I have a 1990 ex uhaul truck with the 7.3l idi. I am experiencing excessive coolant pressure, i know the first thing that comes to mind is cracked head, head gasket or something making the compression gases get into the coolant system. I dont think that is the case and here is why. When i first fire up the truck the top coolant hose will get hard within 15 seconds then after about 30 seconds the overflow bottle is getting air pumped into it then after about a min the overflow bottle starts to overflow with coolant. This all happens while at idle, now heres the kicker. If i start the truck and start driving immediately and keep the rpms above idle none of this happens and the truck does not overheat. While driving on the freeway the temp gauge will be at a normal reading then all of a sudden drop below the cold line then after a bit creep back up to normal and will continue to swich back a forth while driving but never overheat. But if i sit idling for a long period the overflow bottle will overflow leaking on the ground which will lower my coolant then makes my temp go up. I thought i had a head, block or gasket problem so i purchased the blue fluid combustion gas indicator tool. I tested the fluid and tool as suggested by placing the suction end in the palm of my hand and blow into my cupped hand as i siphoned the air up to the test chambers and the fluid turned green. Then i siphoned air in it until it turned blue again. Then i did the test to the radiator as the instructions stated and the test was negative. So i did another test by starting the truck and putting the test tube sealed against the radiator fill and after about 10 seconds air was forcing itself into the test tube and blue fluid making it bubble, still after about 10-15 seconds of this the fluid was still blue. I am at a loss on this. Oil looks fine, coolant looks fine. I noticed one day after the truck sat overnite after driving that the coolant system still had pressure in it. You would think that if the was a crack or leak or bad gasket that the pressure would bleed itself off. Somebody help, I am a little afraid to take long hauls in this thing thinking i might break down. I picked up a car with this truck pulling my car trailer 1 hour each way and did not get close to overheating but i did not let it idle any longer than i needed to . Any help would be appreciated. Thanks
Oh yeah I,m new here so HELLO.
Greg

 

Just out of curiosity, any idea what sort of history the thermostat has? I'm wondering if it's the wrong one in it and it's causing issues.

 

I don't care what tests you take, if you are getting this kind of pressure immediately, you probably have a blown head gasket, allowing compression pressure into the coolant passages. A crack won't allow such pressure immediately, it will just leak coolant into the oil and vice versa.
Advise? Tear the heads off and do a visual. If it's jut bad head gasket, it will be apparent. If not, send the heads in to be pressure checked, then go from there. 7.3's have cavitation issues from the block coolant passages into the cylinder, but they are usually pinholes, unless you have had a catastrophic failure.
Does it seem to have a miss on one or more cylinders?
Oil in the coolant, or coolant in the oil?
Sweet smelling exhaust, like anti-freeze being blown out?
If it is just head gaskets, reassemble with Victor-Renz gaskets, a set of head studs, and you probably won't have the problem again.

 

I replaced the tstat after i purchased the truck, i installed a motorcraft one in it. It is about 8 months old and about 1000 miles.

 

coolant is fine and oil is fine. It idles great and no smoke or smell out the tailpipe even at start up. I have read about the cavitation issues and from what i have read it takes roughly 100k miles to penetrate thru block. I have the service records for the u haul and it has had the engine replaced 50k ago.

 

Do you know what the mc part # is off the top of your head, i will have to look when i get home to see which one i bought, but i did get it at the dealer.

 

If the engine was replaced with a re-built, it could have been on the verge of cavitation pin holes when it was re built...

 

true, but you would think they would have checked for that somehow before wasting time and money rebuilding it.
It almost seems as though something is pluged somewhere. Since coolant is going to flow in the engine until the tstat opens, it seems like something is stopping that flow and the waterpump is just building pressure. Its weird

 

did you already try a new 13 lb rad cap?
building so much pressure instantly though....probably not good.
what happens when you run it with the rad cap off?

 

Yes i have tried a couple of different new caps rated at 13psi.
When i run it with the cap off at idle the coolant will slowly creep up the rad till it flows out. If i raise the idle a bit about 1500-2000 the coolant stays in one spot and does not creep up.

 

I have never seen this, but was the check ball plugged in your thermostat housing, or did you use a bunch of silicone on the gasket when you changed it.

What was the reason that you changed the thermostat.

 

I changed the tstat because like any used vehicle i buy i change all the fluids so i cant start my maintanace schedule. When i changed the tstat the check ball was stuck in the housing, i freed it with wd40 and pushed it off its seat with a punch. It rattled freely when i assembled it. I used very little silicone, just enough to keep the tstat from sliding, which it did once and the housing leaked.

 

You said "air" is getting into the coolant overflow. The only source of air is either combustion pressure or turbo pressure. Since the turbo, if you have one, doesn't connect to the cooling system, that leaves combustion. Since you said the upper radiator hose gets hard within 15 seconds, that kinda leaves anything like steam pressure, which is what you possibly could have if the t-stat was stuck closed. If it was stuck closed, you would have overheating issues, which you said you did NOT have.
A LARGE cavitation hole at the bottom of a cylinder might be to blame, but I seriously doubt it. Coolant would leak into the oil, and you would have hydro-lock on that cylinder.
I still suspect the heads or head gasket.

 

I cant figure it out either, but i really dont want to pull the heads on a hunch especially since the combustion leak test is negative. Truck runs great, pulls like it should and only causes a problem at idle. You would think if it was combustion getting into the coolant system that it would be worse at higher rpms but it is actually fine at higher rpms.

 

I guess my next step is to get a coolant pressure tester and test the system to see if it leaks down which i doubt it will since like i said a day after driving the system still has pressure in it.