I've noticed I have some cooling system issues with my 1996 XJ12. The first sign was gurgling from the expansion tank when the car is turned off.

After a long drive, I've noticed coolant dampness around the cap, and down the inner fender under the hood. All the hoses and clamps are tight. Sometimes I won't get any coolant loss at all, even if the car is completely warmed up.

I replaced the pressure cap with a Jaguar genuine cap. I get the same issue. The car doesn't overheat, I don't have any residue in my oil, nor do I have smoke or sweet-smelling exhaust from the car. The engine runs perfect, without a single miss.

With the pressure cap off, and the car running, revving the engine will cause the coolant level to bubble up to the neck of the reservoir, even spilling over if I rev the engine high enough.

Any ideas? I'm worried I might have a head gasket issue. :/

Thanks,
Nick

 

Possibly a head gasket leak. It could also be a clogged cooling system, like the radiator. You're still on the original hoses which are getting replaced anyway, so a thorough flush is in order. I bet its nasty in there. That would give you the chance to check the steel bypass pipe for a rust clog too.

You can also test the coolant for combustion by products, and of course do a leak down compression check. It is a necessity to get the cooling system tuned up, but you might want to know about the head gaskets before you invest in other repairs! Hoping for the best.

 

I'm going to get a block test kit tomorrow and see where I stand as far as combustion gasses in the cooling system.

The coolant is a yellowish color and seems to have a fair bit of stuff suspended in it when sucked some up with a tester. I haven't had to add any coolant in about a month or so.

The bypass pipe is the black pipe located on the front of the engine, connecting the output of the water pump to each bank of the engine, correct?

Thanks for the help. We'll see how it goes. The car is really so clean and it would be a shame to find a head gasket is shot when the engine runs so perfectly. As an aside, they can be replaced with the engine still in the car, correct?

-Nick

 

Coolant test is a good start, although it might not be a definite positive if the coolant is old. Yellow coolant is probably old! However, a negative test would be very reassuring.

Yes the bypass pipe is black, at the front of the engine. I think the head gaskets can be changed in the car if necessary, but what a bear that would be! Let's hope we never go there.

I did my cooling system overhaul 4 years ago. It was overdue for a changeover. The coolant I drained was was truly disgusting, a yellow shade with lots of debris in it like you described. Better get busy!

We will stay tuned...

 

Also Nick, in case you didn't know, when you start dismantling, be careful of the radiator inlets and lower outlet. Like most cars, the tanks are a non metallic poly material. Hopefully they are still in good shape.

 

Thanks for the heads-up, I'll be extra careful on the radiator connections.

I drove the car about 40 miles down the highway after work tonight, doing about 70 the whole time, much longer than my 8-mile round-trip commute to work.

The engine got nice and hot and after returning home, I didn't have any evidence of coolant leakage from the expansion tank cap. After turning off the engine, though, I got the usual 2-3 seconds of gurgling in the expansion tank.

We'll see how the block test goes.

Thanks again,
Nick

 

So I got a block tester kit from NAPA today, removed a little coolant from the tank and started the engine. After about two minutes of pumping, with the car at operating temperature, the fluid stayed dark blue the whole time. Nothing at all. Hopefully this means I'm out of the woods as far as head gasket issues.

I did notice that with the cap off, I don't get any bubbling in the tank while revving the engine, but the coolant level does change quite a bit. With the tank level lowered below the minimum mark, after holding the engine st 2,000 RPM, the coolant level rises to above the MAX mark, and when I lower the engine to Idle, the coolant level drops to below the MIN mark again.

The coolant I removed was very nasty though, more tan in color than green or yellow. I definitely need a flush, and have two new Jaguar thermostats and gaskets. I just need upper and lower hoses and I'll flush the car out.

Any ideas?

Thanks,
Nick

 

Thats good news, Nick. I've not used a kit like that before, but sounds like a cool invention.

Aside from head gaskets, we are back to system blockage. Notice in the V12, the heater hose has a direct connection to the coolant tank as the line returns to the main lower hose (there is a branch under the reservoir). There might be too much back pressure at the heater hose due to blockage somewhere, which could force a backup into the tank. Perhaps even the thermostats are causing it. You will find out more once you get into it.

BTW if those heater hoses are spongy, they get to be replaced too! I did ALL the hoses back then, about 12 total. I believe Rock Auto has many of them (Uros) including the 3 short bypass hoses and the spider heater hose. They all fit well as I remember. It is tedious, but worth the peace of mind. Long term, the car deserves it, right?

 

You're right SleekJag12, I really should replace all of them while I have the system drained. I did the same with my '92 XJ6; I ordered the 15 or so hoses from RockAuto and replaced them all. Even had to pull the intake manifold to replace the dreaded coolant rail hose. But there's so much peace of mind now, knowing the whole system isn't dependent on decades-old rubber.

I did use the URO hoses as well, and found them to be okay. I think I'll go with them again for this car, too. I figure even a new hose that's aftermarket is much better than an OE hose that's 17 years old.

I did notice there's something funky going on with that pipe that runs under the expansion tank along the fender and down the left side of the radiator, connecting to the hose that's molded into the lower radiator hose, I believe. Its definitely got a little rust on it where it connects to the hose, and it looks a little dented too... I should order that as well, I think.

New antifreeze is a definite... Yuck!!! It's still a bit translucent, and it has never been frothy, (so I'm hoping it's not oil mixing in) so hopefully a few flushes will clear it all out, and I can get clean green stuff in it.




Thanks,
Nick

 

Now that you mention the metal heater pipe section, I bet it is blocked with corrosion! That would surely explain the backup into the reservoir. A bit unnerving to see the coolant come up like that with only a little throttle input. If so, I doubt if any harm has been done.

Thanks for posting the video. The tester seems easy to use, and worth it for the peace of mind!

 

Attempting to revive this dead thread as my XJ12 appears to have started exhibiting the exact same issues. I've tried reaching out to @NTL1991 via DM and email to no avail.

My car is behaving the same way runs fine, stays cool, exhibits no symptoms or evidence of exhaust gasses mixing with the coolant, but it pushes coolant out the cap at a very slow rate when the car is up to temperature. When I increase the revs with the cap off the level will rise from max to the point of overflowing from the chamber.

I changed the coolant when I first got the car and followed the bleed procedure in the manual so I believe the system has been bled properly. I highly doubt its a head gasket or something gravely serious as it appears @NTL1991 was able to drive his XJ12 for several more years before it's untimely end with no relation to the cooling system...

Any thoughts/ideas of where to look?