Coolant Leak (Series III)

Frustrated here. Last time I drove the car (I bought it last year and am trying to recommission it to reliable driving status), I got to my destination and had an active coolant leak coming from the hose between the water pump and thermostat housing. It took a long time for me to gain motivation to do the job, but I finally did it last week, also replacing the thermostat (which was stuck open) and housing gasket.

Today I took it for a drive to see if driving it would bring it up to temperature (the gauge has never read in the normal range, likely owing to the stuck open thermostat). Instead of coming up to temperature, the low coolant light came on. I got it home and had an active leak in the same area as the previous leak where I’d replaced the hose between the thermostat housing and water pump.

When I did the job, I thought that I probably should also be replacing the hose clamps, but I did not. Today I got a pair of brand new hose clamps to replace the old, topped off the coolant, started the car, and I STILL have a coolant leak there.

I cleaned it all up so I could see if I could determine exactly where the leak is coming from. It is coming from where the replaced hose slips onto the waterpump. The hose clamp is tight, the hose is seated all the way on the waterpump, and the hose clamp is on the hose only where it is over the waterpump male orifice (hmm, is that the right terminology…:?) :slight_smile:

I dried everything up and with the car NOT running, just stared at that interface between the hose and the water pump. Everything is secure, but every 30 seconds, I can see coolant start to form a little drip at the bottom of that hose. If I start the car, the leak there becomes more active.

I suppose it is possible there is a crack in the waterpump? I hated taking that hose off – it was ok because I had to replace the thermostat. But this time I don’t have to replace the thermostat, and I don’t think I can slip that hose off without removing the thermostat housing.

Anyway, venting my frustration. Not sure there’s any guidance to be given, but I just wish I knew where the leak emanated from so I could just address it and be done with it. So much left to do on this car that it’s frustrating that something so simple is sidelining me.

You may have a split hose or a pin hole leak in the hose or some rust on the pipe end the hose attaches to. I put some grease on the inside of the hoses before I fit them to the pipes it helps to seal and makes it easier to fit the hose over the pipe fittings and lets you get a tight hose further on the connections.

It sure is a tight fit, Matt - it is arguably easier with the thermostat housing removed. And since you have already done it…

The purpose of that small hose is to bypass a closed thermostat during warm-up - ensuring coolant circulation pump-head- water rail. Have you considered that the leak may be at the thermostat hosing end - just running down along the hose?

As Gary says; for a closer inspection the hose must be removed - and possibly replaced…

xj6 85 Sov Europe (UK/NZ)

Thanks for the replies.

This job happened because a big coolant leak occurred at that hose a few months ago. The hose was ballooned out. I ordered a new hose and requested hose clamps but the supplier didn’t have hose clamps and told me to just get new clamps locally. As previously mentioned, I also got a new thermostat and thermostat housing gasket. One of the bolts (the longest one) that holds that housing onto the front of the head would not come out of the housing, so I had to pull it all out with that bolt still in the housing.

Putting the new hose on the housing and then slipping it over the waterpump inlet was difficult, and in fact I did put a small amount of grease on both ends of the hose to facilitate slipping it on.

When it leaked on my drive yesterday I was dismayed but I assumed that in fact the hose clamps had failed. With new hose clamps it still leaks.

I cleaned everything up and just stared at the area waiting for a static leak, and that leak occurs at the bottom of that hose, as if my lower hose clamp where it secures to the waterpump is too loose. It is not too loose! But there is a tiny static leak there nonetheless, and it becomes more than a tiny leak when the engine is running.

I don’t think it is the hose leaking, and I don’t think it is the clamp. Here is a picture I took of the area after the initial leak. Looking at the rust on the waterpump makes me wonder if it is actually somehow leaking from the pump. I know it is a greasy mess in this picture, but remember, I’m just in the initial stages of recommissioning this car.

The lower by pass hose connects to the water pump by a fitting that screws into the water pump housing this is sealed buy a copper washer looking at all the rust on the housing you may be leaking through the threads of this fitting or the copper washer has failed. you could take this pipe off and put plumbers thread tape on the threads and fit a new copper washer.

To me, the picture depicts that top hose not fully seated on the radiator tube .

A po cut it short? New hose if it will not be long engh to seat fully. And, an old fix i have used Two clamps. Screws apart 180.

And use a socket to tighten., Screw driver at times not tight enough.

At times, I clean the area where I plan to work, works best all the way around.

As Gary says, Matt - picture shows the ‘main’ radiator hose…?

Of course, a leak is a leak, though not visible in the picture. Nothing obvious seems to be amiss, but as Carl suggests; adding another clamp may provide something. The pump body is aluminum and cannot rust, but it can still have defects - which should be visible.

Increased leakage as the engine warms up is to be expected - the system is pressurized as coolant expands.

The long bolt on the thermostat housing sticking to the housing is a common problem - bolt corrodes to the housing. With some coaxing it can usually be freed, but brute force is likely to snap it - which causes much trouble…

xj6 85 Sov Europe (UK/NZ)

To clarify, don’t look at the main upper radiator hose, but instead the smaller hose just below it. It is ballooned out a bit. I don’t know if there was an actual failure in the hose or the clamp or both, but I’ve now replaced that small hose and both clamps. Look at the bottom of that small hose where it connects to the waterpump. Look at the hose clamp there – it is right next to the adjustment rod for the power steering pump (I think that’s right by my recollection). It is leaking right there now at that hose bottom. It is a brand new hose and brand new hose clamp, the hose is seated properly, the clamp is tight. Note in this picture, the rust just below that hose clamp – that’s on top of the water pump. Gary mentions above that this is a fitting that secures to the water pump housing. I suppose that fitting might not be aluminum. There is definitely surface rust there. But as he mentions, perhaps it is the copper washer.

I’ll have to take a look again. I hate to partially drain coolant again, but it is what it is.

The thing is, if I run the car and just let it sit, and I stare at the bottom of that small hose, that is where the drip comes from. It looks like the hose is what is leaking.


Why not try to double clamp the bottom of the hose first you can get a second clamp in there. The tube / pipe you clamp to is long enough to do this and it might just save you pulling it all apart.

A sage recommendation – I will try that before pulling things apart again. What I really should do is replace 100% of the hoses and clamps with high quality stainless steel clamps. This is what I usually do when I “get in there” on my cars, but I have a serious issue with “While I’m in there” to the point where if I start going down the path, I’ll never get the car back on the road anytime soon. At least for a little while, I just want to drive the car, but I need it reliable first. You know, for example, not spewing out coolant while driving or sitting…

Following up, I did add a second clamp but it didn’t help. I had an appointment set up for a few things at Master Auto Jaguar in Rockville, Maryland, so I’m sheepishly letting them figure out my simple coolant leak as well. I’ll update here once I find out what the true culprit is.


maybe there is corrosion damage at the neck preventing a good seal. Apart from that the only thing I can think of is a wrong size hose. If the hose is too big, no stainless or anything gizmo will clamp it down.

It is a tight thing: as much as you like a hose to just pop on the neck like … comparison of your choice … chances are it won’t seal then. If you think the hose will never get on the neck, it will eventually find a seat ,… and seal well.

I’d try another source for the hose.

Good luck


75 Xj6L 4.2 auto (UK spec)

The issue was simple. I’m the guy who replaced the hoses at the thermostat housing as well as the thermostat, but the new leak was from the new thermostat housing gasket that I also replaced. User error. It’s fixed now.

Great - thanks for letting us know!

Keep the spirit


75 XJ6L 4.2 auto (UK spec)