2010 Prius - Regardless of speed/Vehicle vibrates inconsistently

welcome!
are you losing any coolant?

 

Mine did that before throwing a rod

 

Lots of things could cause a misfire. Bad spark plug or coil, fuel injector, low compression or other engine mechanical fault, partially clogged EGR passages in the intake manifold, etc. If there was a problem that required the engine to be replaced- then I would expect the misfire to be present all of the time.

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It has been more common (by far) for cylinder one to report misfire. Still of late there’s been a few cyl four reports. Both cases somehow due to imbalanced* rate the EGR passages in the intake manifold clog?

Low compression could indicate valve not sealing? Or bent piston rod, so TDC is slightly lower? The latter seems to mesh with “vibration”.

* Tends to be cyl one “wins” the race.

Oh: how many miles?

 

The only thing so far established in #6 is that cylinder 4 has slightly lower compression than 1, 2, and 3.

That result seems to have come from a compression test, not from a leakdown test, so it has not indicated why the compression is lower. (In a traditional compression test, the tech would normally then repeat the test with a dollop of oil spooned into the cylinder, which can at least distinguish a few of the possible causes. If the compression goes up after adding the oil, leakage around piston rings is indicated. If the oil doesn't change anything, the leakage is probably related to the cylinder head, valves, or head gasket. A leakdown test will give more information.

If the cause of low compression is narrowed to the top end, and if it is further determined to be a head gasket failure and not something else, then speculations about what might have caused that could be in order.