Cooling System Diagnosis

[sjarrett71]

Here's my problem. I cleaned my radiator and engine cooling system, replaced all the hoses, 190 degree thermostat (times 2), temp gauge sensor, and eventually installed a new aftermarket temp gauge. With both the old gauge and the new, it would get up to about 240 degrees before the thermostat would open. Then go to about 190-200 and start to heat back up. While it's doing this, I can go out and grab the heater hoses and radiator hoses and they are scalding hot but the radiator is so cool to the touch I can hold my hand on it without a problem. I have "burped" the system, flushed the radiator and flushed the engine. What would you guys suggest is my issue?

The engine is a stock 350 in a 72 Chevy with auto trans.

[truckster]

You didn't mention the water pump. That's where I would start looking.

[sjarrett71]

I was hoping to avoid that issue! lol

I will say the water pump sounds fine while listening to it with a stethoscope and felt fine while spinning it with the belt off. There is no fan clutch, just the fan bolted to the water pump with a spacer.

[Stocker]

Originally Posted by sjarrett71

There is no fan clutch, just the fan bolted to the water pump with a spacer.

This is actually what I would do first -- a new clutch fan (with a good shroud).

And even with new hoses, make sure the bottom hose isn't collapsing. Does it have a spring inside it?

[davepl]

FWIW, I had a similar problem years ago and misdiagnosed it as the water pump when it was really the thermostat. You've done that twice, so its not your issue, but in case it helps anyone, the upper hose being hot does NOT mean that your thermostat is working properly.

I've never done it, but you should (when cold!) be able to disconnect the rad side of the upper rad hose, stick it in a bucket, and see flow when you start the motor. I'd pull the thermostat first so you don't have to mess with scalding fluids.

I'm not certain that would prove the pump to be ok (after all, maybe it only fails under pressure) but still should reveal a problem if its bad.

Is there a better way to test the water pump on the vehicle if it makes no noise and doesn't have an obvious bearing problem that gives it away?

[sjarrett71]

Originally Posted by Stocker

And even with new hoses, make sure the bottom hose isn't collapsing. Does it have a spring inside it?

I have looked at the bottom hose and it isn't collapsing. There is no spring inside of it although I did opt for the better Gates brand over the store brand.

[sjarrett71]

And I did test the thermostat in a pot of boiling water to make sure it worked before I installed it.

[68gmsee]

As davepl mentioned, you can test to see if the pump is at least pushing water by removing the thermostat,reinstall housing and place upper radiator hose into a container. Again, that's only to see if you have water movement but the vanes could still be rusted and worn to the point that it's won't force enough coolant through.

Having said that I had a similar problem with my truck and finally give up on generic t-stats and bought a quality one at CarQuest. Tested it in boiling water to make sure it opened at the approximate rated temp, got a new radiator cap and installed an overflow container.

[geezer#99]

If you pull the top rad hose off, leave the thermostat in and start it you won't see any flow till the thermostat opens. That's how it's supposed to work. THe coolant circulates around in the block and into the suction side of the pump with the bypass hole. When the temp comes up the stat starts to open. Top hose will start to get hot. If your fan is working right the rad will stay cooler.
Your temperature might be high due to location of your sensor.
Where's the temp sensor located?

[sjarrett71]

The temp sensor is located in the rear of the head on the passenger side. There is a plug in the original location in the front driver side head that I could not get out no matter what I tried. The original temp sensor had been moved to the rear passenger side head before I acquired the truck.

[68gmsee]

Originally Posted by geezer#99

If you pull the top rad hose off, leave the thermostat in and start it you won't see any flow till the thermostat opens. That's how it's supposed to work. THe coolant circulates around in the block and into the suction side of the pump with the bypass hole. When the temp comes up the stat starts to open.....

Geezer... The reason to remove the t-stat is a quick way to see if the pump is pushing coolant. Otherwise you have to sit there and hold it until the t-stat opens. However, one could do it that way also.

[DesertCat]

Originally Posted by sjarrett71

The temp sensor is located in the rear of the head on the passenger side. There is a plug in the original location in the front driver side head that I could not get out no matter what I tried. The original temp sensor had been moved to the rear passenger side head before I acquired the truck.

Sender in the head is not the ideal location, I prefer on the intake manifold.

Questions; why did you do all this work?
Was it overheating?
Did the temp gauge read differently before this work?

Personally I would replace the pump so the whole system is new and done.

Good luck!

[sjarrett71]

I did it for general maintenance. The truck sat for a few years and I wanted to replace the antifreeze before winter so I just flushed the system and replaced the hoses and thermostat.

The factory temp gauge never really worked right. It has an extra ground wire going to it so I assumed the PO had problems with it, that was my motivation to get the new gauge. But both gauges are reading hot.

[DesertCat]

Originally Posted by sjarrett71

I did it for general maintenance. The truck sat for a few years and I wanted to replace the antifreeze before winter so I just flushed the system and replaced the hoses and thermostat.

The factory temp gauge never really worked right. It has an extra ground wire going to it so I assumed the PO had problems with it, that was my motivation to get the new gauge. But both gauges are reading hot.

Well, you know the thermostat is working because of the readings you are seeing.
I would first move the sender to the intake and see what happens there. If you are seeing the same temps there, you have a bad pump or really bad restrictions of the internal water passages.

[magwakeenercew2jh]

I always have to offer this disclaimer: I'm no mechanic. Just a dude that (sometimes) thinks logically.
But, I DO I know how to "trace wires" and "R&R parts". Hahahaha!!!!

I'm thinking that if the truck sat for a while, I'd even be MORE suspicious of the water pump.

To, me, all this work and NOT replace a water pump? Probably a mistake. Have you checked the price of
a water pump for a 350? Sixty bucks at Summit....Maybe even better from a Forum Seller....In all, pretty cheap insurance...

Maybe a little pricey for diagnosing a problem. But "what the hay"? In the end, you have a new water pump.

[PanelDeland]

I would get a thermometer and check the temp against the temp guages.That said,it sounds like it's not flowing water thru the rad.Even a direct driven fan should keep it from overheatiing.If it runs hot while driving you can be fairly sure you have airflow.Either the rad is plugged(enough to not cool well) or the water isn't circulating.I agree with changing the WP.Then the only thing left is the rad.

[sjarrett71]

I got a new water pump and am waiting for the paint to dry on it before I install it. I also ordered a new radiator. When I took the old water pump off, I took the backing plate off and it looked fine on the inside and the bearing was still tight and sounded good so I guess it wasn't the problem. I'm going to replace it anyway. The radiator does look kinda gunked up on the inside though.

[sjarrett71]

OK, here's an update. I replaced the water pump and the radiator. I apparently needed the radiator because there are areas all over it where it appears it has been leaking throughout the years. Plus the inside of it was absolutely disgusting.

So I get everything replaced and refill the coolant and start that puppy up. The temperature climbed and climbed and climbed and when the gauge hit 240 degrees the thermostat opened and it went back to 200. I've got a 195 thermostat in it for the winter but I think I'll go with the 180 and see what happens.

Ideas anyone?

[68gmsee]

Originally Posted by sjarrett71

..... I've got a 195 thermostat in it for the winter but I think I'll go with the 180 and see what happens. Ideas anyone?

I've always used a 180 year round. No need to change it in the winter. If the t-stat is working properly, 180 degrees is plenty warm. Some say that an engine runs more efficiently at 195-210 but in my case, and since my engines are not performance types, I've never noticed much difference.

[haysonj]

Originally Posted by sjarrett71

OK, here's an update. I replaced the water pump and the radiator. I apparently needed the radiator because there are areas all over it where it appears it has been leaking throughout the years. Plus the inside of it was absolutely disgusting.

So I get everything replaced and refill the coolant and start that puppy up. The temperature climbed and climbed and climbed and when the gauge hit 240 degrees the thermostat opened and it went back to 200. I've got a 195 thermostat in it for the winter but I think I'll go with the 180 and see what happens.

Ideas anyone?

Mine was doing the same except it would only go to 220 then it drops to 195 and would stay there. I drilled a 1/8 inch hole in the thermostat and it evened the temps out now it opens at around 200 and drops to 195

[Andy4639]

Did you install the thermastat correctly in the housing. If you put it facing backwards it will do this. Yes there is a front and back to a thermastat.

[sjarrett71]

Originally Posted by Andy4639

Did you install the thermastat correctly in the housing. If you put it facing backwards it will do this. Yes there is a front and back to a thermastat.

Yeah, I've installed a few.

[Andy4639]

Next Question then. Did you take the radiator cap off and make sure all the air got out of the system? I have had to do this several times to get all of the air out.

If you are sure you did all the basic stuff then get a good temerature gauge and double verify your truck guage.

[sjarrett71]

Yes, I left the cap off through several cycles of the thermostat opening and closing so I'm pretty sure all the air was out. I have also already installed a new gauge but may try another to see if it's defective. I'm going to change the thermostat too.

[sjarrett71]

Another update. I installed another radiator (4 core) and still the same problem. I borrowed a laser temperature gauge from a friend and used it on several spots on the engine and it shows the engine at normal temps, about 190-195. I guess the sending unit/temp gauge is bad. I will replace it and update again soon.