overheating at idle
 

I have a k20 with a 350sbc. When i drive around at speeds over 30 or 40 miles an hour my truck will run at 175 ish degrees on the coolant temp. When i idle or hit city traffic i slowly start creeping until i near over heating. I have a brand new 2 core aluminum radiator, a new hi volume water pump, a new thermostat, new hoses, in stalled a fan shroud etc... I did a chemical flush before i installed all the new items.

with all that im still overheating. I read on here that overheating at idle but not at speed is a sign of air flow like a missing fan shroud. I do have a fan shroud and the 7 bladed fan blades are half-way in like they should be.

the only thing i can think of is that:

1. the fan blades are several inches away from the back of the radiator. im guessing maybe 5 or 6 inches. i read that the fan blades should be almost touching the back?

2. my thermostat i got has a hole pre drilled into it "hi flow" bypass made specifically for hi flow water pumps. maybe too much fluid is seeping through and not allowing the coolant to stick in the radiator long enough to cool? My old thermostat before I got the new cooling system would still creep and it was not a hi flow stat. so i think this may not be the issue.

3. my truck has a big fat a/c condenser in front of the radiator maybe its deflecting air flow? I dont want to rip the condenser off because thats my next project after tires.

Do you think any of these might be an issue? anybody have suggestions on what to look at?

Re: overheating at idle
 

Do you have a clutch fan or fixed ? If clutch it might be not working properly.

Re: overheating at idle
 

Posting some photos of the fan and shroud may help us.

What diameter pulleys are on the crankshaft and water pump? Maybe someone swapped some under driven pulleys some time in the past.

Do you have an overflow/expansion tank?

It's possible you have air trapped in the engine. Have you tried idling the truck with the front end raised 2 feet or so? This helps trapped air bubbles to get out.

Are you seeing a lot of micro sized bubbles in the coolant?

Have you verified the accuracy of the temp guage?

Not the best photos of my fan fitment in the shroud but hopefully it will help.

Whether or not you have a shroud determines the fan distance from the radiator. No shroud means the fan needs to be close to the radiator. With a shroud installed then the fan needs to be at the outlet of the shroud.

Re: overheating at idle
 

If you have a clutch fan how old is the clutch ? My 85 started to creep up at idle I changed out the clutch and it’s all good now . I can hear when the clutch engages as there’s a load whooshing noise didn’t hear it with the old clutch

Re: overheating at idle
 

I have a clutch fan. The clutch works fine it is almost brand new. It’s a heavy duty Hayden cutch.

Re: overheating at idle
 

It's usually air flow at idle that causes heating, though. You can check the clutch by watching what happens when you turn the engine off. If it coasts for a bit, it's toast. It should stop rotating almost immediately when you shut the engine off.

I found a high flow thermostat at NAPA. The valve is much bigger than the one in the standard thermostat. It doesn't have a hole drilled any where. Check it out.

Re: overheating at idle
 

Here are a couple of possibilities.

• 50 years of bugs in the AC condenser blocking air flow
• Incorrect reverse rotation fan
• Fan installed backwards
• Defective fan clutch

Is the fan moving much air at idle? Does the air coming off the fan feel hot?

Re: overheating at idle
 

First things first, you say it runs at 175ish? Is that with an aftermarket gauge? And what temp is it overheating at? My Cold/Hot gauge when pegged is 195/200 degrees. I have verified this with more than one laser temp reader.

Re: overheating at idle
 

https://ibb.co/r2FHbvw
https://ibb.co/4ZCFZy2
https://ibb.co/jT6VjF9
https://ibb.co/5B1Kbqg

I was following some of your suggestions and and I think I may of spotted something. The top of the fan blade has a 2ish inch gap from the blade to the top of the fan shroud. The bottom blades have maybe a 1/4 inch from hitting the shroud. I played with the shroud and it doesn’t seem adjustable up and down. I can adjust it left and right. The shrouds part number cross referenced to a c10 NON AC truck. My truck has ac . I don’t know if the shroud holes are different on an ac truck. I also. Noticed that the fan blades are about 3 1/2 to 4 inches wide. The blades only stick in the shroud maybe 1/2 inch. I don’t know if I can get a spacer to push the fan out an inch or so. Or if getting an ac speced shroud would solve that.


Also to answer some earlier questions. I do have an over flow tank. And the water pump pulley is roughly 6 inches in diameter. Would an over drive pulley help? Or do I already have one?

Yesterday when I was filing in my driveway the truck crept up to 200 and I started driving and it went down to 175 -180 when I came back and idled again it crept up to 225 ish and then started to idle real bad so I shut it off before it overheated, the gauge is an aftermarket one. I did at one point borrow a laser temp gun to see the temp spread of the radiatior .

Re: overheating at idle
 

Which way is the fan blades supposed to be orientated? I reassembled them the same way I took them off but who knows if the PO had it on right to begin with

https://ibb.co/GvckTf7

Re: overheating at idle
 

From what I remember when dealing with LMC on fan shrouds. The only difference between a/c and non a/c shroud was the depth and its like 5/8”. What size thermostat is in it? The hole in the thermostat should be small like 1/8” or so. That used to be pretty common for a guy to drill one and i believe the thought process was to purge air more than anything else.

Re: overheating at idle
 

See my photo above for the correct fan blade orientation.

Your fan should be deeper inside the shroud like mine is. About 5/8 to 3/4 inch sticking out of the backside.

For the comon lengths GM fan spacers are available in 1/2" increments. So getting a longer one shouldn't be a problem.

Having finally figured out how to view your pictures I can see that your shroud has definite issues. First off it needs to be sealed around the radiator completely. The green area in photo needs to be closed up. Is the area near the red arrow open? It looks like I can see the radiator fins there.

Air being pulled by the fan will take the path of least resistance which is not through the radiator fins. The idea is the fan blades create an area of low pressure (A vacuum if you will) and then air rushes to fill the low pressure area back up. The air must be forced to flow through the radiator fins. On yours the air can bypass the radiator fins just about everywhere.

Then the fan blades need to be much deeper into the shroud
I'm not sure but you may not have the correct shroud someone else will know for sure. To me it looks wrong or maybe heavily modified. A last photo from a wider angle of my shroud.

Re: overheating at idle
 

Your fan appears to be too far out of the shroud, but is installed correctly. I don't recall ever seeing a spacer used with a clutch type fan. Maybe there is a different shroud for K Vs C? I recall something about the engine being mounted higher? If the fan is too close to the bottom, it may interfere if you move it though.

Re: overheating at idle
 

Not sure if a 2 core with a V-8 and A/C is enough to start with, but the 4x4 shroud is different than that one. Hopefully this pic shows a little better.

Re: overheating at idle
 

Thanks guys for giving me some direction. I think you guys narrowed the problem down for me. According to the part number The shroud on my truck goes to a c10 2 wheel drive no Ac truck. There is a separate part number for ac trucks as well as 4 wheel drive trucks. The description of the parts at a classic part website shows the 4x4 shroud being deeper than the 2 wheel drive version. I bet if I get the right part the depth of the fan and the opening will align. Thanks for helping me track down this issue. I’ll update when I find and get the new shroud in.

Re: overheating at idle
 

The two row 1inch tube cold case should be more than enough to cool a stock slightly modified V8 . Had a two row 1 inch tube in my Chevelle with a 496 ran nice and cool with the stock clutch fan the surface area of the tubes is greater than a 4 row copper radiator because copper tubes can’t be very wide because they would break . When it got damaged I went with a Dewitt’s 2 row 1.25 inch and spal fans . Runs cool never has it overheated .

I agree that the shroud is the culprit. Looks like the fan isn’t even inside it also it seems to look like the fan has a huge space between it and the radiator.

Re: overheating at idle
 

The temp idiot light in my 67 GMC L6 would come on at idle and go off when moving down the road - the reason turned out to be that I had a small exhaust manifold leak right over the sending unit….

Re: overheating at idle
 

I just now ran across this thread..... I agree that the shroud looks all wrong. What year is your K20? Also, it's hard to tell but your fan looks a bit odd to me. Seven blades = good, but what is the diameter? 18" diameter seems typical.

Re: overheating at idle
 

The fan should be centered in the shroud with a minimum of a 1/2 inch gap evenly spaced all the way around. With a 2 inch gap at the top you either have the wrong shroud (most likely) or the engine is mounted too low. The 2 inch gap is what's causing your overheating problem.

Also a Hayden 2731 clutch is 0.61 inches longer than the 2747. Other than the length both clutches are identical.