I noted on the first page that 200PSI is way out of line. One of the reasons I suggested a redo.

 

yeah, I loaned another one from Autozone and got the same result 210 psi after 4 strokes. One stroke brought up #1, 2 and 4 to around 180 psi.

When you say, many many seafoam to clean up the combustion chamber, do you mean pouring it down through the spark plug holes or put them into the engine oil or spray through the throttle body with the engine running?

 

yes, and in the fuel. Maybe the spray into the throttle body thing, adapter comes on some containers. If SeaFoam is too expensive, some kerosene/diesel fuel is an option old folks used.

Do you have unusually bad gas in PA? Is your oil consumption ok? is your coolant disappearing? Gas or water in the oil?

I would think that high a compression would be caused by close to an eighth inch of buildup. That all needs to be burned off.

 

It's difficult to safely clean heavy carbon deposits, without taking the head off and mechanically cleaning. By safely, I mean that you don't want a chunk coming off and lodging in the exhaust valve seat.

I would think cleaning the 3 good cylinders is secondary. You want to find the cause of the low compression. I think the next step would be a leak down test.
https://www.chevyhhr.net/forums/how-...-engine-56053/

 

Yeah, I have my gas mostly from PA and NJ. No oil consumption. I do the oil and oil filter change every 3000 miles. Never see any observable oil consumption. And no oil leaked to coolant. Coolant looks good.

 

I used the Seafoam Spray clean from the throttle body. After the first try, the misfire got better. Previously, whenever it was idling for more than 2 minutes, P0303 was reported. After the Seafoam Spray, it was reported after 5 minutes' idling. I saw big smoke after that for around 5 minutes, then much less for around 15 minutes and then no more.

I think the Seafoam Spray somehow works so I am going to try again.

 

I repeated cleaning the carbon deposits ( putting seafoam into the spark plug holes and spray though the throttle body three times) but still got the same P0303.

I retested the cylinder pressure using three different gauges. Still got 205-210psi on 1, 2, 4, 125psi on 3 without putting oil and 175 psi on 3 after putting the oil in the hole.

 

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One thing I noticed was when the misfire happened during idling, the coolant temperature increased. The coolant temperature was around 174-178 on high way, 180 local but increased to 190, 200 and even over 210 when the misfire happened and then decreased. Could this be something related to the misfire?

 

Maybe. Maybe not.
Can you actually feel the misfire or just get a CEL when it happens ?
If that 1 cylinder is low and the CEL is always P0303, then I would say the misfire is continuous. Even if you and/or the car only see it randomly, I will bet that misfire is always happening.

FYI. 174-178 coolant temp on the highway is too low. Thermostat is sticking open or someone replaced it with the wrong one.

 

X2 what FMG said

AC Delco 131-158 thermostat, don't go cheap, get the one that works!
And use Dexcool 50/50 premixed , much easier to work with