Coolant system: NO pressure!

[chessy56]

Searched a bunch of threads- no luck. So-
'01 LB7 with 425K miles. My coolant system had been holding pressure as is always has since new. Now, I seem to have NO pressure when hot. The upper radiator hose is soft. I've had the head gaskets replaced @ ~325K miles, then began experiencing coolant puking @~400K miles. Head gaskets replaced again, but this time I bought studs for the dealership to install. Up until a week ago, even when stone cold, the system held some amount of pressure and the reservoir level was constant at ~ the mid (flange) level.

Now, I can run this engine until it's up to full op temps and I feel no pressure (or, a tiny bit- the upper hose is easily collapsed by hand). Engine temps range 190 - 195 throughout a 250 mile highway run. I'm not seeing any loss of coolant from the overflow reservoir (it rises in level when hot and recedes when cold in the AM- to about the flange midway up the bottle). No puking, no low coolant indicator. No smoke at the tail pipe. No puddles on the ground. No smells from inside the cab. No leaks around the water pump, nor anywhere around the radiator from what I can see. No indication that I have anything in my oil (at least by looking at the dipper- it's golden in color, just like my last oil change 1K ago).

Took it to the dealership this morning thinking maybe a hairline crack in the reservoir bottle or a leaking O-ring on the bottle cap (the cap was replaced during the last HG replacement ~23K miles ago). Anyone have any ideas? Dealership didn't have any immediate answers, but said it should be safe to drive. (BTW: it's hot outside- I haven't tried the blend control to see if I have heat in the cab, but my guess it that I would.)

[CoyleJR]

It sounds like you have a bad radiator cap. It is easy to check if a mechanic has a pressure tester with a radiator cap adapter. If a tester isn't available I would just buy a new cap and see if it solves the problem.

Good luck
John

[chessy56]

Boy John, that would indeed be too simple, but I'm overdue for a "simple" fix. And, given the mileage and rust that's developing, a "new ride"! I just have to make this one do for a bit longer however....

[Kennedy]

Easy enough to test just install a pressure tester to the fill cap, pump it up and see what happens. If nothing shows ten test the cap.

Not sure if the 2001 models used the same heater tube, but it's quite common to have these pinhole at the weld by the lower stay bracket and leak. These are typically pretty obvious though.

[chessy56]

JK: By "pretty obvious", I'll assume you mean there would be leaking fluid observed? And where is the "heater tube" you refer to?

[Kennedy]

If you pump up the system it should hold pressure. If not at 15 psi it should drip fairly decent. Could be a simple hose connection. Look for worm clamps that may have been added by the party that serviced the head gaskets last.

As for the alum tube leak follow from the heater core forward. You will see it change to aluminum tube and run near the fuel filter/dipstick and below the FICM as it heads towards the connection in the lower rad hose.

[chessy56]

Thanks John- Not seeing any signs of leakage nor do I smell any coolant like I have before with a leaker. Dealership states they pressurized the system and found nothing. My guess at this point is that the O-ring on the cap may not be holding the pressure. It was a new cap as of last Feb when they last did my head gaskets.

The system appears to be holding some amount of pressure when hot. What I've always noted was the amount of residual pressure that remained in the system after the engine had cooled overnight- the upper hose was not easily collapsed in the morning when I checked it.

It's time for a new ride.