After driving my 88 XJ40 last week I noticed half way through the trip the whole car began to shudder.
At first I thought maybe wheel alignment issue or brake pads need replacing even though were just replaced.
I ruled the wheel alignment out as it’s not just the steering wheel that’s shuddering while braking or hitting a certain speed when accelerating; the whole car shudders when braking, accelerating and idling at any speed.
Any thoughts?
You might check the prop-shaft bearing and the jurat. When I had an XJ 40, the bearing went on me and I couldn’t figure out why I was replacing jurats all the time. Once the bearing was replaced, the vibration went away.
Mr. Wackjark …
When your whole car begins to shudder, that can’t be good, but it’s probably one of the easier things to trouble shoot.
Does it just shudder all the time or can you determine when it’s not shuddering ?
Does braking or not braking have anything at all to do with the symptoms ?
Does this still happen when you’re stopped and at idle, how about if you put the car in neutral ?
Is it speed related ?
Try getting the car up to speed and then put the car in neutral and coast, still shuddering ?
All of this will narrow our search.
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Don’t mean to be the spelling cop here 
but in case the OP doesn’t figure it out Grahame means the Jurid coupling
Hey mate, just took it for a spin and was shuddering the whole time I was going 50km/h. I then went to 60km/h and cruised at that speed for a bit and put it to neutral which is when the shuddering decreased a bit.
I slowed to the lights and it wasn’t shuddering anymore while idling.
I picked up the speed after the set of lights and found now that every time I accelerated over 1000 rpm it would begin to shudder again.
So besides accelerating, it no longer shudders when braking or idling.
But when you are stationary, does the shuddering also occur when you rev the engine above 1000 rpm in Neutral or Park? What I am trying to establish is if the shuddering is engine related or drive line/transmission related.
Yes I just tested it then and it shudders when I’m stationary in neutral revving over 1000 rpm
OK so it is not the transmission or drive-line.
From various reports in the archives of ‘engine vibration’ at higher rpm, it would appear that a defective/broken mechanical cooling fan is one suspect and/or disintegrating rubber front engine mounts and/or the rear engine/transmission centre mount collapsing can result in ‘shuddering’.
My only experience of such a phenomenon (albeit not on a Jaguar) was cause by a fractured fan blade - and I know that the plastic blades on the AJ6 engine are prone to that. Check that the fan blades are not fouling the fan cowling when you rev the engine.
If you suspect the fan, you could always temporarily remove the fan belt and start the engine for a few seconds to see if the shuddering stops.
Hmmm …
I like your idea about a defective fan blade Bryan.
I’m not sure how much shudder a bad engine or transmission mount would produce while in neutral/park and no load on the engine. But it certainly would be obvious as soon as the transmission was selected to drive or reverse.
That’s a nice graphic, Groove.
I had this symptom back in the day, not on a 40.
Turned out to be a burned valve, felt just like a driveshaft vibration.
I think I’d try to throw a rotor in it, just based on known issues. If
that doesn’t work, it might be time to get it on a scope, or the modern
equivalent.
Cheers, Alan
Dennis,
The problem there is the engine torque in neutral or park against the engine mounts as you blip the throttle to increase the revs - but I still favour the mechanical fan theory.
Bryan …
I agree that “blipping” the throttle would bring out the shudder demons…
…but just smoothly increasing the RPM and then holding it there shouldn’t. However you’ve beaten me over the head with the truth so many times in the past that I now wear a crash helmet when I’m posting.
Dennis,
Ah yes, but nowadays in this new DT era, how do you know that I’m not recounting ‘alternative facts’? 
I do recall that after I had replaced the fan and alternator belts on my car, in refitting the fan and the fan shroud I had not correctly ‘centred’ the clip-on fan shroud. When I then revved the engine, all sorts of horrible noises/vibrations emanated from the front of the engine as the fan hit the shroud due to movement of the engine on its mounts under torque.
Frightened me to death until I discovered where the noise was coming from!
Hello gents!
A couple of weeks ago after finishing replacing the rear shock isolators, I started my car going to a test drive when I felt vibrations. Turned out a geco was stuck to one of the blade of the upper fan. I removed it then went for a test drive but still felt a slight vibration. I checked the lower fan blades and I found it’s head stuck on a fan blade. I guess it lost his head when I started my car. So even as small as the head of a geco on a fan blade can be felt inside of the car.
Joseph
A gecko’s head ! … YOU WIN !!!
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I noticed that little dude has been conspicuously absent from the GEICO commercials the past few weeks - now we know why! Such a shame, he was one of my favorite advertising mascots…
What do I win? A discount car insurance? 
Mike, I also heard someone yelling “do not start the car otherwise I won’t be able to help people save money on car insurance”. So I guess no discount for me I suppose.