[xk-engine] Intermittent clunk and shudder, 1984 XJ6 VdP w/ 4.2L EFI XK engine {Scanned}

While driving my 1984 XJ6 VdP (4.2L EFI XK engine, 150K miles) this week I
noticed a new faint intermittent “clunk” coming from the engine. I pulled
into my garage and using an automotive stethoscope observed that the clunk
was very noticeable on the exhaust side of the engine. Particularly if the
stethoscope was placed on the front exhaust manifold. I shut the engine

This morning I started the engine up and ran it for about 5 minutes at idle
and there was no “clunk”. The engine idled smoothly as usual. Then I took
the car out for a drive and warmed it up the engine to operating
temperatures and returned to my garage. The clunk had returned. It isn’t
very noticeable when driving the car, but is very noticeable when stopped
and the hood (bonnet) is raised. The clunk seems to be coming from the
exhaust area, but I am not certain of that. There is also a noticeable
shudder in the engine when the clunk occurs, so I think this is engine
related and not an accessory. The clunk/shudder is intermittent and seems
to be related to engine temperature (at least for now). Also, the engine
runs fine for a few seconds and then the clunk/shudder happens. There isn’t
a regular pattern to the clunks, just every once in a while it happens.
Perhaps 10-20 clunks each minute.

My first thought was it sounds as if a tappet collar has come loose on the
exhaust side, however I installed a stake down kit on the exhaust side of
this engine in 2003 when I removed and replaced the cylinder head due to a
burnt #1 exhaust valve. I suppose that one of the steel plates could have
come loose, or perhaps the problem is on the intake side and just sounds
like it may be coming from the exhaust side. I checked the most forward
stakedown plate through the oil filler port, but it looked fine. However
the other two are not visible through that port. I have decided not to run
the engine any more until I find the cause and fix it. I needed to do an
oil change on this car this weekend anyway, so I am draining the oil now.

I plan to do the following this weekend:

-remove both camshaft covers and inspect the stakedown kit, tappets,
collars, camshafts, camshaft bearing, etc for wear and any other problems
-measure the valve clearances on both sides and adjust if required
-remove the plugs and inspect them
-run a compression check of the engine

I should mention that this engine has typically run lean since the cylinder
head rework at about (115Kmiles) and is susceptible to pinging on hot days
under load. I use 89 or 91 Octane fuels depending on the kind of driving I
am doing and that seems to minimize pinging. CA smog test results
document that it is running lean, although it has passed. I was planning
on retarding the timing a bit, but hadn’t got to that yet.

Are there any ideas of what the problem can be or what else I should check.


Paul M. Novak

1990 XJ-S Classic Collection convertible
1984 XJ6 Vanden Plas
1969 E-Type Fixed Head Coupe
1957 MK VIII Saloon
Ramona, CA

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In reply to a message from Paul M. Novak sent Sat 24 Sep 2005:

Paul you know far more than me, and noises are tough to diagnose
without being there, but your description reminds me of some bike
engine noises at idle, espeically on multicylinder machines.

An engine can sound like a bag of nails when idling under no load,
due to backlash in things like primary drives, cam drives and
clutch baskets, or even just the multiple cylinders not quite
firing perfectly in synch. When you increase the revs even
slightly, or put the engine under even a light load all the
backlash disappears and so do the untoward noises. Assuming yours
is an auto, does the clunk still occur when idling in Drive (with
someone inside of course, to hold the footbrake on in addition to
the hand brake)?

Try having it idle with one plug lead at a time removed and a spare
plug inserted in the cap and resting on the head in case you damage
the amp (not sure if that type is susceptible to burn out from a
missing plug? I expect not). If it’s a misfire you’ll see which one
removes the hiccup and if it’s just backlash then the extra load on
the rest of the engine may cause it to disappear.

If the clunk isn’t some kind of ‘hiccup’ due to a slight erratic
misfire, maybe check for end float on cams (& crank pulley if you
can pry it without damaging anything?)

Just some less obvious ideas anyway.

or slightly higher revs–
Peter Crespin 66 2+2 ‘E’
Buxton, United Kingdom
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