XJ6 & XJ12 Series I, II & III 1968-1992

Series 1 XJ6 4.2 rough engine...

Old 04-25-2019, 04:16 PM
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Default Series 1 XJ6 4.2 rough engine...

OK, I've got one in the workshop at the moment.

1972 Series 1 4.2.

She's come in for a few niggles, Oil change and a tune up.
It's one of a fleet of classic hire cars, so they don't want it off the road if at all possible.
I've done my best without sorting out the tight valve clearances, but thats another issue.

I have a question about the NVH (Noise / Vibration / Harshness) of these engines

I've not got a lot of experience with the series 6 cylinders - My era was the fabulously silky smooth XJ40 sixes and the sublime v12 X300.
But I've worked on a few XK engines, and they have been pretty smooth - especially for something from the 50's..
I've always been able to get them running sweetly.

I know its not going to be as nice as the sixes in the X300's, but still, this XJ6 feels really rough- rough as in "out of balance bottom end".
It's been rebuilt at some time in the past, and I'm not sure by who, but the cam timing being set 8 degrees out does not bode well.

Oil pressure is good, as is the power - doesn't pull that well at high revs, but with the valves not set right, its what I'd expect.
The crankshaft damper looks brand new, as do the engine mounts - I can't see anything untoward, but it just feels really rough and "harsh".

Are these Series 1 engines a bit harsh, or should it be typically Jaguar silky smooth?

Thanks in advance,

Old 04-27-2019, 07:43 AM
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I seem to have the same problem with my 1975 4.2. But the problem didn't start (as memory serves me) until I started mucky about the exhaust system. Changing mufflers, gutting the cat converter, etc.
Old 04-28-2019, 09:11 AM
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Originally Posted by Captainbeaky View Post

Are these Series 1 engines a bit harsh, or should it be typically Jaguar silky smoooth?.
I have 4 S1's at the moment. They are definitely not inherently harsh or unrefined. If it was rebuilt by someone who didn't even set the timing correctly who knows what else they did. Maybe they didn't bother to balance the rotating assembly... Etc, etc...

I have a gorgeous Mk2 in my shop right now that is a nightmare example of incompetent 'mechanics' doing stupid things.

In the US a 72 would have several emission control devices that can cause issues however...
Old 04-29-2019, 04:04 AM
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Yeah, I thought that.
A customer dropped in while passing to pick up some parts in his XK140, and its running really smoothly - I would expect the 4.2 to be the same or better....

Yeah, it's definitely not been put together correctly.
The exhaust has a lot of problems too - resonating at strange RPM's - sounds too loud as well..
I'm going to get it in and pull the head - I have a feeling I will find burned valves from the years of running a bad cam timing.
The customer is prepared for a long job.
That will be a fun one...

It's very hard to tune it meaningfully at the moment.
The AED has been disconnected too.
Also will get a price on a new exhaust, and see what they say.

The customer is a large construction company has a fleet of classic cars that they rebuild and run as a sideline - the owners are really nice people and 100% petrolheads.
On their cars, the bodywork is always absolutely fabulous, but the mechanicals are often questionable..

A great example:- I was over with them fixing the hood hydraulics on their Bentley Azure.
This went very well - they were very pleased.

They have rebuilt a modern XK8 convertible, and it is pristine, but the engine was a mess when they got it.
They spent a lot of money on custom pistons etc (because they couldn't get pistons for it apparently).
Now its running badly, with lots of strange noises, a check engine light and lots of misfire codes in the ECU.
They were repeatedly asking me to fix the "ECU problem", turn out the engine light and therefore get it running right...

The ECU wasn't reporting any specific engine management electrical issues - just misfires.
So as a quick and dirty check, I cranked the engine over with the injector fuse disconnected.
Lo and behold, I could hear the poor compression on some cylinders just on the starter (a good sign that things are really not well with the motor)
Did a compression check, and they are all low (highest was 120psi). One was as low as 60psi.
Did the compression check wet (with 5cc's of oil added to the cylinder).
Nearly all compressions improved greatly, but two didn't change at all.
So we had a freshly rebuilt engine with new pistons showing ring sealing issues and two cylinders with valve sealing problems!

Rather than dare open that can of worms, we got a second hand low mileage engine from the UK.
Did the compression check on the bench - all were at 150 psi dry and around 155psi wet - and all were exactly the same.
It sounded much better just cranking over on the starter!

We will be changing over the engines just as soon as I can get over there.
(We will be changing the timing chains as a precaution, and because it's so easy to do on the bench.).

Since going to see that XK8, and talking them through the faultfinding process, they are sending a lot more "Engine and mechanical" work to me - one new job is a complete chassis and powertrain overhaul on an MGTC which makes me very happy...

It's a strange mentality - they see a symptom and try to fix the symptom rather than work out what's causing it.

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