Hello Folks,

05 Grand Am 3.4L, #4 cylinder low compression.
Dear Son reported engine was running rough/missing. Did a quick diagnostics and determined miss was specific to #4 and OBD code read P0304 (misfire). Further testing determined plug was getting spark so performed compression test on all cylinders.
#1= 180psi, #3= 180psi, #5= 180psi
#2= 175psi, #4= 135psi, #6= 175psi

5psi variation is no problem however 45 psi is a problem! Engine has 100k plus and I cannot determine if it has been overhauled in the past as it was purchased approx. 6 months back. Nevertheless, since the front bank contained the anomaly I chose to pull both heads and have them checked. Results confirmed variation of up to .007 on front head opposed to .002 on the back head. Heads were machined, new head bolts, headgaskets, upper and lower manifold gaskets and new injectors. Put everything back together. Fired it back up and still had a misfire on #4 cylinder. Repeated compression test and everything read exactly what the first test results were with the exception of #4 cyl. It now read 145 psi. Also, I noticed during the compression test on #4 I got some liquid (which appears to be coolant) on the spark plug and on the tip of the compression hose attachment. Also noticed coolant coming out tailpipe and white smoke. More or less confirms that is coolant in #4, but no other cylinders affected.
By the way, valves were good in all cylinder heads so they were not removed which means valve seats were not checked. I have aluminum heads with cast iron block. What are the odds that I have a crack journal in the block, or a crack in the head? Logic tells me the head would be the first to give way to heat opposed to the block... Anybody been down this road before?
Any input/feedback is appreciated.

Thanks,
deucer

You had that all apart and didn't bother to drop the oil pan, disconnect connecting rods and pop the pistons out? You could have overhauled most of the engine still sitting in the car. You might have bad rings on #4

Thanks Mike for your input. It is plausible the compression rings are done on #4. The last thing I want to do is pull that engine out and overhaul, but it maybe the only option. Aside from compression ring, I was hoping it was something up top.

The coolant has me thinking.........I would do an overnight pressure test......then remove the plug on #4 cylinder, and kill fuel and spark.....then crank over engine......if coolant comes out, could have a hairline crack in cylinder wall.....otherwise, take compression reading again......then add a few drops of oil into the cylinder...if compression comes up, as Mike suggested, could have bad rings....