The 'E' Type Forum

Dear forum. I have just completely stripped , rebuilt and installed my 71 S3 engine. It runs sweetly, but , even though the water temp shows normal and the fan switch comes on, the cooling water continues to boil and evacuate through the overflow on the header tank. This has been repeated every time I drive, cool down, refil and start again.

I have taken the thermostats out, but no difference, I have bypassed the heater matrix, again no change.
With the header cap off, I could see bubbles coming up ……so when it was cold, I did a compression and leak down test and all showed normal. The header tank is new, but an older cap on it…. Aside from the cap not sealing properly, what are the other causes of the water boiling and evacuating after the engine is shut down (and presumably when running as well) ? What else could it be or I should check ?
I appreciate any help or experience here.

What value cap are you useing....Steve

Overfilling the header tank will do it every time, just like you have observed. Since this is happening after the drive, what you are reporting is heatsoak.

After shutdown, water is no longer circulating. The water in the cylinder heads is stuck there and the residual heat in the engine block and heads is now transferred to the water. The total volume of air and water inside sealed system is constant and the radiator cap pop-off pressure means that this volume and the temperature it was achieved are now being exceeded. Either a hose has blow out to expand or the excess pressure of this superheated water has to be relieved by the radiator cap and you keep seeing the latter because there is too much water in the system for that temperature and pressure.

If you want a comprehensive explanation of how to deal with heatsoak, take a look at
https://forums.jag-lovers.com/t/solving ... ple/377151
The car being datalogged here is a v12 etype.

kind regards
Marek

Doing some cooling related work on my XJS recently, I filled the cooling system to the brim and connected a transparent soft drink bottle to the overflow pipe.

The idea was to find out by how much the coolant expands due to warming up from about 20°C to 86°C.

Answer: about 400cc (0.4L)

The E-type also takes about 20L, so should also push out approximately 0.4L of coolant, if the header tank is filled to the brim.

What I can't explain is why the header tank level of my car will settle at about 1/3 of max which is quite a bit more than 0.4L.

PAZ wrote: ↑

Mon Feb 17, 2025 11:49 am

I have taken the thermostats out,

Not a good thing to do!

This way the bypass circuit is not closed off! This means that a significant portion of coolant never passes through the radiator.

Also, do you have the correct format (not temp rating) thermostats fitted?

The etype runs 82'c thermostats and the xjs runs 88'c thermostats. They also probably have different pressure caps on the radiator and will heatsoak a bit differently, as the xjs is slightly more enclosed and tightly packed.

f you can, rerun the experiment with some of those variables swapped around and see if the same amount of coolant is lost.

kind regards
Marek

jagwit wrote: ↑

Thu Mar 13, 2025 12:07 pm

Not a good thing to do! This way the bypass circuit is not closed off! This means that a significant portion of coolant never passes through the radiator.

Phil, have you tried this? For me, having no thermostats made my car run cold. Fitting thermostat's fixed this, made it warm up.

PAZ, did you use the correct head gaskets? What was the part number? If you fitted later (HE) head gaskets by mistake, the coolant holes are different, may be blocked by the head gasket?