The following relates to a 1991 XJS Classic Collection with a V-12.
We started looking for this yesterday, I had idled it until the temp gauge read N then shut it off, then it sounded like a steam vent for almost five minutes. Today I ran it until I could see it was leaking, 30 seconds, with the passenger side air box off, then shut it off. I used a mirror and light to look for this but being new to V-12's I'm at a bit of a loss. What I saw was a drop of water hanging from an exhaust bolt and a wet exhaust manifold. This was straight up from the passenger side motor mount. The block and head where dry with no sign of leakage, present or past, at the seam. The exhaust heat shield is still in place, it will come off soon because I saw evidence of an exhaust leak from the front cylinder. Is there a water line under the heat shield or running through the manifold? Or should I be looking at something else?

Thanks for your help.
RTD

 

Yikes.

I think you have a coolant flow/air pocket problem. It's the steam pockets (more than coolant temperature, per se) that cause problems with these engines.






No, but there's a coolant pipe/coolant rail directly above the exhaust manifold on both sides of the engine. They have the various temp sensors and, at the forward end, thermostats. I wonder if your leak is from there?






Any leak needs to be fixed, naturally, but make sure you properly bleed the system afterwards. You can't simply top-off the system. When you're ready we can give you some pointers on bleeding the system.

If the history is unknown you should clean the various small cooling hoses and pipes. All of them. They're often clogged.

Actually, if there's any doubt about the cooling system I'd suggest doing the full monty. Do it once, do it all. Coolant flush, have the radiator professionally cleaned, new fan clutch, new (correct) thermostats.

Cheers
DD

 

As Doug said 100%.

I am assuming by the abbreviated address locale, you are in the USA, and LHD????. soooooo passengers side is RH side, as in "car talk".

The water pipes Doug mentioned, that run between those 2 water castings are made of steel, and rot out very quickly, and often. At each end of those pipes are seals, of a rather silly design. These sit in a machined groove of the casting, and the pipe pushes thru the seal, applying pressure that keeps it is place. Weeeell that is the theory. In practice, these seals pop out.

On the rear casting is the hose that travels TO the heater tap, and is a leaker, as it is neglected until it leaks, coz you cant see it easily.

Exhaust manifold gasket noises are usually, but not always, sorted with simply tightening the manifold nuts. Fun job, NOT.

The integrity of the cooling system of a V12 is absolutely critical, and short cuts will have an engine failure eventually. Neglect is the thing you will be sorting here, NOT design faults.

 

Thank you Doug and Grant. I checked the pipe above the heat shield, no leak. So I took the smog pump and heat shield off. As soon as the shield comes off I see a freeze plug that's leaking, the one between the two manifolds. I thought cool that's not to tough. So I squeezed the radiator hose to check and crap that's not the big one. Out comes the mirror and light. When I squeezed the radiator hose I was able to see water dripping, just short of a stream, off the manifold stud and nut on the under side of the manifold just in front of where it turns down to the exhaust pipe. So I'm wondering if that stud threads into a through hole into a water jacket? I tried to get a pic of the stud and nut, no luck, It is above the pipe in the circle.

 

OK, memory here from waaaaay back.

There are freeze plugs between exhaust port #1 and #2. Then #3 and #4. Then #5 and #6.

My best guess is the one between #1 and #2 has let go, and will be a bugga to do.

Failing that, the head gasket is all that is there to allow coolant out, as this is an open deck engine, the head gasket circumference seal is critical.

 

I don't want to get to excited but I think that's it, I got a different angle on the mirror and now can see water coming from under the manifold on the flat around the plug. Since two of three are bad, I'll change all three. Maybe the other side too. I got the easy nuts off and all but one of the down pipe bolts, broke my swivel, so I'll finish disassemble tomorrow. I didn't even know to look for that, Thank you Grant.