Howdy,
I was searching the forum here for a way to test the water pump on a 1998 XJ8 (V8 4.0L VIN: 842068, Milage: 86k) when I came across this bit of advice...

While you have the thermostat out, put a bucket in front of the housing, start the motor, and rev it; only for a few seconds. If coolant gushes into the bucket, your water pump is ok.

I took this to mean water/coolant would be coming from the thermostat housing, but as you can see in the video below, when I ran the test, only the radiator hose produced fluid...

Jaguar 1998 XJ8 water pump test - YouTube

(I should mention that I didn't follow the instructions exactly - the thermostat was in place - without it, the water ran straight out and I wasn't able to perform a test)

Does the fact that fluid is burbling out of the radiator hose upon revving mean the water pump is working?

Is it possible that a failing Coolant Temperature Sensor could stop circulation?

Thanks for any and all advice!

 

1 belt and 8 small bolts and the water pump is in your hands!!!!!
Put the bucket under the front of the engine to catch the spill and remove it to look for yourself.

Jaguar warranty time is less than an hour to replace.
I must have done a hundred and got REALLY fast at replacement when the original ones were failing in the late 1990s.

bob gauff

 

"Is it possible that a failing Coolant Temperature Sensor could stop circulation?"
The answer id NO. The temperature sensors control fans and engine fueling parameters, and do not have anything to do with controlling heat (except by running the fans) or water flow. Thats what the thermostat does. It opens when it senses hot and closes when the water is less than its operating temperature (about 190 deg F).

 

Thanks!
So, to properly test the water pump / thermostat, the car needs to reach operating temperature - is that correct?

If that's the case - the video I shot is inconclusive since we're beginning from a cold start.

I've got a brand new thermostat in there, so, if there's no water coming from the thermostat housing at operating temperature, that points to a Bad Water Pump?

Thanks again!

 

Given the age and likelihood of plastic impellers, just replace it and hoses too.

Now, some of those 8mm bolts get their heads easily stripped making it more challenging than Bob suggests. And my belt tensioner sheered its spring loaded bolt . . . But still necessary and worth it.

 

Thanks for the input.

I made a new video today while the car was running at operating temp.


Upon revving, you can see a bit of water sputtering out of the thermostat housing. There's quite a bit more gushing out of the radiator hose.

Is this indicative of a faulty water pump? Or is it normal to have more coming from the radiator?

I had just topped off the water before the test, btw...

 

As far as I am aware, most cars have coolant circuits that direct water UP from the bottom of the radiator, not down from the top. It follows therefore that the water being pumped directly from the pump will gush from the top radiator hose as it will arrive there before it reaches the thermostat. Therefore I would suggest it shows that your pump is working, but of course, does not necessarily show how well it is working, it could be pumping a reduced amount. I could be wrong of course, but that is my understanding.