This is a new thread on a problem I have posted before. I am down to the last straw before I sell this for parts.
I have a 1998 XK8 convert. 117k miles.
This sounds very similar to a new post by Platinumaker.
The car started over heating randomly about a year ago. It blew an upper radiator hose, I replaced it. Problem was gone for a couple months. Next the radiator blew the side plastic tank. Old radiator, I replaced it. Problem was gone for a month or so. Next it blew the bottom of the fill tank on the firewall. At that point I replaced all hoses, water pump and thermostat. I chemically flushed the system. Now every time the water temps get to about 185/190 the car pumps about 1.5 gallons of coolant out the overflow reservoir in the right front fender well. If I remove the fill cap and run the car, when it hits 185/190 the water shoots out the fill cap instead of the over flow tank. All hoses are connected correctly. I tested the thermostat on the stove, and it works. There is no smoke or steam from the exhaust. I ran a block test and it showed no combustion gasses in coolant and no foam in the oil. The car runs as smooth as ever and has full power. I am 64 years old and have worked on cars and motorcycles most of my life,(even professionally!). I am stumped.
Last time I posted this I got a large number of suggestions and I tried them all. It just does not make sense. I know when the problem is found it will make sense. I hope I can find it. I love my Jag.

 

My suggestion would be to go back to the basics. Working hypothesis: whatever heat the engine is generating is somehow not getting evacuated, therefore the fluid's temperature rises to the boiling point.

When the engine warms up, is the thermostat opening for sure? Is there hot water going in the radiator, and substantially colder water getting out of it? This can be tested by checking the temperature of the radiator hoses. Is any of this helped by activating the AC (should activate the fans and accelerate the cooling through the radiator).

Separately, is the car kept cool when the heater is activated (just another radiator, I suppose)? Is there heat coming out of the vents? Is the heat constant and really warm, or is it petering out and lukewarm?

There are other working hypothesis, but this is a start.

 

Oops, double post

 

When you replaced your waterpump, do you remember if the old one that came out of your engine had a plastic impeller? If so, do you remember if the new replacement waterpump that you installed also had a plastic impeller? If you said yes to both of these, then that most likely is the problem. The plastic impeller mounted over a metal shaft inside the waterpump comes dislodged after time or loosens up and theoretically slips while the metal shaft spins. It causes an over pressurization of the cooling system and well...you know the results. You need to replace it with one that has a metal impeller. Coventry west I believe sells those. They come in a plain white box with no name but we use them all the time and have no problem.

 

If you are losing that much coolant from the overflow tank, then I would suspect some type of blockage. Either the passages themselves are corroded or partially filled with gunk, or something foreign has made it into your cooling system.

There is an earlier thread where someone had to clean out some horrible goo from the passages to resolve this problem. Search that thread for his fix.

Lastly, make sure you have installed the thermostat correctly (no offense intended). It is correct in only one position even though it will fit multiple ways. The usual culprit is putting it in upside down. The little pin sits at 12 o'clock.

 

I am good on the metal impeller for the water pump. Thermostat is good, vent at 12 o clock. Looking at the inside of the water pump housing there is NO residual goo or any noticeable build up. I see clean surfaces. What is the best method to try and flush any foreign material out? Thanks a lot for any help.

 

There are really only 2 things I can think of that will cause your problems.

The engine MIGHT have a 'slightly' blown head gasket putting enough pressure on the system to overcome the coolant cap and send coolant out the atmospheric catchment tank.......

or..............

The reservoir lines are routed incorrectly. Is it possible that the atmospheric catchment tank and the air bleed lines are swapped??????

The cap has 2 'O'rings and the overflow line port is between the 2. If you are having an air bleed line port on the reservoir routed to the atmospheric catchment tank, then the coolant will flow WITH NO RESTRICTION to the atmospheric catchment tank from lines that are supposed to be AIR BLEED.

An air bleed line to the overflow port will do NO GOOD as it will not overcome the pressure cap to get back into the reservoir.

You can have the cooling system checked for exhaust gasses with a chemical kit. It might detect a 'blown-head-gasket' if that is what the fault is.

Keep digging!!!

bob gauff

 

Hi.


Have you seen this TSB?

 

Motorcarman and Oyster,
My (98) only has one line running to the reservoir from the thermostat housing. See photos please.
I did the chemical block test several times and no combustion gasses present.
If there was a gasket or head/block problem, the gasses would have to go somewhere. If the combustion leak went to atmosphere there should be a noise. If the leak goes to coolant system gas residue would be present in the coolant. If to the oil system, the oil would be contaminated by coolant. Additionally the motor would probably miss a bit and it does not. Spark plugs look good.
Should I pull the fender liner and check the tank and hoses there?

I am not clear on the cap bleed system. Looking at my cap I see the two o rings, but no obvious bleed circuit on the cap or the reservoir neck.

 

As far as I can see your bleed line is correct being a -98.


The bleed port from the reservoir to the recovery tank is in the aft part of the filler neck. (See picture)


The reservoir cap is somewhat complicated, and I don't have the full understanding how it works. But I think the bottom part opens at a certain pressure/temperature letting coolant flow through the bleed port to the recovery tank.
When engine cools, coolant is drawn back from the recovery tank.
There is a metal thing in the top of the cap. (See pic) I don't know what it does.


My best bet now is a problem with the filler cap. I would get a new original cap.

 

Update to my old post. I let the car sit for about 4 months,(too cold to work on it). When I came back to it the battery had died. Fixed that, then the fuel pump had died. Fixed that, then the car has run fine with NO cooling issues. I have put over 2k miles on it and it works great. Soooo... there must have been a bubble in the cooling system that I could not get out by the standard burping process. Glad I did not listen to the Jag repair shop and change out the motor for a cracked block!!! European Motor Cars of Las Vegas. They would not refund my diagnostic charge either.

 

The plastic impeller WP's are just fine. If they go 80-100k from new the same thing will last just as long. I had the metal one and thought the workman ship of it was lousy, no center o-ring (has to be a reason the orig type was made that way) and the cheap *** paper gasket that came with the one I got, I sent it back and get the OEM unit.