Panic reporting - valves expert opinion needed : sudden zero compression on cylinder 2, XJ6 1985

Two days ago I was bragging about the condition of my XJ6 which drove as a new car until… this morning.

Today, for the first time it failed to start at the second turn of the engine and idling very roughly, engine shaking with a metallic valve-related noise. I went for a short drive but had to came back immediately, it was obvious something was wrong. I took off the spark plugs, all clear except cylinder #2. Measured compression, zero on #2 but looked inside and saw no damage to piston whatsoever. Measured head temperature and around that cylinder it seemed 5-10 degrees hotter (but no guarantee on this last observation).

The engine never overheated and the temperature has always been on the spot ,slightly less than 90C.

One thing that may help the diagnose, I heard that metallic noise 3-4 times before time ago, but maybe for 30 sec upon starting the engine . but then it would disappear and the car would drive perfectly . This time it stayed . Link to YouTube below.

I will search the forum for sticky valves, bent valves, burnt valves, dropped seat valves .

If you had any similar experience or dealt with one, please let me know.

Now that I was concentrating on the AC as I wanted to put the car as coming out from the factory :-(((

Many thanks

It is possible that you have a dropped valve seat in #2 cylinder. Another possibility for that noise is a loose tappet collar. At this point you will have to remove the camshaft covers and cylinder head to find out what failed.


Thanks Paul, it was all of a sudden . I suspect loose/ dropped valve seat that could have gone back in place after heating, until it dropped for good, Or a burned valve but then i do not think that it will make that metallic noise.

I now added a video where one can clearly hear the noise and see the shaking . Yesterday it purred .

I watched and listened to your video. If I were you I would not start that engine again until I removed the cylinder head to inspect for the cause of that noise and loss of compression in #2 cylinder.



Indeed . Car now in the garage , I will go on holiday next week and will have a look when back and I will have time .

Leave everything connected, disconnect the exhaust downpipes (two gaskets), fuel line and everything up to the throttle, don’t forget the oil lines at the back, 6 screws at the front of the head and remove the camshafts obviously. It‘ll take you a day if you take your time, without a crane you need a helper, good to have wedges and a bottle jack. I hope it’s nothing major.

As Paul says do inspect the tappet guides before going in; but that may have caused a bent valve as well. Hard to see markings on the piston if that happened.

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Thanks David, meanwhile I also read the following thread How to check if valve seat dropped due to overheating on a 1988 Jaguar VDP V12 5.3L and my problem seems the same, except i had no overheating but I do have zero compression in cylinder #2 . Pity there was no more reporting or follow up from John hriant88

I also see that references to drop valves are mostly related to the V12 , but what about the XK in this respect?


Go in and find out!

Along with others, I would caution you to not start that engine, again, until you at least remove the cam covers.

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The xk is not immune, Rui - but it is not common, and then usually on the exhaust side…

The noise may be a wandering tappet guide, see ‘stakedown’, but that should not really interfere with valve operation to that extent…

The valve seats are press fit and may fall out, but the intermittent character of your fault sounds odd - never heard of a seat falling out and get back in place! Nor bent/broken valves temporarily mend themselves.

Cannot see any alternative to removing the head for a proper fix - but with the valve covers off; carefully check the valve train before removing head…

xj6 85 Sov Europe (UK/NZ)

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my $30 borescope is small enough to fold over so it points up when fed down a plughole

It doesnt really matter though, as the cam covers have to come off, and then 99.99% the head, so just relax & prepare


If you don’t already have them, I recommend that you get these three Jaguar documents that will be essential to your work on this car.

These include the Series III Service Manual, the Series III XJ6 Parts Catalogue, and the Series III XJ6 S57 Electrical Guide. Pictures of the covers of my hard copy documents are attached. Having and reading these documents will answer many of the questions that you may have and they will save you time and money as you work on your XJ6 for this project and the others that will come up in the future. I got these on eBay about 20 years ago and I have used them regularly since then.


Thank you all,

@Frank_Andersen , @PeterCrespin explained it in the other thread and the symptoms in my case seem to confirm: The first time i detected the problem in winter ( i drove very seldom since then) it started by a diesel kind of knocking (metalic) upon start-up that disappeared soon after. Imagine the seat drooped slightly with a valve half closed but it was still in place and moved back up to stay in its place when hot. The issue repeated itself 2-3 times, until this time is was fully off with side effects - shaking engine , zero compression etc.

But as you all say, only an head-off job can tell (folding scope yes but not that tiny - still will try again) . Very frustrating as I have no real pleasure in ´fixing´cars that are in top order, I like to restore to bring things back to its former glory but not work on things that were already perfect and then break down , worse if that happens by bad design. If I restore a car I can enjoy the after and before, in this case the after will be exactly the same as three days ago when the car was driving silk-smooth, not 50 years ago. no joy. I am already to busy with a Renault 4cv from 58, a Ford Y for 32 etc, which needed restoration, the new tiles in the bathroom waiting now for one year… well.

I was actually considering selling the XJ6 as it was too perfect …

As for the material Paul, many thanks, I am also fully equipped on that side :slight_smile: Extremely helpful but some of the Jaguar procedures are wrong or far too simplistic and do need complementary information as the one we can get through this forum.

I will report back in October after my holidays and a work trip.


This thinking only leads to more mechanical trouble.
Three days, break in between for parts to arrive and you have it all back in place.

I was in the same place when I bent four valves due to really old fuel.

Carry on…


Thanks David for the encouragement, that photo kind of makes me willing to head on to the garage right now :grin:. But free time in October only - I promise news by then .

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@Frank_Andersen , what so you exactly mean by check the valve train before removing head ?

My plan is to remove the exhaust valve cover first to see if i see something strange. Is that what you mean or do you have something else in mind?

Its quite a good idea to let your favored penetrating oil soak down between the studs and head for as long as possible

The very first thing I would do is remove camcovers, inspect for visual damage and rotate engine by hand to see if the suspect buckets move


Indeed, Rui; removing valve cover(s) is the first step - unless the borescope reveals something.

The main reason is to clarify, as far as possible, what is the failure - and what was the cause of that failure…before starting dismantling for repairs.

As the exhaust side valves experience higher temps than inlet, it’s natural to start there. However, the fault as yet unidentified, may be ‘something’ else, both in cause and effect.

Check valve clearances. Check tappets, tappet guides, cam shafts, springs and surrounds for sign of damage/anomalies. Specifically for #2, exhaust and inlet, of course - but also for comparisons with others.

The spec clearances is 0,012 - 0,014; too tight may cause valves to run too hot which may cause valve and seat problems. But tight, or to specs, nominally means the valves are closed and seated

Excessive clearances means bent valves, ‘unseated’ seats or possibly broken spring. If one side is normal, it is unlikely the other is - unless you have a head/gasket fault. Or more exotic; failed collets…

These are preliminary; any compression fault almost certainly means a head removal for proper inspection/repairs. But checks may, as said, clarify matters - useful later…

xj6 85 Sov Europe (UK/NZ)

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Holidays over , coming back to this issue .

Removed exhaust valve cover and here’s what I found :

1 - lots of black sludge , not expected as oil was changed every year and at the symo looks clean …

2 - exhaust valve cylinder Nr. 2 ( the guilty one ) has a gap exceeding 0.7 mm (0.028 inches) ! - certainly due to valve blocked

I recall the car has 110.000 km , was not often driven but every couple of months or less or so and he felt like new - performance , fuel consumption etc - before the problem showed up .

I may be wrong but that amound if sludge in the valve train was not expected…<–what are your views on this ? could it be that the sludge in the valve guides is the cause for a non-proper closing?

Anything else I could do before start to take the head off ?

Many thanks

That sludge looks very indicative of some kind of snake oil treatment: that’s precisely that kind of sludging that STP creates.

With the involved cylinder at bottom dead center, gently tap on the tappet guide with a brass drift, and see if you can break it loose that way: after a few tabs – – and I don’t mean whanging on it – – if it doesn’t come back up to something like Standard clearances, you most likely have a bent valve, and will have to remove the head.