250 inline 6 cylinder #1 Low...What to check next?

[shambles]

Hello All,

I just compression tested the 250 in my 1978 GMC C15. #1 cylinder tested at 65 PSI. The other 5 cylinders were in the 160s. I poured a little oil into cylinder 1, and the PSI went up 5 to 70psi. I didn't wet test the other 5 since there were pretty high already.

I'm not sure what to test next. Any suggestions?

[rick1956]

Sounds like it's time to take the head off to check for that burnt valve...

Actually, pulling the valve cover off would be next I would think. That will tell you if anything in the valve train is amiss. If everything under there functions as it should there's not much left but a head gasket or burnt valve. Beings you didn't mention any oil in your water or water in your oil, I'd guess the latter.

Good luck!

[shambles]

Oh yeah, forgot to mention no oil water mixing either. Both are clean. I was going to pull the valve cover today if I get a chance. I had it off couple months ago to fix leak. It was pretty clean in there too. But I didn't dig too much.

Thanks for the suggestion. I'll check there next.

[Dead Parrot]

My first thoughts when I saw the symptoms were:
Blown head gasket
Mis-adjusted valve(too tight doesn't fully close)
Burned valve

Other ideas:
busted ring
ruined cylinder due to past coolant leak not disclosed by previous owner.

[shambles]

I pulled the valve cover last night. Looks pretty good in there. Seems to even have the factory Torque markings on some of the head bolts. I guess I'm pulling the head. The questionable cylinder is at the bottom of the pic. Has nice oiling.

[rick1956]

If only every head removal were as easy as a Chevy six... You've got that going for ya.

[Dead Parrot]

If the head is still on, might be worth backing off 1/2 turn on each rocker nut on #1 and see if your compression improves.

[shambles]

I'll try that. The head is still on. What's the theory? Rockers too tight would cause the valve to stick open?

[Dead Parrot]

Yes on the too tight rockers holding valves open. Had something similar during a repair on a 327(Pre Internet), so I know it can happen.

[shambles]

I tried loosening the rocker nut, but the compression is still low on Cyl One.

I pulled the head this morn. The head gasket is fine.

The valves look odd though. What causes the pinkish finish? Or is that normal? Since I have the head off, I'll take to a machine shop for a valve job:

This is one, two, three (cylinder one is the low compression cylinder):


This is four, five, six.

[44th Miss Inf]

Looks like the intake valve on #1 may be recessed due to a bad seat. Hard to tell from picture angle though

[Dead Parrot]

2nd the thought that #1 valve looks different compared to the other 5. Your shop should be able to tell you for sure.

Anything look odd about the #1 cylinder compared to the other 5? Extra scratches? Anything that might be caused by a broken/bad ring?

#3 & 4 look to have extra carbon/oil

[shambles]

Actually the cylinders look really good. They even look to still have cross hatching. definitely no vertical scratches or gouges. There also isn't a ridge at the top either...well maybe just some carbon build up. I might try to measure the diameter to see if it has been bored.