68 - L79 Overheat Problem

[Richard Dickerson (38987)]

Hi everyone,

Well, my car is pretty much done, and it is running pretty well.

Just ran into an overheating problem, and was wondering if you guys could give me some advise.

Did change over to a GM clutch fan from one of those flex jobs, as per good advise, but stupidly did not change off the aftermarket chrome pulley to the stock GM pulley. Now, I'm pretty sure that the fan is not into the shroud as deep as it needs to be to keep the airflow right during idle. During idle is where the car is not cooling properly, although at "speed", the motor keeps a 180 - 200 temp OK. The fan blade if back from the shroud about 1" or so, it does not look right to me.

Ques. - What depth is correct for a fan to sit into the shroud (it's the original shroud)?

It's an original L79, do these typically run on the "hot" side?

I did get the "seal" kit from the Dr., and had it installed. How tight does the radiator have to be to the shroud, and how tight do all those seals need to be to make everything work right?

Thanks for any advise.

Best regards,

Rich

[Jim Trekell (22375)]

Re: 68 - L79 Overheat Problem

Richard my original 68 L79 does not typically run on the "hot" side and I still have the original seals on the car. Might check how your vacuum advance is hooked up to the carb. Using ported advance could be a contributing factor of increasing the temp at idle. I usually run about 10-12 degrees initial advance when I set the timing and just use pump 93 gas.

[Clem Zahrobsky (45134)]

Re: 68 - L79 Overheat Problem

the fan blade should be half in and half out of the shroud. the end of the shroud should split the fan tip

[Rich Giannotti (38594)]

Re: 68 - L79 Overheat Problem

I second Jim's suggestion. I had been fighting a hot running 66 L79. First the correct radiator made a world of difference at "cruise", but it would still climb at idle. Following advice from this board, I found the vacuum advance not working at all and even if it was, it was getting vacuum from the wrong place. The results are amazing. It never gets over 190 idling on an 85 degree day.

Rich Giannotti

[John Thomas]

Re: 68 - L79 Overheat Problem

What do you mean "vacuum from the wrong place?"

[Rich Giannotti (38594)]

Re: 68 - L79 Overheat Problem

John

I hope I don't get this backwards, but I believe the Vacuum needs to come from an "unported" source, vs. a "ported" source. And I think that means off the manifold, not the carb.

Rich Giannotti

[Richard Dickerson (38987)]

Re: 68 - L79 Overheat Problem

Thanks Guys.

The fan is "half in, and half out" as suggested.

There is no vacuum advance for the dist, it's a Mallory Unilite job.

I will recheck my advance and timing.

I've read before that air can get "trapped" in the cooling system and possibly cause overheating until everythings settles out. I read that you can drill a small hole in the thermostat to help the air move to the top of the system on refills when you drain and refill the system. Does this work?

I've noted in the past that my car would always lose coolant upon refilling until it balanced itself out.

I also had a shut off valve on one of the heater core hoses that the previous owner had put in (and I left in after I got the car, but did put new hoses on it). Maybe this is restricting the overall coolant flow? I will remove it next week and just go with a straight feed through hose to the core.

Lastly, if I run a weaker than 50/50 mix of coolant (using green Prestone), say 35 - 40% coolant to water, should that help a bit? And, anyone out there use Redline WaterWetter product to help reduce coolant temps? Does that stuff work?

Thanks again for the advise,

Rich

[Jim Trekell (22375)]

Re: 68 - L79 Overheat Problem

Richard a co-worker has a 66 that overheats at idle and it has a Mallory distributor with no vacuum advance.

[Duke Williams (22045)]

Re: 68 - L79 Overheat Problem

It's overheating because of the lack of vacuum advance! Happens to virtually all Corvettes that either don't have a vacuum advance or don't have a correctly calibrated vacuum advance that provides full vacuum advance at idle.

Duke

[Duke Williams (22045)]

Re: 68 - L79 Overheat Problem

"Ported" vacuum advacne means he signal line is above the throttle valves at idle,so the is no vacuum signal at idle. Normal vacuum advance means the signal source is below the throttle at all time, so the signal is always full manifold vacuum.

Normal or ported vacuum advance sources are usually on the carb, which is designed for the specific application. If it doubt, just test the source by checking for vacuum at idle. Most pre-emission controlled engines have normal vacuum advance.

Duke

[Richard Dickerson (38987)]

Re: 68 - L79 Overheat Problem

Duke, thanks.

Is there a fix for this that compensates for the vacuum advance for these distibutors that you know of?

Thanks,

Rich

[Joe Ciaravino (32899)]

Re: 68 - L79 Overheat Problem

Richard:

What do you mean by "overheat". You should measure the temp at the base of your temp sender with an IR thermometer to verify. Does the engine actually boil over?

One very common result of insufficient/improper vacuum advance, is overtemp during extended idling. The way to go, would be to switch back to a stock distributor, or an aftermarket unit that supplies vacuum advance (manifold, not ported vacuum).

A test, to verify this, would be to create "artificial" vacuum advance, idle the engine, and see what happens BEFORE changing out your distributor. Set your base timing to 26* BTDC and let it idle (DO NOT DRIVE the car like this, it is just a test). If the engine runs cooler, then reset your distributor back to 10*BTDC (or wherever it was), and drive on over to the parts store and look for a new distributor.

Joe

[Richard Dickerson (38987)]

Re: 68 - L79 Overheat Problem

Jim:

I'm looking into the Mallory distr. specs now.

I believe it's made to mechanically compensate adjustment for the lack of a real vacuum advance on the distr., but as soon as I find out, I'll let you know so you can advise your friend.

Thanks,

Rich

[Duke Williams (22045)]

Re: 68 - L79 Overheat Problem

Not that I'm aware of. My suggetion is that you buy a single point tach drive distributor. Give is a good blueprint overhaul with the proper L-79 vacuum can and install it.

Duke

[Duke Williams (22045)]

Re: 68 - L79 Overheat Problem

There is no such thing as "mechanically compensating" for lack of vacuum advance. A L-79 should have an additional 16 degrees of timing above initial with a proper spec vacuum advance. This TOTAL idle timing of about 25-30 degress results in the lowest idle EGT and keeps the car from running hot.

If you can buy up enough of these old Mallory distributors you might be able to fashion a useable boat anchor.

Duke