Author Topic: Compression: How Low is Too Low?  (Read 161 times)

Offline Ben w

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Compression: How Low is Too Low?
« on: February 21, 2021, 12:25:08 AM »
I recently purchased a 1978 GMC K15 with a 350 that by all reports has never been rebuilt.  I purchased the truck from the second owner who had recently purchased it from his uncle. The uncle had used it for a farm truck for the last 20 years or so.

I'm going through the truck and just trying to make it as reliable as possible. Tonight I did a compression test and the results varied from 97 to 122 PSI. This was a dry test and none of the cylinders seem to rapidly lose pressure or anything along those lines. I realize that these numbers are pretty low and a little too varied but how concerned should I be? Should I just pull it and have it rebuilt now or should I go ahead and run it for a year or two until it gives up the ghost?

Offline bd

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Re: Compression: How Low is Too Low?
« Reply #1 on: February 21, 2021, 12:32:22 PM »
Not enough information to provide a reliable response.  Cylinder compression for a healthy 350 CID engine should hover around 150 PSI.  Does the engine run?  Does it exhaust bluish-gray smoke?  Does it clatter?  How do the spark plugs appear?  Are there deposits coating the insulators and electrodes?  Post pics of the plugs. 

Check the oil level and oil condition then run the engine to operating temperature.  After the engine is warm, recheck the compression or perform a cylinder leak-down test, and perform a wet compression test.  Post all of the results and pics for additional feedback.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Ben w

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Re: Compression: How Low is Too Low?
« Reply #2 on: February 21, 2021, 06:08:10 PM »
Unfortunately I cannot take a picture with a low enough resolution to be able to upload it to this website.

Other than the compression, everything seems good. I haven't owned the truck long enough to know if it's lost power or anything like that but it accelerates just fine getting onto the highway and such. No blue smoke no noise no anything. The truck seems to run well. It leaks like a sieve so I am pulling the engine next weekend and just going through all of the seals. I have not decided for sure that I will pull the heads and do the head gaskets but definitely everything else. The spark plugs themselves do have a light brown Haze on one side of all of them. It is uniform on all the spark plugs. It is not particularly heavy and does not seem to be building up at all. I presume this to be just a normal amount of carbon.

As for the oil condition, it is honey gold because I'm constantly having to dump oil into it. Honestly, it is hilarious how much oil this thing leaks. If I didn't have to drive it I wouldn't until I got it all sealed up. If I can do another compression test after work tomorrow, I will.

Offline bd

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Re: Compression: How Low is Too Low?
« Reply #3 on: February 21, 2021, 09:46:36 PM »
Bear in mind that the disparity in compression readings between cylinders could be an issue with the gauge.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline 75gmck25

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Re: Compression: How Low is Too Low?
« Reply #4 on: March 05, 2021, 04:52:01 PM »
I don't want to doubt your mechanic's skills, but are you sure you know how to do a compression test?

- You need to take the power feed off the HEI so it will not fire.  If possible, clamp down on the rubber fuel line (vide grips?) so you aren't constantly pumping fuel into the carburetor.
- Block or hold the carburetor choke open and hold the carburetor linkage to full throttle so the butterflies inside are fully open.  You want to suck maximum amount of air into the carburetor
- Start out with one cylinder and turn it over with a remote starter until it goes no higher.  This will usually be about 7 or 8 strokes, but keep track of the number.
- Test every other cylinder by turning it over exactly the same number of strokes.
- If any cylinder is more than about 10% lower, put a little bit of motor oil into that cylinder and retest.  If it moves higher that usually means the rings are not sealing well.

If you can consistently get 90 or above on all the cylinders, its a worn, but usable engine.  I wouldn't put a lot of money into it without planning for a rebuild, but it might work fine for many years.

Bruce


Offline VileZambonie

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Re: Compression: How Low is Too Low?
« Reply #5 on: March 05, 2021, 07:21:20 PM »
Some basics to add to doing a compressions test:

All accessories off, battery charger connected and on, consistent cranking @ WOT, choke open, ign and fuel disabled. If you miss these basic steps it's garbage in garbage out.
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Offline JohnnyPopper

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Re: Compression: How Low is Too Low?
« Reply #6 on: March 06, 2021, 12:55:43 PM »
All good advice, essential to performing a accurate test.

The uniformity is interesting, could be that the rings are sticking, even in the presence of golden oil.

I had a V6 that had not been maintained, idle was not smooth, suggesting. I used motor flush, then used cheap Penzoil to continue cleaning up the rings.

Flushed out the Penzoil, replaced with Castrol GTX and the idle smoothed out nicely.

BTW, sealing doesn't always require pulling the motor unless the front & rear seal and oil pan is leaking. Valve covers and the notorious oil sending unit are usually the culprits.
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction