Overheating 72 3/4 ton 402

[w6tractor]

OK, I'm stumped. My 1972 Chevy 3/4 ton with 402 has begun overheating. I pull a 24 foot goosneck with a 4-6K tractor, mower on it. I have replaced the water pump, radiator (new NAPA 4 row brass and copper) and installed a 160 degree thermostat. It still goes to 230+ even on a mild day. Doesn't do much better w/o the tractor and trailer. It runs good, no miss or backfire. It will "run-on" after switched off sometimes when it's hot. It has a 400 auto with a 12"x15" trans cooler in front of the radiator on the passenger side and an auxillary electric fan pushing air on the other side. I also added a coolant recovery tank, set the timing to 8 degrees BTC. What am I missing? Could the trans cooler be blocking too much air? You guys are great with diagnosing.

[Lee H]

If it just began to overheat with out any mods than that's good. It just means one or more of the components has failed. Did it start overheating before or after the rad and WP? How are you measuring the temp? If by gauge then you need to verify with an IR gun. Thermo does not affect high temp, just how fast it gets there. Do you have a good clutch fan? How about the fan shroud? I would bump the timing a few degrees and see if that helps.

[Stocker]

Originally Posted by Lee H

Do you have a good clutch fan? How about the fan shroud?

Ditto these two questions... also, are you running 50/50 antifreeze/water?


As to 'dieseling' when shut off, leave it in gear when turning the ignition off, instead of neutral or park. Not a cure but it helps the engine shut down quickly.

[Stocker]

Originally Posted by w6tractor

It has a 400 auto with a 12"x15" trans cooler in front of the radiator on the passenger side and an auxillary electric fan pushing air on the other side.

Wait, do you mean it's pushing air forward? If so, this could be part of your problem. As you drive, air will naturally try to flow through your radiator front-to-back -- and now your electric fan is trying to push it back through, from back-to-front. You may not be getting much airflow at all, and may be better off with a fan that will pull air conventionally, front-to-back.

Whether you try that or go back to a clutch fan (my recommendation), I would definitely get a fan shroud if you don't already have one.

[Andy4639]

I have replaced the water pump, radiator (new NAPA 4 row brass and copper) and installed a 160 degree thermostat.

Has it been over heating since the swap? Jack the truck up as high as you can in the front and take tha cap off the rad. and starter up and see if you have a air pocket.

[w6tractor]

Originally Posted by Lee H

If it just began to overheat with out any mods than that's good. It just means one or more of the components has failed. Did it start overheating before or after the rad and WP? How are you measuring the temp? If by gauge then you need to verify with an IR gun. Thermo does not affect high temp, just how fast it gets there. Do you have a good clutch fan? How about the fan shroud? I would bump the timing a few degrees and see if that helps.

Thanks for the quick responses. It was overheating before the addition of the radiator and water pump. Temp is by an aftermarket mechanical gauge. Fan clutch was replaced two years ago when A/C was redone. Fan shroud is factory and in good shape. Do you mean bump the timing up or down?

Stocker, it is 50/50 antifreeze and the electric fan is mounted in front of the radiator and is pushing air toward the radiator. It only operates when the A/C is on.

Thanks for the help. I'll try the IR for temp. and keep trying.

[LONGHAIR]

When does it overheat?
Low speed (city) driving is usually an airflow issue
High speed (highway) driving is usually a coolant flow problem.

But, first you need to verify that you really do have a problem. Eliminate things like the possibility of an air bubble or the accuracy of the gauge.

[cdowns]

i'd take an oldschool tmermometer and put it in the coolant and see what the actual real temp of the coolant is before saying it was overheating and throwing parts at something without knowing for sure a problem exists

[geezer#99]

Do you have a support coil in the bottom rad hose??

[w6tractor]

Originally Posted by geezer#99

Do you have a support coil in the bottom rad hose??

Thanks for the continued suggestions. Won't be able to try solutions until Sat. Don't know about coil in lower hose. Its pretty new,so I would think so. I've tried to purge an air pocket, but have not tried jacking it up. I'll keep you posted. I'llalso verify the tem reading.

[geezer#99]

Originally Posted by w6tractor

Don't know about coil in lower hose. Its pretty new,so I would think so.

Most new hoses don't come with one. Just squeeze it. If you can flatten it out somewhat when it's cold it could be collapsing and restricting flow.

[DANTIP]

Believe it or not, my gauge was lying because my voltage regulator was bad. I realized my gauge was lying when I used an infrared thermometer on the radiator. Replaced the regulator and the gauge read correctly.

Run-on/dieseling, makes me think you are overheating.

If you have access to an infrared thermometer (some multimeters have them built-in), check the temp of the radiator tanks (left and right). You should see a significant drop (mine was inlet at 180 and outlet around 150). Low coolant flow would result in a higher than normal temperature difference in the tank temps. Low air flow would result in lower than normal temperature difference (bad clutch or plugged front). Fouling on the inside of the tube (not likely in a reasonably new rad) would also result in lower than normal temperature difference. Get some data and let us know what you find.

Good Luck!

[w6tractor]

Well, I'm back. Still no good news, but I think I have eliminated some things it's not. Changed the temp. gauge, let it run al loooong time to purge any air bubbles and bought an IR thermometer. Still seems to run too hot. Here's the data. After driving long enough to get the gauge to 230 I returned home and took measurements. Water temp gauge = 230+, Air temp.=80, thermostat housing =218, radiator tank at inlet =215, radiator tank passenger side = 200.
Next (last?) option is to replace fan clutch with a hd version. Current one is 2.5 yr old but doesn't make alot of noise. Another possible cause is the a/c condenser. Changed to 134a with new almost everything. A/C shop was concerned with high head pressure and installed an aux fan wired to the compressor. Could this generate enough extra heat to cause my problems? Thanks for all the help.

[w6tractor]

I forgot-the bottom hose does have a wire coil inside.

[geezer#99]

Even though you replaced the WP and it was overheating before that did you put a WP in that was for an A/C truck. Those are made for more flow using a different impeller.
Second thought!! WP speed? Is your WP pulley stock? A smaller one could produce more flow. You have to watch not to speed your pump up too much though. You can cause the WP to cavitate which hurts flow too. And one more. Maybe you need to slow the flow down for more coolant retention time in the rad. Just some thoughts!!!

[DANTIP]

The temperature drop across the radiator seems low. Where were you testing the inlet air temperature? If your A/C was heating the air too much or affecting air flow across the radiator, I could potentially see a problem. I also assume the fan that was added to the A/C system pushes air from the front of the condenser toward the radiator. If it is in the opposite direction, I would expect a problem.

Did you do the rolled-up newspaper test on the fan clutch (the fan should not be able to be stopped with a 5-6 page rolled up newspaper rubbing against the fan while running)?

[w6tractor]

Originally Posted by geezer#99

Even though you replaced the WP and it was overheating before that did you put a WP in that was for an A/C truck. Those are made for more flow using a different impeller.
Second thought!! WP speed? Is your WP pulley stock? A smaller one could produce more flow. You have to watch not to speed your pump up too much though. You can cause the WP to cavitate which hurts flow too. And one more. Maybe you need to slow the flow down for more coolant retention time in the rad. Just some thoughts!!!

As far as I know the pulley is stock. The WP is what was listed in the catalogue for this truck, but I'll check. Thanks

[w6tractor]

Originally Posted by DANTIP

The temperature drop across the radiator seems low. Where were you testing the inlet air temperature? If your A/C was heating the air too much or affecting air flow across the radiator, I could potentially see a problem. I also assume the fan that was added to the A/C system pushes air from the front of the condenser toward the radiator. If it is in the opposite direction, I would expect a problem.

Did you do the rolled-up newspaper test on the fan clutch (the fan should not be able to be stopped with a 5-6 page rolled up newspaper rubbing against the fan while running)?

I tested the temp on the tank by the inlet hose. The fan is a pusher. First I've heard of the newspaper test. I'll try it. Very frustrating. I feel like it's something little that I'm just overlooking.

[JCD]

My Barracuda overheated with a 160 thermostat..
At 160 the antifreeze was being allowed to pass through the radiator quicker, and not giving it ample time to cool.
Once it gets 160 or higher, the thermostat stays open, and the radiator can't work as efficiently.
I put in a 180, and it kept the water in the radiator longer to allow it to cool more.
The higher temperature thermostat made my car run cooler overall.

J.

[w6tractor]

I'm back. Only thing left (I think) is the fan clutch. Mine has a little leakage. Bought a heavy duty (hayden 2747) which is a little longer. Thanks for the help. I'll let you know if this does it.

[AusTx68]

I just went thru a similar situation. Truck started heating up with less than 300 miles on a frame off build. All components are new. I swapped t-stats, flushed radiator, replaced lower hose, ect. I then removed the t-stat all together. When I did that it never got hotter than 150 while cruising. Closer observation turned up bubbles in the coolant. Looks to be a gasket or crack. Still need to tear it down to find out. Took me a week to figure it out. I have no water in my oil and the truck runs and sounds great. I was able to determine the problem using a combustion leak tester. You fill the tester full of blue fluid, connect it to a vacuum source, remove the radiator cap and snug it into the hole. If the fluid turns yellow you have a leak.

[w6tractor]

Originally Posted by AusTx68

I just went thru a similar situation. Truck started heating up with less than 300 miles on a frame off build. All components are new. I swapped t-stats, flushed radiator, replaced lower hose, ect. I then removed the t-stat all together. When I did that it never got hotter than 150 while cruising. Closer observation turned up bubbles in the coolant. Looks to be a gasket or crack. Still need to tear it down to find out. Took me a week to figure it out. I have no water in my oil and the truck runs and sounds great. I was able to determine the problem using a combustion leak tester. You fill the tester full of blue fluid, connect it to a vacuum source, remove the radiator cap and snug it into the hole. If the fluid turns yellow you have a leak.

Never heard of that device before. Where did you get it? May have to try that. Thanks that.

[AusTx68]

I picked mine up at O'Reilly's Auto Parts and paid around $45. I could have gotten it cheaper if I wanted to wait. It's $24.96 on Amazon. Check here: Amazon.com: Combustion Leak Detector: Automotive

[w6tractor]

Thanks for the info. If the new fan clutch doesn't fix the problem, that will be my next option.

[Stocker]

I'd bet the new one will fix it, since the old one was leaking... I've been mistaken once or twice in my life but I hope this isn't one of those times.