# 67 GTO Conv Overheating - Help !



## Centofanti (Jun 21, 2020)

So the customers 67 GTO Convertible overheats to 230 - 240 degrees and does not recover down. Complete rebuilt 400 by a qualified, local Pontiac machine shop. We added aftermarket A/C, stock engine except mild RV cam, stock exhaust, Edelbrock carb/intake, 6 degrees BTDC both pulleys the same size.

1.) Re-cored the original radiator to a 4 row Desert cooler - no change. Took out thermostat no change (with the cap off the radiator is flowing really well)
2. Fan clutch/Fan blade is 1/2 in 1/2 out and all is sealed between shroud and radiator
3.) Changed water pump to a flow kooler with water wetter in the 50/50 green and it dropped 10 degrees - 220/230
4.) Forums said changed to a smaller water pump pulley which we did to a 6 1/2", changed 7 blade clutch fan to a 18 1/2 flex fan and it dropped 5 degrees - 215-225

#4 did today, so drove up and down the road at 190 degrees but at a light is creeps up to 225 and once you take off the temp never comes back down. The inferred thermometer at the thermostat matches the temp gauge. The funny thing is with the inferred thermometer shows the front passenger side head 20 degrees hotter then the front driver side head.??????

We seemed to do everything suggested on the forums so is there anything else you guys could think of? Thanks for your time and look forward to your answers.


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## BLK69JUDGE (Jun 10, 2010)

stock exhuast manifolds with a passenger heat riser flapper ?
67 timing cover still ?? 8 bolt water pump ?? divider plate
correctly clearanced ?
does the lower radiator hose have the inner spring so it wont collapse ?
have you put the 185 thermostat back in ?
hmmmmmmm


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## armyadarkness (Dec 7, 2020)

67 is "my year". Ive rebuilt many and they always run hot... which usually indicates that something was missed or is wrong.

Most pro's here will tell you that little things, like radiator gaskets and inner fender flaps, are a must for proper cooling. I respect that opinion and I agree that they help...

However, my philosophy is that if everything is right, then the car doesnt run hot.

I have no shroud, no gaskets, and no flaps, and I can idle in traffic at 170.

So... First:

Are you SURE that the timing is right? Ive seen where someone thinks they advanced #1, but they actually advanced #2, which is retarding #1.
How did you do the divider plates in the water pump. Were they clearanced?
NEVER run without a thermostat. It hurts, not helps. Personally, I drill three 1/8 holes in mine. That lets coolant circulate at all times, yet it also holds it in the radiator longer, so that it can cool.
Did the radiator shop do a flow check? If everything else it right, then maybe there's an obstruction.
Have you tried backflushing the system since the rebuild? Maybe some sealant or a gasket is lodged in there.
Have you tested the fan clutch?
If the divider plates are good and the timing is right, then personally, Id:

Backflush.
Remove the AC belt and reinstall the thermostat. Try that. If it didnt help.
Try a generic parts house radiator to see if yours has an obstruction. Im a welder and Ive seen that happen several times... I prefer to use Cold Case, Less cores but bigger cooling surface.
I also use a flex fan. The clutches are great on factory cars, if your clutch is good!


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## geeteeohguy (Feb 2, 2008)

What the first two gents said. The first thing I would verify is the water pump plate clearance, and that you have a cast impellor pump. I might also check for hydrocarbons in the radiator to verify that you don't have a leaking head gasket on the passenger side that's allowing hot exhaust into the cooling system.


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## lust4speed (Jul 5, 2019)

Well... A Pontiac V8 will run exactly as cool with no thermostat as when a properly working thermostat is installed. We've done many tests on many Pontiacs and every time the engine ran as cool as a perfectly working thermostat. So until you get the problem sorted out keep the thermostat out. Maybe not running a thermostat makes a Chevy hotter, but not for Pontiac. Just for info, I'm located less than an hour from Palm Springs, CA and we're very familiar with really hot weather.

Also, while I would agree that the cast impeller should work much better than a stamped steel impeller, back to back testing on the same engine didn't show any difference in cooling ability. Everything says that the stamped steel impeller should be inferior, but we definitely couldn't prove it.

67 engines had two plates and both must be installed properly or overheating will occur. Erosion does occur and the two plate system is more prone to allowing coolant to bypass around one or both the plates. It's also a little more difficult to get the plate clearance correct compared to the later single plate 11-bolt pumps, but taking the time to get the clearance as tight as possible without touching is the best bang of all the tweaks. If prepped properly, the 8-bolt pump cools the same as an 11-bolt pump.

Another thing that many people miss is the feed at the top driver's side of the timing cover that connects to the intake manifold crossover. Originally both the manifold crossover and the rear of the pump had a 3/8" hole. Over time this erodes and we've seen some well over a half inch in diameter. Coolant flow is out the crossover and directly to the back of the pump. The pump actually has a negative pressure area here and every drop of coolant shortcutting back through the pump is that much that doesn't go through the radiator. If 20% of coolant flows directly back into the pump, that is 20% of coolant that is not going through the radiator. I enlarge the hole as necessary, tap for 3/8" pipe threads, and install a 3/8" plug that has a 1/4" hole drilled through it. This is enough to keep the bypass functioning properly, and allows maximum coolant flow through the radiator. The plug can be installed in either the crossover or the timing cover, and I will pick whichever is the easiest to do.

Last, don't forget to check gauge accuracy with a laser temp gun. Except for Auto Meter, most gauges promise an accuracy of plus or minus ten percent. So that means the temperature might be reported as 198°, 220°, or 242° and the manufacture is within their claimed tolerance.

Here is a cast iron manifold with the drilled plug installed. This manifold had a hole that needed very little enlarging to be the right size for threading.


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## PontiacJim (Dec 29, 2012)

Have you checked your vacuum diaphragm on the distributor to make sure it is operating correctly? How much advance does it pull?
Is it connected to a manifold source or carb/ported source?


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## lust4speed (Jul 5, 2019)

One other thing about the added AC. My non-AC 67 GTO always ran at thermostat prior to the installation of the Vintage Air. It still ran close to thermostat while highway cruising or light city driving with the air on. If I left the AC running in stop and go traffic or pulling into a car show where movement was at a crawl, temps would go from around 165° to around 210°. I went to the small water pump pulley and 8" crank pulley, added a severe duty fan clutch and all that helped, but temps on hot days crawling along were still too warm. Finally bit the bullet and went with the deep radiator support which allowed a 17.5" radiator core instead of the 15.5" core. With the deeper radiator I was able to go up to the 19.5" fan and shroud. At this point the car runs at thermostat regardless of weather or traffic.


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## djcharliecimino (Jan 5, 2022)

your statement regarding everything working right and NOT needing a thermostat is, by my vast experience, 100 percent correct. whenever there is a cooling problem the first thing i do is remove the thermostat completely then "tune" the cooling system. sometimes it takes a little trial and error until you find the right combination of parts and mechanical changes to get it right.


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