Looking for opinions - compression low on 2 & 3

J

James G.

Senior Member
Engine low power still starts and idles good. Compression test show cyl 1 -> 4 as 65, 55, 45, and 60. What are the more likely issues? Head gasket between 2 & 3? Valves? Or????
Where do I go next?
All, all, opinions are so welcome.
Jim.
 
If you've already done a wet compression test (oil the cylinders) and there's no change, a leak-down test might be the next non-invasive diagnostic step. That's what I did to isolate a valve problem.
 
Sounds like you need to grind the valves. You may be able to hear the valves leaking by removing the manifolds and listening at the ports while the engine is cranked over with the hand crank. Otherwise try the leakdown test as Alexiskai advised.


If you have a leaky head gasket you will find wet plugs, loss of coolant, and/or bubbles in the radiator when full and the engine running.
 
Try looking into the radiator while engine is running and look for bubbles.

Check valve lash. But, if valves are starting to leak and burn they go quickly and compression will drop to 0.

Try leak-down test if able. Thats the tell-all.

It seems like a bad head gasket, but, I'd recommend making sure before replacing it.
 
I would pull the head and check for a Crack in the head between 2-3 this is very common.
Check head gasket,it may not be copper. Last thing is to lap and adjust the valves. This always seems to make a difference on a engine.
If the head is cracked, I recommend a new H.C. head. Service your waterpump and upgrade the fan.
New bushings in the distributor and new hoses. Flush the block real good.
Alot of this is preventive maintenance, good job before it gets warm.
Wick
 
What Whick said. Also take a good look at the cylinder walls with the head off, you can tell a lot. I myself would stick with the stock head on a tired engine, my opinion.
 
Years ago a shop would do an 'overhaul'.. adjust bearings, glaze break cylinder walls and re ring pistons, clean and lap valves. It was an inexpensive way to put fresh life in an old engine that wasn't badly worn. Given how most folks use their cars today, do this once and be done if your engine is a little tired.
 
If this were my engine, I would:
> Takeoff the intake & exhaust manifolds;
> Chock the wheels & shift the transmission into 3rd gear;
> Pressurize the 2 low cylinders and listen for air leakage at the manifold ports;
> If leakage, takeoff the cylinder head for further examination.
Good luck!
 
Worth noting, a leak-down test will also tell you whether there's a leak between 2 and 3 or a leaky valve – but without needing to take anything apart.
 
Wick said:
I would pull the head and check for a Crack in the head between 2-3 this is very common.
Check head gasket,it may not be copper. Last thing is to lap and adjust the valves. This always seems to make a difference on a engine.
If the head is cracked, I recommend a new H.C. head. Service your waterpump and upgrade the fan.
New bushings in the distributor and new hoses. Flush the block real good.
Alot of this is preventive maintenance, good job before it gets warm.
Wick
Click to expand...

i had the head gasket let loose between 2 and 3, zero compression in them
 
Thanks All for your comments/advise/ideas

This should get me started. Likely I'll be back before too long.
Again, thanks.
Jim G.
 
The piston should be at TDC on compression stroke when testing a cylinder. If a valve or two are even slightly open you will be chasing your tail.
 
Bob Bidonde said:
If this were my engine, I would:
> Takeoff the intake & exhaust manifolds;
> Chock the wheels & shift the transmission into 3rd gear;
> Pressurize the 2 low cylinders and listen for air leakage at the manifold ports;
> If leakage, takeoff the cylinder head for further examination.
Good luck!
Click to expand...
Hey Bob - What would I listen for, valve leak, or ? Then what else. sounds like a lot of work to check for bad valve. Isn't there some other way. How about leak down. Wet the cyl w'oil to check rings, wouldn't the same process look for leak down from valve also?
 
Flathead said:
The piston should be at TDC on compression stroke when testing a cylinder. If a valve or two are even slightly open you will be chasing your tail.
Click to expand...

Also DONT use 120psi shop air, use air regulated to 60psi max! Check with a local automotive parts store for a leak down tester for rent!
 
I'm a recent convert to the leak-down test. As alexiskai said, it will tell you not only IF there is a leak, but WHERE it is and how bad it is before you take anything apart.

You test the cylinders one at a time, on the compression stroke and listen for the hiss. If you hear it through the carb: it's the intake valve; through the exhaust: an exhaust valve; oil filler: rings; bubbles in the radiator: head gasket.

It is not a perfect system. You can get confusing results if, for instance, the head gasket is leaking between two cylinders. However, it is a lot of information before any major wrenching.

I built my own leak-down test kit with parts from Harbor Freight for about $25.

Ken
 
You must log in or register to reply here.
Back
Top