Overheating Issue
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Overheating Issue
Hi I have a 1978 F150 with a 400ci 6.6L that tends to overheat while I'm driving but doesn't when I'm idling. It gets close to 240 degrees when I'm driving but at idle it stays at about 200-210 degrees. Heat works great and upper/lower radiator hoses get hot. Do you guys have any advice on what to look at next?
Post Fiend
Joined: Apr 2008
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From: Big Sky Country
A few possible causes come to mind...
1) water pump, it could be moving coolant at idle but cavitating above idle speeds and not pushing coolant through the system.
2) running with a too lean condition (carb jets and/or intake leak)
3) bottom radiator hose collapsing at higher rpms (no internal spring)
1) water pump, it could be moving coolant at idle but cavitating above idle speeds and not pushing coolant through the system.
2) running with a too lean condition (carb jets and/or intake leak)
3) bottom radiator hose collapsing at higher rpms (no internal spring)
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1. Would there be any signs if it was a bad water pump?
3. To see if its a lower radiator hose collapsing all I would have to do is rev the motor up and see if it collapses?
Also, I have made sure that there is no air in the system and also made sure there was enough coolant in the radiator. I have not noticed any loss of coolant.
3. To see if its a lower radiator hose collapsing all I would have to do is rev the motor up and see if it collapses?
Also, I have made sure that there is no air in the system and also made sure there was enough coolant in the radiator. I have not noticed any loss of coolant.
Post Fiend
Joined: Apr 2008
Posts: 8,261
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From: Big Sky Country
Is the water pump making any noise, or is it losing any coolant through the weep hole (bearing going out)
As for the bottom rad hose just squeeze it to see if there's a spring inside or not.
As for the bottom rad hose just squeeze it to see if there's a spring inside or not.
Senior User
Joined: Feb 2013
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From: Idaho
radiator could be partially clogged, thermostat could be stuck half open/closed, clutch fan could be going out or leaking fluid--look at the clutch fan if theres streaks looking like an oil leak its bad. If its an original water pump the pump fins could be rusted and almost gone so the pump isnt pumping/moving coolant...check your fluid level too.
Cargo Master
Joined: Nov 2009
Posts: 2,364
Likes: 118
From: starship enterprise
The 400 needs the thermostat with the bypass block off. Every one I have ever gotten from the parts house is wrong,You need a Robert shaw 16394 or equivelant.
It needs to be able to do this. Take yours out and look to see.
Napa THM 297 Autozone 7223-180 ?? (maybe, you will have to look)
Thm 197
Carquest 3088
3089
Get the right thermostat in there and go from there.
Napa THM 297 Autozone 7223-180 ?? (maybe, you will have to look)
Thm 197
Carquest 3088
3089
Get the right thermostat in there and go from there.
Freshman User
Joined: Jun 2011
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From: Houston
Just a thought. I have a 6.6 and had the same problem. I finally realized I had too much water in the radiator. You don't fill them all the way to allow for proper circulation. Just a thought.
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From: Houston
I can remember replacing the Fan Clutch, thermostat and fluids to finally read the manual only to find I had overfilled the radiator. Sounds dumb but it worked. I have not had any overheating problems. Start there first.
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New User
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I'm not exactly sure but I don't think I have a fan clutch. Correct me if I'm wrong as I'm new to all this. Here's a pic I just took.
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New User
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Also, it should be noticeable if there was a spring in the lower radiator hose? I was able to squeeze the hose from the bottom of the radiator all the way up to the water pump, didn't feel anything like a spring in there.
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New User
Joined: Dec 2012
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The 400 needs the thermostat with the bypass block off. Every one I have ever gotten from the parts house is wrong,You need a Robert shaw 16394 or equivelant.
It needs to be able to do this. Take yours out and look to see.
Napa THM 297 Autozone 7223-180 ?? (maybe, you will have to look)
Thm 197
Carquest 3088
3089
Get the right thermostat in there and go from there.
Napa THM 297 Autozone 7223-180 ?? (maybe, you will have to look)
Thm 197
Carquest 3088
3089
Get the right thermostat in there and go from there.
Senior User
Joined: Feb 2013
Posts: 266
Likes: 0
From: Idaho
You are correct you DO NOT have a clutch fan. Next check your thermostat...also look in your radiator filler neck and see if it looks like its clogged or low on radiator fluid.
Post Fiend
Joined: Apr 2008
Posts: 8,261
Likes: 15
From: Big Sky Country
There's no spring or you'd feel it, have someone rev up the motor as you look under the hood and see if the hose collapses.
