compression test results
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Junior User
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compression test results
Just did a compression test on my 78 f250 400. I'm just looking for a more experienced opinion before I pull the heads. The results were #1 155 #2 100 #3 140 #4 120 #5 160 #6 160 #7 165 #8 110. Are these results bad enough to stop it from starting? It also back fires through the carb (and cracked manifold). Today or tomorrow I'll try repeating the test on 2,4,and 8 with some oil squirted in there.
Laughing Gas
Joined: Mar 2014
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Just did a compression test on my 78 f250 400. I'm just looking for a more experienced opinion before I pull the heads. The results were #1 155 #2 100 #3 140 #4 120 #5 160 #6 160 #7 165 #8 110. Are these results bad enough to stop it from starting? It also back fires through the carb (and cracked manifold). Today or tomorrow I'll try repeating the test on 2,4,and 8 with some oil squirted in there.
Postmaster
Joined: Jan 2013
Posts: 3,512
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From: Panama City
^^ what they said. An old rule of thumb is compression should be within about 10% of each cylinder. More than that indicates some kind of problem. Before you pull the heads do some more tests. Hook up some air into each cylinder and see if it leaks out carb, which indicates intake side. Out the exhaust indicates exhaust side. valve covers or pcv indicates rings. If it is getting blowby there isnt much use in rebuilding heads. Drive her til she quits, then rebuild.
Senior User
Joined: Jan 2015
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From: Vidalia onion,GA
That should run fine... Did you put a couple drops of oil in the spark plug hole and retest. The oil will temporarily seal the piston rings if that's the problem whereas it won't do anything for leaking valves. .
Your heads could be fine and the rings leaking, or some of both. It'd be nice to know before you drop money into heads and your comp doesn't change
Your heads could be fine and the rings leaking, or some of both. It'd be nice to know before you drop money into heads and your comp doesn't change
Elder User
Joined: Apr 2012
Posts: 990
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From: Tamaroa, IL
I think you might be doing your compression test wrong. There's just no way you should be getting 160+psi. a fresh engine is usually around 120 psi if I remember correctly.
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Cargo Master
Joined: Dec 2008
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From: SW PA
I agree 160 is high. Good point.
I just check my 400 the other day and it was between 120 and 130.
I just check my 400 the other day and it was between 120 and 130.
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From: Vidalia onion,GA
I had a 150 on my 460 and 140 on my 351.....
And the 460 was running with 2 bent pushrods, making 2 separate dead cylinders.
So the not running has got to be separate issue
Crazy some of the problems you can have. I broke a camshaft one time in the 351 and it jus backfired and quit running. Could not figure out what the heck it was until I pulled the valve covers and found only the first few rockers moving
And the 460 was running with 2 bent pushrods, making 2 separate dead cylinders.
So the not running has got to be separate issue
Crazy some of the problems you can have. I broke a camshaft one time in the 351 and it jus backfired and quit running. Could not figure out what the heck it was until I pulled the valve covers and found only the first few rockers moving
Elder User
Joined: Jul 2001
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From: Portland, OR
I just did a comp test on my 78 400M and it was 150-158 all the way through. I warmed up the engine, removed coil and plug wires, loosened plugs a bit and cranked with a remote start with the throttle plate jammed open.
If you have weird #'s AND back firing through the carb, I'm suspecting bad cam timing; although I would tend to think that it would make all read lower (not absolutely certain on that).
Good luck,
BB
If you have weird #'s AND back firing through the carb, I'm suspecting bad cam timing; although I would tend to think that it would make all read lower (not absolutely certain on that).
Good luck,
BB
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No I didn't warm the engine up since I can't get the thing started. Does that make my numbers much less accurate? I have fuel, spark, a good carb and module, and apperently compression. I have also brought it to tdc and checked that the dizzy was pointing at 1, it was. So what else is there???
Posting Guru
Joined: Aug 2010
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From: Knoxville, TN
Besides having fuel, spark, and compression, they also have to be there at the right time.
It could be cam timing, as biederboat suggests, or could also be the distributor being 180 out of phase (i.e. if it's lined up with cyl 1 breathing TDC instead of cyl 1 firing TDC) - that wouldn't change compression, but it would cause backfires through the carb and prevent the engine from starting. That would be the first thing I'd check, as it doesn't involve disassembling the engine.
It could be cam timing, as biederboat suggests, or could also be the distributor being 180 out of phase (i.e. if it's lined up with cyl 1 breathing TDC instead of cyl 1 firing TDC) - that wouldn't change compression, but it would cause backfires through the carb and prevent the engine from starting. That would be the first thing I'd check, as it doesn't involve disassembling the engine.
Elder User
Joined: Jul 2001
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From: Portland, OR
I've just always understood it to be that the engine should be warm and if you don't open the throttle it just makes it harder for the engine to pull in air. But I think even without that, the numbers should be closer together.
BB
BB
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Junior User
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I bought this truck as is and never heard the engine run before. I know the previous owner was in there messing with the ignition components. Bare with me here as I've never messed with a distributor before. So you saying I should pull the distributor and turn it 180 and see if that does the trick? Any tips or tricks or is it pretty straight forward to pull?