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  1. #1

    Default What's up with 240F?

    I note after highway jaunts at legal speeds of greater than 5 miles, when I get off the highway the car will ping for a few miles.

    The 66 GP 389 / 4 BBL /Dual Exhaust does have an aftermarket Carter 625 CFM carb. Timing set to factory 6 degreees BTDC. Pertronix point eliminator. Vacuum advance and standard advance springs free and operating.

    Pinging can be caused by an engine running too hot, generally, right?

    No temp gauge in the car, but I do have access to a calibrated IR thermometer. I did a test run and then took a reading off of the thermostat housing. 240F ! After 5 minutes of rest the reading peaked at 250F.

    Theories yet to be validated:

    - The cooling system sucks (define sucks)
    - The carb is too lean
    - The distributor vacuum advance connected to a wrong port and being advanced to aggressively

    Cooling system

    - Shadetree cooling system flush / Fresh Anti-Freeze and new hoses within the past couple of years.
    - Aftermarket coolant overflow bottle. Only a slight rust coating in the bottle.
    - 180F Thermostat per factory
    - 16 Lb cap (2 Lbs over factor 14 Lb spec)
    - Filled to the Cold Fill line, which is a good 3 inches below the top of the radiator

    Lean Carb
    - Just replaced the plugs. They actually looked good. Very neutral appearance with no deposits either way (i.e. not black (rich) not white (lean)).

    Excessive Advance
    - No clue how to characterize total advance without having access to a dyno...

    I've read all the notes in this thread. I'm leaning to inefficient pumping due to excess gap between the impeller and back-plate described by Mike N. a few posts back.

    Any experiences you can share?


  2. #2


    Problem could be one or more. I have found that the aftermarket carbs don't drive as well as an OEM carb without a lot of work. See if you can find a rebuildable original. You should be able to see if the radiator is flowing well and is clean. To check your advance borrow a dial back timing light and check the timing at highway RPM. I have never had a Pontiac in the past 62 years that ran hot at highway speeds. It appears to be retarded or lean. Good luck.

  3. #3


    I finally got around to putting some Stewart Warner Temp and Oil gauges in this past weekend. These are mechanical type.

    A 20 mile cruise mixed highway and back-road during an 85 F day yielded a temperature of 215F after I got off the highway. Oil pressure looked good varying between 20 Lbs at idle and 40 Lbs on the highway.

    These temps just seems on the wrong side of comfortable. And, I have the impression it would have kept climbing if I had kept driving.

    The factory manual indicates my car should have had a 4 blade fixed fan (non-AC). My car has a 7 blade thermostatically operated clutch fan (specified for cars with AC). Not sure who or when that was put in.

    Anyone know of a sure test for those types of clutches? Heat it up with a torch?

    What should be the spacing between the fan blades and the radiator?

    The factory chassis manual provides no guidance. I measure 3-1/8" from the fan blade most forward point to the radiator. Ames sells a 2-1/2" spacer. Anyone try this as a fix?


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