C3 Over heating in High Speed

[Omid Abbarin]

I have 77 Corvette, with new crate engine LM1, new high flow Stewart water pump and thermostat , new 717 3 row radiator, new fan clutch. My car over heats in high speed. If I stay in 30-40MPH I run at 160, but as when my speed goes higher(High RPM) my temperature goes up to 220, during summer was up to 240-250.
I have checked the flow of the water, took the manifold out and checked the gasket. Also, do have the air dam and foams between the condensor and radiator. Please help. Thanks

[Jim T.]

Re: C3 Over heating in High Speed

What is a crate engine LM1?

[Omid Abbarin]

Re: C3 Over heating in High Speed

1969-85 GM Vehicles 350 Engine
Part # 10067353
Engine Name: 350 LM1
Horsepower: 249 HP @ 5,000 RPM RPM
Torque: 304 Ft/Lbs @ 3,500 RPM

[G B.]

Bottom feeding

You said that you have the air dam, but I'm not clear which one you mean.

This model is a bottom feeder; it takes in cooling air through large openings below the radiator. There must be a spoiler extending beneath the body to force the air up into the radiator. Several soft extensions were used on this spoiler over the years by the factory to scoop even more air into the radiator at speed. Look at the illustrations in the Dr. Rebuild catalog to see if your car has the spoiler/extension combination it should. Also, there must be an air cleaner inlet with a seal around it located above the radiator to keep the cool air from flowing over the top.

Please confirm that you've already double checked your ignition timing. A severely retarded spark will cause overheating on the freeway.

[Duke W.]

Re: Bottom feeding

Yeah, spec out your whole igntion map - initial, centrifugal curve, and vacuum curve. If you have access to an IR gun, shoot the manifold, at least at idle. Optimum idle timing (assuming no emission control) should yield a reading of about 500F.

Duke

[Omid Abbarin]

Re: Bottom feeding

I do have the spoiler which allows more air in at higher speed. And foams on top and side of the radiator to keep the air in. I have changed my timing few times and made sure it is correct. I have not changed the spark plugs since I have 1000miles on the engine, will try that. Thanks.

[Roger Legge]

Re: C3 Over heating in High Speed

Check the lower radiator hose to make sure it has a spring in it. This keeps the hose from collapsing during higher RPMs.

Roger

[Roy Braatz]

Re: C3 Over heating in High Speed

Omid! My love is C1 and C2 but I've seen this happen to so many HOT ROD owners and Corvette owners too. Your high flow water pump is pushing the water throught the Rad. TO fast. Not giveing it time to cool down. every one wont's FAST , but with water fast is bad. Change your water pump to match your old one and your troubles are gone.

[Doug Flaten]

Re: C3 Over heating in High Speed

You do not mention if it is truly overheating, spewing coolant or if it is an indicated high temperature. As suggested confirm the temp reading of your instrument panel reading. If you replaced the temp sender, it may not be calibrated to your gauge. Also, I would not bother changing the high flow pump, heat transfer increases at higher flow rates. Basically higher flow rates will pass more heated coolant through the radiator, keeping the radiator tubes and fins at a higher temperature. Therefore, there is more heat and a greater differential of heat between the radiator fins and the air that is picking up the heat. The heat differential is acts like a driving force for the transfer of the heat transfer. Assuming the radiator is adequately sized, it sounds like you need to tackle the source of the heat, possible ignition timing issues, or address the air flow across the radiator.

[Dale P.]

Re: C3 Over heating in High Speed

DO NOT trust the gage itself ...... did you change the temp sending unit?? if you did I am sure that is the issue .. trust me .. been there ... I spent the entire summer trying to figure out why my 71 LS5 ran hot ... well one day I used my wife's cooking pot (please don't tell her) filled it with "boiling water" and took it to the garage .. threw the sending unit into the pot (still connected to the wiring) and placed it on top of the intake (to complete the circuit) ... well we all know that water boils at 212F .. however my gage read 250F + ..... therefor the gage/temp sending unit is a miss match and I have been trying to find the "correct" temp unit since no one offers a true temp replacement unit of any qualitity .. hope this helps .. oh and by the way ... don't spend the money on the high flow water pump .. in this case more is NOT better

[omid Abbarin]

Over heating in High Speed

OK, Installed radiator support seal kit, replaced the bottom hose of the radiator, local radiator shop checked the temperature with Gun and it was fine. Also, added water wetter.
Now it runs at 160 in the city (same as before) and 190 in high speed (60-70) which is 30 degree cooler.
The only part I don't understand why am I losing antifreeze from the expansion tank (new tank and cap). It seems boils out of the cap and I can see it on the wall. Again replaced the hose from tank to radiator and I was reading the pervious posting, someone suggested to keep the hose lower than Radiator cap, this will be impossible for my car.
I do really appreciate for all help.

[Patrick H.]

Re: Over heating in High Speed

If your temps are fine, and the "only" time you boil over is at rest or after stopping, you likely have too much coolant in the system. Let it burp a few times WITHOUT filling it, and you'll be fine.

Patrick

[Ed Jennings]

Re: Over heating in High Speed

Patrick's right. Those things sort of "self level".

[Patrick H.]

Re: Over heating in High Speed

Yep. Had it myself a few years ago. One caveat: I had a defective radiator cap at the time that didn't help the matter. Replacing that helped too.

Patrick