Running Hot at Highway Speeds with A/C On

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Moseph3

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I have a 1987 R10 with the stock 305. My temperature gauge wasn't working properly so I put a new sending unit it and everything seems good now, it will climb to 210° at the middle of the gauge and stay there. But if I cruise down the highway, no traffic and with the AC running, it will eventually climb to the 3/4 mark or about 235°. If I shut the AC off, it will work its way back down and run at 210° until I turn the AC back on, and after a while it will climb back up again. Any ideas on what might be causing this? A friend was thinking that the radiator might be plugged up, but wouldn't you think that would cause it to run warm all the time instead of only when the AC is on?
 
Goldie Driver

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The engine works harder at highway speeds so I could see it running hotter on the highway with a plugged radiator. When you have the cap off and look in does it look grungy ?

Looks like Rusty is thinking it is a fan clutch issue, however.
 
Grit dog

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Goldie Driver said:
The engine works harder at highway speeds so I could see it running hotter on the highway with a plugged radiator. When you have the cap off and look in does it look grungy ?

Looks like Rusty is thinking it is a fan clutch issue, however.
Click to expand...
At least you’re here to transfer rusty into legible lingo for others….lol
 
fast 99

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Fan clutch and or radiator. Assume shroud is intact, fan blades are partially protruding from the rear. Could also add a fan with more blades or HD fan clutch.

First item to check is fan clutch, that's easy.
 
Bextreme04

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I don't get it... the fan should be doing nothing at highway speed. You get more airflow just from the vehicle moving through the air at highway speed than a fan can possibly move. I would bet the radiator is clogged or there is an electrical ground issue that is causing the gauge to read higher when the AC clutch is engaged, even though the engine isn't actually running warmer. You are loading the engine up about 5-10hp more to turn the AC compressor.. if it was a fan clutch issue I would expect to see that show up when not moving. It takes ~75-100hp to move the truck along at highway speed, so you are really only adding about 5-10% more load depending on a bunch of other factors. You add way more load than that just by going up a hill. Does the temp start climbing going up a hill with the AC off?
 
Hunter79764

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Bextreme04 said:
I don't get it... the fan should be doing nothing at highway speed. You get more airflow just from the vehicle moving through the air at highway speed than a fan can possibly move. I would bet the radiator is clogged or there is an electrical ground issue that is causing the gauge to read higher when the AC clutch is engaged, even though the engine isn't actually running warmer. You are loading the engine up about 5-10hp more to turn the AC compressor.. if it was a fan clutch issue I would expect to see that show up when not moving. It takes ~75-100hp to move the truck along at highway speed, so you are really only adding about 5-10% more load depending on a bunch of other factors. You add way more load than that just by going up a hill. Does the temp start climbing going up a hill with the AC off?
Click to expand...
I'm mostly on track with this, but I'll add that with AC on, it's more than just the HP load, as the heat from the system is all rejected in front of the radiator. Basically it's like driving on a 5° warmer day. That said, if the system is that borderline, something is wrong and the clutch is sorta the last item I'd think of when overheating on the highway, but I've been wrong before.
 
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Moseph3

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The days that I noticed this were really hot days in the summer, probably pushing 100° outside. As far as if the temp gauge rises going up hills withour the AC on, I'm not really sure, I haven't taken it up any steep grades in a while. I'm not super mechanically inclined but I wasn't really thinking it could be the fan clutch for two reasons. I would think at those speeds that most of your cooling would be from air flowing through naturally instead of with the aid of the fan, and also I got stopped in traffic on one of those days and it cooled back down while sitting at an idle after I shut the AC off.

I appreciate everyone's input! You guys have definitely given me some ideas of things to look into.
 
fast 99

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Yes, bad radiator is a possibility. I suggested checking fan clutch first because it is very easy to do.

I speak from personal experience on this one. 74 Elcamino bad fan clutch caused it to o-heat. New clutch, o-heating was no more.

All I am saying is check it.
 
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An excessive number of bugs committing suicide and plugging air flow through a/c condenser and or radiator.
 
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Just had another member with overheating issues.at road speed, aluminum radiator dual electric fans etc. The fix was a factory fan shroud,heavy duty fan clutch and 7 blade fan. Search engine wind tunnel fanshroud will lead you to the right thread. So obviously more air flow is needed at road speed than what comes through the radiator on its own. I'm with @fast 99 check the fan clutch. Easy check with a rolled up piece of cardboard. Remember with A/C on you are having to cool the condenser also,many vehicles have an electric fan that just comes on when the A/C is on for that reason.
 
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After 30MPH your fan clutch is useless. Also check your high side pressure at 2000 rpm. Excessive amount will cause higher temps. Should be 275 to 325 max on a hot day with low side around 30. My guess is a clogged lower part of the radiator if all the other checks out. Hope this helps.
 
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Knuck55 said:
After 30MPH your fan clutch is useless. Also check your high side pressure at 2000 rpm. Excessive amount will cause higher temps. Should be 275 to 325 max on a hot day with low side around 30. My guess is a clogged lower part of the radiator if all the other checks out. Hope this helps.
Click to expand...
I used to think the same thing,but in some cases insufficient fan causes overheating even at highway speeds. This guy was having problems at 65mph. A new fan fixed this guys problem and there's a link somewhere on that thread where it fixed another guys problem. GM Square Body
https://www.gmsquarebody.com › ...
Overheating climbing a mountain
 
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