*sigh*

I'll try to provide a timeline here.

Replaced all hoses some time ago, waterpump was new, full flow spin-on coolant filter installed, numerous flushes (with and without chemicals, with rust-lifting agents, and a shopvac in every direction) new fan and clutch now I've replaced the thermostats with proper ones ordered from Jaguar - 82c with jiggle pin...

One previous thermostat look like they came out of a WW2 tank, discoloured 87c no pin on one side and a new looking one 74c on the other side.

A new Temp sending unit is about 15 dollars (cnd) and an aluminum radiator is quite abit more after currency exchange...that being said has anyone here got a recommendation for how best to verify the temps the gauge is reading? I'm having a hard time believing it's accurate and I'd like to try installing an aftermarket probe with digital readout.

I'd also note that the Aux fan never kicks in, bridging the wires does make the fan run though.

 

Have you ever removed the radiator? often there is quite a bit of debris between the rad and condenser and that blocks the airflow. I hold the rads up to the sky when I pull them, there is often an amazing amount of junk in the fins. You should be able to see daylight through a rad!

For measuring temp, I use a IR thermometer from Princess Auto. I'm sure Canadian Tire has them too. I use them on the hoses, or on the side tank of the rad. They will be fooled by shiny metal, so aiming it on the heads or intake manifold will give an inaccurate reading, unless you put a patch of matte black paint on them first and measure off that.

https://www.princessauto.com/en/deta...er/A-p8500589e

 

Hi Vancouver

It might pay to check the Thermostatic Switch that is 'supposed to cut in' to operate the Aux Fan with a Test Light

Or a Water Circulation problem, where no Hot Water or Very Little Hot Water is getting to the Thermostatic Switch

to make the Aux Fan cut in

Or maybe the Thermostatic Switch has gone bad

 

Vancouver
Your original post did not say the car was overheating, why do you think it is?
The easiest way to be 100% sure of all temps is to buy one of these, with 3 metre long sensor wires, and tape one sensor end to the thermostat housings and one to the water pump inlet, run the wires through the door (the seals will allow it) to the unit, then go for a drive.

 

As Greg has said.

I used a CHEAP electric accessory style gauge, waaaaay back in the early 90's, and screwed the sender into the RH housing at the REAR of the cylinder head, and ran the wire through the door, and went for said drive, and noted what was going on, and it WAS NOT overheating in any meaning of the word.

I lived in the bush at that time and there was NO Internet, so one my own with this one.

82c stats, will give APPROX 95c top hose temps, and APPROX 81c bottom hose temps. ALL based on a proper working radiator etc, which I had.

Odd temp stats is sadly a USA thing, with the theorists going all out about LH and RH side running at different temps, FAIR DINKUM, it sounds good, but really, think about it over a beer or 3 or 4, and will all clear up.

The factory temp switch in that bottom hose spout is 85c, SOOOOOOO, if that silly fan is not switching on, then the bottom hose temp is below 85c, and the engine is happy as is the rest of the cooling system.

It was at this time, and like you, had replaced, rebuilt all and sundry trying to find the reason for above N running temps, that I found the WRONG spec stats in the beast.


This might clear the mud a tad.

V12 thermostat fiasco explained.pdf


Once I got the stats sorted, the fan exploded and vented the bonnet, BUGGA, so thats when the Efans went in and I have NEVER looked back.

 

Thank you , Great Francis for that article , very informative.
My XJ12 has always been prone to overheating , whereas my XJS is not. Maybe you have solved the problem for me.
I also found your comment on the B bank running hotter somewhat comforting , I had thought of it over a beer and wondered if it was just I who couldn't take it seriously.
My XJ12 had , at one point , different temperature thermosats in the A and B banks ( lower on the B side ! ) : you could see the temperature gauge ( on the A bank ) " notching up" , as the car got warmer.

 

My car recently started overheating again. I replaced one thermostat a few months ago, which cleared up the problem (it wasn't fully opening). Went with the other tstat, since I already had both in storage, but nothing.

I have an electric fan that is always on, so no need to consider the aux fan cut-off.

I'm suspecting the fan clutch. In, my 8 years of daily driving, it seems new fan clutches only last 2-3 years. So it is probably about time. This is usually despite the fans passing the "spin test". Perhaps I will try to gather data on the clutch to report back with.

In the last two weeks my car will sit at the top of the N, which is about 200 deg. Not insane overheating as many report, but not correct nonetheless.

You can get a radiator for pretty cheap if you look around. They are as much as $700 US, but you can now buy them as cheap as $200 US for a 56mm, $300 for a 62mm, and $400 for a 74mm. Look for a large temp differential in a small area to see if your rad is clogged. As others have said, a cheapo IR thermometer is a must.

 

Thanks. This is what I need before I invest in proper aftermarket (permenant) gauges. I've consumed all available knowledge on the Tstat issue and short of flying to Austrailia to buy the tridon? brand I've got the next best ones. The outside temps were a muggy 28c and I was flooring it up the mountainside here, but usually the needle drops back when coasting down. It did not this time around...given how much of the system is new(ish) and im running mostly water/water wetter now I don't kniw what to think other than the gauge or sending unit is failing. Wasn't this bad last summer in 35c heat.

 

Sounds like a lack of airflow to me. I'd start by pulling the radiator and examining the fins. Unfortunately, there is really no other way to determine the condition other than to remove it.

 

Just to mention...it's not the right solution for everyone, but a few years ago I ditched the engine driven fan, the tiny electrical fan, and the engine driven water pump. Installed an electrical water pump, two big fans, and a pump/fan controller. I also got rid of the coolant crossover pipe at the front, and also pitched the thermostats.

Our yearly temps range from +95F to -20F. Overheating issues are in the past. When it's really cold, I might have to stick a piece of cardboard in front of the radiator to get the engine up to temperature, but that's about it.

I also had the exhaust manifolds ceramic coated...how much that helped keep overheating down I don't know.

Thanks,
John
1987 XJ-S V12

 

JCR,

Davies Craig or Meziere? Any tips/advice for others going this route?

 

Davis Craig (or is it Craig Davis...just kidding).

Their newer pump/fan controller is a vast improvement over the last one. LCD display showing water pump and fan operation, water temperature, a few more things I can't remember, and even a system voltage indicator (which means I can stop trying to make the in-dash volt meter work properly).

The only thing I didn't do in my garage was braze the plumbing to the radiator (I can weld but not braze...go figure). The water pump is in the LH front fender well, and I had to cut two holes for a water line to go out to the fender well and then back in. So I took the radiator and my copper pipes to a radiator shop and they brazed it all together for very little cost.

Where the water pump was I just bolted in a piece of 3/8" aluminum plate with a 90 degree elbow welded into it (again done at the radiator shop for a song). The elbow and pump are connected with a radiator hose.

Hmmm...the above all seems a waste without pictures. I'll take some tomorrow and post them. But all in all, while it requires some work, it's much less time consuming than it is "hard". Compared to getting the exhaust manifolds on and off it was super-easy :-)

Thanks,

John