Engine temp problem

[Joe Martin]

I live in Dallas, TX area and have a numbers matching '65 coupe with 350hp 327, 4 speed, stock 3.70 rear end and factory air. Just completed a total frame on restoration and everything under the hood is new. Engine was bored .040 over and we put in flat top pistons to cut compression and runs great on 93 octane gas. Installed a Griffin aluminum radiator (a tad bigger than the original Harrison I think), standard 180 degree thermostat and 6 blade clutch fan (car originally had 7 blade fan but long since gone). Car ran beautifully at about 185 degrees with ambient temp 80 degrees or less, no matter what speed.

Finally got air conditioning system hooked up in June. Immediately noticed that engine temp was creeping toward 210-215 with air on, especially in traffic with ambient temp in upper 80's to high 90's. Decided to install twin 8" electric fans (800 cfm each) in front of condenser. They kick on at 180 degrees or when air is on. This helped my slow speed temp regulation immensely. However, took a 40 mile trip and noticed that the faster I went, the higher my engine temp. At 97 degrees outside and 70 mph, temp was running 230-235 with air on. Slow down and temp went down. Come to a stop light and temp dropped to 215. Start when light turned green and temp shot up about 10 degrees.

Made two alterations. Replaced fan clutch (it wasn't locking up properly) and put in a 195 thermostat. With ambient temp about 90 or 95 car runs about 210-220 with air on at 70 mph. Drops to 205-210 with air off. Still cools down quickly to about 200 with air on and speed below 50 mph or sitting at red light.

The increase in temp when I speed up baffles me. Shouldn't have anything to do with fans. You shouldn't even need a fan at 70 mph. I can't figure out why this car won't run at about 185 at any speed, with or without air on, using a 180 thermostat.

Has anyone else encountered this problem? Oh, yes, I have original shroud and fan sits half in, half out at back of shroud. I also don't believe stepping down in thermostat temp will help. It would seem water would flow too fast, not staying in radiator long enough to cool down.

I used to own a '66 big block roadster with factory air back in the late 60's and I don't EVER remember having any engine temp problems. And it was just as hot here in Dallas back then.

Any and all suggestions and comments are welcomed. Thanks.

[Ed Jennings]

Re: Engine temp problem

Check some of the posts here and on the Corvette forum board concerning vacuum advance and overheating at speed. Check to see that you actually have all the advance you're supposed to at 3000+ RPM. I think that number is 50+ (something like 53-54?) on that engine with the advance connected.

[Jim T.]

Re: Engine temp problem

I recommend going back to a 180 thermostat. The car came from the factory with this type. The higher 195 thermostats are more for cleaner emissions. My 68 L79 doesn't have air so I can't compare my highway running temp, however one trip back from Bowling Green through Nashville and west on I40 a few years ago it ran warmer than it usually does. The afternoon ambient temp was about 100 and the 68's guage was 210 running about 70-75.

[Joe Martin]

Re: Engine temp problem

Thanks for the input Ed. I'll pursue the vacuum advance advice. That may be the culprit.

[G B.]

Maybe it's a tiny head gasket leak.

If you're running the rubber a.c. air dam above the raditor and you've installed the lower a.c.-only foam strip as well, then I think you've got an engine problem.

The Griffin aluminum radiator isn't as efficient as a new Harrison, but I think it's efficient enough. I've put several Griffins in air conditioned mid-years. They are handling the southern heat load just fine without any auxilliary fans.

[Joe Martin]

Re: Engine temp problem

Thanks, Jim. After I look into the vacuum advance I'll try going back to the 180 thermostat.

[Joe Martin]

Re: Maybe it's a tiny head gasket leak.

I hope to God it's not a head gasket leak Jerry. I think I'll cross my fingers and toes for the vacuum advance to be the culprit. The last thing I want to do is pull heads. I appreciate your response.

[Peter Ansted - #32760]

Re: Engine temp problem

Joe - I had temp problems with my 327/340 bored .030 over and could not solve the problems until I purchased the best aluminum radiator I could find. The best Griffins radiator is their HP Series, which will give you the needed cooling capacity. They also offer a Pro line, which is a step up from copper/brass, but may not give you enough reserve. With a bored engine, AC and bigger cam / compression ratio you will need all of the cooling capacity out there.

[John H.]

Re: Maybe it's a tiny head gasket leak.

Joe -

If the two electric fans are running at highway speed, they are actually reducing ram airflow into the condenser and radiator, which will contribute to the highway heat-up symptoms you describe. I'd put a 180 thermostat in it, make sure the radiator is sealed to the radiator support and the shroud is sealed to the radiator, remove the electric pushers, and see how it runs. I'd also have a shop "shoot" the thermostat housing with an I.R. gun and compare that with the temp gauge reading to see what the gauge is really telling you - they're notoriously inaccurate (read hot), especially if the temp sender was replaced during the rebuild. The real cure is a DeWitts repro Harrison radiator, even more efficient than the original, both of which are more efficient than the one you have.

[Joe Martin]

Re: Maybe it's a tiny head gasket leak.

Thanks for the info, John. Everyone's been great to respond. I'll be sure to check sealing of shroud. I don't know that the electric fans are causing the problem at higher speed since the problem arose before I ever installed the fans. I've also checked engine temp with a mechanical guage and it's exactly same as my dash guage. I put both 180 and 195 thermostats in pan of water on the stove and they open exactly at right temp. Also, some have mentioned a head gasket but I would think it would run hot all the time. The most logical hint given yet is checking the vacuum advance, which I haven't had time to do yet. And I absolutely cringe at the thought of replacing the radiator.

If and when I get to the bottom of this problem I'll be letting everyone know. Thanks for the input.

[G B.]

I don't think it's your radiator

Joe -

A relatively new Griffin mid-year aluminum radiator is adequate to cool an air conditioned small block in the southern summer in all kinds of driving without supplemental fans. I know what I'm talking about here based on experience with more than one car.

If you have all the a.c. radiator seals in place (there are extras that are not on the non-a.c. cars), and your gauge is reading right, and your lower hose isn't collapsing, and your thermostat is working, and your water pump is good quality, and you have the original type shroud/fan/pulleys on your engine, and your fan clutch is working, and you have all the air out of the cooling system, then you probably have a small combusion leak into the water jacket. It can be from a cracked casting or a leaking head gasket, but it won't necessarily leak under all driving conditions.

I am sorry to suggest such an awful problem, but you've done a good job of ruling out most everything else.

[G B.]

Engine timing and cooling.

If you've set your initial advance with the vacuum advance canister disconnected, then your timing is close enough to prevent overheating even if the canister later fails.

A non-functioning mechanical advance system, sometimes caused by a gummed-up weight plate, can cause you to set the initial timing incorrectly. To check the mechanical advance, take off your distributor cap and twist the rotor with your hand to see if it snaps back into place when you let go.

[Dennis Mucci]

Re: Engine temp problem

First of all, check the basics.
1. If the engine is overheating at speed, then make sure that the lower radiator hose has a wound spring in it. If it doesn't, the hose will collapse from the suction of the water pump at higher engine and vehicle speeds.
2. Make sure you are using the correct seals between the radiator and the radiator. If the the seals are missing, hot air will be pulled from the engine compartment back around and through the radiator. Dr. Rebuild and others sell the correct seals.
3. Here's a tip for a good water pump. I use a Stewart CI pump on my 75 and it is the best pump for the money, which is about $70 direct from Stewart. The NASCAR guys use the Aluminum version which runs about $150, but the CI one works great. Go to their website and see for yourself.
Good Luck!