Hi, I'm experiencing new overheating troubles with my XJ6 SIII 4.2 of 1979.

I noticed something strange during a run early in September, when I saw that when in a queue in downtown Milan I had 110°C or so on the thermo gauge. Outside temperature was rather high, so I didn't pay much attention - my Jag had always suffered from city queues.

I was away then so it wasn't used for a couple of weeks except for the usual short standstill startups for keeping the battery ok.

Last week and today I noticed that after about 3 to 5 mins after startup the engine goes to 110°C, even moving slowly in my neighborhood, and outside temp is not high. It had always slowly come to about 80°C under similar conditions. Also, while going at about 110 km/h on the highway temperature reached a more worrying 115°C or so. Between the runs I checked the cooling fluid tank (I use purified water), it was not totally filled but also far from empty. There are no evident leaks to the outside (not puddles/drops of water under the car). I checked the level of engine oil and it is ok.

I also noticed that listening from the outside with the engine running there is a low freq noise coming from the mid-back of the car, probably connected with the exhaust pipes. So I'm guessing whether there might be a connection between exhaust pipe occlusion/damage and oveheating. I guess that failing to eject exhaust gases the engine might burn in overpressure, causing the same effect of a poorly balanced mixture, in turn causing overheating.

Is that reasonable? Should I try checking/changing the exhaust pipes? Or this cannot be the reason?

Thanks for the help!

 

hi,
when was the last time you replaced the Thermostat ?

 

I agree that the thermostat is a likely culprit.

A relationship to the exhaust noise seems less likely.

If a new thermostat doesn't solve the issue I'd consider having the radiator professionally cleaned and replacing the fan clutch...as it may well be slipping.

Cheers
DD

 

Thanks for the clue, I hadn't thought it may be just a problem with the gauge. The engine has been removed and totally overhauled in 2003. The computer and some of the sensors were changed in 2009. I can't remember whether they also changed the thermostat. Of course I will try checking it's working correctly.

I also inspected the exhaust pipes and noticed there are many points of rust. It's strange they got consumed so fast, but for sure they need a replacement.

Thanks for the help, gentlemen.

 

de nada.

if replacing the thermostat, make sure you get the correct one for your region, one thermostat is for northern climates, one for southern climates, although some say it doesn't matter, you might confirm with your parts supplier. Also you need a new thermostat housing gasket.

the studs at the thermostat housing are known to break in two when loosening the nuts, so it is important to spray a rust-busting lubricant on the nuts for a few days before attempting to loosen the nuts.

 

If the cooling system hasn't been touched since 2003, then i'd start with inspecting every part of it, the thermostat, the water pump, radiator, hoses.

Could be a blockage, could be an air lock that needs bleeding, water pump impeller could be corroded/broken, faulty thermostat stuck closed, blocked radiator, viscous fan hub old age, gauge sensor faulty.

 

As I remember it, you also have to install the correct type of thermostat that has an extension on it that gradually closes off the bypass hose as it opens, thus making all the coolant pass through the radiator. Any thermostat without this may open, but much hot coolant will still pass through the by-pass thus not cooling the car properly.

 

I have that kind of thermostat but in my '65 S type, I don't recall it in the '84 XJ-6, or maybe it does have it and I wasn't paying attention last time I replaced it which was years ago, fortunately it is still going.

 

Thank you everybody for the many replies. I have contacted my mechanic for a check on the system, he will wash the cooling circuit and change the rusty and rotting exhaust pipes. Based on the suggestions I asked him to evaluate the correctness of the thermo sensor reading. All this will be next week.

During the w-e I will try a short run to check whether the water in the radiator is really boiling or not. This should tell me if there is only a problem with the gauge or not.

I will keep you updated.

In the meantime thanks again.

 

A couple of ideas:


1. If the T'stat sticks closed, an overheat condition comes quick. and obvious. Lots of gurgling in the system


2. Purified water is good. But, not enough. Corrosion inhibitors are necessary. Antifreeze has them, but they degrade. If freezing is not a risk in your clime and you choose not to add antifreeze, add an inhibitor. I think they are still available.
They also provide a degree of water pump lubricant. Increases seal life.


3. Get an infra red temp reader. Cheap at Hf. Very useful to read real tempss at various locations.


4. If your exhaust pipes are rusty and leaking, excess back pressure is not an issue
Changing them will tell. As I recall, dimly, there are two plugs at the manifold outlet. Why, I forgot, if I ever knew. Removing them will result in bp relief and a lot of noise!!!!


5. Yes, I suppose the pipes might be occlude from internal rusting...


6. A small hole drilled in the low side of the mufflers will allow condensation to drain and extend their life. Some come that way...


Do not drive the car hot. Disaster a virtual certainty.... and the gauges may not tell you so til it is too late. don't ask!!!


Carl

 

Here is an update about this thread.

My mechanic went through a careful inspection of the cooling circuit and concluded that it's a probrem with the head gasket. He suspected this from the beginning as he noticed that the engine was slightly but noticeably underpowered. He found that there are leaks from the cylinders through the head gasket to the cooling circuit, lowering the pressure in the cylinders and causing a reduced power performance, and raising the temperature and pressure of the cooling circuit. The low-freq noise I could hear was due to a slight a alteration to the idle rpm also due to the loss of pressure in the cylinders, causing a form of resonance with the exhaust pipes and silencers.

Clearly this brought the car to a complete halt. I gave my mechanic a green light for changing the gasket after I noticed from the mechanic's note from the engine overhaul (that was another shop) in 2003 that this part had not been changed, somewhat surprisingly. So after 36 years it is not so strange that this part has come to the end of its life...

The odd thing is that it will take some time for the change to be completed, as the spare parts need to be ordered from abroad (so things go for old Jags in Italy...).

Thank you everybody for the interest! I'll keep you updated.

 

You got a good run from the engine then if it is indeed original!

Not sure what the previous mechanic classes as an engine overhaul, if they didn't even change the headgasket.

Now would be the time to go over the whole engine bay, and fix/repair anything else that is looking old and suspect, wiring plugs that have become brittle, gauge sensors, vacuum and cooling system hoses, radiator flush.

Any obvious issues will highlight themselves anyway when the mechanic is removing the head and accessories.

In regards to the cylinder head, it is worth doing a complete rebuild, initially check it for any warpage and cracks, (also check the block around the head bolts for cracks) then a full strip and rebuild of the head, plenty you could do to restore lost performance, fresh valve guides, reground valve faces (even 3 angle grind) cylinder ports cleaned up of dags and match ported to the inlet and exhaust manifolds, and even ported further if you desire.

You can very easily spiral out of control if you listen to me

 

Hello, yes I was a bit surprised too about the fact that they hadn't changed the headgasket, but at that time the main focus of the work was on the valves (totally changed, not really cheap btw!), piston rings and spark plug/ignition system. This was due to age and also followed the banning of the correct fuel for this engine in Italy, which required some small alterations to make it work acceptably also with the current "green" (?) fuel.

A few years ago I had a problem with the cooling circuit (broken pipe, it was my fault), resulting in a revision of the whole piping of that circuit as well as the radiator.

Unfortunately, I don't own a mechanic shop myself, so I can't always do all that I want, and I need to plan in advance to avoid halting the car for a long, long time waiting for spare parts. This time the fault just popped up and was unexpected, so I could not plan a major operation on the engine in advance, but in case they find an important fault when inspecting it, I will surely ask them to refurbish the corresponding system.

Thank you for your help!

 

To change the valves the head needs to be removed, so the headgasket would have been changed.

 

Methinks a mechanic has been a bit untruthful??? and, that is as mild as I can put it!!!!


Hopefully, the present mechanic is a different one!!!!!


Aye, aye, pain, or is it anger....


Carl

 

What exactly are you reading that brings you to the conclusion that the head gasket was not changed?

Cheers
DD