Temp gauge not working

Hi Gary,

I think that you’re absolutely correct. I’ve bought a car that has been altered from the original and the deeper I look into it the more that I find. It’s a bit like an archeologist trying to find out what happened somewhere years ago by digging deeper in the ground. For example the heater control valve has been moved from the original position - but why?

Thanks,

Regards,

Bob

Hi Bob, You will get there it all takes time to sort out I am in North Waikato so if you need any advice where hear to help.
Once you have your workshop manual you will be fine.
Cheers,
Gary

Robert,

I am not familiar with the 6, as I have a V12, but the principle is the same.
As mentioned before, the cap at the thermostat housing is a total seal cap and the hose leading to the header tank should be under the cap seals.
If you have an expansion tank under the fender, the header tank should be full of coolant.
But then, it should also have a hose, coming out between the two seals of the pressure cap, leading to the expansion tank. Excess coolant will go to the expansion tank and will return back when it cools down.

If you don’t have an recovery tank under the fender, then your nice and shiny tank becomes the recovery tank, and should be half full of coolant.
The tank in your car it’s missing the hose connection to a recovery tank. This would probably mean that your pressure cap is not a recovery type, so excess coolant just flows overboard.
It would be simple enough to add a spigot that will connect to it, or get the original tank.

The second hose I believe is there to remove air from the radiator.

You must have a pressure relief somewhere or things will blow up…
Maybe this explains your coolant loss, something leaks under high pressure.

Thanks Gary

Regards,

Bob

Hi Aristides,

The cap at the thermostat housing was not a pressure cap and so the coolant was able to go to the overflow tank (the nice shiny one!) at any time. I have purchased a pressure cap today for the thermostat housing and I will try it out when the auto electrician has fixed the wiring but I am fairly confident that I have the solution to my problem. If you look on the official Jaguar parts list, it shows a pressure cap there (I think 15 PSI).

I just want to be sure that I know what temperature my engine is running at before I drive any further, so I will test it perhaps next week hopefully.

Thanks once again,

Regards,

Bob

That’s exactly what it’s supposed to do.

The pressure cap should be at the header/expansion tank, not the thermostat housing.

All correct and how my Series 1 operates. Paul.

I can see what you mean in regards to the standard set up but the one that I have has been modified with a different expansion/overflow tank. If the pressure cap is fitted to this tank the coolant cannot flow in or out of it because it is completely sealed. When I have driven the car previously there has been no coolant flowing in, today when I loosely put the cap on the coolant went in the tank when the engine was hot and was sucked out again when it cooled.

I have to admit to being very confused as to what to do. But as I say I want the gauge correct before I do anything else and then I can experiment

Regards,

Bob

Of course if the pressure cap on the expansion tank has the wrong setting (PSI too high) then it might explain my issue.

Regards,

Bob

**
The square tank showed is a custom/aftermarket replacement, Robert - hopefully functioning like the original.

here were basically three versions of the cooling system lay-out - your 74 has the header tank, top front of the engine. The cap on header tank is sealed and not used after the initial fill of drained system. In addition; you have the expansion tank as shown.

As coolant heats up, it expands - into the header- and expansion tank. Any air in the engine is funneled into the expansion tank by hose from the header tank to the expansion tank - originally at its bottom.

At initial fill; remove the header and expansion tank caps - and set cabin heat to ‘hot’ position. Pour coolant into the header tank until full, fit cap. Check that the expansion tank is also full - then fit that cap. Idle the engine for some minutes - stop engine and recheck levels. Refill expansion tank(!) if required.

As engine is run up to temp, the expansion tank cap opens as pressure rises. As Gary et al says; the expansion tank cap opens at 13 - 15 psi. It vents out excessive coolant through a vent - usually in the expansion tank filler neck. Either directly to open air or through an attached hose. This venting is very important - and must(!) function…

As the engine cools down after the full heat cycle, the coolant contracts. Checked cold, the level in the expansion tank is now at the ‘normal’ level - but it may require a couple of heat cycles to stabilize. Always check coolant level at the expansion tank with the engine cold - against the ‘normal’ level. Do not add coolant beyond this level - it will simply be expelled; giving the impression idea of a leak…:slight_smile:

As an aside; the S3 system is different; there is no header tank and cap - just the thermostat housing. Two vent hoses, from radiator and thermostat housing goes to the expansion tank. Venting air that may have collected at high points - preventing air lock in coolant circulation…

In both cases; the expansion tank, like engine, is pressurized to cap pressure - raising the boiling point of the coolant to some 120C. In both systems a hose can be connected from the expansion tank vent to an atmospheric recovery tank - catching ejected coolant instead of loosing it to ground.

Note; checking coolant level in the expansion tank repeatedly with the engine cold - without adding coolant - is the first step in verifying that you have a leak…

Frank
xj6 85 Sov Europe (UK/NZ)
**

Don’t do that, Robert; it is plain wrong - the venting must be at the expansion cap…

If not; expelled coolant will spray all over the engine in the fan slipstream…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)
**

I have attached 2 photos of the different caps. The first one is a blank cap as you will see which was fitted. The second photo is, as it says a ‘recovery ‘ cap and when the pressure builds it is designed to allow the coolant to enter the pipe(s) between the 2 rubber seals in the neck of the casting. As I understand it it will not allow the coolant or steam to go to atmosphere.

image2.jpeg

Regards,

Bob

Hi,

This is the YouTube video that I watched this morning which seemed to make sense to me when I was trying to understand how these systems work.

https://youtu.be/OPKaFcOwemI

If the Jaguar XJ is different then I must bow to your knowledge but I would welcome your thoughts on what this guy is saying and why the XJ is different

Regards,

Bob

Robert,

XJ coolant systems are no black science and the simple and straightforward original documentation is all it takes to get it in good working order.

The SII 4.2 litre engines have a header tank (filler cap not to be used) located in front of the engine and an expansion tank located at the left front wing (filler cap to be used for changes and refills, but only to a certain level). In original shape the expansion tank looks like this:

They are available in reproduction, not cheap, and still will need some enhancement in terms of internal and external coating to keep them functional for some decades or two. There is a reason why Jaguar switched to plastic containers for SIII.

If your stainless replacement part works, it is good. If not, I’d revert to original parts.

Good luck

Jochen

75 XJ6L 4.2 auto (UK spec)

Thank you Jochen.

I am going to absolutely ensure that my temperature gauge is correct before I drive the car again and I require professional help for this as I have neither the tools or knowledge for this job. I have ordered an infrared thermometer in the meantime.

When I know that all is well I will carry out some experiments with various options. I think that when the replacement expansion tank was fitted it altered the entire system and what worked originally doesn’t anymore, but we will see what happens.

Regards,

Bob

Robert,

a tank is just a bowl or jug and won’t alter much to the system, except for its pressure. If it is tight, the cap will determine the pressure. The crucial variable elements for the cooling system are the t-stat and the pressure. Anything else should just be tight and not clogged. Start with the sender (cheap and easy) and the t-stat (cheap, easy, but tedious). Verified info on real temps (IR thermometer) will help to determine fault and success in rectifying.

Good luck

Jochen

75 XJ6L 4.2 auto (UK spec)

Jochen, just further to my earlier comments.

I have owned the car for a little over 6 weeks now and I have done quite a large number of trips (max 100 Km’s). Because I couldn’t understand the cooling system I always put my hand on the expansion tank at the end of my journey. Not once was it remotely warm, always cold. This indicated to me that the overflow coolant wasn’t venting into this tank. All through this time there has been coolant loss but where from I’ve no idea as there was never any smell or other indication of a leak but it was going somewhere. All hoses are in good condition by the way.

Yesterday for the first time I unscrewed the cap on the expansion tank and basically taped it into place meaning of course that it was open to the atmosphere. I then drove maybe 20kms and checked it and sure enough it was very warm which indicated that it was doing its job.

I think that as the system was changed by a previous owner I have to find a way to make sure what I have work.

Regards,

Bob

But that sounds good. The purpose of the expansion tank is just to take care of the expansion of the coolant due to the heat. There is no coolant flow through the expansion tank. Feeling the temperature at the expansion tank is fruitless, as you say.

With a cold engine the expansion tank should be about one third to half full to make sure that the coolant circle of the engine will always remain full. If the expansion tank is too full it will throw out excessive liquid. That is all it is supposed to do.

All critical issues relate to the engine block and most importantly the head. The inlet manifold is used as a coolant pipe. The water temp sensor is sitting front right under the t-stat housing. You should check temps there to make sure what’s going on. For lack of a thermometer you might check with your hand at the rubber tube to the header tank in front of the engine.

Best

Jochen

75 XJ6L 4.2 auto (UK spec)

**
You have a point, Bob - POs do the strangest things…:slight_smile:

However, to function safely; there must be venting. However, if the system is not full, at or below, the ‘normal’ hot level, there is no venting - and a seal cap will work. Unless higher engine temps cause further coolant expansion that wants ‘out’ - in which case; with ‘sealed caps’ something will blow!

To clarify ‘pressure cap’ function; a disc with rubber seals are fitted to the cap by a spring. The disc is pressed against a ‘step’ inside the neck - blocking passage until the pressure forces the disc up. Between the disc/‘step’ and the cap lid - there is an opening for the coolant to vent out of the cooling system. This is the principle, and essential - but there are some variants in execution…and yours may be one of them…:slight_smile:

However; there are two different lay-outs for expansion tanks; pressurized, as used on the xk engine - and unpressurized. In the latter case; the pressure cap is fitted before, not on the expansion tank. In this case; unpressurized overflow is delivered to the unpressurized expansion tank…

In theory; a pressure cap may be fitted to the header tank on the xk - provided(!!) the neck has the necessary ‘step’ (which i doubt). In which case; unpressurized coolant will flow into the expansion tank…

As an aside; pressure is irrelevant for the cooling effect - the sole purpose of the pressure is to raise the boiling point of the coolant. Ie, an unpressurized system will cool equally well - but will boil at 100C, instead of 120. Which is the red sector of the temp gauge…

Sort out your temp, using the infrared thermometer - also verifying your gauge. Sorting out your cooling system lay-out is an interesting sideline…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)
**

Hi Frank,

Well I’ve had the wiring fixed, the gauge works fine (when you connect it to ground the gauge goes hard over to the right). Just waiting now on a new temperature sender which should be here Monday.

What I’ve decided to do is this. Fit new sender, fill thermostat housing to just below lip of filler. Return system to Jaguar spec (why wouldn’t I?) ie blank cap on thermostat housing, pressure cap on expansion tank with maybe an inch or so of coolant in expansion tank. Make a dipstick for expansion tank so that I can check level accurately.

Run car for a week or so, checking levels in expansion tank and also thermostat housing after every run (when engine is cold of course) and then see how it goes like that. The critical thing is to ensure that when cold there is sufficient coolant in thermostat housing (at least during this ‘testing phase’).

I am now thinking that my mistake in the past is having too much coolant in the expansion tank and when the engine gets hot and coolant expands there is not enough room in the expansion tank to take the overflow. That’s my theory for now.

Regards,

Bob