Hey guys, I got another problem with my 99 XJR, that might be a deal breaker If a reasonable solution is not found. The car has 102K miles on the odometer.
The car is not mine yet, but If I solve this I will keep it.

I have spent the last couple of days reading all postings I can find about overheating Jags, specially XJR.

So far the following work has been done
Most heating hoses have been replaced
New thermostat (autozone)
New water pump (autozone)
The fans are working
The electric SC pump is working strong
Coolant has been purged several times with the electrical pump.

The car can idle for hours on Park and will not overheat.
Some times I will work fine for a week, and you will get your hopes up.
It has a steady strong engine, no smoke on the tail pipe and when is working
Fine (no overheating) is feels like a brand new car.

Then all of the sudden will star to overheat.

It feels as some blockage on the system, is there a check valve that might get stuck?

The heating is not working, should I eliminate the heater loop? Is this area a possible blockage area? I know there are two check valves back there, can they interfere with the cooling system when not in use?

All of the sudden will start overheating, at speed, on traffic, no specific condition that I can pin point the cause.

Any suggestions from people that have gone thru this?

On monday I plan to do a chemical test on the coolant and test compresion on the cylinder to verify the condition of the head gasket.

Any sugestions welcome, I will hate to part this car appart.

Thanks
Ivan

 

Couple of checks on your story:

1. How do you know it is over heating?
2. What are the symptoms?
3. Did you check it with a live OBD reader, or the idiot light on the dash?
4. Do you loose coolant?
5. Did you replace the coolant temp sensor?
6. Are you sure the hoses on the expansion tank are crossed?

Seems you did all the obvious things already, and I can't rally think how a blockage should occur.

Let us know on the above first.

 

The lack of cabin heating could be caused by a blocked heater core, broken heating pump, or malfunctioning valve (all not unusual). All those components are past the "heater supply/heater return" lines in the following picture. Since they tap off the main cooling loop, and are not inline, they shouldn't affect the rest of the system, unless the heating system is throwing chunks of gooey old coolant or something (?) into the rest of the system. You could also try backflushing the heater core, or bypassing it completely, to see if that affects anything. The heater pump is a little pump under/near the driver's side false bulkhead, you should feel it humming if you put hands on while the heat is turned on.

 

Thanks for the reply:

1. How do you know it is over heating?
Drive it around with the OBD scaner temp goes from 220F normal to 280F when
The idiot light on and gauge all the way up.

2. What are the symptoms?
You loose AC and coolant smell

3. Did you check it with a live OBD reader, or the idiot light on the dash?
OBD Snap On Scanner

4. Do you loose coolant?
Yes from the overflow tank on the front driver's side

5. Did you replace the coolant temp sensor?
No, I am getting temperature readings on the OBD, is it still necessary?

Y6. Are you sure the hoses on the expansion tank are crossed?
Yes

 

What codes? Almost certainly must be some.

 

These were the only codes.
The transmision related codes are no more since I replaced
The conector plate on top of the valve body of the transmision.

 

Broken water pump blades?

Those codes look serious BTW.

 

He mentioned he changed the waterpump.

Right now I am baffled, seems he did replace parts by the book.
He''s got real overheating with all signs and proper OBD2 indication, not just an electric gremlin.
And then fact that it comes intermittent ...

My only guess could be the thermostat at this point, either reversed or wrongly installed, installed upside down, or just a non function one new out of the box.

Hope that others have some alternative suggestions.

 

I would flush the system and replace thermostat with oem part. Just my 2¢

 

Which is why I mentioned broken blades!

 

If you start the engine cold, with the filler cap of... rev the engine a bit and check for bubbles.... if it bubbles, or if it push cold water out.. one of the cylinderhead gasgets may be blown.


What can happen is that the engine blow gas in to the coolant and cause the waterpump to stall and it do overheat, but quite locally.. it depends a little where the gas tend accumulate..


If the engine tends to leave a puddle of water at all times.. this is IMO the most likely cause...


/E

 

I am waiting on an OEM thermostat.

I started the engine cold, full of coolant and I do not see bubbles on the top reservoir.

The coolant has been flushed and the radiator was taken to be internally cleaned.

I hooked the scanner again and the engine has no codes, only the transmission
gives a P1632 CAN message.

Now here is the interesting thing I just notice. I can leave the car at idle for hours and will not overheat.
I can accelerate the engine on park or neutral up to the rev limit (2,900 rpm) for extended time and it will not overheat.

Only when I put the car on drive and apply the accelerator + break pedal at the same time it will then overheat immediately.

The overheating is not RPM, speed, or outside temperature related. Is transmision load related.

What factor can cause this overheating only when the transmission is engage. What is the relation between engine cooling and transmision that might be causing this? The transmision in the past had several messages but since I replaced the connector plate, all have cleared with the exception of the P1632.

This is really weird.

 

From the 99 XJR Transmission Codes pdf:

 

I am just waiting to receive the thermostat and I will take out the water pump for inspection. But I still at a loss with the relation with the engine load vs. idle.

 

I am not a specialist on transmissions, but I can hardly see how a healthy cooling system should not be able to handle any heat thrown at it from the transmission.

I was first thinking electric gremlins, but following my questions, seems you have real overheating issues, with all the symptoms.

I hope someone else with more transmission knowledge and its cooling / piping will step in.

 

My memory is not perfect, but I think I recall a thread in the past with a similar story. I believe it was determined that when the transmission overheats the ecu reports that temperature in place of the engine temp. Maybe motorcarman or one of the other experts can confirm or say I'm wrong.

 

This afternoon I did some more testing.

1. I topped the coolant thru the supercharger water port(again).
2. Started the engine with A/C on at 60 degrees. Ambient temperature 95 degrees.
3. Left the engine idle for about 1/2 hour. ( no overheat)
4. I can hear the fans going thru hi-low cycles.
5. No sign of coolant from the overflow tank.
6. I put the transmision on drive, parking brake on and let idle for about 15 min. (No overheat)
7. Placed the transmision back on park and took the engine to 2,900 rpm for a couple of minutes ( no overheat, no cooland on floor)
8. Put the transmision on drive, parking break on, and accelerated the engine to about 1,500 rpm with the break on. After a couple of minutes coolant started to drip on the floor from the resevoir on the front driver side.
9. Placed the transmision back to park and eventually the coolant stopped leaking and temperature did not increased further.
10. Left the car idle on park again, temperature never increased again, and coolant stopped leaking.

 

So it works fine now then? The water that comes out after a while is related to the fact that it was topped up (1.), the water volume expands when hot and the exessive amount pushes its way out and then stops..

 

Sorry, but why the heck should you do that, what do you try to simulate there other then destruction?

It should not happen, as the secondary tank in the fender is designed to hold that excessive coolant, then let it suck back in from there if the system cools down.
However, if the secondary tank is full to start with, then yes, it will start to overflow (designed to do so).
If that tank has a crack, it will start leaking too, but you will get a low coolant alarm at a certain point, due to the fact that coolant is pushed out, but can't be sucked back in.

 

I learn something every day here!... going to check that tank for cracks...