Okay, as I read your description, you did NOT remove the spark plugs?

The point was to get the SeaFoam onto the rings in force. I guess I should have been more precise I.E.: remove the plugs, then pour SeaFoam into the spark plug hole attempting to saturate the piston rings.

 

After one day's driving, the P0303 was still reported when in low speed, especially during idling. Other than swapping and/or rebuilding the engine, is there any cheap way that I could try?

 

Yes, I did remove all the 4 spark plugs first and added the seafoam into the 4 wells. I could see the seafoam covering the piston top (black). When I checked in the middle night, the seafoam was still covering the tops. After the whole night, all seafoam were soaked.

 

I put the seafoam to the #3 well again last night. I could feel the engine run smoother than before. But in terms of the misfire P0303, nothing changed.

 

Do the compression again just for kicks. It was a long shot.

Your terminology is what is confusing me. The depression in the valve cover that the plugs go down is a well; the area that the piston lives is a cylinder; to me anyway.

 

So are you putting the Seafoam in the spark plug we’ll with the plug installed or down into the combustion chamber with the spark plug removed, just to verify

 

Sorry for confusing you with the wrong terms. My bad. Here is the process I did:

1) Removed all the spark plugs
2) Compression test
3) Put the Seaform into the spark plug holes, it went all the way down where I could see some carbons at the "bottom" of the hole. I stopped until the Seafoam fluid covered the bottom.
4) Waited for the whole night, rechecked each spark plug hole and didn't see any Seafoam fluid.

 

I retested the compression again. One or two strokes brought the pressure measurement of #1, 2 and 4 to 180 psi. Four strokes brought it up to 210 psi. For the #3, One or two strokes brought the pressure measurement to 115 psi. Four strokes - 125 psi.

I added engine oil to #3 again. Then four strokes brought it up to 150 psi. No big jump as before. But it is still much lower than other three.

 

I am wondering if this could be some carbon deposits on the intake valve of #3. I noticed my throttle body and even the intake air pipe connecting to the throttle body had a oil/fuel like black layer. Is it possible the intake valve of #3 couldn't seal the cylinder well due to the deposit on it?

 

Over 200 PSI, if your tester is accurate, might indicate heavy carbon deposits in the combustion chamber. I mean a lot, it might take many, many seafoam treatments to clean it up.

I would try a different tester, auto parts stores have them in their loaner program.

But, yes there does seem to be a problem with #3 that the seafoam might be able to help with.