Need help with a coolant problem

[barryjackson]

I've got a problem that has stumped my mechanic and he has since sent it to another mechanic who hasn't had time to look at it yet but seemed to indicate he wasn't sure what the problem could be either. The problem is that coolant is being forced out of the overflow hose once the truck gets much beyond idling speed. To this point, my mechanic changed the radiator cap (twice), ran multiple pressure tests and hasn't found any leaks in the system, and ran a combustion test to see if there is a crack in the head which may be forcing air into the system (this was also negative). Our intitial thought was a crack in the head or a gasket problem, but both the tests were negative and the engine doesn't overheat - which the mechanic indicated usually if you have a head problem the engine temperature will rise. Does anyone have any ideas what the problem could be? Thanks for the help.

[rustypig]

Not sure which pressure tests you refer to (leak down test), or what a combustion test is(compression test?). It could possibly be a cracked cylinder wall (usually #8) that expands once the engine is hot and allows air to be pushed into the cooling system. I would think that if the #8 piston was at the bottom of cylinder after the engine was hot, you may get some leakdown during a test. Hard call though. Sound like there looking in the right direction though.

[HowieE]

Dr.Edgerton, who I beleave came from your area, would ask for a bit more information.
Is it pushing enough air into the system to cause the radiator cap to releave pressure and thus lose coolant? How old is the truck and what milage? Tell us all you can.

[HowieE]

Dr.Edgerton, who I beleave came from your area, would ask for a bit more information.
Is it pushing enough air into the system to cause the radiator cap to releave pressure and thus lose coolant? How old is the truck and what milage? Tell us all you can.

[barryjackson]

It is a 95 K2500 with 138,000 miles. It does appear that there enough air in the system to force the cap to relieve pressure. The first mechanic who looked at it said there was a considerable amount of air in the system but he thought he was able to bleed it all out, and even after that it was still forcing coolant out of the overflow. I do wonder if it's possible he didn't get all the air bubbles out.

[ucdavis]

1) It is very important to run with a minimum amount of air in cooling system for 2 reasons. First, if running permanent coolant, air in system causes an accelerated rusting problem in the block due to a sort of galvanic corrosion reation; there is an air bleed screw to assist in getting to zero air (the 'right' amount). Second, air in system will allow increased temps and possibly hot spot(s) depending on volume.
2) There are only two ways to push out coolant. First is overheating (or at least expansion of improper coolant mix). Second is leakage of compression; could be thru head, head gasket or block. If you get a choice, choose overheating.

[gerickson]

I had this very same problem. In my case after living with it for nearly a year, (merely a slight nuisance) I eventually found my problem after sniffing the coolant reservoir with an EGA (exhaust gas analyzer) Only then did I finally believe that the added air was actually combustion gasses. Causes were classic GM 6.5L; failed head gaskets, cracked heads, cracked block, you name it. She ran so well that I was surprised with what I found during disassembly, and yet all symptoms were well documented here on TDP and elsewhere.
Good luck

[twaddle]

I would get them (the mechanics) to try as Gerickson suggested sniffing the expansion tank with an exhaust gas tester.
If they can't do that get a container with water in it, remove the hose at the expansion tank and put the hose into the bucket of water. Rev the engine untill pressure opens the rad cap if large amounts of bubbles (probably ex gas not air) show up in the water you have problems with cylinder pressure entering into the cooling system.

They might be able to ID which bank of cylinders is causing the problem by removing the cross over tube with the temp regulator (also remove the fan belt), run the engine and which ever side has bubbles coming out of the water port should put you in the right direction.

I would get the problem repaired ASAP as the longer you run it like this the more damage will need to be repaired later.
Good luck

Jim Twaddle
Biggar, Scotland