Compression results-Now what?

[Thor]

I did a compression check on my 94 6.5 TD-Readings where between 385-400 on all 8 cylinders, however #3 and #8 did not hold pressure, dropped to zero quickly. I also noticed coolant residue in the cap, and have been losing coolant slowly. I know I am going to have the remove heads, but wanted to get some advice on what might be causing this in cause there is something else I can check before I tear it apart, mostly want to know if it would be better just to pull the whole engine. Also engine has 205,000 miles on it, was a GM replacement engine covered under warranty, it has the larger oil cooler threads in the block. Truck has 300,000 miles total. Thanks guys

[simon]

Sounds more like the compression gauge check valve leaked on these 2 locations. compression readings sounds fine.
the traces of anti freeze could point to a sweating head gasket or a cracked head.
worst case a cracked main web up into the water jacket.

[DmaxMaverick]

Did you do the test with the engine up to operating temp? It must be hot, or the test won't be accurate. It also allows valve seats and valves to expand fully and seat properly. If the engine was cold when you did the test, I'd expect such results, and discard them and do it again on a hot engine.

If the engine was hot, and you can repeat the results, I'd suspect a head gasket. Could be a cracked head or block, but either way, the heads come off.

[JohnC]

If you mean the gauge went up to 385 then droped down to zero, that's the gauge. The cylinder won't hold pressure very long regardless, and if it's turning over, not at all.

[Robyn]

Coolant loss can be from a failing head gasket (usually the end ones)

The number 8 cylinder can crack at back about 3/4" down and around the lower back quadrant. It will be evident by staining on the cylinder wall from leaking coolant. This is game over.

The heads can and do crack between the valves but usually this is not to water. Cracks on the fire deck are game over.
Cracks in the precup port are a non issue as long as they dont cross the fire ring of the gasket.

The main webs can be checked for cracks by cleaning with Brake Kleen and then warming with a propane torch until hot. Look for a thin line of oooozing oil. This can and usually starts though the outside main bolts.
Cracks can be almost impossible to see at times all the way to very obvious.

This is a game over issue no matter as they will propagate down into the can journal area and also into the cylinder wall and water jacket.

Hope this helps

Robyn

[Thor]

I re-tested with a digital gauge and found out I had a bag gauge, Have 330-350 psi in all 8 cylinders (cold). What is the best way to check for leaking head gaskets using the "look for bubbles" test. Do you unhook the belt, start engine and then look for bubbles? Thanks

[rhsub]

Warm engine, remove belt, thermostat, fill coolant as high as possible, start engine, check for bubbles at t/stat opening, you may have to load engine (have buddy put in gear and rev engine), don't run engine any longer than necessary
Ron

[Thor]

I have a aftermarket SS diesel supply dual stat system, which mount both stats on there sides. Can I unbolt the housing from the intake manifold and llok for bubbles there? Thanks