Overheating - need help diagnosing

[johnwaynejeep]

My 75 C10 overheats when driving under 20mph. above 20mph, it's fine, even in 115 degree weather on the freeway.

I flushed the radiator, heater core and engine, replaced the water pump and upper/lower radiator hoses as part of regular maintenance and there is no fan clutch. I'm taking it in to replace the thermostat next week because I think the bolts are going to break and will need to be drilled and tapped.

My one concern is the fan. I have a 19" 6-blade flex fan. There is 1.5 inches of space between the fan blades and the radiator shroud. Could this space be the cause of the poor air flow? And how do I fix it? I cannot find a replacement fan big enough to fill this shroud. In fact I can't even find a fan bigger than 18".

[Bandit130]

Take picture of your fan setup and post. It could be there is too much space between fan blade and radiator. Also measure distance. Also, I have been told that half of width of fan blade should be inside shroud.

[tucsonjwt]

I recommend a genuine AC Delco fan with clutch. In my experience, flex fans never worked well.

[Bandit130]

Originally Posted by tucsonjwt

I recommend a genuine AC Delco fan with clutch. In my experience, flex fans never worked well.

X2. I saw at least one video on youtube, may be more, that showed they do not cool as well. Also heard stories as they get older, can come apart and cause injury and/or damage. Have never used one myself.

[Dead Parrot]

What is your clue(s) that it is overheating? If you are using the factory gauge, keep in mind that a 42 year old gauge paired with a 42 year old sender are rarely accurate or even consistent. Aftermarket senders rarely match the factory resistance curves. If you can, verify your temps with a 2nd source. Laser temp reader, meat thermometer, etc. Might not have an over heat problem but a gauge problem.

+1 for reverting to stock 7 blade fan with standard fan clutch.

That fact that it reads cooler at speed then slow does indicate an airflow problem or an under performing water pump.

Check your radiator and condenser coil(if equipped) for excessive bugs and crap.

[jkk]

Sounds like you have air in the system you need to purge the air out of it you be amazed at the difference this will back at slow speeds

[jkk]

Make sure the air is purged out of the system if you just did all of that it can have air pockets in it

[johnwaynejeep]

Thanks all for the inputs. I'll purge the air and make sure the gauge is reading accurate. I'll post a pic of my fan setup once I take care of these other things.

Thanks all

[johnwaynejeep]

I think I may have found the problem. Dead Parrot, you were right, my gauge fluctuates quite a bit. I took the truck for a drive and took readings with an infrared thermometer at various stops and temp readings along the way.

My gauge does not always reflect the actual temp and my thermostat does not open until 215 degrees. Once open it keeps the engine at 180-200 depending on load. It's obvious I need to replace my thermostat and troubleshoot the inconsistent readings in the temp gauge.

Thanks all

[SparkyRnD]

if the gauge and sending unit are functioning properly, then it's either the fan clutch or the fan distance from the radiator. If you overheat at low speeds, it's typically the fan (all else being equal). It if overheats at higher speeds, then it's either too small of a radiator, or you have something restricting air flow over the fins.

[andyh1956]

When you replace your thermostat be sure to drill a 3/32" or so hole in the flange to let the trapped air purge