I have been sinking a little bit of money into a 98 xj8 Long, I just had knuckle bearings replaced on a wonky wheel that was limiting ability to drive last summer. I was going to throw some paint on it and drive a few thousand miles this summer. But before I put $1,000 worth of paint on it I need to make sure she doesn't have a head gasket issue.
Last summer driving her on limited 30-50 minute trips I was every 100 miles getting low coolant warnings and on the first occasion saw a very little amount of steam coming out the front hood - sure enough coolant was low - so I topped it off. but then needed to keep adding coolant every 50-75 miles.. While recovering from a bad back I had a local garage do an oil change and I asked him to look for any coolant leaks. He said he couldn't find any but he said my 2ndary over flow tank was full of coolant.
that made me nervous about head gasket pressure pushing the coolant --- so I spent the end of last fall trying to nose sniff all over the engine compartment and the exhaust. Every MWF I was convinced it was coming from the engine (valley heater hose ?) and every T, TH,S I was convinced it was coming from the exhaust (head gasket).
I told the guy fixing my knuckle bearing not to do anything to the car if it had a bad head gasket.
After charging me $1,000 for the bearing job (seemed pretty steep) I asked him about the head gasket and he said coolant system passed his pressure test and the car was running so good he was sure the head gasket was fine - he said he has been doing foreign cars for 20 years and he is sure car is fine - drive it and don't waste any money trying to track it down. Drove it 50 miles (25 miles round trip with a break in time) and checked coolant level --- it was down - decided to measure it and it was 20 ounces that disappeared in 50 miles - no smoke out tail pipe, no smell, no overheating, no visible signs of coolant anywhere on road or drive way.- no signs of coolant in the oil
Any thoughts ?

 

Valley hoses are notorious for failure, they leak down the Vee to the transmission, or pool in the V.
The full overflow tank could be as simple as an air lock from standing, maybe bleed the coolant system and empty the expansion tank. This could be where the 20oz went.
There's also plenty of reports of hairline splits in the header tank, their plastic welded seem around the middle can leak with heat and pressure, if you can pick up a new one to fit and see how that goes, not expensive.

A "SNIFF" test would probably confirm if it is a head gasket, either a dye is added to the coolant or some other means of testing. Is there any white smoke on startup after standing, if not it's more likely coolant component failure.

 

It appears the coolant is going to the atmospheric tank when the temp. rises, but is not being drawn back during cool down. The pressure cap needs to be replaced.

 

I agree with Sean and RJ along with replacing the expansion tank and cap. Plastics that are 26 years old can definitely be compromised. Two things that can contribute to the overflow tank receiving coolant is 1) The original water pump impeller made of plastic was mounted on a steel shaft and when the engine was up to temp, the impeller had a tendency to slip on the steel shaft as they both had a different rate of expansion. There is a tsb concerning this. I’m going to assume that at one time or another your water pump has been replaced. They had a thick aluminum gasket where it meets the block. 2) With the engine cold, I would fill up the expansion tank no more than 1” below the bottom of the cap. Filling it up to the brim then installing the cap will only lead to over pressurizing the system when the engine comes up to temp and then the cap would do its job and allow some of the pressure to bleed off into the overflow tank.

 

Are your coolant pipes connected to the header tank as shown on the picture?

 

Thanks for all the replies, I did confirm my hoses are crossed correctly
and it looks like the previous owner replaced the plastic thermostat housing with the metal one

I am not sure the guy that told me my overflow tank was full of coolant was on the up and up - can you even see that tank without removing covers and such ?

There doesn't seem to be definitive smoke or smell out the exhaust
Last fall there was a solid coolant odor - I thought it was coming from the engine bay but I might have been wishing that into my brain and nose ---so far with limited starts this spring I dont smell any coolant smell yet.

I will probably check the cheap things first (caps, coolant flush, etc) I was hoping there was a nice quantitative test for the gasket - I thought my last mechanic said he had a fancy device to analyze the exhaust to check for coolant But if he does own it he definitely didn't do it

 

It's the water pump that A2B was referring to. There's no way to know which version you have just by looking at the water pump. You'd have to remove it.....and once you've removed it, you might as well replace it. If your old water pump has a plastic impeller, it might be slipping on the shaft as A2B mentioned. That means it's not pumping coolant like it should, which could cause overheating - even partial overheating, which might be pushing too much coolant to the overflow tank.

Here is probably what I would do:
1. Replace the cap on the expansion tank.
2. Inspect the overflow tank, which is ahead of the left front wheel. You have to remove the wheel and the inner fender liner to get a look at it. Check that the hose enters the tank from the top, and extends down inside the tank most of the way to the bottom, so it can suck coolant back into the system from the overflow tank. There is an open port on the overflow tank. If coolant gets that high, it will run out the open port onto the ground. Check all of these things, as well as the level of coolant in the overflow tank. There might be some coolant in it, but it shouldn't be 2/3 full or something.
3. If those two things don't fix it, replace the water pump itself, and probably the thermostat while you're at it.

 

Most of the current generation of coolants have built in UltraViolet die that will provide
a green or yellow glow when exposed to a black light. I would get the engine up to
temp and then look the engine bay over with the engine off. It's very possible you
will find a leak at either the T-pipe near the radiator, the vent line from the motor
back to the top of the expansion bottle, or out the front of the water pump. It will need
to be dark and you will probably see tracing of the coolant that runs back from it's
source. Black lights are not expensive and can really be helpful especially if looking
underneath so you can trace the source of the leakage.

 

The guys gave great advices
i Believe it's not head gasket check under the intake manifold it may leak there and just disappear because of high temperature it may also be the water pump gasket inspect the radiator tanks both sides it may leak also
If you will replace water pump go with motorcraft

 

Below the throttle body on top of the transmission is this plastic one way coolant valve. It’s part of the octopus hose. They develop cracks, slowly leak coolant on top of the hot transmission and can evaporate the coolant prior to hitting the ground.

On the passenger side of the engine bay is this quick coupler. It to is also part of the octopus hose. It can crack and drip coolant on top of the passenger side catalytic converter and evaporate the coolant before hitting the ground.

There is a plastic (what a surprise) 3 way tee on the drivers side of the engine below the cable side of the throttle body. They can also crack and drip coolant.

Check the plastic coolant pipe above the water pump for any splits in it.