How to check if valve seat dropped due to overheating on a 1988 Jaguar VDP V12 5.3L

My 1988 VDP V12 5.3L got overheated due to the failed fan switch. Coolant temperature went up to the red zone before I noticed. The next day I was able to start the car with no problem but got very loud tapping noise at idle from the front end of bank B. If I give a bit gas, the noise will decrease a bit but not going away. It seems the noise comes from the cylinder 1 or 2 area. So I pulled the spark plugs out and check the compression. It seemed OK as all cylinders got roughly the same readings around 185.

I thought the noise was from the dropping valve seat(s) due to overheating, but the compression testing results seemed not supporting this. Is there any chance that the compression can still hold with the dropping valve seat? or if any other things can cause this loud tapping noise (it sounds just like a tapping valve but mush louder, like the noise from a car first started after being parked for a long long time)

Any comments and/or helping tips are very much appreciated!

Not sure but could it be the old ailment that the XJ engines suffered, a loose bucket under the cam?
Never been inside a V1 so only a thought.

A dropped valve seat will inevitably cause no compression, John…

While the V12 differs somewhat from the xk engine, I don’t disagree with Robin. Remove the cam cover and check for tappet anomalies in the suspected area…

However, the noise might also be from a bust connecting rod bearing…

xj6 85 Sov Europe (UK/NZ)

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V12s run tappets direct in the alloy - no tappet guide to lift up.

A valve seat could dislodge without dropping completely if it came loose during shut down heat soak with the valve partially open. Assuming there was no loud noise before shut down, this is the main one of very few options.

A seat which dropped, say, a 1/16” would cause a loud noise but little ot no compression loss or piston mayhem.

A borescope will confirm engine integrity - along with compression check on all 12 in case of a wrong guess as to cylinder number. Then intake removal and clearance check to confirm .

Been there in a different engine!!!

My 63 Corvair camperized van. At times it would run only 5 of it’s 6. usually when away from hoe and not able to stop and diagnose !!

Aha at last, in my drive!! On 5 only!!! Pulled the plugs and did a compression test
One at 0!!!

Pulled the heads. Yup, on seat "floating’.
At times it would align and the seat would be forced back into the head. At times remain “cocked”. Hydraulic lifters adjusted and no noise resulted!!!

Beat up head welded. New seats machinist promised a much tighter insertion

Corvair back on 6!!!

Carl .

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The noise you probably heard was the valve seat moving up and down the cylinder head.
When those valve seats are inserted into the cylinder head they are inserted with far
less interference then they should have and work themselves loose.

The thing to do is to remove the camshaft and check that valve seat for any loose
carbon which found ints way under the valve seat ring.

Thank you all for the comments and tips.

I double checked the compression for all 12 cylinders, they were consistently at around 185 reading.

Hi Walter, how I can check the valve seat by just removing the camshaft? Do I have to remove the entire cylinder head to get the access to the valve seats?


Definately a head off job to check the valve seat if the compression tests ‘consistently’ show good figures.

Yes, John…

You may or may not detect something by checking valve clearances - which likely would be excessive with a loose seat. That a loose seat stays in place without falling out is ‘very lucky’…

xj6 85 Sov Europe (UK/NZ)

Thanks again for all the replies.

Two more questions before I dissemble anything further down:

  1. In the case a loose seat, why the compression can still be hold at high around 185? What would I do if it’s a loose seat? just “push” it back?
  2. Do I have to remove both cylinder heads or just the one with the tapping noise?


If the seat dropped while the engine was stopped and heat-soaking towards a temperature spike (due to no water circulation) then the seat may only have dropped say a millimeter two before it rested on the part-open valve. The engine then cooled and the seat was gripped again by the head, leaving a gas-tight seal but a huge tappet clearance and resultant clattering noise.

Since you will need to take the intake manifold off to check clearances you should check all of them. If only one is out then just do that seat. Some would peen the alloy over the seat edge but it is better to bore for an oversize seat insert. If the head was totally bare you could weld it and recut a standard seat OD but if you have only one dud seat the others have survived extreme heat and are likely fine.

So in this case, would I be able to reduce the clattering noise by just adjusting the valve clearance with a different thickness shim, instead of removing the head and reinstall the vale seat?

Not really. If the seat worked it’s way back into position there would be minus clearance and the valve would burn. If it is a valve seat you have been extremely lucky so far. Don’t push your luck too much further. Repair what is wrong.

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Hi Peter - can you please list out the exact step I need to do to avoid pushing my luck too much further?

As I stated earlier, so far I checked the compression for all cylinders and got the same readings across at around 185.

I am going to remove the intake manifold and the valve cover to check the valve clearance. From there, what are the steps I need do to avoid mistakes?


If you find a large gap at one of the valves (not necessarily the one you expect, as sounds travel) then you should remove the head and investigate that cylinder. A large gap may even be visible without feelers and there is no other reason why a valve would suddenly sit low except a foreign body under the valve head (which would destroy all compression) or the seat moving. I would check the other bank clearances while you were there but not expect to find anything wrong if it sounds like one valve.

To answer the question about what to do to prevent further damage, obtain a good workshop manual that will have the step by step instructions, to be honest it would be a long post to list here.

Just wondering, is there any value at this point in listening with a stethoscope or long screwdriver in the ear to pinpoint the source of the noise?

My car has no ears so the screwdriver thing wouldn’t help :slight_smile:

Working with an abundance of caution—knowing the consequences for the seat breaking off/breaking the valve/ falling onto the piston, were it me, that engine would not be run, till the cause of the noise is determined.

I’ve already tried. I used a long extension bar and listened, narrowed it down to the right bank cylinder #1 area where the tapping noise coming from, thus I thought it’s related to a dropping valve seat. However, after I confirmed OK on all cylinders’ compression readings I got confused, as if it’s due to a dropped valve seat, there should be zero or very low compression.