1967 L-79 a/c overheats

[Tom W.]

With a/c on above 85deg. overheats. Blows water out overflow. Tryed two different 160 t-stats. Fan clutch works great stops just as soon as engine shuts off. Plugs are med. tan in color. This will happen at highway speeds. I would not worry much if it were an in town traffic thing. The heat riser seems to work fine. Engine is strong and seems very healthy no noise. No smoke at all. Coolant clean and green.Has had three new rad caps all #26 15 lb caps. Any ideas as to what to check next. This is an AZ. car 85 deg. is an average day.Thanks

[Ray G.]

Re: 1967 L-79 a/c overheats

With a/c on above 85deg. overheats. Blows water out overflow. Tryed two different 160 t-stats. Fan clutch works great stops just as soon as engine shuts off. Plugs are med. tan in color. This will happen at highway speeds. I would not worry much if it were an in town traffic thing. The heat riser seems to work fine. Engine is strong and seems very healthy no noise. No smoke at all. Coolant clean and green.Has had three new rad caps all #26 15 lb caps. Any ideas as to what to check next. This is an AZ. car 85 deg. is an average day.Thanks

Hello Tom;
Sounds like you have had experience working on this.
Is Air flow through condenser and radiator adequate ?
Just the first thing popping into my pea brain while reading your question.
We backflush against the air flow w/ a water stream not strong enough to damage cooling fins. Requires removing the radiator to get at the condenser.
When clean you should be able to see light through most of the cooling fins.
Hopefully in the next year we will be permanent AZ residents. Look forward to meeting the AZ NCRS members.
Ray

[Gary K.]

Re: 1967 L-79 a/c overheats

My '68 was running at 210 and then slowly heading higher after about 30min in our Phoenix weather last May, Early June when temps were 95 - 100. Cleaned stock radiator, total system flush, new fluid, special wetting agents nothing worked. 95 degrees isn't that hot outside - not for us here in PHX.

The Fix: DeWitts aluminum radiator with dual-electric fans. Runs at about 200 and never gets to 210, even in August while hot-rodding around at higher RPM;s. Obvious drawback is that you're losing the entire stock fan assembly which is a big deviation from original, but for daily driving without constant overheating worries it's the best thing I've done. Cost about 900 bucks.

[Timothy B.]

Re: 1967 L-79 a/c overheats

Tom,

When the engine warms does the system go under pressure (top hose get hard), that's a sign the radiator cap is sealing against the overflow tank. You may want to check the pop off pressure on the cap just to be sure it's sealing and not releasing under 15lbs.

When this overflow happens what's the temp reading on the dash. Are you sure it's not overfilled and seaking it's level when hot. Expansion tank half full when cold..

Over heating at speed is a sign of a radiator problem but if your car has the retarded AIR timing you may want to advance timing to see if the engine cools some. Checking the air flow through condenser is a good suggestion.

[Tom W.]

Re: 1967 L-79 a/c overheats

Thanks. I have checked cap. Have not checked hose for being hard when hot. Tank has about an inch in it cold. I may remove and clean rad, and cond. but they look clean. The rad. looks to be newer. I have had several big blocks, but being that Az. is like living in an oven I opted for a small block so this would not happen.

[Duke W.]

Re: 1967 L-79 a/c overheats

Do you have an original 316 type aluminum radiator? If not, install the DeWitts reproduction. Aftermarket brass radiators do not have the heat transfer performance of the OE type aluminum radiator.

Also check that your vacuum advance is functioning to spec.

The expansion tank should be about half full when cold.

Us Zerex G-05 antifreeze. The obsolete "green" is responsible for clogging up a lot of vintage OE radiators with silicates and other precipitates.

Also, wire the heat riser valve open, and install a 180 degree thermostat.

Duke

[Michael J.]

Re: 1967 L-79 a/c overheats

I just spent the morning replacing my '67 stock radiator with a DirectFit DeWitt aluminum, as the last resort to get the heating up under control. Works like a charm, but nothing else really did.

[Domenic T.]

Re: 1967 L-79 a/c overheats

When I was working at the chevy dealership and installed radiators in the 60'S Vette's THERE WAS A WARNING THAT THE FOAM SEALS needed to be installed to get adequate cooling.
Many ignored it but with the radiator angle, it was important that the seals were installed.
I ignored it and had to install the seals to get proper cooling to get mine rirght.

DOM

[Tom W.]

Re: 1967 L-79 a/c overheats

I Have Installed The Top Rubber Flap On The Rad. I Will Check The Foam Seals. Thanks T.w.