I've regularly had to top up the coolant recently, after replacing the water pump, thermostat and belt. I found out the thermostat was leaking though, and replaced the aftermarket rubber seal with an OEM jaguar seal, that stopped the leak at the thermostat.

However, after another weak, again, low coolant level. Halfway my journey (after ~30kms of high way) I had to pick someone up and decided to top up the coolant level just to be sure. This is where I found out why the caps always say "don't open when engine is hot". I opened the cap within ~1 minute of turning off the engine, and a fountain of coolant came out. It looked like boiling, but the first coolant to come out and touch my hand was only luke warm?

I don't really know what the proper way of diagnosing this is, I do have a first question though, the direction of the thermostat. A thermostat has a side with big and small spring (the "charge cilinder"), and a side with a valve and a "bridge" holding it there. I put the side with the spring in first (towards the rear of the car), so when you take off the thermostat cover (with hose) you're looking at the bridge / valve. Is that correct?

If it is, I would really like some help in diagnosing / first things to check. We're talking about a 1998 XJ X308, 3.2L V8, also see my welcome thread.

Some more maybe valueable info: when replacing the waterpump / thermostat I flushed the coolant system 3 times with water (until it came out clear), and then flushed it once with the new coolant, before filling it up.
As a "bleeding" procedure, I filled up the car, waited 3 minutes, filled it up again (barely anything), started the car (level), let it run for ~10 minutes until it was nice and hot, let it cool down, and topped it up again (~250ml).

Currently the coolant is still very nice and clear (in the expansion tank), the engine does not smoke and the oil cap is nice and clean.

 

If you've fitted an OEM thermostat there is normally a little grub or locator on the face of the unit, this locates in an indentation on the thermostat cover. In other words the spring part goes inside the tower as you have done.

You should replace the coolant cap with a new one, it's an £11 part from Jaguar and it's something that is meant to be replaced every so often during servicing. One minute is not enough time to allow the cap to be opened safely, even 10 minutes would give you the same result. You should give it at least 20 minutes and then open very slowly to allow it to release the pressure safely.

 

I wonder if those 2 thin black Norma hoses are crossed, an easy mistake to make that's been done a thousand times....I talk of the thin hoses going onto the coolant tank, many have got these 2 mixed up leading to low coolant, can you have a look at the expansion tank - this is tucked low at the front left of the car behind the front wing - your missing coolant might be there.

 

I replaced my left most (radiator) "norma" hose because it broke due to accidentally leaning on it during water pump removal. The two thin black bleeder lines are connected in the same way as on this photo (not my car):

 

Your pipes are connected the correct way, although your coolant should be orange in colour if you've used the Jaguar specified stuff. Have you checked there isn't a weep from any of the joints around the pump, thermostat tower or from the bottom of the expansion tank?

 

During the past week troubles worsened and the car would smoke (water vapor) after a drive.

 

When you get the new thermostat assembly, do not use any of those paper gaskets. Only the rubber ones and I'd highly recommend using silicone sealer where those rubber gaskets are as well. Nothing worse than having to take it all apart again just to have to apply silicone sealer because there's a small leak. You might have an issue getting to the back 2 bolts on the thermostat housing. You should determine if your going to pull the intake manifold or buy a VERY small 1/4 inch drive 8mm wobble socket from Snap-On. $$$

 

Tons of threads on replacing the thermostat housing on this engine, in this forum and the XK8 forum. Read each: some folks had to raise the manifold to get at the back bolts, others could break the old housing and pull the bolts (seems some assembly guy was using two different bolt lengths). Putting in the new ones is a bit testy if you have not removed or raised the manifold. It is really, really tight (bought six different very small 1/4 wrenches, never could find a good match). There are special bolts that allow an allen wrench or (what I did) cut a slot in the original bolt with a dremel tool and used a screwdriver to tighten them down (they don't have to be too tight).

 

I replaced my plastic housing last fall with the metal housing. I made a crows foot wrench by heating and bending the open face 90degrees to get to the tough bolt. worked out very well and saved me the trouble of taking the plenum loose. Be careful and try not to get the knock sensor plugs wet. I did and started getting restricted perf. knock sensor fault.

 

Thanks for all the great tips, I'll make sure I read some extra posts. I was planning on raising the manifold a little, kinda afraid it might develop a vacuum leak afterwards though. I'll post some pictures of my process.

 

I had to lift the manifold a tat to get the bolt out (after breaking the plastic tower to pieces - the easiest way to get to those 2 bolts IMO).
It was only 5 min of work, just turn all bolts some turns, but leave them in place.
I had no vacuum problems afterwards.

 

About breaking the thermostat tower, isn't there a big chance a piece of plastic will drop into the engine?

 

I put an old rag into the hole to prevent just that.
Then, using a big vice, it came loose in big junks, not the hundred thousand pieces I expected.

 

Since you've opened up the coolant system, possibility there is air still trapped.


Try doing this - with engine cold, take thermostat housing top cap off. Take expansion tank cap off. Check level at thermostat housing, coolant level should by close to the top, if not fill at expansion tank till it reaches. Replace T-Stat Housing cap. Start engine, turn heater on high. Expansion tank should be about half full at this time, let it run till operating temp. If there is air in the system, it will overflow and bleed itself. Once at operating temp, and if its not over flowing, check level and retest.


Also when you changed the water pump - did you make sure none of the impeller blades weren't broken off and missing? I've had several go into block and cause a lot of issues.