Lowtech

Cut to the chase...I have a 94 4.0 Auto. Cooling system doesn't seem to be pressurizing. While running hoses are soft and the overflow system doesn't seem to be working. And the cap stays cold to cool after running it for 20 minutes or so. Ran it with the cap off and was getting bubbles in the coolant. Once the 'stat opened level dropped and I filled back up to the top while it was running. I get bubbles while its running...not a lot but biggish ones. I'm afraid I have a head leak and want to know what else I should be looking for...haven't pressure tested it yet but know I'm going to have to. And an odd thing...when I shut off the truck I can hear what sounds like water running in the engine compartment but there is no observable leak.

I need a little guidance and some idea how to proceed.

tjwalker

A quick compression test would likely verify a head gasket problem. Low compression on adjacent cylinders is the smoking gun there. The compression spec. on the 4.0 is 120-150 psi, with no more than a 30 psi variation between cylinders.

Was the engine overheated recently?

Lowtech

Not at all.

Paul111779

Your water pump could be warn out. The fins inside could be mostly warn out still working but hardly.

Lowtech

Water pumps 3 years old.

Paul111779

Any milky residue on the dipstick or oil cap

alpine.adrenaline

Water pump. The sound you hear when you shut the engine off is probably boiling coolant that's sitting stagnant around your cylinder walls.

Lowtech

No milky residue on oil cap or dipstick. Going to drain some oil off tomorrow and see if it's there... supposedly coolant sinks to the bottom of the pan and should be first out...we'll see.

Lowtech

Except it's not overheating...let it idle today for about a half hour and it never went past middle line on temp gauge.

BlueRidgeMark

It's not overheating... up where your temp sensor is located.

masteroguitar

my old caddy would do this and it was because my water pump wasnt working. as soon as i fixed that the heater got super hot and all my hoses were hot and pressurized. good luck.

Waynerd

That water running sound may be the oil running back into the pan. All 4 of mine do that when I shut them off.

Also, does it still have heat?

freegdr

Pressure test system dont forget cap.

Lowtech

And the winner is...water pump. Cap was OK and the system was OK(which surprised me). New water pump and all is well. Thank you to those who responded.

Tyler Steez

a tip to diagnose a water pump is to squeeze the top rad hose shut with your hands and release it, you should feel a surge of fluid when you release it, if there's no surge then the water pump isn't pressurizing.