1968 L36 Overheats at Idle

[Charles F.]

I have a 1968 Coupe: L36 (427/390), with a M-20 (4 speed), factory A/C and a 3.08 differential. It overheats at idle (ONLY). I have not tried it with the A/C running.

Everything on the car is stock, except the heads have been re-worked to accept modern fuel. The Rochester Carb has been rebuilt. The hoses are new, the fan clutch seems fine as does the water pump. The foam pieces that are supposed to be around the shroud are missing. The coolant is a 50/50 mix. Not that it matters, but the thermostat is new, 180 degree.

The car runs at 170/180 degrees when it is moving, even on the highway at 80 mph. It can creep up to 200 in town, in stop and go. At a traffic light, however, it moves up fast (i.e., at idle) to the point it boils the coolant.

I have replaced the stock radiator with a Dewitts Direct Fit (Aluminum). It brought the operating temps down (as indicated above) but did nothing for the idle overheating.

I changed the timing: 4 degrees advance of TDC, with manifold vacuum providing another 10 degrees advance (14 degrees total). Didn't help.

When it overheats at idle it is strange. The temp gauge never goes beyond 22-230 (accurate at the radiator as per an infrared temp sensor). The gas in the carb bowl is actually vaporizing. The radiator overflow hose first starts dripping coolant, then pure steam, then "pukes" coolant.

Don't know what the problem is nor what to do next. I am not a mechanic. HELP !

Thanks.

Chuck Faillace

[Joe L.]

Re: 1968 L36 Overheats at Idle

Chuck-----

Well, I can tell you that the problem is not the DeWitts Direct Fit aluminum radiator. That radiator has a cooling capacity significantly greater than your original. By installing that, you've eliminated the "radiator deficiency" possibility from the equation----you should have way better cooling capacity in the radiator now than when the car was new.

Anyway, idle cooling problems are usually traceable to a fan clutch deficiency and/or shroud/radiator sealing problems. You definitely want to make sure you have all the seals installed. Dr. Rebuild sells perfect radiator seal kits for your application with respect to both form and function.

Regardless of how the fan clutch "feels", I'd replace it. If it's an original unit (post a photo of it front and rear and we can probably tell you if it is), make usre you keep it for possible future rebuilding. Purchase a new GM fan clutch of GM #3916141 and install that. It will fit and function perfectly.

Another thing that you might try. All 1968 applications use ported vacuum for the vacuum advance Try changing this to full manifold vacuum. This will often reduce idle engine temperatures significantly.

[Clem Z.]

is the fan clutch working properly?

when the engine is hot is the fan stiff to turn by hand?. remove the fan clutch and replace it with a spacer of the proper length and see what happens.the fan blade tip should be half inside the shroud and half outside the shroud

[Terry F.]

Re: 1968 L36 Overheats at Idle

I would look at the fan clutch also. Chuck, does your engine still have the engine lift brackets on it?? I am trying to identify any unique features of 68 bb engine lift brackets. Thanks, Terry tfringo@aol.com