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135 cold cranking PSI. Low?
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Author:  Reed [ Wed Aug 08, 2012 1:04 pm ]
Post subject:  135 cold cranking PSI. Low?
A few years back, I checked the cold dry cranking compression on the motor in my brother's van. The 225 in it had compression of 135 PSI across all cylinders. If I remember correctly, this was with an unmilled head, .060 oversize pistons, and a stock cam.

Recently, I noticed the van blows a bit of blue smoke on hard acceleration (mostly lugging itself up hills). Most figures I see for cranking compression are near or over 150 PSI. Should I be concerned that the rings or bores in the motor in my brother's van are starting to go? The motor has had less than stellar maintenance and hasn't had an easy life. The last time I had my hands on the motor i discovered it was pinging pretty bad going up hills. I don't know how long it has been pinging like that, so I don't know how much, if any, damage has been done to the rings or pistons. I backed the timing off and the pinging seemed to go away.

Thanks for the input. The next time I go up to my brother's place I will bring a compression gauge and get a current reading.
Author:  Rug_Trucker [ Wed Aug 08, 2012 3:52 pm ]
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Oil is a low octane fuel.
Author:  Pierre [ Wed Aug 08, 2012 9:09 pm ]
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That sounds fine to me. Per the 75 manual it says 100 +/- 25 between cylinders. Valve stem seals?
Author:  Reed [ Wed Aug 08, 2012 10:59 pm ]
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Possibly. The seals are over ten years old. I hope it is seals and not bores or rings.
Author:  SlantSixDan [ Wed Aug 08, 2012 11:01 pm ]
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Compression readings of 135 are excellent, but cold/dry compression readings don't give much useful info. Much more informative: engine at normal operating temperature, all six spark plugs removed, choke and throttle wide open, battery fully charged.

Whatever the present compression readings might be, it does sound like the oil rings are clogged or gone. As Rug_Trucker mentions, oil will drag the effective octane way down and cause ping, which can fracture and otherwise damage rings, which lets more oil into the combustion chambers, lather-rinse-repeat. And at the same time, oil craps up the combustion chambers with carbon, which itself causes or aggravates ping, lather-rinse-repeat.

It's probably not valve stem seals (or guides) if the smoke is occurring under heavy engine load—low vacuum, up hills, etc. That's usually rings.
Author:  Reed [ Wed Aug 08, 2012 11:08 pm ]
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Quote:
Compression readings of 135 are excellent, but cold/dry compression readings don't give much useful info. Much more informative: engine at normal operating temperature, all six spark plugs removed, choke and throttle wide open, battery fully charged.

Whatever the present compression readings might be, it does sound like the oil rings are clogged or gone. As Rug_Trucker mentions, oil will drag the effective octane way down and cause ping, which can fracture and otherwise damage rings, which lets more oil into the combustion chambers, lather-rinse-repeat. And at the same time, oil craps up the combustion chambers with carbon, which itself causes or aggravates ping, lather-rinse-repeat.

It's probably not valve stem seals (or guides) if the smoke is occurring under heavy engine load—low vacuum, up hills, etc. That's usually rings.
Sigh. This is what I was afraid of. The motor doesn't seem to burn oil idling or under average driving conditions, but backing up my driveway or going up the step hills near my house is cause for noticeable blue smoke.

I need to get my hands on his van again and do some more in depth diagnoses. Power and MPGs are down too. At least I have collected enough parts in my garage to rebuild a whole motor. :roll: At this rate his Duster will NEVER get finished... Image
Author:  SlantSixDan [ Thu Aug 09, 2012 9:04 am ]
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The other thing to mind: oil smoke is hydrocarbon particles. The catalytic converter will eat a great deal of it, so what looks like a little smoke is probably a fair amount of smoke before the catalyst.
Author:  Reed [ Thu Aug 09, 2012 9:09 am ]
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The catalytic melted down years ago and was removed. The only thing between the exhaust manifold and the tip of the tailpipe is a muffler.
Author:  SlantSixDan [ Thu Aug 09, 2012 9:51 am ]
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Ah, okeh. Then what looks like a leetle smoke is…

…a leetle smoke.

(catalyst meltdown could be related to this what you're seeing; overly-rich fuel mixture washes oil off the cylinder walls and rapid wear results)
Author:  Reed [ Thu Aug 09, 2012 10:25 am ]
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You forget Dan, this is my brother's van. It has had two camshafts, two heads, and about three carburetors since the catalytic was removed. :roll: Not to mention four transmissions, three windshields, and a driveshaft. I don't know why this van is so cursed, but it seems to break parts or wear them out much faster than normal. The BBD carb that was on the motor at the time of the catalytic meltdown was removed over a year ago and replace with a NOS Holley 2280. However, I did find that the 2280 was running rich at least at idle, so I wouldn't discount fuel wash wearing out rings at this point.
Author:  SlantSixDan [ Thu Aug 09, 2012 10:46 am ]
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You're reminding me of the main blue Cadillac in "Tin Men".
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