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1985 k5 overheating while driving not at idle

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chevyluv

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I have a 1985 k5 with the original 350 engine and i am having overheating issues, but only when driven, at idle the temp stays normal. I have changed thermostat, heater core, water pump,coolant temp sensor and all hoses and still the same problem, if anybody has any ideas on what steps i should take next it would be very much appreciated.
 
Stomis said:
Clogged or rotten radiator
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Yep, or undersize.
The problem is obviously not enough flow for higher power use, at idle it doesn't need much but once you start making the engine work, it needs more.
 
I would suggest the belt was slipping at the higher fan speed, which would cause that, but I doubt that you changed everything else and left an old worn belt on.

If it were serpentine, I would suggest that the tensioner was bad, but I don't think they had those in 85.

If you even suspect the belt, you can take it out and drive it a few minutes at the speeds it gets hot at, then stop and feel the fan belt and pulley. If its slipping, it should be shiny and hot.
 
Fordum said:
I would suggest the belt was slipping at the higher fan speed, which would cause that, but I doubt that you changed everything else and left an old worn belt on.

If it were serpentine, I would suggest that the tensioner was bad, but I don't think they had those in 85.

If you even suspect the belt, you can take it out and drive it a few minutes at the speeds it gets hot at, then stop and feel the fan belt and pulley. If its slipping, it should be shiny and hot.
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Yeah could be fan clutch too.
 
Are you basing the over heating off of the factory gauge or are you taking a temp reading with a laser?

Just figured I would throw that option out there since the gauge could be reading wrong. But if your getting a reading from a seperate source then I would agree with the radiator being to small or possibly the tensioner/belt
 
Stomis said:
Yeah could be fan clutch too.
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I was going to say that when Fordum talked about the belt, because a belt would squeal and would be obvious but the fan clutch is actually a possibility, although IIRC while driving, the fan is pushed by the air so not much pressure on the clutch, usually it's the opposite, it overheats when not moving.:dunno:
 
If they get loose or glazed enough they won't squeal. Had that proven to me the hard why a long time ago.
Belt looked fine, nice and tight, and nary a sound.
But it would overheat.
I took it to a friend of mine to get his help changing the waterpump. He was this old genius mechanic that had built it in the first place.

Told him what it was doing, he looked at the belt, looked at me kinda disgusted, and said let me show you something.
Cranked it up, said feel how much air its moving. I did. He sprayed a shot of belt dressing on the belt.
The air slowed down for a couple of seconds and you could see that the fan had slowed.
Then the solvent in the dressing dried up, and the fan took off. Easily blowing twice as hard as before.

I said that fixed it. He said, no, that patched it. Go put on a new belt. Cured the overheating.

When I blew out the fan clutch in my 79 by driving underwater, it would do fine at speed, but overheat like crazy at idle.
 
My guess would be a collapsing lower radiator hose.

Verify that there is a spring in the lower hose.

A quick test would be to rev the engine while looking at the lower hose to see if it is collapsing when the engine at higher RPM's.
 
Fordum said:
When I blew out the fan clutch in my 79 by driving underwater, it would do fine at speed, but overheat like crazy at idle.
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I knew that was how that worked, so no on clutch fan but of course, I forget the other thing that happened to me and took me a while to figure out why the flow wasn't adequate for cooling on one of my trucks... someone just beat me to it, radiator hose collapsing, mine didn't have the steel coil in it, so I got a new one with, and problem was solved.:doah:
 
chevyluv said:
yes checked all vacum lines, Im also not gettinn very much pressure at the top radiator hose
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If you are not developing much pressure on the radiator hoses, then you either have a leak, or the radiator cap is bad.
Cap is cheap, and might fix the problem.
 
Have you looked into the radiator? Pop the cap off WHEN IT'S COLD and look inside. All rusted out? Gooped up? If so, you may want to try one of those flush kits.

But I second the collapsed radiator hose. I've had that happen. Just squeeze it. If you can crush the hose by hand, it's probably in need of replacement.

You can also get a system pressure check tool. Put it on where the cap goes and see if you build pressure.

Finally, take the cap off WHEN IT'S COLD and then run the engine. Do you see any bubbles? No coolant? This could mean a blown head gasket. Also check for coolant in the oil to confirm this.
 
Also look into replacing your thermostat again. I have on more than one occasion had a thermostat that was bad right out of the box.


I usually drill a very tiny hole near the edge of the thermostat to allow air to go by when first bleeding the system. But not a big enough hole to affect the cooling.
 
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