Cooling (Overheating)

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26 Aug 1996, John Spence:

All Jag trained mechanics here in Phoenix recommend cutting out the central moving portion of the thermostat and blocking the bypass hose with a wooden plug on XK engines. There is a high temp Jag testing facility here. (Our summer temps are 110 - 120F, in winter over 90F in dec. Frost very rare). My car doesn't overheat (i.e. boil), but the gauge goes so far up into the oil gage after turning off the engine that I worry (I had to repair it once from this - ether leak). This suggests fitting an electric fan to reduce heat soak, but this obstructs air. It seems to me that a fan in front or behind the radiator should beequivalent, but an air scoop to reduce pressure behind the radiator should help - avoid deflecting air into the space between radiator and engine, increasing pressure. With an external temp of 100F, my car (XK140 SE FH) runs at 95 -100 C in overdrive at 70mph on level ground. Is this reasonable ? Other suggestions I've had are an aluminium radiator core, or to fit the more efficient water pump from later cars. I'm interested in your comment about core plugs. After having my engine removed from the car and rebuilt recently, the first thing that happened was a burst core plug. I assumed this was because they had not filled the heater before the block with water, but now that you mention it I found that they had also fitted the wrong thermostat. They fitted a simple one which did not close the bypass line -it had no slotted collar on the side. (I've since replaced it with the correct one). Could this have caused the failed welch plug ?

13 Aug 1996, from Rob Reilly:

I guess I got lucky with mine, because when I noticed a core plug in the block starting to weep, I pulled off the intake and exhaust manifolds and radiator and removed the core plugs and flushed with the garden hose and the car runs reasonably cool now. Sounds like you'll have to pull off the head. When you take off the cam covers stuff a rag down around the timing chain to catch any stray bolts or tools. Note there are 8 core plugs in the block, 3 big ones on each side, 1 at the back and a little one down by the starter motor. I believe early blocks also had a little one on the left hand side. Remove the gearbox tunnel to get at the rear one, and even then it's a pain to reach. Better do the ones in the intake manifold as well.

Aug 96, from ? Dick White ? - (no, wasn't me)

Hearing your story about your engine becoming a coolant pump after 30 miles reminded me of my XK150. When I bought it several years ago, it had not been driven for the preceding 13 years. After I cleaned the carburetors and repaired the gas tank, I actually was able to get it running. My problem was that it kept overheating. I finally removed the head and found a blown head gasket. However, I also had a chance to look down and around the water jackets. The corrosion was not too bad, but I found a lot of gunk that must have been the residue of coolant that eventually evaporated. That was as much of my problem as the blown head gasket. I used boiling water to flush the water jackets. I did this by simply boiling water in the tea kettle and then running out to the garage with it and pouring it in the openings next to the cylinders. I eventually got it cleaned out. Since then I have removed the engine and rebuilt it. When it was apart, I had the block boiled just to make sure there was no residue of gunk in it.

23 Sep 1996, from Tom Veale,

Hi John, In looking over your latest letter, I had a few thoughts about some of the over-heating problems at which were hinted. In our race cars we try every possible way to remove heat and most of the methods wouldn't be acceptable for those who show their car. Here's some thoughts that might help:

1) On our race cars we make up 1/2" blocks for the bonnet hinges. This raises the back end of the bonnet enough to let a significant amount of heat out of the engine compartment. Since it's not a permanent modification, one could use the car this way and then for "show" take the blocks out. This is about as effective in most cases as louvering the bonnet.

2) Most new cars have "burp tanks" (a non pressurized reservoir) installed on their cooling systems. Old Jaguars like them too! Because the cooling system on the XK engine is large in volume, the tank capacity needs to be large as well. A half gallon would be minimum with a gallon being better. The reason burp tanks work so well is twofold: 1) You don't get air back into the cooling system (air doesn't cool anything) and 2) the oxygen from the air doesn't make hard water drop out as "boiler scale". The water jacket of the engine must be free of scale to transfer heat well. You will need a new radiator cap that allows the drain tube to draw water back from the burp tank. Then you'll need a canister of adequate volume with either a long tube that goes to the bottom of the canister or a bottom take-off on the canister itself. After filling the system with an appropriate antifreeze/anticorrosion material and water you should only need to top up the burp tank. When the engine is fairly hot the burp tank will be nearly full but as the engine cools, it sucks back the fluid keeping air out of the system. If it empties the reservoir when the engine is cold, then it's not big enough! On our race cars we've usually mounted the reservoir off in the left wing. A look under the bonnet shows nothing unusual other than the changed radiator cap and the longer overflow hose.

3) For cars that are driven and not shown, I'd suggest adding an engine oil cooler. This takes the heat away in a much more direct way than usual. Instead of heat having to pass from the oil to the water to the radiator to the air, it goes straight from the oil to the air. This takes quite a bit of heat load from the radiator.

4) If you've already dispensed with the old mechanical fan on your Jag. then there is usually room for a deeper radiator core. We have had deep cores made for our race cars and then add an electrical fan in "blow through" mode on the front of the radiator core. Again this might not be acceptable for show cars, but really makes for happy old Cats that are driven and not shown.

24 Sep 96, from Tom Veale:

You are exactly right on the function of the burp tank. It's a "draw back" reservoir. As the liquid expands with heating it must go somewhere or hydraulic action will break something or pop freeze plugs. When the radiator cap bleeds off this expanded liquid in older cars, it just gets wasted. With a burp tank it gets re-used over and over. My co-racer David Hinton at TwinCam Sportscars says that the XK150 may have an unusually deep filler neck and fitting a new style cap may be difficult. With a normal cap, the pressure relief valve is that part at the lower end of the cap. It has a spring that will lift when the internal radiator pressure reaches the rated amount. This then lets the liquid or steam out. There's also a vacuum break valve there as well. It opened to keep the radiator from going to negative pressure on cooldown and opens to draw back air. At the top of the radiator cap is a flange seal as well. On the older style caps this is just there to keep water or steam from coming out, but won't seal tight enough to get water to draw back thru the relief tube. The new style caps have a rubber gasket at the upper flange as well. You may be able to create a "new cap" out of your old one by taking some inner tube matl. and cutting a gasket to fit inside the cap. I've done this successfully on a A-H Sprite. It would be better to get a new cap with the proper depth seal for the neck of the 150, however. I'll ask around to see if there are any available. The pressure rating can be the same as stock. Terry's Jaguar Parts has an oil collar adaptor kit for XK 150 cars. It uses some parts modified from the XJ6 series cars I think. It mounts to where the normal oil filter goes. As a bonus, you use Fram PH8A spin on oil filters in place of the old cartridge types. That big Fram is available almost anywhere. Mounting the cooler would be dictated by your own ideas.

I'd suggest mounting it behind the right hand cowl vent on the front of the car. You may require a bit of shrouding to contain the air flow thru the cooler, but not much more than that. This is how the cooler is mounted on our XK 140 rally car. David Hinton mounted his this way on his XK 120 racer and he's reporting water temp of 170F to 180F down from 190F to 200F for the same ambient temperatures. He's not using an oil temperature gauge. On my 120 roadster, I'm running similar water temps and the oil temp (on the return from the coolmy 120 roadster, I'm running similar water temps and the oil temp (on the return from the cooler) is at 210F to 220F under race conditions. This is great. If you can take heat out of the oil, you will maintain better oil pressure with regular petroleum based oils. They tend to thin with heat and so oil pressure drops. Modern plate-type heat exchangers (oil coolers) have very little pressure drop across the cores. So whatever you gain from cooling the oil stays as a gain to the system. On my Fiat 2000 Spyder I actually have a thermostat to keep the oil temp at or above about 160F before it starts to flow to the cooler. This can be added to any street car. On that car at highway speeds my return oil temp is 80F above air temp. in warm weather (90F outside + 80F differential = 170F return temp, which is very nice for the bearings).

One more idea that I use on my race car is to make sure that the shrouding around the radiator is as complete as possible. I have even used hi-temp silicone to glue a piece of hot water pipe insulation (like you would use in your basement around the pipes) the width of my radiator top to the inside of the bonnet. When I close the bonnet this compresses between the bonnet and the radiator top tank and stops air from going around the radiator instead of thru it. On my 120 there's a space in front of the bottom of the radiator that has the drain valve attached. In that space between the frame and the radiator I've cut another piece of that same foam insulation and fitted it in there. Again it forces t same foam insulation and fitted it in there. Again it forces the air to go where it will do the most good. The shrouding and use of the foam rubber was good for between 5F and 10F lower temps at race pace.

Hope this helps.

29 Oct 1996, from Bill Schorse, Ontario, Canada:

I replaced my original rad core with a modern type. The chap who did the job for me used the type of core that hot rodders use on high performance engines. Another problem is the availability of the original type of thermostat. It has an external ring that shuts off the bypass hose when it opens. If you use a standard design thermostat some of your coolant will not go through the rad but return back through the bypass hose to the block. I have blocked my bypass hose to stop this as I cannot find the proper thermostat. It runs about 10 degrees cooler than before. The Mark9 sounds interesting but I live in Northern Ontario about a half a world away. Too bad it isn't closer. As I want to get a closed car to take the grandchildren out in. Got to get them appreciate the finer things in life when they are young.

30 Oct 1996, from Cleo Bay, Texas, XK120, XK140:

I got an original type of thermostat from XKs Unlimited not so long ago. (6 months?) They do work. Hopefully you haven't totally blocked off the bypass. You run the risk of having hot spots in your motor before the thermostat opens. Those hot spots could cause accelerated engine wear or head warpage (worst case). Until you can get the correct thermostat, you could use an insert in the bypass that has a hole in it to allow some water flow.

16 Nov 1996, from Jim Canedy:

Adding an expansion tank to a 120 does not require any changes to the radiator. Modify your existing cap to seal completely above the overflow tube and attach the tube to the overflow tank. I used a series II E type on my 120 racer and it works well.

Mike Plechaty, 17 Nov 1996:

Before strapping on external electric fans and whatnot, be sure the cooling system is clean (consider having system professionally power flushed). Be sure thermostat is a *stock* Jaguar unit, not a generic aftermarket unit from the local parts store, and it is working properly. Be sure the radiator cap is new. I would also recommend a stainless steel flex fan in place of the stock fan; it made an amazing difference on my 120, as it pulls much more air thru the radiator at low speeds. Pricey, but worth it (XKs Unlimited).

If you have any questions or comments send e-mail to: ted@jag-lovers.org
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