Engine Running Hot

[rumbac]

I have a 71 truck with 350 and auto. At highway speeds (55) the truck runs hot. I have replaced the thermostat and had the radiator flushed.

Can someone please tell me what else could be cause it to run hot.

Thanks

[hjalbert]

is your water pump working? also if your timing is off some I think it could casue this, If you dont have a fan shroud it will get hotter then with one, but at idle also (alot hotter at idle) is there any water in your oil?

[GloryHound]

Well the first Question is what do you call hot?

The second question is are we dealing with stock engine, Transmission and rearend?

Also do we have a load on the truck?

If it is not stock I think the 350 used a 4 core radiator while the 307 used a 3 core. If you only have that three core you don't have what the engineers at GM deemed nessesary for a 350. Also if you put a different kind of auto transmission in you might actually be running hot in the transmission and transfering some of that heat through your radiator. Also make sure you have a fan shroud on they don't look like they do much but they really help in directing airflow around your engine and through the radiator for proper cooling.

[rumbac]

Thanks Guys

The truck is all stock. 3 core radiator. Fan shroud is in place. The needle on the stock temp gauge will go 3/4 of the way over. If I turn the heater on it will move to a little over half. Timing is good.

How do I check to see if the water pump is good?

Thanks

[GloryHound]

To see if the water pump is circulating you can remove the radiator cap while the engine is cool. Start the truck. when the trucks thermostat opens you should be able to look down in the radiator and see the water moving through it. But before you do that look at the weep hole on the pump and look for water marks. If you are not loosing water this is probably not the problem but if you are slowly loosing water you could be loosing it through this weep hole that is supposed to start leaking when your pump bearings go out. Another check is to remove the belts and try and wiggle the pump shaft if you have play it is probably worn out. Stepping up to a four core radiator would help but to do that you need to get four new radiator mounting brackets also since the four core will not sit in the three core brackets. (Tried that, been there, was really peeved.) Something else that you might want to do is temporarily hook up a thermostat that gives you an actual temp reading. That way you can compare it to what your thermostat is. It could be a simple matter of a worn out gauge if it has pegged before it could have damaged the copper windings in the meter.

[Mike C]

Your waterpump seems to be fine. By turning the heater on, you are effectively increasing core size and airflow. If the pump wasn't, then turning the heater on wouldn't matter because there would be no water being pumped.

In my Jimmy I replaced the radiator, thermostat, hoses, and water pump. It still ran hot on the road. Turned out the fan clutch had gotten weak. With the new thermostatically controlled clutch ($29 at a local parts store vs. $150 from GM) it stays right on the thermostat of hot days and actually 10 degrees cooler on cool days.

[Capitol68]

I agree with fan clutch,,,sometimes when they go out you can see a little oily blow by around the bearing seals

[rumbac]

I am going to replace the fan clutch tonight. Is it very hard to do?

Thanks for all the help.

[bpmcgee]

It's a piece of cake.

Brian

[Mixup]

Check the bottom hose. It will collapse at speed and restrict flow. Some hoses have a spring in them to stop this from happening.

[Blazer1970]

Also, how old is the radiator? Was it flow tested, or just flushed? Overheating at highway speeds is a sure sign of a plugged radiator, and flushing it out won't normally help. You have to have it flow tested by a radiator shop to tell for sure. You get plenty of air flow through the radiator at highway speeds, so the fan should not be the issue.

[rumbac]

Tim,

I took the radiator to a local shop. The guy said that he flushed and pressure tested it, and the flow was good. It is the factory radiator.

Thanks

[tom hand]

First thing to aways do is verify it is indeed running hot. The simpiest thing to do is go to Kroger and buy a cooking thermometer. Duct tape it to your upper radiator hose and go for a ride...when the gauge in the truck says it is hot pull over and see what the tempature really is. Since you have said nothing about it puking antifreeze on the ground I bet it is not even running hot. If it is running a little hot, fans and fan shrouds do nothing at hiway speeds so check for water flow and you may want to do a compression test or test the pressure on the radiator to see if you have a leaking headgasket that is pressurizing the cooling system.

[GloryHound]

I just remembered if you do not have the right kind of radiator cap you can have heating problems. The two major differences to look for is one is made to run with a resivior and the other is not. And sometime a radiator cap can just go bad!

[rumbac]

Gloryhound,

You may be on to something. The old man I got the truck from took a wash bottle and mounted it as a reservoir. There is a hose running from the radiator to the bottle. I will check the cap tonight.

Thanks

[Longhorn Man]

Normally freeway oveheating can be traced to poor coolant flow. The rad is getting all the air it can take (onless you have a crappy A/C condencer in front of the rad).
I do agree with checking the actual temp. You are looking at a 32 year old guage.
The rad cap isn't going to be your problem either. There are 2 kinds of caps, the kind that will let coolant out, but not back in, like our trucks had originally.
Then there is the kind that will puke into a tank, and then slurp it back up as it cools.
If you install a later cap, and a puke bottle, then there is no problem. If you install a lateer cap, and no puke bottle...still no problem.
If your cap was bad (regardless of which style it has on it) then you would be boiling over. The primary function of the cap is to raise the cooling system to a certian pressure, which increases the boiling point.
Looking at what has been said, and asuming you are actually running hot, and not just dealing with a faulty guage...
Fan clutch...would not cause it to overheat at highway.
Fan...even if it was missing all together, you would still run cool once you are on the freeway.
Rad, just been flushed...since it is not overheating at lower speeds, we know it will transfer heat. Speed would have a greater effect at cooling since there is more air being forced through it...rule that out too.
You are not boiling over, so we know the cap is OK...or at least still good enough.
As stated above, the lower hose may be collapsing at speed. Open the hood once the engine is warmed up, and rev it up to about 2500 - 3000 RPM and look at the lower rad hose. It will squeeze a little, but if it collapses, then replace it.
No one has mentioned a slippin belt. I'm sure you have double checked it, and you'd most likely (but not always) hear it squeal.
Also no one has mentioned the thermostat. But, since it is doing a good job at lower speeds, we know it works.
We have eliminated everything but the pump.

I say verify the actual temp. You could even mount some aftermarket guages for peace of mind. Everyone here knows I don't trust stock guages...esp when they are 30+ years old.


If you verify the temp is actually high, then I would replace the pump asuming the lower hose is not collapsing.

[Southpa]

Original temp sensors may not be reading properly. If you want to find out what the actual coolant temp is go buy a mechanical sensor/gauge setup. They aren't that expensive and are easy to install. Plus, you can see some real numbers. If your temp consistenly reads over 200 then you might have other problems.

[Hotrod70C10]

I had this same problem not too long ago. I replaced the water pump and was fine after that. The only thing is the bearing at the front of the "new" (reman) water pump is making noise. I've gotta pull it and get another from the parts store. Like most everyone else I would check the weep hole on the pump. good luck

[rumbac]

Just wanted to thank everyone for the help.

Hopefully I will have sometime these weekend to work on the truck.