1969 BB Overheating?

**Joe L.** · July 30, 2004, 10:19 PM

Re: 1969 BB Overheating?

Tony------

The temp sending unit location for all 68-74 Corvette big blocks was the left side cylinder head. If the sender proves to be the cause of inaccurate gauge readings, then I'd replace it with an accurate one.

Changing the thermostat will do VERY little, if ANYTHING, for a true overheating problem. As I've said on numerous occasions, the thermostat only controls the engine's MINIMUM operating temperature. Unless the thermostat is defective, changing it won't do any good, at all.

**Dick W.** · July 31, 2004, 06:22 AM

Re: 1969 BB Overheating?

I would borrow an infared thermometer and take the temperature at the thermo housing. If the car is not losing water, I owuld be very suspect of the temp guage. As Joe says, a 160 thermostat is not going to accomplish much in the way of reducing the temp of an engine that is overheating

**Tony Roussos** · July 31, 2004, 06:52 AM

Re: 1969 BB Overheating?

OK. What would cause overheating?

Not a dead pump, but a poorly circulating pump? Replace with another? Is there any way of testing the pump?

Air flow problem? Install a flex fan? Something impeading air flow through the radiator? Blockage in the radiator? How do you test that?

Exhaust blockage? No catalytic converter, but maybe a overly restrictive muffler?

Even with 11:1 (or so-we're not sure) compression, engines don't run 240 degrees as their norm, do they?

We'll take any possibility into consideration.

Thank you. Tony

**Jack H.** · July 31, 2004, 10:33 AM

Re: 1969 BB Overheating?

I think I'd hold off on the cause side of the 'hot Corvette syndrome' until you've verified the accuracy of the temp guage.... Several comments follow.

(1) If the car was actually running at 240F, I'd expect to hear you say, "AND my friend experiences numerous instances of the rad cap popping, steam & coolant blowing all over the engine compartment and on-road engine shut down episodes....

(2) If the guage is actually reading wrong, do NOT try to 'fix' the problem by 'shimming' the temp sender with serier resistance. Yep, you can wind up fixing the problem (accurate mid-range reading on the temp gauge), but creating another 'monster' of a problem--now the gauge will read low/false in the event of a bonafide engine overheat episode....

(3) Most are very quick to blame the temp sender without taking time to understand there are TWO components in the system (the sender and the gauge). Each gauge was hand calibrated when it was originally built....

On later cars like this Shark, the pointer needle was installed on the shaft and the guage was inserted into a calibration fixture that generated precision voltage and current to emulate an impending OVERHEAT situation. The cal operator picked from an assortment of precision wire-wound resistors to 'shunt' the gauge windings and FORCE the pointer needle to indicate the correct temp at the HIGH end of the dial. Remember, Chevy didn't really care about the exact precision of the gauge under 'normal' temperature conditions--this is why there aren't any hard mid-range gradacules on the gauge; they cared about accuracy at the HIGH end of the dial where you'd use the info to shut down and prevent engine damage!

I've seen cars that were 'put together' and the calibrating shunt resistor was discarded by 'Bubba' (a useless 'extra' part). I've also seen cars whose temp gauge had a blown calibration resistor because somewhere in time a fella 'tested' the gauge by shorting around the temp sender and the cal resistor acted as a 'fuse' to protect the gauge....

Bottom line here, you want high accuracy at the TOP end of the dial and should be willing to accept some level of inaccuracy in the mid-range portion of the gauge. That's how things worked worked back then.... Many force a late model technology perception on these classic cars wanting them to work like current new cars do. Sometimes, this is an exercise in pushing on a rope and 'shimming' the temp sender with external resistors can REALLY backfire unless you know what in (*&& you're doing!

**Dick W.** · July 31, 2004, 11:03 AM

Re: 1969 BB Overheating?

Ok, let's assume (you remember that ass/u/me) that your car is actually overheating. First check, are fins, front of radiator, area between radiator and condenser clean? Is thermostat opening properly (test in a pan of water on your stove. Use candy thermometer to check. Is the water pump functioning properly? Some aftermarket water pumps have impellers that do not pump water properly, or can cause caviation. Is the radiator clean internally, if not it will impede water flow, and the crud in the tubes can insulate and not let it shed heat? Is the bottom radiator hose collasping? Haven't seen that one in a looooong time, but it does happen. Just a few major areas to check.

**Tony Roussos** · July 31, 2004, 11:33 AM

Re: 1969 BB Overheating?

We would never have thought of the lower hose collapsing. Thank you for the tip!

Tony

**John H.** · July 31, 2004, 11:44 AM

Re: 1969 BB Overheating?

First, determine if you really have a problem - "shoot" the upper radiator hose just above the thermostat housing with an I.R. gun and compare that against the gauge reading at the same time so you know what the gauge is really telling you. Don't shoot the head, intake or the thermostat housing adjacent to the sender, as painted/polished/glossy surfaces will give incorrect I.R. readings.

**Dick W.** · July 31, 2004, 08:41 PM

Re: 1969 BB Overheating?

Good point John. I might add one other possibility if the car is heating. The fan clutch.

**Mark #28455** · August 1, 2004, 04:03 PM

Re: 1969 BB Overheating?

One of my '69's was missing the black plastic chin spoiler. Once I replaced it, the car ran about 30 degrees cooler. Is yours missing?

**Wayne W.** · August 2, 2004, 08:04 AM

Re: 1969 BB Overheating?

Ben through this so many times that I cant count. Ass/u/ming that you have a real problem, THE most common cause of overheating is the radiator. I dont care what the fins look like or what it looks like inside, they get tired over time. The fins crack away from the tubes and the radiator looses its cool so to speak. I just chased this problem again on a 68 BB with everything added, shroud extension, new everything with no luck. We had changed the radiator some years ago, so we thought it was good. It was clean inside and out and looked new. Welp, it was wrong to ass/u/me that! With a new radiator it is cool as a new breath of Spring.

**Dick W.** · August 2, 2004, 09:43 AM

Re: 1969 BB Overheating?

Very good point Wayne. My memory has gone to the dogs (NCRS disease). When I ran a radiator shop 25 years ago I ran into that several times. The radiator would flow according to specs but would not cool. Best I remember, for some reason, it occurred more often on Harrison radiator.

1969 BB Overheating?

1969 BB Overheating?

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