XK8 Temp Gauge went up to red but engine not overheating

[davidt34]

Hi Everyone
I recently had the thermostat and water pump changed on my 97 XK8 to the latest type. I was driving today very slowly and the temperature gauge suddenly climbed up to red and the throttle response felt odd - like it was running out of petrol. I pulled over right away and switched her off. The fans were running so I opened the bonnet expecting to see steam everywhere but the engine was cool to touch on the top and even the hoses around the rad felt cool.
I waited a few minutes and started her up and the engine temp gauge went back to the centre and I got her home (only a mile or so).
If the engine had overheated surely it would have felt and smelled really hot.
The mechanic that did the work is taking a look on Friday and he suspects an airlock could be to blame.
Any thoughts please?.
Thanks
Dave

[Gary220262]

Hi Dave, you could do worse than have a look on the S Type index under cooling System and Mike Kennedy's (J44 EAG) exploits, could well be same problem, thanks, Gary

[davidt34]

Hi Gary
Many thanks for the heads up on that article.
I'll be delighted if that's the problem and I've just ordered the part from British Parts UK for £6.95.
I'll let you know what happens!.
Cheers
Dave

[J44EAG]

For easy reference, here is the link to the saga which unfolded with this spoof overheat condition.

http://www.jec.org.uk/forums/viewtopic.php?f=201&t=6228

It isn`t funny when it occurs. At least if you have read about this, forearmed is forewarned. Best advice if you have a 4 litre V8 in any Jaguar model is to at least hold a spare sensor in the car together with a spare Dowty washer and a 19mm spanner. A road side fix can then be done in a couple of minutes.

My mail bag over the period of time since this happened to J44 at Weston Park 2012 has been significant. Probably a dozen or so members clocking in with me to ask more about the issue which i suspect will become more publicised as these models become older.

Dead easy fix but a real screamer if you don`t know about the problem.

Mike K

[Gary220262]

Hi Dave, No problem mate, its nice to be able to point someome else in the right direction (fingers crossed). Without the help of certain people on this forum ive no doubt that i wouldnt be able to run a Jag with the cost of problem diagnosis at dealers / specialists etc. Gary

[davidt34]

Thanks again both Gary and Mike
I certainly couldn't afford to run my xk8 without help from you guys.
My local mechanic has been brilliant too and charged me only £600 labour to replace the water pump,thermostat,complete timing chains and tensioners and also the throttle body and that included an oil change with oil.
He enjoyed the job and he let me look in as he went along.
I'm sure a main Jag dealer would have charged me a whole load more for all that work.
Because of all the info I've learned from the forum I knew which parts to buy -priceless info.
With regards to the coolant sensor is this easy to fit myself as a novice with cars?.
Best regards
Dave

[J44EAG]

Dead easy to fit, Dave.

Buy the new sensor and a new Dowty seal.

Fit the Dowty seal to the sensor and put to one side.

Unplug the wiring loom plug from the old sensor. Take a 19mm spanner and slacken the old sensor in the manifold. Then screw out the old sensor with your fingers. The thread is not a tight one. As the sensor comes out of the manifold, put a finger over the hole to stop coolant poring out. Dis-guard the old sensor, pick up the new and quickly screw it into the manifold and nip up with the 19mm spanner. Then reconnect the loom plug. Job done. I replaced my sensor loosing only a tea spoon full of coolant. Turn the key ignition on and you should find the gauge, the message centre and the coolant level lamp back to normal operation.

As the sensor is right at the front of the engine, access is very easy. You won`t even need to bend over. The whole job from start to finish should only take two minutes and is perhaps the easiest repair you will ever do to a 4 litre V8.

Mike

[davidt34]

This will sound like a daft question but I've been trying to find out where the coolant level should be in the header tank.
I cant see a level marked on the tank anywhere.
The coolant in mine is about two inches below the top of the expansion tank and level with what looks like a plastic level marker inside the tank itself.
Most searches I've put into google say the coolant should be right up to the top of the expansion tank.
Also to put my mind at rest if the engine had overheated would it have felt really hot and would there havs been steam etc visible?.
Many thanks
Dave

[mike020150]

Hi Dave,
Most Jaguars have a max fill level marker inside the expansion tank, often marked as such, so that is probably what you have been looking at. Check your handbook for the precise details relevant to your specific car or buy the JDHT disk with the handbook information.
An overheated engine would normally show some wisps of steam, dependent upon the cause of the overheating. However, if you caught a sudden rise in temperature soon enough, it's possible for there to be no external signs. Being optimistic, a faulty sender is indeed the most likely cause of the problem. Good luck. Mike.

[J44EAG]

Agree fully with Mike H on this.

The thing about the problem is that when we see a full deflection temperature gauge, a message centre flag up and perhaps also a red low coolant indicator light come on, we tend to panic.

What then happens is we open the bonnet and madly look for signs of overheating. When you find that the radiator, the engine, and various pipes are at normal temperature, you still can`t quite believe that all is ok because the car dash display tells you otherwise. One goes further and carefully releases the expansion cap on the expansion tank. we then expect the coolant to be thrown out as indeed happens when a true engine overheat condition is present.

If the classic signs of an overheat are not present, then we have to take a deep breath and have another think. In the case of J44 and the recent sensor failure, that was all that was wrong with the situation. Replaced the sensor and all came well.

true overheat signs are all to apparent. Pressurised cooling system, perhaps steam and coolant from a hose joint or hose itself, a hot radiator and extremely hot thermostat housing tower, hot smells and much ticking of scalding engine metalwork. If you have non of those signs then the sensor is the most obvious unit to be at fault.

Mike K

[davidt34]

After the first apparant overheat I checked the expansion tank the next morning and it was almost empty!.
I topped it up and took it to the mechanic who did all the work and he was sure an airlock had occured.
He had some other little jobs to do for me so kept her for a week.
The heater had never worked properly so he fitted a recon heater pump and I picked her up three days ago. The engine had been running for quite some time with the heater on full and i got in and drove off only for the gauge to shoot

Agree fully with Mike H on this.

The thing about the problem is that when we see a full deflection temperature gauge, a message centre flag up and perhaps also a red low coolant indicator light come on, we tend to panic.

What then happens is we open the bonnet and madly look for signs of overheating. When you find that the radiator, the engine, and various pipes are at normal temperature, you still can`t quite believe that all is ok because the car dash display tells you otherwise. One goes further and carefully releases the expansion cap on the expansion tank. we then expect the coolant to be thrown out as indeed happens when a true engine overheat condition is present.

If the classic signs of an overheat are not present, then we have to take a deep breath and have another think. In the case of J44 and the recent sensor failure, that was all that was wrong with the situation. Replaced the sensor and all came well.

true overheat signs are all to apparent. Pressurised cooling system, perhaps steam and coolant from a hose joint or hose itself, a hot radiator and extremely hot thermostat housing tower, hot smells and much ticking of scalding engine metalwork. If you have non of those signs then the sensor is the most obvious unit to be at fault.

Mike K

CIMG0048.JPG

CIMG0031.JPG

right up to overheat again. We inspected the engine again and once again there was no sign of a true overheat.
Anyway I've been driving her for the past two days and have had the heater running on full most of the time and 200 miles later all problems seem to have gone!. The heater is now lovely and hot and the level in the expansion tank hasn't moved at all.
It would seem that the airlock moved through the system and when it passed the temp sensor the temp gauge shot up to overheat.
Without tempting fate she is running beautifully now and I had a great drive to a job in Maple Cross yesterday and I even enjoyed sitting in heavy traffic on the M25!
As a side note I'm really impressed by the handling of the car in the icy conditions.
Thanks to everyone for all your advice.
Dave

[mike020150]

Happy endings are always the best! Mike.

[oldtimer]

Hi Mike and All,
I have read about your saga with Temperature senders (well several times) and how the same problem it has effected other people.
I have also read that folks have replaced the sender in olders cars, XJ's for example, only to find that the indicator needle of the Temp Guage does not sit in the position that it previously did.

Now, I can only talk about back in the "dark ages" when I was involved in such things.
The sender is a Thermister or temperature sensitive resistance.Now in the case of the old XJ the Temp gauge has a bi- metal strip inside with a piso wire( the heater element) wrapped around the bi-metal strip.When the ignition is switched on a supply goes to the instrument voltage staberliser from there to the input side of the heater element, from the element to the sender(thermistor). As the coolant warms up the resistance of the thermistor reduces,the electric flow increases and the heater temp in the guage increases ,the bi-metal strip deflects and with the neddle attached a reading is given.

Hundreds of thermistors for hundreds of applications are manufactured by differant firms around the world. A time comes when stock levels of all these have to be looked at cos the cost on the shelf becomes prohibitive.So rationalisation programmes are carried out to reduce the number of Part Numbers. In doing so the resistance values tend to drift- hence the needle of the gauge is not quite were it should be.

The reason for writing this is that I noted that David had purchased the sender from a non Jaguar franchised dealer. As time passes I don't doubt that the same rationalisation programme continues to this day. So, my concern is that folks buy a product from a rationalised range and run the risk of building in a problem that was never there

Unless you can be positive that the resistance values throughout the heat range is identicle I would advise sticking with the OE (original equipment) as shown in the photgraph in Mike Kennedys article on this subject

I better stop "rabbiting on" but I hope this creates food for thought

Regards Oldtimer

[Nik]

Hi Oldtimer ,
Food for thought on that one , answered a lot of questions I had years ago (why does the pattern sensor not behave like the OEM one) ,
thanks for the explanation , it's guys like you that make this BB what it is

Best wishes

Nik

[J44EAG]

Very nice appraisal of the state of the nation as regards the whats and whys of these units Oldtimer. (Phil)

I have it in mind that with sensors and similar units, that OE manufactured and supplied units are possibly the only reliable way to go. Odd ball after market units could well be "off the charts" and the hassle and nause of having to part out, recover costs and then have to source and fit replacement parts for a second time, makes the idea of having to do the job twice, a no brainer.

I paid around £13 for a genuine replacement temperature sender so the cost is hardly worth trying to diminish by further window shopping. I took the view of order from a reliable source, install, check as operational and then forget for the next ten years or 77k miles.(Which ever comes the sooner!) This important but boring part in my opinion has to be the correct item but after that assumes pretty mundane interest.

Further to my posts on the S-type BB in relation to this identical part, I would certainly advise carrying or fitting a replacement unit in any V8 model Jaguar (of in my case 77k and ten years of age) as preventative maintenance and prevention of the issue as shown in the posts above.

Mike K