Hi Everyone.
I have a 1967 MK2, to which is fitted a 4.2 XJ6 engine, together with inlet manifold and HD8 Carbs from a 1971 XJ. Engine runs great but gets VERY hot (not up to the top of the gauge, but just within the upper
section of 'Normal'. After a short run (engine warm) it dumps coolant through the radiator overflow (The engine has been completely rebuilt, new radiator and water pump by the way). I know MK2's were never easy
to keep cool, due no doubt in part to the small grille and engine bay. At the moment I'm using the standard metal pump-driven fan - I did look into fitting an electric 'pusher' but haven't got round to it yet. I'm not sure
what thermostat is fitted (It's been a while since I put the engine together) but suspect it's either an XJ6 82 degree or the 88 degree one.
Looking at the MK2 parts list I see that a 74 degree thermostat was fitted to later cars. My train of thought is to fit one of these (or an 82 degree XJ one if not already fitted) but I'm unsure whether the MK2 one will fit inside the XJ6 (series 1) housing. Does anyone have any thoughts on this, either from personal experience or from a professional perspective. Any help would be appreciated.

 

I have a series 2 4.2 litre XJ6. It was dumping water at a low rate!! It had suffered from cracking between the block cylinder liners. The cracks are very very fine. I suggest you get your coolant water tested for combustion products to either eliminate, or confirm exhaust gas contamination. I hope it does not have these cracks!!

 

Thanks 'littlelic69' . I hope I haven't got such cracks - I know the XJ blocks were prone to this. Compressions are all good and there's no sign of oil in the water (or vice versa). The engine has only just been put into use
following a full rebuild. It was thoroughly examined and all was good, so I don't think this is the problem (fingers crossed !). Dumping of coolant (particularly in warm weather) seems to be a common thread - some owners
have fitted expansion tanks to the inner wings to catch same. If I can't resolve the issue I'll probably end up doing the same. Since my original post I ascertained that I have the 82 degree thermostat fitted (the other XJ
option was an 88 degree one) so I'm hoping that one from a MK2 or 420 (both fully open around 75 degree) will fit. Thanks again.

 

CaptainQ
The thermostat does little to effect the operating temperature, that is a common misconception. The maximum temperature an engine reaches is dependant on the load, and the ability of the cooling system to remove this heat.

The thermostat is there to help the engine to get up to operating temperature quicker as it stops the flow around the radiator until the thermostat opens, if your car is running over 82 degrees then reducing the thermostat opening temp will have no effect on the running temperature. Reducing the thermostat temp will only lower the temperature if the existing setup is operating below 82 deg as the existing stat will be closing and stopping further cooling.

The manufacturers specify the thermostat for the operating temperature of the engine and the stat controls that minimum temp and not the maximum.

Check the actual temperature with a laser thermometer, these are cheap and you can check the temp either side of the stat and at the top and bottom of the radiator.

A fan shroud is important for efficient cooling with the manual fan, do you have one fitted ? Reducing the amount of antifreeze in the system increases cooling efficiency, but raises the freezing point, you can also get additives that help with cooling efficiency, but check they are suitable with the coolant you are using.

 

Mark 2 cooling was always a bit marginal, and will be more so with modern fuels that generate higher temperatures. The radiator may be partly silted up, thus preventing efficient heat transfer. People say, "But I flushed it out and put a hose on it and the water flowed perfectly" Yes, of course it did, because the flow is less important than the heat transfer area. Coolant can rush through a rad without giving up heat if loads of tubes are blocked or there is a build-up of limescale and crud. The plain fact is that rads rarely block up, they just stop transferring the heat to air. Adding an electric fan will be beneficial but only when the rad efficiancy is dealt wit.

 

Hi Jon,

Thank you for your observations and advice. Your explanation regarding the thermostat make sense now - in which case there's no point in going down that route. I do have a hand-held laser thermometer, so will check
tomorrow after a run - it may be that my car isn't 'overheating' at all - it just seems much hotter in the engine bay compared to all the modern cars I drive (there again, the MK2 engine bay is quite a tight fit and the grille area
is quite small). In answer to your question - no, there isn't a shroud fitted at the moment, which certainly doesn't help (I'll have to have a dig around the garage and see if I can find the original). I was going to drain the cooling system soon anyway (I'm not sure of the current mix) so will try perhaps a 60:40 mix (Water/Antifreeze) and see if that helps. Thanks again Jon.

 

Thanks Fraser for your response. The radiator was recored at the same time as the engine was rebuilt, so I don't think that is the problem. As you rightly say, the MK2 cooling system was marginal, hence the number
of owners who have fitted expansion tanks (or more correctly water collecting tanks). I have a number of things to try now (see Tilleyjon's reply above) so that should keep me busy over the Bank Holiday (weather forecast
looks rubbish anyway so I've got an excuse - lol !!). Thanks again.

 

The engine bays get really hot on the Mk2, there is no airflow through the actual engine bay except for the hot air from the radiator and fan.

Primaz added an air scoop under the front valence and outlets in the wheel arches which I am also going to incorporate in my resto, (which hopefully will be up and running shortly,) take a look through his S-type restomod thread, or PM me and I can send you the pics of what he did and what I have done so far.

Here's some interesting reading for an evening if you are bored !

 

Good Morning Jon,

I note that you are restoring a 340 model. One thing I've noticed (my car started life as a 240) is that the by-pass hose on the 240 & 340's goes straight to the filler neck of the radiator, whereas I believe the earlier
cars had it going to the pump to improve warm-up and heater use. Do you know why they changed - as it seems now to serve no purpose. Am I missing something here ?? I presume this change means that there's
no longer a need for a special thermostat that opens/closes the outlet for said hose. I wondered whether they just continued to use their stock of thermostat housings (the Series 1 XJ used the same one, and I believe
they used a standard thermostat, as opposed to a 'sleeved' one to block off the outlet). Kind regards Keith

 

 

Hi Rob. Not sure to be honest. I'm taking it for a good run today so will know then. I'll report back. I'm hoping it's just expansion.

 

since the engine is carburetted, not fuel injection, try removing the thermostat. Depending on the region where you live, you might not need it at all. In the Caribbean where temperatures are hot people remove thermostats from their housings, not needed. Engines run cool without them.

note that the thermostat housing studs in the 4.2 engine are easy to crack, so spray a rust busting lubricant for a few days BEFORE attempting to loosen those nuts.

 

The bypass hose is a bleed to remove air from the housing. It works on the basis that the air rises along the tube and vents to the filler neck, there is a valve inside the stat housing behind/in the tube. not sure what happens if the system over pressurises I will look at that some more, as I can't get my head around that completely at the moment !

Attached supplementary info re 240/340

 

Thanks for your post 'Jose'. I'm not sure whether removing the thermostat is a good idea - from what I've read, it can disrupt the flow resulting in hot spots, particularly at the reamost part of the block (not good as that
can result in a warped head). Thanks anyway.

Jon - thank you for the pdf link - as you say, it's an air bleed NOT water bypass. Luckily I had an inlet manifold in my garage from a Series 1 XJ6 (which I believe is the same as fitted to 240/340's). Upon taking it
apart (see photos attached) things become much clearer. Although different in the way it looks, these thermostat housings do indeed have a bypass to enable quicker heater warm-up. The thermostat is double-acting in
operation - When cold, the disc on the end leaves the bypass hole open (visble in centre of housing - picture 1) and, as the main thermostat opens, it also extends the small disc to block off the bypass hole, ensuring that
ALL coolant goes to radiator. Much easier to understand when you can actually see it. Kind regards.

 

Forgot to mention the actual bypass, that would have been more informative of me ! I used the same reference from your post.

You can see the bleed valve on your first picture, I never delved into how that actually works, maybe you can have a look as you have one to hand and see what the mechanics of it are, the valves are no longer available, but it would be good to know what they consist of (not a lot by the look !), It does not make complete sense to connect it to the radiator filler neck, I can see how that would vent the air, but not how fluid would escape if and when the rad cap opens.

 

Hello again Jon,

I've now taken the thermostat housing off my car to check the thermostat (which seems to work correctly when immersed in boiling water - although I wasn't able to check the exact temperature at point of opening). On
the housing, although there is a bleed hole, it doesn't have a little valve as seen in the earlier picture (which I think was a Series 1 2.8 XJ, a slightly later version of my manifold but with HS8 carbs). When I removed the
bleed hose at the housing, coolant came out, so is it an air bleed or coolant bleed ?? And would the lack of this little valve cause a problem such as mine ?? Doubtful, I should think ? The small valve shown in the
pictures seems to be a small ball (plastic) with with the prongs sticking out - it appears to be a brass insert (see attached picture). Presumably, when pressure is applied from the ball side the hole is closed - perhaps it's purpose is to return water from radiator neck back to circulation ?? Following your advice I used an infra-red thermometer yesterday after a ten minute drive and the top of radiator showed 103 degrees !! The areas either side of the thermostat were around 93 degrees - much too hot I think.

I'm going to try several options during the next few days - firstly putting a brand new 82 degree thermostat in (just in case the current one is faulty), secondly putting a 74 degree one in (at least that would give the radiator
cooling a head start before the coolant gets too hot, and thirdly, trying without a thermostat at all (I know everyone says 'don't do it' , but it's just an experiment at this stage). I'm also going to change the antifreeze to a
70 percent water and 30 percent antifreeze mix, to see if that helps. A fan shroud is also another option.

I'll post the outcome of my finding in the next few days. Cheers everyone.

 

The bleed is supposed to be for air, the valve I am assuming is designed to let past air but not fluid to do this.
Some assumptions here, but plastic would float, so maybe the valve floats and seals off the outlet, but in the presence of air opens up and releases the air until water appears. However after 50 years I would imagine that corrosion would pretty well have stopped that function working, and without either seeing or being able to get a new valve we are probably all stuck in the dark to some degree.

The temps you are getting are a little on the high side, as I said before the thermostats will make no difference on a longer run, on a short hop it would help a little as the rad starts cooling earlier, but it simple physics, the engine generates heat, the cooling system tries to dissipate this heat, if the cooling system cannot give up enough energy, the engine temperature will rise. The fan shroud will make the biggest difference, so much air is being bashed about rather than being sucked directly through the radiator that the efficiency will be compromised.

IMO you would be better served doing the fan shroud as early as you can. Does the new radiator have a modern core, what type/design is it ?

 

Hi Jon,

I'm off to David Manners today - first thing on list is, as you rightly suggest, a fan shroud. I've already drained the coolant so tomorrow I'll remove radiator and fit shroud (the radiator itself was recored with a more
efficient version a couple of years ago so should be up to the job). I've also managed to find a new replacement 'jiggle pin' so that will also be replaced, together with a new thermostat. Thanks for your input - I'll
post updates tomorrow (fingers crossed). Kind regards.

 

the confusion here is that the car is LUMPED with an engine it never came with.

you need to look for a 4.2 liter thermostat, either from a 420 or XJ, not a "MK2" thermostat because that would be a 3.4 or 3.8 liter thermostat which is a totally different thermostat.

In other words, the body or model of the car is irrelevant in this case because it does not have the original specification engine.

 

Hello Jose,

The car has been fitted with an XJ6 thermostat (82 degree centigrade). However, from engine No 7J 50001, the MK2 (240/340) had a change of theromstat housing and thermostat, which is the same type as the
XJ's (changes were made to the way the bypass operated - it now had an internal orifice as per my picture - which dispensed with the sliding type thermostat, and instead the extended part of the double-acting
thermostat closed off the orifice when the main part of the thermostat opened, ensuring that all the coolant passed onto the radiator). This later MK2 (240/340) version had a 74 degree thermostat as standard, or an 82 degree one as an option for colder climates. The last thing I want to do is to 'cook' my engine, so I intend to do anything (including trying out an 74 degree thermostat) that reduces this possibility. Obviously,
the MK2 engine bay is much smaller than an XJ one, hence much hotter I know that some Jaguar enthusuasts will disapprove of my 4.2 engine, but this is a car for me to enjoy and, at the end of the day, it is still an XK engine. Thank you for your welcome input.