White XKR, Thanks for the tip on the thermostat. I know this is the case as the cars do not operate properly at temps under the required level. As I have been testing the car has never gone above 185°. The shop I took it to said the engine "immediately" failed the "block test". and suggested a used engine. After reading all the great feedback on this thread, I thought I'd better test it myself. I bought a test tool that detects combustion gasses in the coolant. IT uses a blue colored fluid that turns yellow when combustion gasses are present. I bled the air (AGAIN! I'm getting real good at this). Warmed the car to 180° and ran the test. No leaks detected. I ran the car in the garage, with the reservoir cap off for another 20 minutes and tested again. No leak. I put the cap on and drove for a mile and then back in the garage. Checked temps at thermostat housing, 186°. Car started puking coolant. I decided to remove the cap (very carefully with many towels and gloves!). I kept unscrewing and the cap would not release. As it turns out the system had a VERY strong negative pressure! It took maybe twenty five pounds of force to get the cap to release! When it came loose a lot of coolant came back into the reservoir tank and on to the floor. So now I come to the realization that my radiator and reservoir tank "imploded" as opposed to exploded. I ran the test tool again and still no combustion gasses present. Now I am totally confused. I have been working on cars and motorcycles for over 40 years now (no Jags though), and have never seen this.

 

White XKR is not addressing the issue of thermostat-controlled temps and how they might relate to engine sensors, etc.; but rather he is saying that the thermostat itself, physically directs coolant throughout the cooling system passages... and that by eliminating the thermostat, you have altered the pathway(s) of the coolant and by changing the flow characteristics of the coolant you've created an overheating condition. Put in a known good t-stat and start again.

 

Everyone is right and leaving the thermostat out is not a good thing . As the thermostat allows the engine to warm up but it also provides the coolant the time necessary in the radiator to cool down. If you wanted the air out of the coolant system try squeezing the top coolant hose with the coolant cap off and watch the air dissipate and the coolant level go down.

Link WHAT DOES A THERMOSTAT DO?

Remember that the radiator cap has 2 functions to allow the coolant to move freely in both directions to and from the overflow tank.

 

Ok, thermostat back in. Tested its operation, it is good. Refilled and bled cooling system (ran heater). Checked temps with Lazer thermometer. Taking readings on therm housing, at 170° water started coming out of reservoir filler hole. Temps went up quickly and water was being forcefully expelled several feet into the air. water temp as it came out was 200°. It spewed the same gallon and a half approximately. If the reservoir cap was on, the coolant would have expelled from the catch tank behind the right front wheel well creating the extreme vacuum I found on the cap yesterday (I think). Thermostat made no difference in this situation. By the way, the engine still runs fine.

 

Check for any collapsed hoses. Specifically the installation of your TOP Radiator hose. I once installed mine backwards and it collapsed at operating temps causing an overheating situation.

 

Did you say you checked the water pump?

 

ditto on the WP?

 

Something is blocked. Have you tried to check the radiator for flow? Like out of the car with a garden hose?

Check you hose routing against an OEM chart or another Jag. Maybe a hose is crossed or connected wrong?

 

This might be completely irrelevant but I have a vague memory about a similar situation when an owner crossed over the two pipes to the expansion tank. The correct installation looks wrong (the pipes DO cross over), but the 'obvious' way to connect them is NOT to cross them.

If crossed over, a pressure lock builds up.

 

Now - my statement above is a perfect example of 'confusion creation'!! Let me have another go ....

This might be completely irrelevant but I have a vague memory about a similar situation when an owner uncrossed the two pipes to the expansion tank. The correct installation looks wrong (the pipes DO cross over), but the 'obvious' way to connect them is NOT to cross them.

If not crossed over, a pressure lock builds up.

 

New hoses.

 

Only 1 return to tank. No crossover possible.

 

I will double check hose routing.

 

The thermostat in the xk8 is a dual acting type. As the thermostat opens to allow coolant through its aperture, the rear of the thermostat acts as a valve which seals against a seat in the housing closing off the bypass path. With the thermostat removed this dual acting valve doesn't work.

On other car types the thermostat just closes off coolant flow, but not so on the xk8. Make sure you have the correct type installed, you should see a flat round plate on the rear with a spring to keep I seated in the housing when the thermostat has opened.

Also check the water pump impeller as the plastic gets brittle and the vanes shear off. Are the fans running ok ? With the aircon switched on they should run all the time, so a good way to test them. Does the coolant still overheat with aircon on ? If not, maybe the fan thermo switch has failed which would mean the fans don't switch on properly at the right temperature.

Good luck with it.

 

Thanks for your thoughts on that I appreciate it I did install the thermostat back in and the problem still persists double checked all the hose routing and the problem still persists I'm going to flush the cooling system next and see if I get a big blob of something to come out. And then I think I will do a leak down test.

 

I said inPost #29 that it is possible to cross the pipes over - but you said there is only one pipe. Look at the pic (from the Manual) ------>

 

Ok, did the flush. Nothing came out, no blobs or hard parts. I just pulled the water pump to be triple sure, and it was the original plastic impeller;however it was totally intact! I just ordered an new pump and a brand new thermostat. I have little confidence that they will solve this problem at this point. As soon as the temp at the thermostat housing gets to 180 it pukes the coolant, all the time. Weird!

 

Something is not computing based on all you have done. How about pulling the plastic covers off the engine and post a few pictures. Something is amiss in the hose/tube arrangement.

 

After all you've done and checked and replaced, my money is on an obstruction inside either the radiator or the heads/block.

The radiator would be easy to flush with a garden hose. However if something were stuck in the coolant passageways it would probably take a pressurized flush to dislodge it. If someone with knowledge of the coolant flow path would help, we could probably ascertain at what inlet we should begin to flush and what zones to block off.

 

I would remove both hoses to the radiator and with a garden hose I would force water from the top hose and see what comes out the bottom hose if that is ok then do the same from the bottom host and see if you have the same flow. If that is ok then remove the thermostat and cross over pipe and with the hose in the thermostat opening see if water flows out both sides where the cross over pipe was connected see how the flow is. You should see flow on both sides. It seems you have an obstruction and this may help in finding it.