2001 XJ8 VDP Cooling System is building up way too much pressure and is now running hot. The water temperature reading is 220, and the temp. gauge now at 3/4. The hoses are expanding and are now filled with pressure and antifreeze even after car has cooled down when hoses should be empty.


This is what has been done:


(1)The 3 way connector for the left side heater hose (heater return) was replaced (nipple had broken off) along with the feed and return heater hoses---all under the intake manifold. Old hoses were very mushy from all the excess pressure.
(2) New intake manifold gaskets
(3) New fan water pump housing polished and new gasket
(4) Fresh Antifreeze


Head gasket is fine. Why is all this pressure building up?

 

Well, your temperature is abnormally high at 220. The gauge on these cars will always hover near the middle, unless extreme overheat is iminent. The thermostat should keep things at 195, if the system is working properly. These thermostats can and will stick without warning. Although that would spike the gauge. Are the fans working? You can see and hear them without much trouble. Also, many of these engines have water pump impeller failure due to the plastic design. These are easy to replace; might not be a bad idea if it's original. Radiators don't typically have issues except aging of the plastic which makes the outlets kind of brittle. Any other ideas out there?

 

Have you checked your pressure cap? How do you know the head gasket is good?

 

How old is the thermostat? I would start by replacing it. By the way, the thermostat will fit both ways, so make sure it is installed with the temperature bulb facing rear.

 

Might be a good idea to test for hydrocarbons in cooling system.

 

Not sure I agree on the thermostat fitting both ways....

+1 on checking for hydrocarbons

 

I was having an overheating and rough running problem (turned out to be tin connector having trouble with the gold plated throttle position sensor pins).


I had taken the car to a local foreign car shop. Technician insisted that the thermostat was in backwards, blew the plastic expansion tank apart


Took the car away from them and solved the TPS issue, but car would overheat. Changed the thermostat again and installed it correctly, so it is not idiot proof meaning that it can be installed backwards.


My car has the aluminum replacement thermostat housing and it may have to do with how the housing is machined.


That is just what my experience with the thermostat was.

 

Pressure builds from excess heat or blown head gasket. Thermostat might be stuck, it will overheat (thermostat should open about 183*F). The fans might not work (turn on the AC and see if they come on). But most importantly, why are you confident about the head gaskets?

 

All typical signs of head gasket being blown are not there, but will do a compression test.

 

Chopr, you were on the money, turned out this the thermostat was sticking. The cap to the thermostat housing was also cracked. Not sure if it cracked from the pressure or the crack created the pressure. For now I have just put the aluminum cap on the plastic housing but will change it out as soon as possible. Thanks again!!

 

MikeBaker3, you were on the money, turned out this the thermostat was sticking. The cap to the thermostat housing was also cracked. Not sure if it cracked from the pressure or the crack created the pressure. For now I have just put the aluminum cap on the plastic housing but will change it out as soon as possible. Thanks again!!

 

Thanks for getting back to us. For the sake of archives, you might mention that the cap was apparently sticking also, as reported in the other thread, since there is confusion about the relationship between overheat and pressure build up. The cap should alwasy let the pressure out, even if the engine is overheating