Coolant pressure/leak

[12pilgrim]

Hi friends,

My '86 6.9 f250 with 79k original miles has been resting this winter and not started in 3 months (batteries are charged). Before putting her away for winter, I filled the coolant reservoir to the proper level. The reservoir is now empty and there's pressure in the system (top hose is rock hard and 13lb Motorcraft rad cap leaks if opened). I've noticed before that days after driving the coolant system retains pressure. There are a couple drips under the right side of the radiator (haven't diagnosed that yet).

Why would I have pressure in the coolant system? Is this normal?

 

[Big Bart]

Some thoughts.

1) If you filled the system to the brim on a 0* day, closed the system, and now it is 65* out then yes you might expect expansion. But if your system started off a little low, you drove around, when it cooled the hoses should not be holding pressure. You release the pressure and a week later it comes back at the same temp, that would sound like something is off gassing in your cooling system.
2) Your rad cap should allow 2 way flow of coolant to and from your coolant reservoir. Perhaps the cap only is letting coolant suck in but not allowing it to return until past 13psi. Perhaps try a new cap.
3) Many of us have cracks in these old and brittle coolant overflow tanks. Maybe you have a small leak and most of it leaked out. Might be the drips you are seeing, if you are suggesting right side is driver side.
4) You might have a bad head gasket. Start the truck and see if have a constant flow of bubbles up the the radiator neck with the coolant cap off. This could explain the system holding pressure after shutting off the engine. You are building a lot of pressure, even as things cool it is still pressurized.
5) If you have a large aluminum radiator perhaps it has considerable expansion. Then it cools and shrinks building pressure.

Let us know what you find. Send some pics too!

 

[12pilgrim]

Great thoughts Big Bart.

1. I dont think i overfilled the system. It was a couple inches under the top of the radiator. Is it strange that it would suck the coolant out of the reservoir over a couple month period?
2. I replaced the original 13lb Motorcraft rad cap with a replacement 13lb Motorcraft. Could have been a bad cap though?
3. Great idea, but the overflow for sure hasn't leaked. The drips under the radiator appear to be from a small pinhole leak at the top that was soldered with lead. Very minimal leak, a couple drips.
4. I tested for combustion gases in the radiator and have none, there is a very slight external oil leak at the rear of both heads. I had the oil tested and they found very small amounts of coolant in the oil (maybe head gasket, maybe oil cooler?). They said to retest after 5,000 miles.
5. Original radiator.

I'll take her for a spin tomorrow and see what happens.. thanks for the ideas and please keep them coming!

 

[Big Bart]

If we theorize about it. Maybe at night the temps say drop to 15*. That cold air at the top of the radiator contracts and causes a slight vacuum and draws in coolant. The the next day the temp goes to 50* and the air expands in the top of the radiator, pressure say rises to 11psi (Not enough to vent at the rated 13psi.) and the hose feels harder from the pressure in the radiator. Then it leaks a little from the radiator pin hole as the day warms up. Over weeks and months you loose a couple quarts of coolant. Just a theory.

Coolant in the oil, more likely the oil cooler than a head gasket. You may just want to fix it, this is a common issue. Having to clean out oil from your cooling system is a PITA, so maybe just plan to do that job once it warms up.

 

[gnathv]

If you have that much coolant unaccounted for, I would turn the motor over by hand to insure it’s not hydro locked before I hit the starter.