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Topic: Hot at high RPM

in Forum: C3 Cooling and Heating Systems

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Hot at high RPM (1/32)
1/26/07 7:13pm

bytor
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Simpsonville, SC - USA

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1978 25th. Silver Anniversary Edition 350 L-82 Stock

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1st. post and 1st. problem on my 78. The engine seems to over heat when running at or above 2500-3000 RPM after a few minutes. As long as I stay under 3000 RPM, no problems. When I got the car it had a new radiator, water pump, tstat and fan. I'm suspecting the lower radiator hose or catalytic converter. Any other thing to check? I'm ready to get her on the road!

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Hot at high RPM (2/32)
1/26/07 7:29pm

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Is the fan clutch working properly ?

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Hot at high RPM (3/32)
1/26/07 7:36pm

bytor
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Simpsonville, SC - USA

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1978 25th. Silver Anniversary Edition 350 L-82 Stock

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Not sure, it looks new.

Hot at high RPM (4/32)
1/26/07 7:44pm

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Lanoka Harbor, NJ - USA

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also is the shroud in place and all the gaskets. the shroud directs the airflow to radiator. maybe a lower temp stat.

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Hot at high RPM (5/32)
1/26/07 8:23pm

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VENTRESS, LA - USA

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If you have a front liscense plate on try removing it. I have electric fan so slow driving it cooled fine but after a while at 70-80 temp would start to rise. After removing plate it runs cool at any speed.

Hot at high RPM (6/32)
1/26/07 9:59pm

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North Charleston, SC - USA

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You probably have a radiator hose collapsing under load. Put it in park, parking brake engaged, and crank it up. Open the hood and have someone raise the rpm while you watch the hoses. it will most likely be the lower hose closing shut. I have seen this many times. Your symptoms are exactly as I would expect with this problem.
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ranger32007-01-26 19:00:17

Hot at high RPM (7/32)
1/26/07 11:40pm

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Eastern Oklahoma County, OK - USA

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I had exactly the same thing that ranger3 is talking about. I put one of the unifit hoses with the spring molded into the rubber (looks like the section of a bendable straw when it's pulled out) and the problem went away.

Follow the rangers advise on checking cause you will be able to see it collapse on the framerail if it is the hose.

Good Luck!

Hot at high RPM (8/32)
1/27/07 12:03am

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How do you know it's overheating? You can't always go by what the gauge says...they lie!

You may very well have a hose collapsing at that speed. That would slow down the flow of coolant thru the engine, and create a hot running condition. That's why the lower hose almost always has a spring inserted in it. See if you can mash the lower hose together in the middle. If you can, the spring is gone, and that is probably the source of the problem.

At highway speeds, and that RPM, the fan clutch is really just along for the ride, as there should be enough air flowing thru the rad. to cool it. That is, if the air is actually going thru the rad., and not around it! Check to see if all of the foam seals are still around the shroud, and the radiator support.

Removing the license plate on a '78 wouldn't do any good, either...there's a huge rubber thing behind it that the plate bolts to. It's part of the bumper system.

But that idea works well on the chromies!

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Hot at high RPM (9/32)
1/27/07 12:04am

bytor
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Simpsonville, SC - USA

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1978 25th. Silver Anniversary Edition 350 L-82 Stock

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I will check it in the AM. That's what I'm thinking also..

Hot at high RPM (10/32)
1/27/07 12:33am

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Moses Lake, WA - USA

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Also make sure your air dam is inplace. It directs the air coming under the bumper up into the radiator. As stated above, make sure your foam seals are inplace around the radiator. That way the air flow is forced through radiator and not around it. Wink

Wink

Hot at high RPM (11/32)
1/27/07 1:17pm

bytor
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Simpsonville, SC - USA

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1978 25th. Silver Anniversary Edition 350 L-82 Stock

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Ok, put a lower hose with a spring in it. Took it for a spin, still gets hot at high RPM then cools down. After looking around, I noticed that the upper hose is the one collapsing. I thought the upper hose was the return and not the suction, strange. Has someone installed the wrong water pump?? Any ideas?

Hot at high RPM (12/32)
1/27/07 2:10pm

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Lanoka Harbor, NJ - USA

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bytor from a car i had a long time ago check this. had a 72 caddy with a 472 in it and would do the same with upper hose. what it turned out to be was the overflow tube going to the tank was bad. it would cllapse on the inside. i replaced the hose and upper hose did not collapse anymore. also i think they even make hoses with springs in the upper.

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Hot at high RPM (13/32)
1/27/07 2:44pm

bytor
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Simpsonville, SC - USA

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1978 25th. Silver Anniversary Edition 350 L-82 Stock

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You know what, I have noticed taht the water level never changes in the overflow tank never changes..... I'm on it.

Hot at high RPM (14/32)
1/27/07 6:12pm

bytor
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Simpsonville, SC - USA

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1978 25th. Silver Anniversary Edition 350 L-82 Stock

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Posts: 85

Didn't have a chance to chance the reservoir hose today. I assume a collapsed reservoir hose acts like a bad radiator cap. It could prevent the coolant flow to and from the reservoir. I don't understand how that can cause the upper radiator hose to collapse. I'm no cooling system expert though either. :) By the way, nice forum.. I'm a 1st. time vette owner and this site is a good resource for information.

Hot at high RPM (15/32)
1/27/07 7:04pm

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Lanoka Harbor, NJ - USA

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i am far from one either bytor but had the experience with the caddy of upper hose collapsing and could not figure it out. a top notch mechanic i knew told me that might be the problem and as i had nothing to lose except a few bucks for the hose and time it took to do it as on that car had to remove top covering the radiator up to the grill. it cured the problem. i would think not having a way to vent itself on a closed system had something to do with it. eddie208752007-01-27 16:04:58

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Hot at high RPM (16/32)
1/27/07 11:11pm

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Duncanville, TX - USA

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Eddie is probably right on this...if the hose for the puke tank(overflow) is collapsing, then it wouldn't be able to "suck" the coolant back into the engine as it is needed. Check to make sure there is no restriction on either end on the hose first, and that the fitting that it fits onto on the tank is clear.
That's why the upper hose collapses. Then, as the upper hose collapses, you have an overheat condition due to the restriction.
I think if ya'll will take the time to study any of your cars, you'll see this action take place a lot, mostly on initial start/warm-up. As the engine warms to the opening temp for the t-stat, the upper hose will collapse for a few seconds, while it pulls the coolant back from the puke tank. This isn't a problem tho, because it only happens for a few seconds. If the hose is staying collapsed while driving....that's a problem!
Replace that overflow hose, and make sure your rad. cap is the correct rating(13-16 lbs) for the car.

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Hot at high RPM (17/32)
1/28/07 3:11pm

bytor
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Simpsonville, SC - USA

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1978 25th. Silver Anniversary Edition 350 L-82 Stock

Joined: 1/21/2007
Posts: 85

Yep, had trash in the reservoir clogging the line. Pulled the tank, cleaned it and installed a new reservoir line. I went ahead and replace the upper radiator hose and cap (16lb). I was able to drive the car at 3000 RPM with out over heating. However, when I came to a stop it went up to 220 or so but then dropped back to 180 while driving. Is this normal? Thanks for your help with this..

Hot at high RPM (18/32)
1/28/07 3:29pm

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Lanoka Harbor, NJ - USA

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only thing i could think of is maybe a lower pressure radiator cap. if pressure is too high it can't release it and water will heat up more before opening the cap.

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Hot at high RPM (19/32)
1/28/07 3:39pm

bytor
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Simpsonville, SC - USA

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1978 25th. Silver Anniversary Edition 350 L-82 Stock

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Posts: 85

What yould you recomend? 13LB?

Hot at high RPM (20/32)
1/28/07 6:11pm

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Duncanville, TX - USA

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I'd go 13lbs...but check the service manual, or owner's manual...

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Hot at high RPM (21/32)
1/28/07 11:53pm

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Eastern Oklahoma County, OK - USA

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Did the new rad hose have a spring in it?

Hot at high RPM (22/32)
1/29/07 11:50am

bytor
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Simpsonville, SC - USA

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1978 25th. Silver Anniversary Edition 350 L-82 Stock

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Posts: 85

Yes, both the lower and upper hoses have springs installed. I still do not understand why the "upper" radiator collapses at high RPM. After the car warms up, I could rev the engine and watch it suck shut before I installed the spring. What's the likelihood that I have a reverse direction water pump installed. I contacted the previous owner and he had replaced the water pump within the last year because it leaked. He never had any overheating problems before that. Humm.... I not sure how I could check to see if it's pumping the correct direction or not.

Hot at high RPM (23/32)
1/30/07 1:09am

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Duncanville, TX - USA

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Unless you're spinning the pump backwards, it ain't like to be the problem.
When you rev the engine up like that, it creates a vacuum in the water passages. It's caused by the pump pumping the water in quicker than letting it out. The thermostat is essentially a restriction in the system. The water going out has to go thru the thermostat, and that slows it down, but the pump is still trying to pump. So...it "sucks" on the intake side of the pump. Once there is a vacuum, something has to give, and rubber hoses being what they are, that is the give.
If the cap has the correct rating, and the overflow is not restricted, this bit of suction shouldn't really be an issue. The cap should allow the vacuum to pull coolant back out of the overflow as needed.

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Hot at high RPM (24/32)
1/30/07 10:41pm

bytor
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Simpsonville, SC - USA

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1978 25th. Silver Anniversary Edition 350 L-82 Stock

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Ok, pump is not running backwards, duh. I went ahead and replaced the thermostat. This time I �burped� the system afterwards. With the radiator cap off started the car and waited for the tstat to open. Upper hose began to get warm and a minute later coolant burped out the radiator cap and the coolant level dropped out of sight then came back up. I could see the tubes in the radiator and they had water flowing through them fine. My question is, how may time do I let it burp? Once or continue till it stops?

Hot at high RPM (25/32)
1/30/07 10:48pm

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Once eveything has warmed up be sure to open the heater to full blast to get some coolant flowing through the heater core. There could have been an air bubble hiding in there.

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Hot at high RPM (26/32)
1/30/07 11:05pm

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Duncanville, TX - USA

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Burp it till she pukes! Oh wait...that's fer babies...never mind.

A couple of times should be fine. Then put the cap on, make sure the overflow is FULL, and then drive it a while. As the engine cycles thru the temp range, it will pull the coolant it(the radiator) needs from the overflow tank. Once it no longer empties the overflow, it should be full to spec. Then, just see that the overflow tank is at the correct level for "Hot". Drain or fill as needed to get it to the "hot" level, and you're done.

If all is well, once it cools back down to cold, the level in the overflow will be up to the "Cold" mark.

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Hot at high RPM (27/32)
1/31/07 12:34pm

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it's the hose

Hot at high RPM (28/32)
2/1/07 10:53am

bytor
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Simpsonville, SC - USA

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1978 25th. Silver Anniversary Edition 350 L-82 Stock

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Posts: 85

This looks like a neat effective way to purge the air out of you cooling system. http://www.allhead.com.au/cooling.html

Hot at high RPM (29/32)
2/1/07 1:36pm

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Victor, NY - USA

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GOOD INFO. I PRINTED IT FOR FUTURE USE.

THANKS.

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Hot at high RPM (30/32)
2/3/07 1:12pm

bytor
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Simpsonville, SC - USA

Vette(s):
1978 25th. Silver Anniversary Edition 350 L-82 Stock

Joined: 1/21/2007
Posts: 85

Bypassed the heater core and used the method above to get the air out. Presto, no more overheating..

Hot at high RPM (31/32)
2/3/07 1:48pm

gs77vet
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The best thing to do is replace the radiator, water pump and hoses and maybe a heater core and the resourvoir and hoses .This is expensive but you should be done with overheating. I replaced all these things except the heater core.
The overheating was a downer, everytime I went on a trip I had a heating problem. Gene

Hot at high RPM (32/32)
2/3/07 2:34pm

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Lanoka Harbor, NJ - USA

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only problem with that gs besides being expensive is that you really don't know what the problem was and what you did to correct it. as in bytor's case it seems to have been air trapped in the system which could have still happenned if he replaced everything.

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