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Discussion Starter #1
I'm stuck. Bought a 71' F350 motor home with a 390. 77,000 miles. It is overheating but not all the time. Drove up a 5% grade about 3 miles long and it overheated. Two days later, same outside temp, drove up a 5% grade about 6 miles long. No overheating. I have replaced the thermostat. The radiator is huge and very clean. Running 50/50 coolant. The timing is on and the advance pin in the distributor is still on. (had a 390 lose this before and overheated constantly) The water pump feels tight when I loosen the belts (no slop) so I don't know where to go from here. It's a cool old motor home and I want to keep it on the road. Any suggestions?
 

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I think that I would pull the water pump and look at the impeller then replace it with a quality new one. I have seen impellers rust to the point there only sorta good. 馃檪
 

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What was the RPM you were running when it got hot?

Water pump is a smart idea, but I would also look at ignition timing first, make sure you aren't dealing with a stuck advance, or someone who may have set initial with vacuum line connected, retarded timing builds a ton of heat under load. You said you checked timing, but I would run it as advanced as it can handle with the load. Stock is in the single digits, even running heavy they can likely take 10 degrees initial

Second, depending on the fan you are using, you can move a lot of air or not much. If you are going to work it hard, recommend as big of an aftermarket not flex fan as you can fit.

Finally, the water pump is a great step after.
 

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Discussion Starter #6
Don't know exact rpm. No tach, but it is geared really low so the rpm was pretty high. Timing is at 6 BTDC now with advance tube disconnected. Easy enough to up it a little and see if it helps. Thanks for offering some suggestions.
 

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I would expect some additional timing to help a little, assuming it doesn't ping. (BTW to RoseCity427 I think he meant set at 6 with it disconnected for the setting)

I would likely go to 10 and make sure the mechanical advance is working, bring it up to 3000 and see what the timing is reads. You can go as high as 40 total, read with no vacuum then reconnected after with the slow curve that is in it
 

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I'm with My427stang and worth advancing the timing a few degrees and check vacuum advance as well.

I'll add might check the fan clutch. Take a good look at all the hoses as old ones can get weak and collapse restricting flow. And when was the last time the system had a good flush?
 
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