So my daugther away in college in one of my xjrs calls me to say she is on the freeway with the temp gauge in the red, and what should she do? I find out she is 107 miles away using mapquest, so I call a tow truck from AAA. Two hours later, the car is home and she now has a new friend with prison tatoos who drove the tow truck.

I start the car and watch carefully until I see dripping coming from the bottom of the header tank. I can't really see where the drip is coming from, so I remove the tank. I clean the tank up and find a three inch crack on the side of the tank. Suprisingly, it was not near a nipple or seam, but in the middle of the tank. I take the tank out of my other xjr and put in in her car. I fill it up, and go for a test drive.

Two miles from my house, the temp gauge pegs again. I won't talk about how I took the cap off my the side of the road and got a blast of radiator backwash in the face. I get the car home again and think.

Must be a thermostat. I pull the thermostat and put it on the stove. It is marked 84C. I heat up the pot to 100c. It never opens. I get a new thermostat, and fill the car up, and go for a test drive.

About two miles from my house, the temp gauge pegs again. I stop and let the engine cool off. I think.

I replace the radiator and waterpump last year, so I can't imagine it is either of them, so I go for the air pocket solution and spend an hour bleeding the system. I go for a test drive, and about two miles from home, the temp gauge pegs. I think again. I call a friend. We think together. Must be a head gasket.

The next morning, I think again. The temp does not go up quickly when I accelerate. I use a torque pro gauge, which updates every second. I would think that a head gasket would cause a quick increase in temp when accelerating.

So I test the temp of various parts of the cooling system with a lazer temp gauge. Oddly, the temp of the radiator is never more than 110 F when the engine is 190 F. Hot water must not be getting to the radiator. The hoses are full, the system is bled, and the fans are both working.

So I pull the water pump to look for a clog. No clog.

In fact, no teeth. There are no impeller blades. They are all gone. The AC DELCO part I used has no blades. So I reassemble the car with a new pump, pour in my six fifteen dollar bottle of antifreeze in two days, and start the car.

Works great.

Who would have guessed. A thermostat, header tank, and water pump all at once.

 

Did you find the broken blades within the cooling system?

 

yes...many of them, but certainly not all. I suppose some resisted being flushed out in the radiator. I did get a handful. I wonder what caused them to disintegrate.

 

I don't think all this happened "at the same time". Most likely the thermostat failed first, and she was running hot (but not seen because of the crappy gauge), the hot coolant "melted" away the impellers, and all the hot and pressurized system, cracked the old and weak tank.

The "real" temp gauge is a MUST on these cars. If your gauge was "pegged" few times, as you said, I would be VERY concerned with the state of the head and the head gasket.

 

I agree that the thermostat was the likely first harmful event. The water pump vanes were not melted, but appeared to be broken off, like there was an object that got in there....but nothing was found. I guess the blades might have become brittle from heat. As for the head gasket, this is not my first rodeo with overheating. It has been my experience that a warped head, or blown gasket shows itself immediately. Right now it is running fine, no exhaust gas in the water, no water in the oil, normal temps. As for the temp gauge, you are correct. It is worthless. It holds the centerline until you get to 235F, then it pegs in about five seconds. It is really just an idiot light with a needle. For those interested, even here in Tucson, where temps get to 115f, the car, when running properly, never exceeds 208f. The cooling system generally holds the car between 204 and 208. The first sign of a developing issue is when the temp goes higher that 212.

 

Installing whitexkr's Realgauge mod. is a cheap form of insurance.

 

For information see
RealGauge - TheJagWrangler and hit me up with your questions.

 

Now I have an even more interesting issue with my other XJR overheating. I am perplexed.

It is a 99 xjr with 135k miles. The problem is that a minute after you shut it off on a reasonably warm day, it boils over through the overflow tank. It does not run hot, nor does it appear to overheat. I have done the following:

Replaced the thermostadt
Replaced the water pump
checked the fuses
seen both fans run on high and low
replaced the header tank
replaced the fan relay
replaced the antifreeze a bunch of times
added water wetter
bled the syste
checked the fan wiring harness

I must say, I have driven the car in hot temperature and noted that the temp never goes above 100c when driving, and is generally in the 90s. When I shut off the car, the temp rises to 103, and the fans do not run on. Normally, when the car overheats in the past, the fans would run on after shutdown.

I used a thermal temp gun and never took a reading above 95c on the exterior componets of the cooling system, with water leaving the radiator in the 80s.

So the only thing I have not done is replace the fans and the radiator. Any ideas?

 

Did you changed also the cap? If it looses pressure, the coolant boiling point is lower.

 

Check the two small hoses at the front of the reservoir. If they are reversed, the symptoms you see are what happens.

Hard to explain, but they should be crossed when properly installed.

If the bleeder hose from the radiator is at the nipple that leads to the opening at the top where the gap above the o-ring seal is on the pressure cap, then it has a direct path to the overflow tank. It is intended that this hose dump into the reservoir and the overflow hose only get coolant if the system goes over pressure. The cap vents coolant to that area above the o-ring and the coolant flows out the over flow hose.

BTW, it is likely that the reason the fan does not run on at 103*C after shutdown is because it was below the threshold *at* shutdown.

 

Omg....I don't know whether to laugh or cry.
So how could this happen to a guy who has worked on jags for more than 20 years?

I robbed this car of it's header tank to get another one on the road while I waited for it to arrive. A week later it put it in and it worked great. It was December. Now it's heating up in tucson.

This is exactly why this forum is great.

 

And to answer Flay...three times....I bought three new caps, thinking it must have been a pressure problem and the caps were faulty. Now I'm crying.