Dear Members,

Last week I inspected a 1994 V12 6.0L convertible, 59,000km. Unlike the previous 5.3L 1992 XJS I inspected, this engine was entirely familiar to me. The car has not been driven much in the past two years and will require all of the usual fixes that I undertook on my 1993 Daimler Double Six (suspension, fluids, belts, hoses, brake accumulator, pads, distributor etc). The engine ran smooth; acceleration was the same as my Daimler, transmission fine.

BUT, the car presented with a few symptoms that has led me to get a full, third party inspection which will test engine compression etc before I make any offer.

The symptoms were:
a. AC unit did not work at all. (My AC unit is progressively putting out less cold air, but still works at 65,000km; this one not at all).
b. After a test drive that included a highway drive, a lot of heat was coming off the engine when I opened the hood. More than I have ever experienced with my Daimler. The temperature gauge never went above the middle mark, but the heat was noticeable.
c. After I turned the car off, the cooling fan kept running for about two minutes, then it turned off. Then it turned on again for about a minute. The car is running warm to hot.

My question:
What are the classic symptoms of a late model V12 6.0L XJS that has overheated in the past? What should I look for?

Best wishes,
Charles

 

Point A: AC failing is fairly common, it just means the refrigerant has leaked out. A very common leak point is the fill ports themselves, there is a spring loaded rubber ball that provides a check valve, and over time the ball doesn't provide a seal anymore and the refrigerant leaks out. The ports are standard General Motors parts and are available quite cheaply. They just unscrew to replace them. I have put the GM part numbers in posts in the archives somewhere.

Point B: The engine bay of the XJS is tighter than in your Daimler, and the engine does get hot. If the gauge is staying in the normal range, that is good, but you will feel quite a bit of heat coming out. I currently have a 1994 6.0 XJS and I used to have a 1992 Series III V12 car and the XJS heat is more noticable when the hood is opened.

Point C: There is a thermal switch for the smaller electric fan and it is designed to run after shut down until the temperature of the coolant drops below the set point of the switch. It's to prevent heatsoak of the engine when the mechanical fan isn't running. This is perfectly normal, and operating as designed.

One thing that is very common on the XJS is there can be a large amount of debris between the radiator and condenser that gets pulled in by the fan. I have seen radiators more than 3/4 blocked on their face with leaves, grass, cat hair - all sorts of things. The only way to properly clean it is to remove the radiator. It's a big job, but worth it.

The other thing that is common, perhaps almost guaranteed is that the thermostats are wrong. They need to extend to block off a bypass port, when the port is blocked all the coolant is forced to go to the radiator, otherwise it circulates through the engine block and never goes to the radiator. I have measured brand new dealer supplied thermostats and they do not go far enough, such that at least 50% of the coolant will never go to the radiator. There have been a number of posts about that here by Grant Francis in particular. Changing the thermostats will result in the car running cooler, and should be on your "must do" list if you buy the car. Again, part numbers are in prior posts.

There is no way to tell if the car has moderate overheated in the past, only if there has been gross overheating and a valve seat has dropped - then the car either won't run at all (most likely) or will run very badly. It would be immediately obvious and would make a racket; it's not subtle.

 

Hi Charles

(b) + (c) Sounds perfectly normal to me, as these Engines do get hotter than many would imagine 'as in hot enough to fry eggs' which I actually tried to do as a 'Myth Busting' experiment which turned out to be a great success!

Or at least it would have been were it not for the Flies who found the GF's Cooking too delicious to resist!

Although if you follow 'Jagbois' advice, then you won't go wrong

How to Cook Eggs on The Engine Of a V12 XJS (Don't try this at home!) Near the bottom of Page 87 on my 'Cherry Blossom' Restoration Thread

How to Cook Eggs on the Engine of a V12 XJS Don't Try This At Home!

 

This is so true. The difference in cooling system performance with the proper thermostats is remarkable.

Regarding the AC issue, low mileage -- but older cars -- are very prone to compressor seal failures due to lack of use so that could be the issue.

Cheers

 

The car is fine. If you like it buy it. Then all the fun starts as the above posts have pointed out!

 

I agree,

BUT IT, enjoy it, life is short, as we all know.

The radiator will need to come out and be flushed, just to be sure, and all that sitting will have settled sediment in the lower tubes, so no surprises there at all.

Good luck

 

Is the coolant level full? Any consistent overheating, or head gasket leakage will result in a low coolant level. Additionally, if the head gasket is leaking, you might have pressure in the cooling system when the engine is cold.