Compression Test Question

[middleagecrisis]

Working on a buddy's stock carbureted 360 and have a question(s) regarding compression test numbers. The motor is in bad need of a tune-up, but I thought I'd check overall health first. Engine was tested cold with the carb fully open. Numbers ranged from 118-140 psi. Lower numbers were on the 4 "inside" (3,4,5,6) cylinders. It doesn't blow any oil smoke when running (Rings), or at start up (Valve stem seals). Plugs are seriously carbon fouled, but don't appear oily. Besides taking the heads off, is there any way to tell if the valves are leaking? I'm not sure a leak-down test would eliminate the rings or valves. Maybe the head gaskets aren't sealing? No water in the oil or overheating issues. I seem to remember an unsteady vacuum gauge at idle would indicate bad valve seals. Thoughts on how to diagnose?

 

[Scampin]

If he is in a position to rebuild it, do it. If not, don't go looking for problems... Do the tune up and send it on its merry way.

 

[Ant]

118PSI! Barf!
It's not smoking, so that is a good sign. Do a tune up and an oil change, then run seafoam through the intake via a vacuum hose. That will clean the carbon that has been piling on the valves and combustion chamber. This process will make a huge cloud of white smoke, so don't be alarmed. Then once you are done with that, go for a drive and run it hard enough but not abusive. After all that, put a can of seafoam, or stp, or BG fuel treatment in the gas tank and then fill up the tank with gas. Drive it 500-600 miles with the fuel treatment to give time to do its work.

 

[TT5.9mag]

The answer to your question is yes. A leak down test is the next appropriate diagnostic step. You can hear where the leak is coming from when you pressurize each cylinder. If you hear the leak from the intake manifold it’s an intake valve. If you hear it in the exhaust it’s an exhaust valve. Head gaskets are a little harder to detect but you get the point.

 

[middleagecrisis]

118PSI! Barf!
It's not smoking, so that is a good sign. Do a tune up and an oil change, then run seafoam through the intake via a vacuum hose. That will clean the carbon that has been piling on the valves and combustion chamber. This process will make a huge cloud of white smoke, so don't be alarmed. Then once you are done with that, go for a drive and run it hard enough but not abusive. After all that, put a can of seafoam, or stp, or BG fuel treatment in the gas tank and then fill up the tank with gas. Drive it 500-600 miles with the fuel treatment to give time to do its work.

Two cylinders where 140 psi and the others ranged between 118-120 psi. I've done the SeaFoam trick before, so the smoke won't surprise me. Actually, probably be quite a bit of smoke! Hopefully, the SeaFoam will free up the rings, as well. I'm surprised there's no oil smoke when it's running.

 

[Desoto291hemi]

I’m not sure what year it is
But I’ve seen brand new engines back then that had maybe 135-140 psi

So. It might not be in terrible condition really
140 - 10 % is 126
So it’s pretty close really
Might need a timing chain to bring back the pressure a little ?

It depends on what he wants to do
Most smog engines back then were terrible on compression ratio

 

[middleagecrisis]

"Might need a timing chain to bring back the pressure a little ?" I can remove the belts and plugs and see how much slack is in the chain by moving the balancer the opposite direction of rotation, just enough to make it tight again.

 

[TT5.9mag]

Timing chain will affect all cylinders

 

[Desoto291hemi]

Timing chain will affect all cylinders

Yes,,,you are correct .
But a simple timing set might bring the overall power up enough that they don’t feel the need for a rebuild .
I would advance the timing 4 degrees while I was in there .

Running some Marvel in the oil for a few thousand miles can do wonders for the ring sealing .
If the numbers were more equal between cylinders,,,and the timing set added maybe 20 psi or more ?
That would add a fair amount of power and help the owner save some money .

Tommy

 

[AJ/FormS]

FOR ME
That engine is long past it's best before date. Unless it belonged to my wife, it would already be hanging on the cherry-picker.

 

[RustyRatRod]

Really, testing cold isn't the best. You really should get it up to operating temp. And not "Its there now" and test it, but let it run a WHILE. Drive it a ways. Then come back and test it.

 

[Bossanova5]

The quick way is to spray some oil in the spark plug holes. Then retake your compression on the low cylinders. If the compression rises your rings are worn, if the compression stays the same or close the valves are leaking. It definitely is not as precise as a leak down test, but it will give you a good idea.