Ok i have a 1997 chevy express cutaway van with a 6.5 diesel. engine runs great doesnt overheat but when i drive down the highway and get off the ramp and slow down the expansion tank lets coolant out till it gets low. truck doesnt overheat or anything just looses coolant after driving on highway. there is a brand new 15 pound cap from chevrolet but a friend of mine said go to a higher pressure cap.. any ideas would be great

 

Sounds like a head gasket has let go,or the block cracked.

97 was not a good year for 6.5 engine blocks.

 

If it was a headgasket wouldnt it overheat all the time? i can drive it for miles and miles in the city without problems just on the highway

 

Might want to give this a read and see if the details match up:
6.5 blowing coolant everywhere (Total Rebuild) - Diesel Place : Chevrolet and GMC Diesel Truck Forums

 

Ok on his he Blew all the coolant out and had no heat but i can drive 40 miles on the highway and i lose about a cup of coolant. the truck has 353,000 miles on it and the expansion tank is the orig to the rig and the temp never goes up. im at a total loss here

 

Not sure you read far enough,the next day driving gently and still towing with a total load of 18000lbs he was able to drive fine and continued to do so for another week with normal non towing driving.
Then we tore into it.

 

Mine has done this ever since i had the injection pump replaced it has a new water pump new t-stats fan clutch and rad only thing in the system orig is the tank. coolant stays green and i lose about as much coolant as a can of pop. i have a welding system on the truck that works off the alt and i have had it running for 4 hrs and it doesnt lose coolant or overheat . ive added dye to the system and the only place i see coolant is from the tank. it blows no white smoke even at start up runs cool down the road what size is the rad cap supposed to be.. mine is a 15 pounder

 

15 pounds is right,could just be a faulty tank not holding pressure and allowing the coolant to boil off?

Pressure test the system and see if can hold pressure.

 

ill pressure test the system tomorrow and see what it does and go from there . this one has not only me stumped but my father who's a retired diesel mechanic of 40 years.

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update.... ok i put a pressure tester on it about 4 hrs ago pumped it up to 15 pounds and it hasnt moved at all. im letting it sit all day and part of the day tomorrow and go from there i used a pressure tester that blockes off the bottom of the tank so it doesnt let air out of the vent on the tank so theres 15 pounds of pressure in the system

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Update... i have gone back out and its still at 15 pounds. i have also cranked the engine over without it starting to let the valves open and for the pistons to move around and its still at 15 pounds. im going to let it sit another 2 hrs and go out and crank it over again and see if the pressure goes up or stays. i have unplugged the pdm so it wont start but build pressure in the cyclinders.

 

Don't forget to test the cap,might be just evaporation causing your trouble.