Need cooling help please

I was in my garage and found a GT40! It is one I have owned for some 17 years, but rarely drive. I made my mind up to change that this coming year and decided to move it to another outbuilding where I will see it more often and give it the attention it deserves. Basically a "barn find" in need of love. It is a SPF with Roush carb'd 427.

I got it to start, run, and to idle (it had been months since the last startup). While watching the coolant temp it took a little while to register (a normal amount of time as I remember it), then it started it's normal "slow-ish" climb rate. As it approached 70c it rapidly climbed to 90c within a few seconds (way faster than it should from my memory). I reached for the fan switch and the fans had no affect, so I shut it down. I waited, while doing other things, and tried again with same result. The temp needle again rose rapidly to 90c in a few sesconds, so I shut it down. I left to do other things and came back...the engine is warm, I can see coolant in the expansion tank, but the radiator is stone cold. I am thinking thermostat is stuck shut and/or maybe there is air in the system causing the temp gauge to act this way.

I was hoping, based on my limited experience with such things (my other play cars are aircooled), I thought I would ask for some suggestions on the proper approach to diagnose and sort this out. I see the thermostat is behind the seat and could be reached through that bulkhead panel, but if I am just dealing with air in the system, I am hoping there is an easier solution.

Your help would be appreciated!


Randy V

Staff member
Lifetime Supporter
Possibly an air bubble….Coolant temp sensors do not work well if only reading the air temp of a bubble. Depending on the size of the bubble, it can also be drawn to the impeller of the pump and now your pump stops working.. If you do not have a vacuum cooling system filler tool, you should borrow or buy one - particularly with a complex cooling system like that of your 40. It will also tell you if you have a leak.
The reason I would send you in this direction first is because you will first eliminate an air bubble. If that doesn’t do the trick, you’re going to have to start looking elsewhere which will necessitate draining the cooling system.

Randy V

Staff member
Lifetime Supporter
I appreciate your advice Randy. Is there a particular vacuum fill system worth considering, or are they pretty much generic?
Brian has recommended a good one. I got mine from Mac Tools and it was about $250 as I recall. They’ve now ripped off the design overseas and you can get the clones (questionable quality) for $50.

David Garton

The Air Lift unit Brian suggests is a very good unit. You have to have a air compressor to supply air to it causing a vacuum to the system once system is in a vacuum state have a pre-mixed five gallon jug ready. Close the gate valve and disconnect the air supply then connect the long clear hose at the suction port and into the pre-mix of coolant. open the gate valve and watch it pull the pre-mix into the system. It will fill the system before the vacuum is depleted and the system is full and ready to start. I use the same system to fill diesel trucks with eight to ten gallon systems. Once you start the vehicle monitor the top radiator hose going into the thermostat housing. If the temp starts to climb to hot and the hose is cold them the thermostat is not opening and pulling the cool water into the block . Hope this helps. Please keep us posted on your results.
I appreciate the instructions on how to do this David. Am I to assume that the 5 gallons is for a complete fill of the system if it were empty? If I am just trying to get rid of an air bubble, I can just use a gallon container of premix I suppose.

Also, how do I know when the all of the air is out of the system?
You own the car for 17 years, so basicly that rules airbubble out. Even with airbubbles in thevsystem, the rad should be hot & fans should be kicking in.
Rad still cool while engine temp hits 90 'C.
Thats stat not opening or waterpump failure (uncommon).
Ill guess you'r running a mechanical waterpump.

So, if the engine hits 90 'C & rad is still cool, next check is upper rad hose, if thats still cool, there is no flow meaning stat is still closed (common for stat failure).
Bedankt JP!

There was a coolant leak a few years ago when one of the main cooling pipe 90 degree connections behind the seats loosened up.

I also see weepage from the mechanical water pump seal...very slight though.

I will bleed any air out and check the pipe you are mentioning.

The way the temp needle accelerated from 70 to 90 in a couple of seconds was very unexpected.

BTW, we lived in Amstelveen for a couple of years, and my wife and I were married at Kasteel de Haar. We have a great fondness for The Netherlands!


David Garton

It takes hot water on the thermostat for it to work. Hot air won't do it. Your system needs to be low enough when using the air lift tool so when pulling into a vacuum it's not pulling coolant from the resivor. Water is heavier than air so the air will come to the top. Yes a gallon will work as long as it's not lower than the gallon.
JP, thanks for your kind offer to help, however, we lived there some time ago. We now live thousand of miles away from you...on the west coast of the US.

Howard Jones

If the water temp gauge works then there is water up high enough in the engine to submerge the sensor in the intake manifold. If the water in the engine is hot and there is no hot water anywhere else then one of two things are happening. 1. the water pump isn't pumping or 2. the thermostat is stuck closed. The easiest thing to check is the thermostat, so do that first. Remove it and replace it with a new one. This is too much work to put an old and relatively inexpensive suspect part back into the car. Recheck to see if the radiator now heats up as well as the engine. If you still have a problem then it is possible that the water pump impeller is spun on the shaft and even though the pully seems to be turning the impeller isn't thus no pumping of coolant. This is unlikely but not unheard of.
I understand Howard. Thank you.

The consensus seems to be air in the system, followed by stuck thermostat, followed by pump in that order of likelihood. Once the tool gets here I will start the process of narrowing it down. It seems the easiest to do it purge the air first and see if it acts differently. If it doesn't, then I am draining and replacing thermostat. and perhaps pulling the pump and fixing the weepage. Not looking forward to it, but it needs to get done.
As I mentally process my upcoming task, I have a Q or I pulling vacuum from the expansion tank or the radiator? With the car level or raised at the end of the vacuum draw?
Firstly you DRAIN the system. You then pull a vacuum on the system with the device. The position of the car does not matter as you are pulling a vacuum on it. HOLD the vacuum on the system for 20 minutes to ensure the system is not leaking. If leaking and loss of vacuum, STOP and find and fix the leak. If good, introduce your coolant from 5 gal pail (it may take more - if so, as volume of liquid gets low, STOP system and pour more coolant into your pail. Vacuum will be replaced by coolant. Again, does not matter of car is tilted up. Easiest way is to leave it flat on ground. BUT, you do not know if pump is pumping or Thermostat is stuck. I would remove thermostat FIRST and fix it as it is a $20 item (as Howard has already stated. Do that FIRST. Second, drain system. Third refill system with vacuum device.

You can watch how to do the vacuum device on YouTube.

Since we are attempting to "pull a tooth" over the phone (my joke) what motor are we talking about? What water pump, etc. - do you know? How is it plumbed?
It is a SPF with Roush carb'd 427.
Hi Lee,

It has a big Holley and standard coil/disti ignition on the pax side in the way of the thermostat.

Not sure how to describe how it is plumbed, The thermostat is on front top of the engine right behind the bulkhead. It should be accessible, I hope, by the removal of seats and cockpit rear bulkhead panel.

Given the thermostat is below the expansion tank and there is fluid in that tank, it makes sense to drain some to get at the thermostat without a mess. I can't quite see where the expansion tank is plumbed into the system...yet.

I have watched some YouTubers on the basic process and it looks relatively straight forward. I just am wondering if there is anything special to consider on these beasts.