First, thanks for all the help over the yrs. never had to ask because everything was usually covered in a previous thread. My degas overflows just after start up even before up to operating temp. Rad hoses start to swell and have no heat through the heater core. Air is building up just below the t-stat and the t-stat will not open until i bleed using the plug which is found just below.

I know, head gasket, right. But here is what i've done so far. Changed rad cap, new t-stat, new water pump. tested for exhaust gas using the blue juice meter at degas and found no exhaust. Flushed coolant system anyway and ran head repair kit. filled back up and same trouble. Found no blocked hoses and radiator flows nicely. While running at an idle with the degas cap off it overflows even before up to temp, but then it will change and start to empty, quickly. I'll add water to keep up with it and it will change again and proceed to overflow.

Here is what i'm thinking. There is a restriction in the engine block somewhere which causes the fluid to boil. This causes the air found at the t-stat. After the engine burps so to speak it causes the overflow tank to empty untill it starts all over. Currently I removed the t-stat and i'm allowing it to flow freely without trouble, but could there be partial blockage which would cause this while using the t-stat and with the t-stat out still have enough pressure to circulate though the engine? If so what is the path of coolant flow through the block and heads? And what is the best way of finding that blockage? Also does anyone think that an acid flush could cure my troubles? Im going to try that tomorrow after the part store opens.

The truck has many mods and gauges. Motor overhauled around 60k ago and has new heads using head studs. Thanks in advance for any advice.

 

By the way the truck is an early 99, 7.3L, 250SD, crewcab, LB with 214k

 

Have you thought about an injector cup? You say it overflows, so I take it you verified there is no diesel in the coolant? You may want to pull the injectors and take a look. If the crack is low on the cup, there is a chance it is not mixing fuel with the coolant.

 

I dont notice any crossover of fluids and the exhaust seems to be clean with nothing visable. Even after a cold start up there is no white smoke and no sign of hyd lock up in any cylinders. Would the cracked cup cause air to build up at the tstat? I'm having trouble getting the tstat to open at all. I tested the new tstat using measured heat and it opens about when it should. The injectors were newly replaced at time of rebuild, not that they can't go bad, with some single shots

 

I've also noticed with the tstat out i have no air bubbles in the degas tank and the hoses do not swell. I thought with a partial blockage the coolant would boil causing steam to flow up to the tstat, however, if that much heat is getting to the thermostat why wont it open up and allow it to circulate through the radiator. Unless the tstat needs to be in constant contact with the water and not with air.

 

The hoses are swelling due to pressure. With the t-stat out and the degas cap off, you will not build pressure. The injectors are probably fine, the cups are what often cracks and if they were not replaced when it was rebuilt, that may very well be the problem.

 

Thanks for the idea anyway, always good to get another perspective after pulling my hair out for the past 2 days. Wasn't sure though on the best way to start looking for blocked ports and channels running through the block and heads other than tearing the whole motor apart to nothing. Any ideas on how to seal off and test individual paths from the front of the engine with the water pump off while the motor is still in the truck?

Tomorrow is acid flushing day, if that doesn't work then i guess its time to pull out some tools and get dirty. When I get this figured out I'll through up a follow up thread to let others know.

 

Here is a picture of the cup in a cutaway head. I think you will see what I am talking about.

 

Now that i can see the coolant port in relation to the injector cup I wonder, if I'm not getting fuel in the coolant or coolant in the fuel than should I have exhaust in the coolant? It seems if the cup were to be cracked and causing excess pressure I should have one or the other. After combustion would the exhaust push up into the crack of the injector cup and into the coolant channel causing pressure? When I performed the exhaust test in the coolant it never showed up on the gauge. Any ideas? The truck is running ok now that the tstat is out, I put the cap back on the degas and the hoses seem to have the normal amount of pressure to them. But I still have concerns that maybe a part of the engine is not getting the correct amount of coolant.

 

It could be a copper washer on the injector is not sealing and a bad cup. Or it could also be a pin hole in the cylinder wall and combustion gasses are being pushed into the water jacket. Are you 100% sure your gauge or what you are using to check the coolant is meant for diesel or is accurate?

Here are some pics.

 

Which radiator hose is swelling? If my thinking is correct the water pump sucks from the bottom of the radiator and pushes back into the radiator through the thermostat and top hose. If the hoses are swelling it sounds like the water pump might be running backward? Did you do anything to the truck before this problem started?

 

Nothing was done to the truck in the past couple months, the problem just happened all of a sudden. When the degas cap is on and the tstat is in both the top and bottom rad hoses pressurize enough that you can't even squeeze them and they look swollen. The vac set up which checked for exhaust in the coolant was from a commercial garage but i'll make a call to verify that it is used for diesel.
At the time the hoses are pressurized is when I find air at the plug below the tstat and at this point the tstat will not open. It is only when I bleed the air off and allow hot water to start flowing out the plug that the tstat will open, but it will not stay open. Within a few miles while doing a road test the heater will start to blow cold air and the degas will be overflowing again.

Also as mentioned before with the degas cap off and the tstat in, the degas overflows while at an idle and within a split second it will start to empty the degas tank only to start overflowing again after a few seconds. It sucks coolant down almost like when you fill a new coolant system and the tstat opens for the first time but my truck heater doesn't blow warm air at this point. If I have pressure from a bad head gasket or injector cup shouldn't it be constant? Why the differance with the tstat out and with it in when the degas cap is on? With the tstat out and the cap on are the gases flowing out the degas tank cap with out notice of any exhaust smell or bubbles in the coolant?

 

You have the correct t-stat and housing right? I realize it just started happening, just want to rule out that it wasn't swapped before this.

 

Problem started with the housing put on new when the motor was rebuilt and never had any trouble. I also put on a new water pump yesterday just to rule it out. Since yesterday i tried 2 additional new tstats. At this point i removed the tstat and put in a modified one to allow it to flow freely just to get the truck back on the road for short runs till i can pull the motor, injectors, or whatever i need to do. Just trying to make sure i have to pull the motor before I start doing it.

 

Do you have an autozone around or a shop that would pressure test your cooling system and cap? I wonder if you have a bad cap that is allowing air to be sucked in. Check the system and the cap to see if they hold pressure. I am not ruling out an injector cup or head gasket issue but some of the symptoms you describe just sound more like a cooling system issue to me. I would rule out the cooling system components first.

Are you getting any coolant in the oil? When you replaced the water pump did you see any evidence of cavitation behind the water pump in the aluminum cover?

Is coolant flowing through the radiator? I have seen many tractor radiators completely plugged but have never seen that on a truck (although I am sure it could happen). We had a tractor last year that we had to have the radiator boiled and there was 10 pounds of crap clogging the radiator.