Need a lesson on engine tempatures and cooling system.


Detroit, MI
Let me start with saying I just got my truck back from having the engine replaced and it runs fantastic. I can't believe the power it has now. However, when I got it back the shop said they flushed the radiator and a bunch of junk came out. Well that junk was plugging the holes just fine until the flushed it. Now I've got leaking problems again. I put a bottle of Bar's Copper Radiator Seal in it and it's been holding pretty good.

Now I took it out for a leisurely drive and some at a decent speed. When I got to a stop or slow spot, the idiot light would slowly start to glow on. Once I got up and running at a decent speed again, the light went out. Stop, light comes on, get moving, light goes out. When I stopped I checked the coolant reservoir and there's fluid in it, so it's not like the leaks are draining it dry. I also know that the shop installed a new 180 thermostat.

So I'm trying to figure out how much of the leaky radiator is contributing to the idiot light. Or is there something else along the way that could be the problem?

Or should I get a mechanical temp gauge, pull the sensor, and just run that for exact temps?


Active member
Yuma, AZ
Have you checked the fan clutch? It should have resistance when spun by hand, engine off of course.

Sent from my SM-G955U using Tapatalk


Well-known member
  1. Lower temp thermostat does not equal lower running temperature. It doesn't work that way. (Unless you are doing nothing but very short drives.)
  2. A new fan clutch will not repair a leaky radiator.
  3. In various studies done in the U.S., Canada, and Europe, radiator replacement has been found to be cheaper than engine replacement 100% of the time. Your mileage will NOT vary.
  4. Idiot lights are for idiots. Get a gauge.

You don't need a lesson in engine cooling. You need a new radiator.
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Super Moderator
Super Moderator
Edmond, Oklahoma
Recommend you install a gauge.
The 6.2L does not do well if it gets really hot.

Was the water pump replaced on the current engine?
All belts tight?
As mentioned, fan clutch worn?
The water pump and fan clutch should be replaced at same time.

Too much antifreeze (not enough water added to the mix) will also cause overheating.
What brand thermostat was used?
I had several bad Gates thermostats (Very unusual) and switched to ACDELCO.
Problem solved.

I installed on mine Water and Transmission temp gauges.
Engine oil pressure and Engine RPM gauges.
Best money ever spent.
No more guessing.
The factory water temp idiot light is still installed.

I also installed a new copper/brass radiator just over a year ago.
Been flawless and temp stays where it’s supposed to.
External Transmission cooler is next.
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Well-known member
Jonestown Pennsylvania
Like most vehicles the CUCV is equipped with a pressurized cooling system. It requires the proper mix of coolant and water in pressurized cooling system to maintain the correct temperature. If the cooling system is unable to maintain pressure it is unable to maintain the proper cooling as designed. The addition of stop leak just complcates the cooling issue. The small passages in the engine and the radiator are now subjected to a compound that coats and resticts proper flow. As mentioned get a radiator. It don't have to be copper and brass one. A plastic and aluminum will work just fine. I personnaly prefer the plastic and aluminum. But now you will need to / should flush the system and repair/recore or replace the raditor. The damage could be done already if you keep subjecting the engine to high temperatures. My late Father was always amazed that my new trucks ran at 210*. They are designed to run at that temperature. The CUCV is not. But 212* being boiling. The pressurized cooling system is able to control that boiling point with the coolant chemicil mix. Good Luck. I see radiators for $300. I doubt the engine was that inexpensive. Have a Great Day and Stay Cool and Safe.


oklahoma city, ok
2nd on the thought that stop leak is a temporary fix. It sometimes allows you to defer a fix for a few months but in most cases you will be doing a more permanent fix.

When the shop flushed the radiator, did they also blow out any crap filling the air passages? Plugged air flow gives the same result as plugged water flow. Your working fine at speed and overheating while stopped is either poor water flow or poor air flow.

Don't overlook the heater core as a leak source.

Unless the leak is pretty fast and not allowing the system to pressurize, it won't impact the temps until the coolant level gets to low.


Rochester NY
You NEED to get a radiator, plaine and simple. Like said above its a closed and PRESSURIZED system as the engine gets hot the coolant expands into the overflow jug, but in your case it leaks out of the holes. Then as it cools and instead of drawing the coolent BACK from the overflow jug your's is sucking air IN! Check the jug all you want it may be full and your rad may very well be low as in REAL LOW! The rad cap also is importent, ever wonder why they are listed as 10LB or 12 LBS? For every pound of pressure it raises the boiling point of water 2.5 degrees so a 10 lb cap raises the boiling point to aprox 237 degrees, water is a much better coolent than steam the last thing you want in an engine is for the coolent to boil!


Oh look I am back here for 5 min... so... wow let's address everything here...

Old tech antifreeze (that 2 year stuff) coated things... new (long life) is more like soap so it's normal no matter what to have crap in the rad... at 35 years... it is likely done, first thing the shop should have done was told you you need a new rad... if the rad is leaking it is done.

Now for the light, truck could be overheating (for many reasons) sensors could be bad, wire could be bad... million options... first thing is to stop driving it if you have any chance of a overheat...

What you should be doing (imo) flushing the cooling system (get that stop leak out) replacing the rad, and putting in a real gauge.

At this point, fire up the engine and check the gauge temp... if she is still running high look into water pump or fan clutch...

Another test you can do, turn the heater on full blast see if the light goes back out at all... this would not confirm much except it thinks the temp is too high...

A leaky rad won't do crap for the idiot light... it's a TEMP light... unless your low coolant light is on too... (and you are that low) leaky rad means nothing (cept see below)

Now let me help correct some info post here...

A lower t stat 100% does result in a lower operating temp (rofl?) T stats are rated at the temp they *start* to open... if you run a 180... it won't be fully open till like 195... unless your cooling system is not working, if you put in a 160... guess what, that truck is *never* gona run 195 like it can with a 180... so in what universe a tstat does not effect engine op temp... idk... In this one with a working cooling system it does...

A leaky rad will *not* cause a overheat HOWEVER you will lose some boil protection, I would not recommend driving with a leaking rad but you can... it may allow boiling (air) in the system not the best but... if you need to you need to... with a new motor however... that need better be like life or death...

Stop driving the poor thing, get on it with the repairs... and go. Keep in mind these trucks have *large* rads... and unless you are just highway cruising don't be getting a 2 core rad for it...

Why was the motor replaced?


Active member
Steel Soldiers Supporter
San Diego, CA
Sadly Stop Leak, to stop leaks also plugs up radiator and heater cores, which will reduce your cooling potential. That is a product of desperation of last resort, like you are in the middle of nowhere and need to get to somewhere to fix your problem. Never use it if you have ANY other option (i.e don't drive, have it towed, fix in parking lot with new parts, etc.)

As for idiot lights, they are just that. I overheated an engine in my '64 Nova as by the time the idiot light came on, I had a piston knock and caused me to rebuild the engine. Installed good quality gauges before driving again after rebuild. Now all of my older non factory gauge vehicles get full gauges.

Good luck.
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