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#1 |
Junior Member
![]() Join Date: Jul 2010
Location: Goodyear, AZ
Posts: 59
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250 inline 6 cylinder #1 Low...What to check next?
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? |
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#2 |
Registered User
Join Date: Jan 2009
Location: Edgewood, WA
Posts: 107
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Re: 250 inline 6 cylinder #1 Low...What to check next?
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!
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I may be old, but I still enjoy the feeling of the earth rotating beneath my wheels... |
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#3 |
Junior Member
![]() Join Date: Jul 2010
Location: Goodyear, AZ
Posts: 59
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Re: 250 inline 6 cylinder #1 Low...What to check next?
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. |
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#4 |
Registered User
Join Date: Aug 2012
Location: Oklahoma City, OK
Posts: 2,509
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Re: 250 inline 6 cylinder #1 Low...What to check next?
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. |
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#5 |
Junior Member
![]() Join Date: Jul 2010
Location: Goodyear, AZ
Posts: 59
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Re: 250 inline 6 cylinder #1 Low...What to check next?
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.
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#6 |
Registered User
Join Date: Jan 2009
Location: Edgewood, WA
Posts: 107
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Re: 250 inline 6 cylinder #1 Low...What to check next?
If only every head removal were as easy as a Chevy six... You've got that going for ya.
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I may be old, but I still enjoy the feeling of the earth rotating beneath my wheels... |
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#7 |
Registered User
Join Date: Aug 2012
Location: Oklahoma City, OK
Posts: 2,509
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Re: 250 inline 6 cylinder #1 Low...What to check next?
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.
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#8 |
Junior Member
![]() Join Date: Jul 2010
Location: Goodyear, AZ
Posts: 59
|
Re: 250 inline 6 cylinder #1 Low...What to check next?
I'll try that. The head is still on. What's the theory? Rockers too tight would cause the valve to stick open?
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#9 |
Registered User
Join Date: Aug 2012
Location: Oklahoma City, OK
Posts: 2,509
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Re: 250 inline 6 cylinder #1 Low...What to check next?
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.
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#10 |
Junior Member
![]() Join Date: Jul 2010
Location: Goodyear, AZ
Posts: 59
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Re: 250 inline 6 cylinder #1 Low...What to check next?
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. ![]() |
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#11 |
Registered User
Join Date: Sep 2002
Location: Liberty, Mississippi
Posts: 671
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Re: 250 inline 6 cylinder #1 Low...What to check next?
Looks like the intake valve on #1 may be recessed due to a bad seat. Hard to tell from picture angle though
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#12 |
Registered User
Join Date: Aug 2012
Location: Oklahoma City, OK
Posts: 2,509
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Re: 250 inline 6 cylinder #1 Low...What to check next?
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 |
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#13 |
Junior Member
![]() Join Date: Jul 2010
Location: Goodyear, AZ
Posts: 59
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Re: 250 inline 6 cylinder #1 Low...What to check next?
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.
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