I have a slight, slow coolant leak from my XJ8. I have inspected thoroughly and can see staining below the plastic cap at top of the engine. I'm not familiar with XJ8 engine but from my classic car days, I suspect this is the source and I am hoping that I can just remove and re-tighten the cap... Perhaps with some PTFE tape (or other product e.g. PFTE paste) put in place.

Photos attached to show the suspected site of leak (as if coming from the thread of the plastic cap in centre of picture). If this is the source then it hasn't been undone in 8 or more years and I'd hope it would be a release and re-tighten job !

I thought I should ask before I touch anything that I don't know about !!!

Hence the questions:
1. What is this cap ?
2. Should I remove and re-tighten ?
3. Should I use PTFE tape or some other product ?
4. or Should I put a bit of Rad-Weld into the system like I would on my 1970's Rover V8 ???

Apologies for something so simple, but I'd hate to go in there with 1970s knowledge and screw up an expensive engine !

Thanks

Graham

 

Thermostat housing cap, either replace it with a new cap(they crack in the center), or buy a metal complete housing from ken at motorcars and replace the entire housing

 

This is from a 99 xk8 but should help http://www.gusglikas.com/AutoRepairTherHousing.htm

 

Thanks Brutal and thanks Gus,

Housing looks OK. As predicted by brutal... the cap has cracked in the centre as I have tried to refit it. I suspect the rubber ring seal is too old.

I will buy a new cap. part number NCA2270AC as per this link

http://www.britishparts.co.uk/.sc/ms...0HOUSING%20CAP

Would you know if the rubber seal ring comes with this or if I need to order separately ????? If so would you know the part number ????

Thanks,

Graham

 

It comes with it, at least in the us

 

Thanks again for the advice.

I have the new cap on.... and the leak looks the same !! ie A slight leak down outside of housing thread identicle to the original photo above. The housing and its thread look fine.

To summarise.... I took the old cap off due to a slight leak down the outside of the thread (photo above). Then I re-fitted the old cap but when re-tightening it found I needed an extra tweek with the hex wrench which finally got rid of the leak. Hwever this final tweek fractured the inner plastic of the cap at the site of the hex slot.

Now I am re-fitting the replacement new cap CAREFULLY. I am having to estimate the 9Nm torque as my torque wrench does not go below 40 Nm. I estimate that tgbis is hand tight plus a firn tweek with the hex wrench... I then find that i can easily tighten further once system is hot.

I am concerned as to whether I am tightening the cap enough and I am concerned that I may be tightening too much and stressing the thread !! The worry is that if I over-tghted I will damage the plastic housing thread and then have a bigger job.

I am considering RAD-WELD. Is this advisable ? (works nicely for similar on classic cars !)
I am considering silicone sealant. Is this advisable?

The jag part department tried to sell me a metal cover to push over the plastic cap. I assumed this was cosmetic and turned it down (as I didn't have one previously). Could it be that the metal cover adds strength to the plastic cap thread for when the collant system becomes pressurised ??? Should I but and fit ???

Any practical description of how far to tighten would greatly help.

Help !!!

Thanks

Graham

 

Had the same issue. You can either replace the entire plastic housing and cap with an aluminum one from Ken. Or you can do what I did. I took the new "leaking" rubber seal out of my new cap and replaced it with a rubber gasket I made out of gasket material. If you look inside your new cap you'll notice the rubber seal is smaller than the inside diameter of the cap. If you overtighten the cap you will squeeze/pinch the seal and it will leak. The gasket I made fit completely inside the cap and has worked well for 30k.

 

I had the same problem with mine. I started with replacing the just the cap and over torqued it damaging the threads. So yeah, be careful with that! The next step was to replacing the whole housing. I opted for another plastic one but if I had to do it again, I would buy the metal housing as stated above.

 

Thanks,

this looks like the probelm. Ii'm going to try two of the supplied rubber seal vs one rubber seal plus PTFE tape.

 

Thanks,

This has helped me get a handle on how much torque. A bit of experimenting seems to indicate: Less Torque causes less leak.

I'm going to start a new thread with new title to ask about a possible nighmare cause of my symptom. (See new thread)