Hey guys,

Just purchased an '89 XJS 3.6 coupe manual. She is losing coolant - first alerted by low coolant light coming on. Now needs topping up every 100 or so miles. The fluid is exiting ahead of the driver's door (RHD) between the sill and the front fender/wing. I thought it might be from the expansion tank overflow, but the coolant is COLD. I also ran the engine with the cap off for 15 minutes and there is no fluid exiting the overflow. The top pipe from the rad to the engine feels empty when squished. Any ideas? With the heater/fan on full blast, air from the vents is at best tepid. Could it be leaking from the heater matrix somewhere? No leak is visible in the engine bay.

The previous keeper recently replaced two hoses under the inlet manifold and topped up the coolant. The temperature gauge sits perfectly just under halfway at all times. No evidence of overheating but am now concerned to drive the car. She just completed an 800 mile drive to get to her new home, topping up regularly en route, so I would imagine a head gasket would've let go by now. Running is smooth. Any advice gratefully received.

Thanks!

Jason

 

I am not familiar with the 3.6 cooling system. On the LHD V12, there is a second coolant tank behind the LF wheel. Does the 3.6 RHD have that behind the RF wheel? If so, it could be as simple as a bad pressure cap, allowing coolant to be pushed into the tank when hot, but failing to draw the coolant back when it cools.

 

Hi Jason,

As said above, the 3.6 has an overflow tank behind the front right wing (drivers side on RHD cars) between the wheel arch and door edge. This is a plastic bottle and is not pressurized, the hose that goes into it is a fairly loose fit. The bottle sits above the sill and any leaks from this area will exit from the wing / sill gap.

The role of this second tank is to act as a reservoir to collect water from the expansion tank when the engine heats up. As the engine cools, the metal expansion tank in the engine bay will suck coolant back in from the under wing reservoir. So, the expansion tank in the engine bay should always stay full. This expansion and contraction even occurs with ambient temperature change even when the engine is not running.

Suggest you take off the inner guards under the rear of the wing and have a look at what is happening in the overflow tank area. Could be a leaky bottle or hose. Also worth doing some rust prevention work whilst you are in there as this is a classic rust prone area on the XJS.

As suggested by Dleit 53 check you have the correct pressure cap that will allow coolant to be drawn back in from the under-wing overflow reservoir through vacuum in the metal expansion tank. It is not just the pressure rating that is important.

Heater issue may be unrelated.

Good luck and let us know how you get on,
LeeP

 

Hey! Another Jason... This is good.

I'm with D-53!

If there is an overflow type plastic jug buried in the back behind the splash shield in LH wheel well (I don't know if it's different on LH RH cars) like on the 5.3 v12 - there are many and stern warnings to dig in there and pull the tank out, clean or replace the lines and completely clean out the jug.

Could even be a crack in the jug and when coolant gets pushed into it it just leaks out...

On mine, from memory, with the wheel off, it's a matter for removing a few phillips type screws or small bolts, usually caked in rust proofing and debris... Mine was pretty clean amazingly but I hear they can get completely filled up with mud, rust, gumm and such and not properly accept or release coolant as it's trying to push in and out.

 

I just discovered that tank on the facelift 4.0....right behind the (passengers) right side wheel well.

Its entirely plausible you have coolant entering that tank. My plastic tank appeared about 1/4 full, if not less.

 

Probably only want the under wing tank about half full at cold. Much more and it may overflow as the engine reaches temperature.

Cheers,
LeeP

 

Guys - thank you all for your responses. Haven’t had a chance to follow your sage advice yet, soon as I get a window I will jack her up and investigate. Indeed, there is more rust on the front of the driver’s side (right hand) sill than the other - could be this has been dripping for a while now. Not to worry, some paint/metal tlc was already on the list Rather this than a head gasket - though of course, you never know...!

Good to know the cat-loving brotherhood is strong out there. Best of motoring to you.