This car is my daily driver(25,000 miles in two years)It overheats at idle on hot days(80+ degrees) Numerous technicians have gone through all of the possible causes in the trouble shooting section of the shop manual. The engine was just re-built and it's still overheating. I can drive all day long with no problem but when I stop and idle it overheats. I'll try anything, and will really appreciate your thoughts.
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Re: Overheat at idle 67 300H.P Automatic and A/C
Check the intial timing and proper operation of the fan clutch, centrifugal advance, and vacuum advance. Is the engine California spec (with exhaust emission controls) or 49 state with no exhaust emission control provisions?
Duke -
Re: Overheat at idle 67 300H.P Automatic and A/C
Timing has been checked and re -checked. I have a new clutch fan and fan.My Tech. said centrifugal advance and vacuum advance would not be a factor solving this problem since the car only overheats at idle. This car was originally delivered in Texas and it's first driver was the Chevrolet Zone Service Manager. I also have side exhaust.Comment
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Re: Overheat at idle 67 300H.P Automatic and A/C
Paul-----
The most frequent cause of the symptoms which you describe is a deficient fan clutch. You mention, of course, that you have a new fan clutch. However, you didn't mention what type of fan clutch that you have installed. Is it a GM thermal type unit of GM #3916141 or is it an aftermarket unit? If an aftermarket, is it a thermal or non-thermal design?
Basically, the only difference between idle and cruise operation is the fact that at cruise, the forward motion of the car generates the air flow through the radiator. At idle, there is no motion, so the fan is primarily responsible for generating air flow. So, if your engine overheats at idle but operates at a satisfactory temperature at cruise, when engine heat rejection is even greater than at idle (albeit with a greater water pump speed), there almost has to be a problem with the idle air flow. The main factor here, of course, is the fan clutch.
I should also mention that in high ambient temperature conditions, the cooling system of a mid year with Powerglide and A/C is going to be taxed to the outer limits. Don't forget that mid year small blocks WITH A/C use exactly the same radiator as mid years WITHOUT A/C. Plus, mid years with Powerglide use a seperate transmission cooler which, because of its orientation, somewhat impedes air flow through the primary radiator. So, it may also be that your EXPECTATIONS of appropriate idle temperature under such conditions are unrealistic.In Appreciation of John HinckleyComment
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Re: Overheat at idle 67 300H.P Automatic and A/C
Hi Paul:
Joe is, as always, correct to a fault. You mention this is a daily driver, in this case you might consider the addition of an electrical coolant fan to increase air flow, this worked well on a similarly equipped 67 herein sunny Florida.
jerComment
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Re: Overheat at idle 67 300H.P Automatic and A/C
Paul, I think Joe covered most of the problems with your car. I want to add an exclaimation point to the problem. You have the hardest car /engine/options etc. that can be imagined; ie. 300hp,AC,and automatic. That is why Corvette moved the front license plate to the drivers side only on AC/automatic cars. Also you said that you had the engine rebuilt. It is my contention that rebuilding an engine (boring out the cylinder walls .030 or.060 over causes more heat to transfer to the coolent faster and hotter than before. I don't believe that it is possible to run at idle without some high readings on your stock set up. About the only thing you could do is buy a new alum. radiator. This always helps.All the small quick fixes are a waist of time.
Texas Chapter NCRSComment
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Re: Overheat at idle 67 300H.P Automatic and A/C
Joe--
When I bought this car it had a 7 blade fan.
I replaced it with part # 3931002, after finding part # 3863137 or part #3863137 were not available.
This fan would not bolt to the the original clutch fan.
I purchased part # 3916141, clutch fan and installed the new fan and clutch fan.
It seems idle air flow must be the problem. All of the fan shrouds are in place as well as the lower radiator seal and the radiator to hood air dam.I've ordered a fan shroud extension, but I doubt that will solve the problem.
The point is I drive this car every day. These 1967 Corvettes with Powerglide and A/C had to function in all climates and in all driving conditionswhen they were new.
I'm just trying to make it the way it was when it left the factory.Comment
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Re: Overheat at idle 67 300H.P Automatic and A/C
I'll do it. -BUT-
The question remains. Why is it overheating? There were no electic fans in 67.
I feel it's like a band-aid.
I'd like it to function properly(the way it left the factory)Comment
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Re: Overheat at idle 67 300H.P Automatic and A/C
Bob-- Noland Adams says the side mounted front side mounted license plate was only used with the 427 engine in l966. He goes on to say it was probably never installed on any l966 cars but was factory installed on 1967 corvettes.(no mention ofengine option)
He also mentions a lower fan ring added to the shroud on small blockks with A/C and a lower seal.
I agree with the logic of boring of the cylynder walls, but this car overheated before the rebuild.
I'm willing to buy the new radiator and the electric fan, but I'd like to keep it original.
Although its a daily driver it's a pretty nice triple black, with side pipes, bolt-ons and redlines.
I just hate to compromise with the fan and an aftermarket radiator.
I will if I have to, but maybe someone has a better solution.Comment
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Re: Overheat at idle 67 300H.P Automatic and A/C
Paul-----
The GM #3931002 (which is now also discontinued) and the GM #3916141 fan clutch are the correct GM replacement parts for your car. The thermostats on current-issue 3916141 clutches may engage at a slightly higher temperature than original versions, but, without making significant modifications to your car, this fan and clutch combination is about as good as you can do.
You mention that you would like to get your car to be "the way it was when it left the factory". Well, it may very well be just exactly that way now. In other words, I expect that your combination ALWAYS was VERY MARGINAL with respect to idle cooling, especially with the ambient temperatures high and the A/C on.
Also, your original radiator may have sufferred some loss in cooling efficiency. Which, actually, brings up another question that I should have asked earlier. You are using an original or reproduction aluminum radiator with the number 3155316 stamped on the top, aren't you? A brass replacement for this radiator provides marginal cooling capacity for a mid year WITHOUT Powerglide and A/C. Using one WITH Powerglide and A/C would leave one at a hopeless disadvantage.
As I also alluded to earlier, your frame of reference for idle cooling performance may be modern-day cars. Most of these have electric fans and cooling systems which are vastly improved technology and designed around installations which virtually all include A/C. You can't expect a 67 Corvette equipped like you have to provide cooling efficiency like a modern-day car.
Most early Corvettes had marginal cooling both when they were new and, obviously, the way they exist now. Stories about Corvette cooling or lack thereof are legion. My original owner 69 with 300/350 and WITHOUT A/C or auto trans NEVER operated without a somewhat elevated temperature at idle in high ambient temp conditions. And, my car uses exactly the same original radiator as your car. I did, however, improve the cooling efficiency by adding an aluminum water pump (with 3/4" shaft), GM #343721 (now GM #12529342) fan clutch, and GM #469580 fan. The latter two parts, which were used on later C3 cars with A/C and /or HD radiator, are the highest capacity units ever used on a C1-C3 Corvette.In Appreciation of John HinckleyComment
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Re: Overheat at idle 67 300H.P Automatic and A/C
No vacuum advance at idle will definately cause the car to run hotter and/or overheat. You might need a new mechanic. Check that the vacuum can holds vaccum and is fully deployed at idle. If the car has ported vacuum advance or TCS, re-port it to a full manifold vacuum source.
DukeComment
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What do you mean by "overheating"
Is the coolant actually boiling over? What is the highest coolant temperature reported on the gage?
DukeComment
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Re: Overheat at idle 67 300H.P Automatic and A/C
Paul:
It is possible that it is functioning the way it left the factory, you wouldn't be the first.
jerComment
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Re: Overheat at idle 67 300H.P Automatic and A/C
Paul,
Your description of the problem is consistent with the fan clutch not functioning correctly as Joe suggests. In reality, the clutch is probably doing what it was designed to do, however, new clutches purchased over the counter are calibrated to engage at around 212 deg F. Originals engage about 20 degrees lower. One thing you can do is disable the thermostat on the clutch. This is done by removing the tail of the coil thermostat from it's retaining slot and unwind 90 degrees counterclock wise. Just let the tail of the coil float in this position. The clutch will now be pumping fluid into the clutch plates regardless of the ambient temperature.
This may not completely cure the problem as it's possible that newer clutches use lighter viscosity fluid internally. I've never verified this.
With regards Joe's statement about the trans lines adding extra thermal load to the radiator, try placing the car in neutral when at a stop light & see if your temperature still increases.
Hope this helps.
Fred OComment
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Re: What do you mean by "overheating"
I've installed a temporary guage to make sure I was getting a true reading. After letting it idle (from 5 minutes to 30 minutes-depending on ambient temperature) I shut it off at 230 for fear of damaging the new engine.
It has boiled over (coolant on the ground ) once since the rebuild.Comment
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