MK IV radiator problem

I noticed a while ago a small water leak around the gasket of my MK IV thermostat, so while I had it off I tested the thermostat itself - which operated OK in a pan of water heating on the cooker (operating at around 75 degrees I think from memory). Assembled back together and got my helper to refill with water, which for the 1.5litre car is 20 pints. Ran it today and all was well until the temperature gauge rose to about 80 degrees when the poor old masket on the radiator cap was showered in steaming water.

Question: how do you know the water is topped up to the correct level? I suspect my helper filled it up to the brim of the upper expansion tank, and if my thermostat opened then there would be no room to expand heated water. If the pressure built it could breach the seal on the cap. This rubber seal does not look that brilliant and could need replacing. Am I correct in my theory? If so, how much gap should you leave in the expansion gap when Topping up with water?

It sounds like your overflow pipe is blocked. The cooling system is not pressurised. It should be open to the atmosphere. The pipe runs from the side of the filler neck and exits down the side of the radiator at the bottom.


The coolant level may find a comfortable highest level when cold by your experience of overflow as it warms up. As Peter says, the overflow pipe may be blocked, leading to the excess warmed coolant coming out the cap. Or the overflow pipe may be clear and the warmed coolant still does the leak through the cap as the rise in the radiator coolant level is faster than the overflow pipe can accommodate.

On my Mark V the cold coolant level which works well for Southern California is about 2.5 cm below the overflow pipe inlet end which also is at the bottom of the cap threads. When I first bought this car, a radiator filled higher than that gave me some spitting out the cap concerns on the first few drives until I figured out how to let it finds its own best cold level.

It just struck me that we don’t know how original your ccooling system is!
If you don’t have an original bellows thermostat but a modern one instead then you need a significant modification to the thermostat housing to ensure that the bypass hose is shut-off when the thermostat opens.

If you are using a modern thermostat without a modified housing then you would need to block-off the by-pass hose. This is not a good thing to do but if the by-pass remains open when the engine has warmed up then a large part of your coolant will be flowing through the by-pass instead of the radiator and your car will overheat.

Ed Nantes created a method for modifying the housing for use with modern thermostats.


Since late side-valve cars, Jaguar have used a thermostat that is a 2 way switch. When cold the water runs through the bypass hose and as the temperature rises it is diverted through the radiator core.

When the originals got old and gave up the ghost, a variety of fixes were tried.

Removing the internals altogether… not a good thing as the water will prefer to take the easy route through the bypass rather than fight through the radiator core.

Blocking the bypass and putting in an on/off thermostat… better , but takes longer to warm up which increases wear.

Making a new housing to incorporate an XJ6 thermostat… good but not cheap.

This conversion is suitable for 6 cylinder Mk4 /5 thermostats which have the bypass entering the housing in the direction of the centerline. i.e. not SS and 1 ½ litre cars where it enters at a tangent.


The original mechanism blocked the bypass by blocking the feed which faces the rear of the car.

In the conversion, the bypass is turned 180 degrees so that the XJ6 thermostat can push from the front and close it that way.

The first step is to remove the old brass mechanism… unsolder the nut and remove the screw at the front to take out the closing plate and the rest is pretty much force. Cut the 4 tabs which hold it in at the back but don’t damage the housing.

It’s good to clean the housing by sandblasting as you need it clean to braze it.

Mount it in a vice with the bypass vertical, and use a 1 1/8” hole saw [ a good one not the hardware shop cheapies ] in a pillar drill. Remove the pilot drill BEFORE starting.

The saw will cut around the bypass, being guided by it, to separate it from the housing.

The saw will bottom out before it cuts completely through but a light tap with a hammer on the end of the bypass will sheer the last 1/8” tags of metal remaining.

The bypass is now free and can be rotated 180 degrees.

It has a 3/8” hole through the centre and a with a piece of allthread and washers and nuts it can be held in place with the large opening squarely facing forward while it is brazed. It will sit slightly forward up against the edge of the hole but not to worry, the bypass hose has enough give to compensate.

Start brazing at the front edge and finish at the back.

Tap the 3/8” hole remaining in the centre to 7/16” [ UNF ] and shorten a set screw so the it is just long enough to block the hole when held in with a spring washer.

The XJ 6 thermostat mounts in the front opening. To get the correct spacing between the flange and the bypass opening, I insert a ¼” [6mm] spacer. The local laser cutters make them for me as a ‘washer program’, but if you haven’t access to one they can be cut from brass or steel with a hole saw, turned off the end of solid bar or if your local steel supplier has 2 ½” thick-wall tube he might cut a slice off for you.

Braze , silver solder or soft solder the spacer in place and mount the thermostat with 2 1/8” [preferably] stainless steel screws tapped in to the spacer.

Here’s what it looks like finished with one of the spacers shown for illustration.
Total cost , here thermostat about AU$20, spacer lasered AU$10 , screws and a bit of gas and time.

Oh, and while not mandatory, drilling a 1/8” bleed hole in the thermostat will make it easier to fill the radiator.

Could your overflow pipe be too high?
The overflow pipe end should be at the top of the top tank but below the threaded socket.

So there is about 3/4" or 20mm from the pipe to the top of the threads.

My '38 SS came to me with no thermostat at all, and the bypass was not blocked. I wonder how long it had been running around like that. It had an aftermarket heater, and a lot of extra jute insulation in the cabin, suggesting it had been run in cold winters, all of which I have removed.

I blocked the bypass with a through bolt and nut and a big flat washer so I could run it around a bit.

I wonder, what would be the effect of running with the bypass blocked and no thermostat, i.e. all the water circulating all the time. Not driving it in winter, I’m not really all that concerned with getting a fast warmup, and it warms up pretty fast anyway in summer to 70-80C.

Hi Rob,

I don’t think there would be any problem from blocking the bypass with no thermostat other than the extended warm-up time. My comment about not good to block the bypass was just because the thermostat would open late because of the lack of circulation and could cause overheating.


I have checked and the internal overflow pipe under the radiator cap is clear with no blockage, and is about 1/2" below the neck of the rim (so that seems correct).

My 1.5litre car is using the original thermostat, which I took off, cleaned, and tested in a heated water pan off the car. It worked OK, so was refitted - be it some 12 months ago and the car not run since then any distance to get the engine hot (until yesterday when I had the problem).

It looks like I’ve lost about 2 pints of water from the system, from the geezer of water spurting out of the mascot cap. Clearly the new rubber seal on the cap does not appear to be tight against the threads, so I suspect that is the leakage point (so much for modern replacement parts, particularly the rubbers).

I’ve topped it back up the water, but leaving it about 1" below the neck line (before my helper filled it to the top of the inner overflow pipe) - there are no visable leaks, but not yet started up.

Before I strip off the thermostat housing or restart the car does this possible cause sound feasible: engine warming up with increasing temperature, thermostat sticking because its not operated for some time, engine water reaches near boiling point (I think my dashboard guage reached almost 90 before the leak out), thermostat suddenly releases to divert boiling water to the radiator, overflow pipe not large enough to cope with surge of boiling water, radiator already filled to the maximum, rubber seal on cap not fully made, geezer of water comes out!!!

Under normal operation if you run the engine until it is fully warmed up then any excess coolant should be expelled via the overflow pipe and once cooled the level in the header tank will be the correct cold level.

I agree that you shouldn’t disturb the thermostat given that it worked when you last tested it. You can tell when the thermostat opens just by feeling the temperature of the top hose and header tank. So I’d expect them to be getting warm when the temperature gauge was up to about 70C / 160F. You can also warm the engine up with the radiator cap off and sense the water temperature directly. The bypass hose should be at the same temperature as the water manifold during warm-up.

Are you sure that water flows out of the overflow pipe onto the road?


Water exits nicely down the overflow pipe when I’ve tried some water via a small funnel in the top end. The water pump seemed OK when the engine was being worked on, and I can’t see this producing a sudden surge of very hot water to erupt as I experienced (even if it was stuck and then suddenly started rotating). Certainly the top of the radiator and top pipe was very hot when I stopped and whipped open the bonnet, but then again hot water had been running over them. My assumption is that to get a burst of hot water exiting the filler cap (and the rubber seal on it I’m not happy with) then the thermostat must be either working intermittently or perhaps at an over temperature setting - it must be open or opening at some point as the radiator expansion tank was hot (be it after I’d stopped the car).

I’m still wondering if a sticking or too high temperature setting thermostat (if it’s reached the end of its operational function) is the issue and most logical problem - suddenly releasing what could be potentially boiling water from the engine block as a surge into a full expansion tank.

When you get that all sorted out, you should drain and flush your system then refill with EvapoRust. On advice from a private collection care taker, I filled my system with it and ran it for a month. It dropped the cars temp by 15 degrees. I then drained it down, flushed and refilled with antifreeze. Best thing I ever did! The inside of the radiator looked brand new.
I then put it into my 1930 Cadillac for the rest of the summer and it cleaned that system also. I still use it to soak rusty parts in though it’s loosing its potency. Great thing is, there’s no acid to eat aluminum or seals. Has no effect on anything other then steel. And since no acid, it can be safely poured down the drain without hurting the environment.
I have no affiliation with the company…I just really like the product.
Bought it at Home Depot. $85 for a 5 gal. pail.

I think it’s best to save the original thermostat if at all possible. The temperature is adjustable by screwing or unscrewing the valve disc on the central shaft. If memory serves me correctly you need to unsolder the little cap (arrowed in the photo) in order to make the adjustment.

It is possible that corrosion between the shaft and the housing is causing it to stick. You do need to remove the valve disc before attempting to remove the bellows / shaft from the housing. The bellows are slotted into the housing bayonet fashion so you need to grip the housing and by applying torque to the attachment ears, rotate it to get it free from the housing then push the shaft to remove the assembly.


I use a rubber o-ring industrial size 339, 3-1/4" ID x 3/16" diameter, medium durometer high resiliency Viton which is resistant to ethylene glycol.
no. 1288N322 at $9.09 for a 2 pack at McMaster-Carr.

David, a next step to try is letting the engine warm up from cold while completely at rest and with the radiator cap off.

Top up the radiator water to just at the overflow pipe level with the car parked where excess water loss on the ground is not a concern. As the engine warms, in normal operation at first there will be no expansion of fluid up towards the cap threads and overflow pipe. As the warm water begins to circulate in the radiator a rise of fluid level will be observed and water may drain via the overflow pipe during this expansion phase. You may also experience some water flowing over the cap threads and down the car. If the water level rises enough to feed the overflow pipe, you also may observe whether bubbles are seen in the warmed radiator water. If bubbles are a relatively constant presence then there likely is a head gasket leak into the radiator system which will lead to excess heat in the water as well as unwanted gas expansion in the cooling system leading to overflow.

If you observe no bubbles when the engine has warmed to operating temperature and the radiator water indicates the thermostat is open, then you may let the car completely cool down and learn about where the high level for cold coolant fill is for your car.

Thanks everyone for the information and photos. I will fully investigate everything when I have some spare time in my workshop and then start it up for a test run in the garage so I can closely monitor everything, and then if all is well a short test drive around the block.

I intend to keep the original parts if I can, but I may need a partial strip down (again) and check the suggested items more closely.

Hi Wayne,

When you run the liquid through the system do you need to have some type of filter in line, such as a nylon sock, to catch any debris?


I use a stocking foot on my 1930 Cadillac but didn’t use it on the Jag. Most times the scale that comes away from the block and radiator is steel or rust from steel. That eventually dissolves. The Evaporust starts out a yellowish liquid and ends up dark brown from dissolving the rust. I didn’t on my Jag but on my Cadillac, I back flushed with water and air from my compressor through the bottom hose on the radiator just to dislodge any debris.
If you go to the AACA web site forum and follow edinmass he’s just purchased a 1917 White Touring Car which he is using EvapoRust to clean the engine and radiator. He did this 8 months ago on a 1932 Duesenberg that they spent $1.6mil on. He wouldn’t take chances with these cars!

Hi Wayne,

Thank you very much for that information. How often did you drive the car over the month and roughly for how long each time?



I drove it every day as it had become my current favorite. I probably drove it an hour or two stopping from time to time running my errands. I’m thinking that the constant temperature helped out making things swell and shrink with the temp changes.
At first, I couldn’t get up the mountain next to me without overheating. By the time the month was over, I could run up at 50 mph (speed limit is 35 so I didn’t want to drive crazy) and it wasn’t anywhere near the red zone.
It acts just as well as antifreeze but I’d rather not try it in the winter tho the bucket didn’t freeze last winter.

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The shaft in the thermostat was not very smooth - so could have been the problem of a part sticking then sudden release of water flow to the radiator (at higher that normal temperature).

Tested in a pan of water on the cooker - photos when cold, with the bellows begining to open at about 50 degrees C, and then fully open at about 85 degrees.

Assembled back together
5 6

Re-soldered the end plunger cap

Tested with heat of blowtorch to check it opens OK

Then left to cool off and the bellows closed the plunger OK

It seemed to work smoothly on the bench with no water passing through, so just need to get it all refitted before an actual engine start up and check out of slowly warming on tick over before I actually drive it on the road.

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Well done on getting it all apart! I think as long as you have a reasonably smooth shaft with some clearance then all will work well when using an antifreeze with corrosion inhibitor. Perhaps you could aid things by cutting a small chamfer on each end of the shaft hole in the casting.

Looks good.