360 v8 Compression check

Dave,

The first thing you are looking for is uniformity across all cylinders, which you have. I wouldn't worry.

Earle

Got me. 160 is "normal" for the I6, but it has a higher compression rating than the V8.

Consistency (psi +- 10%) is what your looking for in a compression test.

- Run the engine till it gets warm to the touch.

- I find it easier on the battery with all the plugs out.

- Hold the throttle all the way open or the engine won't breathe.

- Cranking time varies, but most go with 5-10. I usually do 5.

FWIW, I do a compression test each time I change plugs. Except this time, the hose on my compression tester sprang a leak. :( 160-165 is what the I6 gets when warm.

Thanks Doug & Earle.

Dave

Dave,

Was the engine warm or cold? I'll second the uniformity. So long as they are all close to 8 bar, I wouldn't really worry about it. You can try putting a few drops of oil in each cylinder and trying the test again. If the numbers rise your rings are a worn.

However, with 8bar and uniformity, I wouldn't worry.

Carl

Hi Carl, Engine was warm. Lowest reading was about 7.75 bar on one cylinder only. Trouble is, I think something is pressurising the coolant and I can't work out what it is. Cylinder pressures are uniform, no steam coming out the exhaust, no mayo in the oil, no oil in the water. Everything points to not having a problem were it not for the fact water is coming out of the expansion tank ! Occams Razor says it is me replacing the inlet manifold 3 weeks ago, but I've just redone the inlet manifold gasket yesterday (new Felpro gaskets both times with RTV around both sides of the gaskets around the water jackets and exhaust as per instructions), and that didn't solve the problem.

"Slightly baffled" Dave

Dave,

Try filling the cooling system and running it with the radiator cap off. If the coolant has a lot of small bubbles once the thermostat is open, you're getting cylinder pressure in there. A cooling system is only designed to hold 15psi, so it doesn't take much to over pressureize it and blow the cap off. Please note that you should fill the radiator when the engine is cold, and start the engine with the cap off.

Also try getting a cooling system pressurizer tool. It hooks up to the radiator cap and has a pump so you can pressure test the cooling system. Pump it up and watch the gauge, then check your oil.

Let us know what you find,

Carl

bad radiator cap not holding pressure?

If you are burning the water, the spark plug in that cylinder will look like new.

How old is the rad cap? A bad one can let too much water into the overflow and not suck it back when it cools down. So can a leak in the line from the rad to the overflow. It might not drip when the coolant is coming out of the rad as it warms up, but it might allow air to be sucked back in when the system is supposed to be sucking coolant back as it cools down.

Even if it fills the overflow, when the rad cap is working, it will 'unfill' it as it cools down. If not it is a bad cap or air leak.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail >

Air in the cooling system can do that. V8 intake manifolds can be bled by removing the temperature gauge sensor, a heater hose or as a last resort the thermostat housing. Pour in coolant until you are absolutely sure that the engine is full, and this should eliminate air as a source of the problem. Make sure that whatever you use on your gaskets says "works with ethylene glycol" somewhere on the container.

Earle

Pretty well what I was doing, so will continue a bit longer. Can't tell on the gasket - I used the black RTV which came with the Felpro gasket the first time round, and the second time round it didn't come with any so I used the bog standard Orange Hermetite RTV.

Dave

Cheers Mike - inline !

No, they are all a nice shade of light brown.

The rad cap is approx 2 years old. I only use the vehicle at weekends, so has done less than

600 miles in that time (was doing it up). Don't think the expansion line is bad, but it is cheap enough to replace on a whim.

That didn't happen.The top of the radiator was low by 1/2". Will get another cap as well, I guess.

Will replace and find out !

Dave

I am using Permatex Hi-Tac (not sure of spelling) since some antique pipe unions I put together six years ago with RTV started leaking. This stuff is red (pretty), sticky and claims ethylene glycol compatibility on the container. It also claims to provide the ability to move gaskets around after you have stuck them, but I see this is with some difficulty.

Earle

compression

That has the AMC 360 right?

Here is what the coolant will look like if you have a blown headgasket and warm it up with no cap on:

Nothing like that fortunately. When I start it up, water just overflows the top if the caps not on.

Dave

One thing I don't think anyone mentioned. Did you have the throttle open when you did the test? If not, the reading will be low.

Chris

They make 'dip sticks' to test for combustion gasses in the coolant.

They make 'dip sticks' to test for combustion gasses in the coolant.

Have you pressure tested the radiator?