed RPMs in real time.
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Did not realize you had a vacuum gauge. Smooth sound in an exhaust suggests the vacuum gauge should show a constant vacuum in the manifold at idle. However confirm this. Intake manifold vacuum at a constant above 17 inches should be observed. If the vacuum gauge needle vibrates, then the problematic cylinder is indicated when the gauge point dips to a slightly lower vacuum.
Assuming the vacuum gauge is contant at idle. Then this is puzzling. Vibration at idle implies a missing cylinder. And yet the exhaust pipe sound suggests no missing. Vacuum gauge will probably report the same 'no problem'. If so, move on to some other suspect. This engine appears to be operating smoothly on all cylinders during idle mode. And gas mileage suggests it is also working properly during normal operation.
Every part of a drive train and engine must be isolated from the car's body. Even a radiator does not make a direct connection; connects to the body via rubber isolators. Is it possible that something (ie exhaust pipe) is touching the body as it gets warm? For example, an exhaust pipe was hung slightly twisted? Or a heat shield place moves slightly with heat?