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I have a 1990 E350 decommissioned ambulance that was overheating when I got it about a year ago. It would run cool at about 35+ mph, but anything less than that it would overheat. I had the thermostat replaced about six months ago and that made a noticeable difference. Pretty much on a level-ish road I can run indefinitely at any speed now. But if I’m pulling a long hill in a lower gear it still heats up fast. I’ve had it riding that red line a couple times but then the road leveled out a bit, the tranny shifted up, and the temp came down noticeably, not a lot, but enough to make me feel a little more comfortable about continuing down the road.

I’m not much of mechanic so I’ve been reading a lot of forums. What I’m getting is that it could be the water pump, it could be the fan clutch (Fan clutch is actually malfunctioned, but it’s stuck in the perpetually engaged position and the fan is turning all the time), it could be the thermostat (which was recently replaced), it could be the radiator cap, hoses, etc., (there’s no evidence of any leaks here), It could be the wrong coolant (I’m assuming the Fire Department’s mechanic knew the right coolant to put in it, and there doesn’t appear to be any contaminants in the coolant), or it could be the radiator – what have I missed?

My question is, how do I diagnose what the problem actually is? Do I begin replacing one element at a time? Do I tear it all out and put in new parts throughout? What’s the work flow here? Or is there something obvious I've missed completely?

Thanks,
  • Owen
 

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First, is it turbocharged? Second, try draining the radiator until you can see the core. What does it look like? Nice and clean? Lots of mineral deposits? I had one that did the exact same thing. I fought it all the way from Kalispel MT. to Topeka, Kansas doing all of the above. Then everything leveled out. Not many hills around here. A couple of years later it broke a piston skirt, (I suspect ether starting to have been the cause) made one hell of a racket and started missing. I had another 10 miles to go to get home. It got me there, and died never to restart again. Cheaper to replace than to rebuild, the old truck hauled its last load to the scrap yard behind a John Deere tractor. I would strongly suggest you run a compression check, see if there is one or more low cylinders. The reason I asked about a turbocharger, is because if there is too much boost, it can cause it to run hot especially under a load. Too much fuel (rolling coal) will cause that too. If you have heavy mineral deposits, rod the radiator, and back flush the engine. I have heard it said, that the fan stuck in always on can cause that too, something about it restricting air flow with the forced turning of the blades . That doesn't really make a lot of sense, but when you're grabbing at straws... it might be worth a try.
 
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