Gents,


I have a 2002 XJR-100. After a thermostat and coolant change, I noticed the temp needle fall slightly when driving on the motorway. My temp needle always sat a needle's width low of the centre mark. The needle would return to this point if sat in traffic, but on a motorway cruise would drop by 2-3mm.


The car's sat for a month, so I decided to take her out today as the sun was shining.

On the way home, I noticed the needle was exactly at the halfway point while on a light cruise on the motorway, again, it was always a needle's width down on this centre mark, so this is higher than normal.

This made me nervous so I tickled her home with hawk eyes on the gauge and the heaters on full.

When I got home I popped the bonnet. No fans were on and the thermostat hoses were not overly hot. It also didn't smell hot. The other oddity is that the supercharger header tanks were cold to touch. These are usually warm-hot after a run.



So, why would the temp gauge suddenly be reading higher with the supercharger tanks a lot colder than normal?


Any help or advice gracefully received.



Cheers,
- Richard

 

Because the thermostat is stuck closed. The water does not flow to the radiator when the thermostat is stck closed.

 

Thanks for the reply.

Both sides of the thermostat housing was hot, so surely water was passing through?

Also, if it was closed for a 30min drive, wouldn't the temp gauge have been through the roof, as in hitting the red?


Cheers,
- Richard

 

Don't know if this applies to your car - but mine has an extra pump for the SC coolant circuit. If it failed I'm not sure what temp the intercoolers would be.

I'd be looking at actual coolant temp via OBD (the ECT value).

I'm wondering what would happen with a failing water pump (also, are they plastic / can blades come off?).

 

Waterpump is new. It went in with the thermostat.

I checked the electric pump. I can hear it going and feel fluid flow in the hoses when on coil. I believe it is functioning.

I have a reader for my phone. What temperatures should I be looking for?


- Richard

 

OK, guess I am misreading your original post. YES, the temperature gauge will read hot if the thermostat is stuck closed. I thought that is what you said was happening. Is you only concern that teh intercooler is cool?

 

The gauge was exactly at centre. This is a needle's width higher than normal and enough to concern me as these things are so buffered.

The 'charger intercoolers are also always warm/hot after a drive. This time they were cold.



- Richard

 

The temp. gauge is a glorified idiot light. Once you reach 185 F it reads mid scale +/- a bit unless you get to about 230 F when it quickly moves to red.
The intercoolers staying cold is a mystery if the pump is functioning unless there is an air lock. Was the 19mm socket hex plug on top of the intercooler removed to bleed off air when the main pump and thermostat were replaced?

 

Wait - you're freaking out because the needle is reading at the halfway mark instead of the ever-so-slightly-less-than-halfway mark?

 

Yes.

I've had three years of rock steady ever-so-slightly-less-than-halfway. Since doing the thermostat change, I've had lower readings and now higher readings. And now cold charge coolers after a run.

I, like God, do not play with dice and do not believe in coincidence.



No, I never undid the oversized hex nut and I didn't have anything that would fit. I ran the car with the expansion cap off and topped off multiple times. The level stayed consistent after this, so I had assumed (wrongly?) that I was free of air locks.


- Richard

 

What is the outside temperature? Get an OBD reader that will display running data while you drive: plug it in, keep it plugged in and observe the actual temperature at the sensor, it should begin to open the TST at about 185*F (you can convert) and have the fans on by 205*F. You should find that in warm weather, the fans come on sooner and cool it back towards 195*, while in the winter, the fans come on later, allowing it to run a little warmer (not sure what that corresponds to in millimeters on the crappy temperature gauge, but you need real data!).

 

Well, it would seem that maybe your new thermostat opens at a slightly higher value that your old one! As to the intercoolers, I guess you could pull a lone and check to see you have water running.

As to the recommended filling of the coolinbg loop via the cap, I have seldom used that method and have never felt I had a problem getting it burped by using the method you mentioned, although it does take a little running amd some "massaging" of the top hose to get the intercooler loop to fill quickly

 

A hex bolt (3/8" diameter shaft) turned upside down will fit the hex bolt on the intercoolers. As long as your intercoolers are cool or cold then that cooling circuit is working fine. Otherwise they would be as hot as the rest of the engine

 

Cooling to the intercoolers is by radiator coolant. How can they be cooler than the radiator when the system is up to temp?

 

Although the loops are connected and the fluid is the same, there are two separate cooling loops.

 

It would be worth separating the circuits and uprating the ICs but it's quite a bit of work and where do you put the extra stuff? (Not for me, too much hassle.)

 

The intercooler circuit and radiator are independent of the engine coolant radiator. they are interconnected only for pressure relief. When the car is moving the intercooler coolant temp is a few degrees above ambient outside temp dependent on boost load and residual heat soak

 

For the needle on the gauge to have moved above exactly half way means the engine is running slightly hot. As I recall it doesn't even budge from halfway until about 118c

You said the fans weren't running? That might be something to check out, if they are even working.

It's still cold in Switzerland at the moment? Could explain why your charge coolers are seem unusually cool. But could also be that the engine bay is hotter than normal, but the chargecoolers at normal temp, so they feel colder.

Read the engine coolant temp via OBD, this will tell you if it's really hot or not.

 

Good points!

Imagine the thermostat is not working properly. I expect the radiator would be doing less than it ought, charge coolers would be cooler and engine hotter. OBD can read ECT and is far better than the dash.