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Matt, Mechanical Engineer
Category: Jaguar
Satisfied Customers: 22763
Experience:  BEng hons Mech engineering, in auto industry 22 years
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I got a December 2003 S Type FH53 MBO overheating. PROBLEM:

Customer Question

I got a December 2003 S Type FH53 MBO overheating.
PROBLEM: For six months now the temperature gauge has been rising then falling and occasionally going into red. When in red the engine is pressurised and water comes out from the radiator cap.

The mechanic has so far done:

New thermostat
New water pump
New radiator

But the problem persists and has got worse. Other than that the car runs well it is not missing, the oil is clean and it is not using oil and the car is not losing any water.

One last thing I put RADWELD in the old radiator.
Submitted: 3 years ago.
Category: Jaguar
Expert:  Matt replied 3 years ago.

the thing to try is a head gasket / block test kit as these take a coolant sample and analyse it for any signs of combustion gas - if present this would indicate the head gasket is on its way out

Also it may be worth replacing the header tank cap as again you can lose coolant as steam if this is blowing off too early ( the cap should hold pressure of around 1Bar)
Customer: replied 3 years ago.
I don't think it is the head gasket:
1. No gas in exhaust
2. No missing
3. clean oil
4. Hasn't been losing water (until now from the cap)

Today my mechanic fitted the water pump and tested the car by letting it tick over for half an hour. He said he tested it as if he had just installed a head gasket and it ran exactly as he expected.

Only just noticed steam coming from the cap will get a new cap tommorrow.

Could it be a combination of steam from the cap and a false reading from the sensor?
Expert:  Matt replied 3 years ago.
Hello John

a new cap may just fix it by the sounds of things

and once the system can't hold pressure then it'll produce gas inside the engine due to localised boiling, the gas then creates airlocks in the cooling system and this will give high sensor readings

so try the new cap as the very next thing