# Gto Overheating



## williamp302 (Jun 8, 2007)

I have a 1965 GTO with a 1967 Gto 400engine. If I am cruising down the road my temperature stays a constant 205 to 210 degrees. If I stop in traffic the temp rises to around 240 to 250. I put a 18inch 6 blade flex fan, a new shroud, and a new radiator and it lowered everything a very very little bit, about 5 degrees lower. I added a can of coolant additive called 40 below and the temp is still the same, when you open the hood it is like opening an oven and the hoses are so hot I don't know how they are not blowing..Please help me with some suggestions. before I go broke..thanks


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## Cottonfarmer (Jul 11, 2005)

williamp302 said:


> I have a 1965 GTO with a 1967 Gto 400engine. If I am cruising down the road my temperature stays a constant 205 to 210 degrees. If I stop in traffic the temp rises to around 240 to 250. I put a 18inch 6 blade flex fan, a new shroud, and a new radiator and it lowered everything a very very little bit, about 5 degrees lower. I added a can of coolant additive called 40 below and the temp is still the same, when you open the hood it is like opening an oven and the hoses are so hot I don't know how they are not blowing..Please help me with some suggestions. before I go broke..thanks


Possible causes:

(1) The radiator may be too small for the heat load the 400 is putting up.
(2) I'm sure you checked the fan belt to be sure it's tight?
(3) The thermostat is sticking and not allowing full flow of coolant through it. Try removing the thermostat and run without it for a short time (not a good idea to make this a permanent fix).
(4) The flex fan is not pulling sufficient air through the radiator when stopped.
(5) Possible blown head gasket into the water jacket. Take off the radiator cap while the engine is running (and not hot) and look into the filler neck and see if you have bubbles circulating around in the coolant. If so, head gasket is likely blown.
(6) If you have A/C, ensure that the condenser core is not plugged up with bug guts or other debris restricting air flow.
(7) Ignition timing if advanced too high can cause problems tho I doubt this is your problem.
(8) Check the bottom radiator hose to ensure that it is not collapsing. Most have springs inserted to prevent this. Also, and I have seen this one time before, is a partial internal delamination of the top radiator hose which can restrict coolant flow.
(9) I've never seen this before but have heard of air pockets in the engine coolant jacket that prevents a full level of coolant being added.

To my mind these are the most likely causes tho I'm sure there are other possibilities. -Jim


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## lars (Sep 28, 2004)

Most common source of low rpm overheating is incorrect timing setup. Check, verify and set up the following:

Make sure you have timed the engine for 36 degrees total timing (with vacuum advance hose yanked off). With 36 total, you should be in the mid-teen range for initial. Make sure the centrifugal advance is bringing in the timing smoothly and fully. Total timing should occur below 3500 rpm. If not, fix the distributor.

Suck on your vacuum advance control unnit and make sure it's operational. If not, replace it with part number VC1765.

Run your vacuum advance from a manifold vacuum source (not ported vacuum). This will pull in a bunch of timing at idle and low speed. Advancing the timing lowers operating temperature significantly.

Setting up timing in this way will typically lower engine coolant temps at low speed by 20 to 30 degrees.


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## DaveH (Jan 20, 2006)

Don't know if this water pump may help or not. I haven't needed to try it, but it seems reasonable if a water pump with excessive clearances is installed. 
http://www.wallaceracing.com/water-pump-mods.php


Also, I think that the original clutch style fan is probably more efficient than a flex fan, providing the clutch coupling is good.


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## Cottonfarmer (Jul 11, 2005)

DaveH said:


> Don't know if this water pump may help or not. I haven't needed to try it, but it seems reasonable if a water pump with excessive clearances is installed.
> http://www.wallaceracing.com/water-pump-mods.php
> 
> 
> Also, I think that the original clutch style fan is probably more efficient than a flex fan, providing the clutch coupling is good.


That reminds me of a problem I had with one of our farm tractors a few years back. We tried nearly everything to solve an overheating issue before a replaced water pump finally solved the problem. Seems the old pump had too much tolerance with the press fit between the pump impeller and the impeller shaft. Damned impeller was slipping on the shaft and I guess the coolant provided enough lubrication between the two to keep obvious wear from being apparent when the pump was disassembled the first time it was checked out. The mechanic was using a 20 ton press to take it apart and didn't notice the lack of resistance between the two parts.

On a side note and a bit off topic: We just got a tractor out of the shop for the third time because the tractors power shift transmission would simply stop transferring torque to the rear end differential. The newer tractors are completely computerized right down to all engine operative controls (injection timing, etc.) and even control of the shifting of gears under full load without clutching. The problem turned out to be the FM music radio (which had only recently been replaced with a new one) which was sending out spurious signals back through the power connection wiring and interfering with the networking between the three computers that deal with engine, transmission and hydraulic systems. Technology is making shade tree mechanics obsolete. -Jim


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## 68goatconv (Jun 12, 2007)

I've got a '73 455 in a '68 that also runs 200-220 degrees at 70MPH in hot weather without A/C. Shoots up to 230 when I pass someone. Any idea what "normal" might be? Or what temp an "idiot gauge" light might come on?
Thanks, Ed


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## 68goatconv (Jun 12, 2007)

lars said:


> Most common source of low rpm overheating is incorrect timing setup. Check, verify and set up the following:
> 
> Make sure you have timed the engine for 36 degrees total timing (with vacuum advance hose yanked off). With 36 total, you should be in the mid-teen range for initial. Make sure the centrifugal advance is bringing in the timing smoothly and fully. Total timing should occur below 3500 rpm. If not, fix the distributor.
> 
> ...


Lars,
What would be the best "manifold" vacuum source rather than a "ported" source?
Thanks, Ed


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## BobG (Dec 20, 2006)

would a 160 degree thermostat help any? or is that a no-no for the old high compression motors?


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## 68goatconv (Jun 12, 2007)

I've been told I'm using a 180 degree thermostat. Book says 190 for a '68 & 195 for a '73 455. I was also wondering about using a 160 degree, but understand that the higher temp helps efficiency. (???)


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## DaveH (Jan 20, 2006)

In Jim Hand's book on Pontiac Engines he makes an argument for using a 160 degree thermostat, claiming a cooler engine compartment provides for cooler intake air, that offsets the improved thermal efficiency from running the engine with a 180 thermostat. I am running a 160 in my 66 and 180 in my '70 and both run fine. 

Anyway - I don't think it will help the overheating issue being discussed here unless the thermostat in use is bad. Both the 160 and 180 thermostats would be wide open at the temps being discussed.


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## lars (Sep 28, 2004)

68goatconv said:


> Lars,
> What would be the best "manifold" vacuum source rather than a "ported" source?
> Thanks, Ed


There is only one ported vacuum source on the 1967-1974 Q-Jet, and it's the forward driver's side nipple on the carb float bowl. All other vacuum nipples on the carb are manifiold vacuum. You can use any of them, as well as any vacuum nipple on the manifold. On 1975+ carbs, the ported nipple is in the carb baseplate on the forward passenger side corner, pointing out at a 45-degree angle. All other ports on those carbs are manifold vacuum.




68goatconv said:


> I've been told I'm using a 180 degree thermostat. Book says 190 for a '68 & 195 for a '73 455. I was also wondering about using a 160 degree, but understand that the higher temp helps efficiency. (???)


As DaveH stated above, a lower thermostat will not lower the temp. The thermostat only controls *minimum *operating temperature, and has no control over maximum temperature. At any temp above its rated temperature, any thermostat will be wide open, and no longer having any control. Maximum temp is controlled by engine heat output, radiator size, pump volume, outside air temperature, and airflow through the radiator. Since the biggest factor to this equation is the engine's heat output, you can make a huge difference by setting up a proper timing curve and vacuum advance curve. Correct timing has a dramatic effect on engine temp: retarded timing (from improper advance curve or non-functional vacuum advance) will run the engine temps and exhaust gas temp (EGT) WAY up. Advancing the timing, and running a proper vacuum advance, lowers EGT significantly.


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## 05GTO (Oct 6, 2004)

williamp302,
1) Set the timing,
2) Replace the T-stat,
3) Replace the water pump, clean or replace the water pump divider and rear housing.
4) Add rust inhibitor to the coolant to protect the new radiator.
If the above items don't correct the problem could it be a bad or mis-matched temp sending unit? Buy a cheap gauge to confirm the accuracy of your existing gauge;

http://www.amazon.com/dp/B000FJS4MW...e=380333&creativeASIN=B000FJS4MW&linkCode=asn

More than likely, the t-stat is bad or rust is blocking the coolant flow.


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## 68goatconv (Jun 12, 2007)

Thanks, guys.
The engine has a little over 3,000 miles. New thermostat, waterpump, hoses, etc.
Radiator is 10 yrs old, but only 12,000 miles (most time just sitting empty on my garage shelf while I was restoring the car). Flushed today with two cans of Prestone flush, two fresh water purges & refilled with 50% mix. Minor improvement (5 degrees).
I'm trying to get someone to work the throttle so I can try to determine the advance at various rpms with vacuum disconnected & plugged. Then I'll try to see what the vacuum advance does. The '73 (?) distributor has Pertronix electronic insides and Pertronix coil, but has never been curved (nor rebuilt). Currently around 10 degrees BTDC at 750 rpm on centrifugal alone. Not sure what total is yet.
Appreciate your help (& understanding).
Ed


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## 05GTO (Oct 6, 2004)

68goatconv said:


> Thanks, guys.
> The engine has a little over 3,000 miles. New thermostat, waterpump, hoses, etc.
> Radiator is 10 yrs old, but only 12,000 miles (most time just sitting empty on my garage shelf while I was restoring the car). Flushed today with two cans of Prestone flush, two fresh water purges & refilled with 50% mix. Minor improvement (5 degrees).
> *I'm trying to get someone to work the throttle so I can try to determine the advance at various rpms with vacuum disconnected & plugged. * Then I'll try to see what the vacuum advance does. The '73 (?) distributor has Pertronix electronic insides and Pertronix coil, but has never been curved (nor rebuilt). Currently around 10 degrees BTDC at 750 rpm on centrifugal alone. Not sure what total is yet.
> ...


Ed,
Call me, I'd be happy to stop by, missed you at Riverstone today.


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## lars (Sep 28, 2004)

68goatconv said:


> I'm trying to get someone to work the throttle so I can try to determine the advance at various rpms with vacuum disconnected & plugged. Then I'll try to see what the vacuum advance does. The '73 (?) distributor has Pertronix electronic insides and Pertronix coil, but has never been curved (nor rebuilt). Currently around 10 degrees BTDC at 750 rpm on centrifugal alone. Not sure what total is yet.
> Appreciate your help (& understanding).
> Ed


You don't need anyone to work the throttle - you can do it easily by yourself by holding the timing light in one hand and working the throttle with the other. Just yank the hose off the vacuum advance and rev the engine up until the centrifugal advance pegs out and stops advancing. Timing at that point needs to be 36 degrees. Then, let rpm fall down to idle and make note of the initial timing, still with the hose yanked off. Hook the hose up to a manifold vacuum source and see how much vacuum advance you're getting - should be 16 degrees. Replace the vacuum advance control unit with part number VC1765 if it is not performing to spec.

For detailed info on setting up total timing, drop me an e-mail request for the Timing Paper:
[email protected]
Lars


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## bg2m8o (Jul 25, 2006)

It is really hard to guess without seeing the car. Of all of the guesses here, I'd say check the tstat and water pump. Sounds like the car is not flowing coolant when idling. A bad thermostat will lift under higher pump pressure caused by higher RPM. If the pump impeller to body clearance is too high, the effeciency of the pump will be crap at low speed. Is the intake manifold the same temp as the radiator ends? (especially the top) Coolant flows bottom (hot) to top (cool) Probe the delta from top to bottom of the radiator when this happens. 

With it running OK at speed, I would check this first then proceed to timing curves, etc. It may also be lean when cutting over to the idle circuit on the carb, but I think it would idle like hell if it was so lean to get that hot when idling. In fact - scratch this, can't really happen

My guess is that the coolant pump and/or the thermostat is the issue, new or not.

Good luck.


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## pwoolford (Mar 22, 2007)

Make sure the shroud fits well. I had similar problems with my camaro years ago and part of the problem was an aftermarket shroud that didn't fit very well. The problem got worse when I dropped in a stronger 350 and I had to replace the 3 core with a 4 core radiator. I don't know how many radiator options there are for GTO's but could it be undersized for a 400? 

Although this is an expensive option I made certain I wouldn't have overheating problems in my 67 any longer...


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## teaswift (Jul 13, 2009)

where did u get that radiator setup I am having overheating issues in my 66 just put in a 73 455


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## GTO JUDGE (May 14, 2005)

teaswift said:


> where did u get that radiator setup I am having overheating issues in my 66 just put in a 73 455


He hasn't been on in over 3 years.


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## Eric Animal (Oct 28, 2007)

It looks like an ENTROPYTEC....


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## dimitri (Feb 13, 2009)

68goatconv said:


> I've got a '73 455 in a '68 that also runs 200-220 degrees at 70MPH in hot weather without A/C. Shoots up to 230 when I pass someone. Any idea what "normal" might be? Or what temp an "idiot gauge" light might come on?
> Thanks, Ed


I am pretty sure the idiot light comes on at 230 degrees

Dimitri


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## injn37 (Nov 16, 2008)

OK, my two cents. Found a thread on PY forum last year. Did it. It worked great.
Take water pump out, and 'flatten' the diverter plate so that the plate is almost touching ( a little barely touch is ok, because gasket will space it away) the impeller. Do this carefully on a flate surface. Hammer in the middle firsts, than areas that need it. You want an even space around the entire impeller. You are looking for the barest of clearance. My diverter plate and impeller had about .300" clearance to start. Now about .020" ! That's it! I am running a 455 (030 over) 3x2's, with a Desert Cool aluminum radiator. Have a 160 T'stat. Temp on motor on a 90 degree day idling for over 30 minutes, never went above 175-180. Kind of freaked me out.... a Pontiac motor not running hot!
Just did the same to my 505 IA Aluminum block , but havent fired it yet. Clearance on this motors pump was originally at about .350". Got out the big hammer.
Oh, use a cast iron impeller pump. They seem to work better.

Hope that helps.

rich


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## 68greengoat (Sep 15, 2005)

injn37 said:


> OK, my two cents. Found a thread on PY forum last year. Did it. It worked great.
> Take water pump out, and 'flatten' the diverter plate so that the plate is almost touching ( a little barely touch is ok, because gasket will space it away) the impeller. Do this carefully on a flate surface. Hammer in the middle firsts, than areas that need it.rich


Did the same thing on an outboard. Made a world of difference. Pumped much better.......


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## dimitri (Feb 13, 2009)

injn37 said:


> OK, my two cents. Found a thread on PY forum last year. Did it. It worked great.
> Take water pump out, and 'flatten' the diverter plate so that the plate is almost touching ( a little barely touch is ok, because gasket will space it away) the impeller. Do this carefully on a flate surface. Hammer in the middle firsts, than areas that need it. You want an even space around the entire impeller. You are looking for the barest of clearance. My diverter plate and impeller had about .300" clearance to start. Now about .020" ! That's it! I am running a 455 (030 over) 3x2's, with a Desert Cool aluminum radiator. Have a 160 T'stat. Temp on motor on a 90 degree day idling for over 30 minutes, never went above 175-180. Kind of freaked me out.... a Pontiac motor not running hot!
> Just did the same to my 505 IA Aluminum block , but havent fired it yet. Clearance on this motors pump was originally at about .350". Got out the big hammer.
> Oh, use a cast iron impeller pump. They seem to work better.
> ...


When I make this modification I bolt the dividerplate to the pump without a gasket then start tapping the dividerplate with the hammer till it touches the impeller. Then I unbolt the divider plate and put the gasket in place and this gives me the clearance I need.


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## Eric Animal (Oct 28, 2007)

injn37 said:


> OK, my two cents. Found a thread on PY forum last year. Did it. It worked great.
> Take water pump out, and 'flatten' the diverter plate so that the plate is almost touching ( a little barely touch is ok, because gasket will space it away) the impeller. Do this carefully on a flate surface. Hammer in the middle firsts, than areas that need it. You want an even space around the entire impeller. You are looking for the barest of clearance. My diverter plate and impeller had about .300" clearance to start. Now about .020" ! That's it! I am running a 455 (030 over) 3x2's, with a Desert Cool aluminum radiator. Have a 160 T'stat. Temp on motor on a 90 degree day idling for over 30 minutes, never went above 175-180. Kind of freaked me out.... a Pontiac motor not running hot!
> Just did the same to my 505 IA Aluminum block , but havent fired it yet. Clearance on this motors pump was originally at about .350". Got out the big hammer.
> Oh, use a cast iron impeller pump. They seem to work better.
> ...


Rich, Where's my pictures???????


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## injn37 (Nov 16, 2008)

Caught me. Wrote this thread while in Canada. Back now and will post them ( I hope ) this weekend. 

Rich


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## bullet20cc (Apr 4, 2009)

*Heat*

i have a 67 with a 455. I have electric water pump but the fan was and after maket 12". I went to a flex-a-lite electric 16" shrouded 3300 cfm fan and my temp dropped 25 deg.


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## GTO JUDGE (May 14, 2005)

There's a really nice article on overheating goats in the Feb issue of Amos Auto Enthusiasts.... some of us recall Pontiac Enthusiasts Magazine, now Pontiacs are just a VERY brief mention. Feb's brief mention focused on overheating issues.


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## goat670 (Aug 10, 2008)

sounds all too familiar,the only way I was able to cool my 67 400 +.30 was electric fans,
dual pushers fit perfect in the 15 1/2 core support got them thru jegs, you can't even tell they are there, and I left the stock fan also. I put a toggle switch under the dash and flip it on when the temp goes up, or just leave them on, it is wonderful not to run hot anymore,


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## olde-goat (Feb 1, 2009)

Similar problem occurred with a friend's recent 389 rebuild in a 65 GTO. Went to break in the engine, and with the Rally gauge set up couldn't tell exactly what the engine temp was, only that it was extremely hot. Damaged the engine. After the tear down, found that the reman water pump impeller was pushed on too far, which resulted in cavitation. Purchased a new pump with properly located impeller and addition of after market temp and oil gauge. Engine now runs cooler.
I am now close to starting my recently rebuilt 389 in my 65 GTO and am having second thoughts about my reman water pump. After reading the above other posts how do you accurately measure clearance between the impeller and divider plate?


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## dimitri (Feb 13, 2009)

olde-goat said:


> Similar problem occurred with a friend's recent 389 rebuild in a 65 GTO. Went to break in the engine, and with the Rally gauge set up couldn't tell exactly what the engine temp was, only that it was extremely hot. Damaged the engine. After the tear down, found that the reman water pump impeller was pushed on too far, which resulted in cavitation. Purchased a new pump with properly located impeller and addition of after market temp and oil gauge. Engine now runs cooler.
> I am now close to starting my recently rebuilt 389 in my 65 GTO and am having second thoughts about my reman water pump. After reading the above other posts how do you accurately measure clearance between the impeller and divider plate?


I use a feeler gauge.


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