My friend has a 88 F-250 7.3L diesel with 400,000 miles on the clock. Recently he has had a problem with it overheating underload. (When pulling his camper) He has had the water pump replaced. He has replaced the fan clutch. We talked to a mechanic and he said that maybe the trans needs to be rebuilt. What the HELL?

The trans shifts fine. I see no sign of it overheating. Could it some how cause the engine to overheat? But only when underload? The only other thing that we can think of is maybe replacing the raidator, but thats 300 bucks.

I appricate any suggestions. Thanks guys!

On older Diesels such as this 400k miles, their is the good chance that the one or more of the cylinder walls has a hole to the water jacket. This is caused by cavation. Not an uncommon thing if the coolant is not maintained with the proper balance of addatives. Before your friend spends any more money have the cooling system pressure checked. Good luck.:beer: :beer:

I concur on the cavitation guess....however it could also as easilly be a head gasket which is repairable.

Thanks for the info guys. That seems to make sense, but wouldn't it overheat all the time?

At idle the temp is normal though. Its also normal at 75 to 80 mph. It only heats up underload. That doesnt make sense.

Thanks again.

Well, very often when people say an engine (esp a for diesel) is overheating, it isn't. The radiator is overflowing because air is being forced into the coolant through leaks to the combustion chamber.

The air can escape through the radiator cap if the leak is small enough.

Higher load=more pressure, so if the leak is only a tiny pin hole,
then it may require significant load before the leak can really foam the coolant and cause it to overflow.

HOWEVER, if it is actually overheating, as indicated on a gauge which is known to be good (the gauges are less reliable than the engines they monitor) then look for exactly the same problems you would on a gasser:

-Fan viscious clutch
-radiator built up with internal deposits
-radiator or condensor built up with external deposits (bugs mostly)
-bad thermostat
-collapsed lower hose
-spare tire mounted in front of radiator

Diesel radiators can plug up quickly if the anti-cavitation additive (a good thing) is overdone (a bad thing).

Unlike a gasser, diesels produce very little waste heat at idle, and the amount of heat they produce is directly related to load, so it is not suprising that any true cooling problem would only rear it's head under load.