Compression
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Junior User
Joined: Jun 2000
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Compression
my truck has been running bad lately and i put in a new distributer and timing chain just to name a few. i believe i could have bad compression on a few cylinders. i know you need a compression gauge but i was wondering if you could tell if a cylinder has any compression? i put a tumb over the number one plug hole and felt no pressure. now when i get a gauge and find out there is no compression how do you get compression? is that what a rebuild kit is for? the truck only has 80,000 original miles so i've been worry lately. thanks for your help.
Postmaster
Joined: Mar 2001
Posts: 4,968
Likes: 3
From: So Cal
Compression
Hoo boy, lots of things...
I assume the engine was being cranked while you had your thumb over the plug hole. Should have felt a lot of air even if the rings are almost shot.
Get a good compression gauge and use it. Remove all plugs and make sure you don't get in the way of any moving or potentially moving parts. Make sure the engine CANNOT START, disable the ignition(depends what ya got). Put the gauge in each plug hole and crank the engine with the starter. Try to make sure each plug hole gets the same number of cranks, maybe 3 or 4 full revolutions.(Follow the gauge instructions to be sure) Each cylinder should have about the same psi reading. Uneven(say 20% difference between cylinders) may indicate 1) Bad rings, 2) Bad valve guides, 3)Bad valves. I do not know what engine you have but in general you should see compressions around 150psi and up, but all within a reasonable range. Two cylinders next to each other with low readings may indicate a bad head gasket. Any cylinder reading 0 or close to it probably has a bigger problem than just piston rings or guides. A valve may be burnt broken or stuck open, or worse a piston may have a hole in it.
To check if a ring is bad squirt about a tablespoon of oil into that plug hole and check the compression, if it goes up, it's the rings, if it stays about the same then valves or guides or a hole.
A rebuild kit is for rebuilds. A complete engine rebuild is probably not a good project for someone with limited experience, unless you are a decent mechanic and can follow directions from a good book.
80,000 miles is kind of a grey zone for wear and tear, some engines are worn out at that mileage, others are just broken in, all depends.
Good Luck, and depending on the results of the test you probably should find a friendly mechanic and get ready for a rebuild.
Jim Henderson
I assume the engine was being cranked while you had your thumb over the plug hole. Should have felt a lot of air even if the rings are almost shot.
Get a good compression gauge and use it. Remove all plugs and make sure you don't get in the way of any moving or potentially moving parts. Make sure the engine CANNOT START, disable the ignition(depends what ya got). Put the gauge in each plug hole and crank the engine with the starter. Try to make sure each plug hole gets the same number of cranks, maybe 3 or 4 full revolutions.(Follow the gauge instructions to be sure) Each cylinder should have about the same psi reading. Uneven(say 20% difference between cylinders) may indicate 1) Bad rings, 2) Bad valve guides, 3)Bad valves. I do not know what engine you have but in general you should see compressions around 150psi and up, but all within a reasonable range. Two cylinders next to each other with low readings may indicate a bad head gasket. Any cylinder reading 0 or close to it probably has a bigger problem than just piston rings or guides. A valve may be burnt broken or stuck open, or worse a piston may have a hole in it.
To check if a ring is bad squirt about a tablespoon of oil into that plug hole and check the compression, if it goes up, it's the rings, if it stays about the same then valves or guides or a hole.
A rebuild kit is for rebuilds. A complete engine rebuild is probably not a good project for someone with limited experience, unless you are a decent mechanic and can follow directions from a good book.
80,000 miles is kind of a grey zone for wear and tear, some engines are worn out at that mileage, others are just broken in, all depends.
Good Luck, and depending on the results of the test you probably should find a friendly mechanic and get ready for a rebuild.
Jim Henderson
Postmaster
Joined: Mar 2001
Posts: 4,968
Likes: 3
From: So Cal
Compression
Oh one other thing, when you put in the new distributor, did you make sure to "line it up" or even better, use a degree wheel? If the timing gear is out of sync, you could also have low compression and bad performance.
I was reminded of this by a guy under the engine section. You might post your question there too.
Jim Henderson
I was reminded of this by a guy under the engine section. You might post your question there too.
Jim Henderson
