Son's TJ 3.5 started shuddering now & then, mostly when the engine had done some continuos miles. Been doing it for about a month.
Finally found the (middle - grill side of engine) spark plug lead sittng up & disconnected from the plug.
Connected the lead & drove for about an hour before the engine started doing the same. Reconnected & drove for 3 hours, checking now & then. It never disconnected & engine ran right :) Though I won't consider it fixed till a bit more time has passed.

Also seems to me to be way too much oil in the (outside of the) hole that the plug & lead sit down in, though it doesn't seem to be using oil (hope you understand what I mean).

Is it possible for something to be wrong that could force the lead off the plug? - haven't touched the plug yet, or tried to pull it out.
What could be consequences of this fault if any?
What will I need to check for?

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Ok, the discovery of the lead problem happened while I'd used the car for a trip away. The 3hr drive was the trip back home.
My son used his car to go to work today, and he's just arrived back home. I checked, & the lead is off again :(
This is a strange 1 for me!

Spark plug tube seals need replacing, common problem on this engine. They go hard and allow oil into the tubes. This bad seal may be causing the ht lead to pop off - perhaps the breathers are blocked as well

Might just be a loose plug allowing combustion pressure to push the plug lead off, or the plug lead may be defective.

A few years ago my old TE V6 suddenly went bang and started running rough as guts.

Pulled over and spark plug #3 (middle of front bank) had blown right out of the head and damaged the plug lead.

Limped home on 5 cylinders (only 1k), got the GF to go & grab a new lead from a Mitsu dealer, fitted an old plug and the new lead, good to go.

I suspect it was due to a slightly loose plug.

Point is, I reckon if the plug is loose it will gradually get looser and eventually blow the plug out of the head rather than simply blow the lead off.

I guess in the OP's case the lead and/or the plug head was defective.

@ MadMax & kj380
Thought it might be that as well, haven't got a plug socket thin enough. So at 1 point, I stuck a screwdriver down & moved it about.
Spark plug felt solid enough, though I know that doesn't prove it's not lose, but I figure if it was lose, vibration over the period would have it lose enough to allow slight movement.
Will know for sure when I can get to buying a socket thin enough to fit down :)

@ Dave
Could you explain more please re: spark plug tube seals?
I've never come across this design before. My experience with spark plugs are, they screw into the head, protruding out/standing proud with the lead pushed on, never in a recessed channel.
I don't understand how oil could get past the plug, unless plug was lose (stripped thread - cracks around plug hole etc)

Checked plug today, wasn't lose.
Took rocker cover off, couldn't see any splits/cracks in either cover or tube(s) the plugs sit down in.
What could be going on???

Check the engine breathers. If the crankcase builds up pressure because the breather hoses are blocked, the pressure will escape wherever it can. It a tube seal is cracked oily air will escape up it and pop the plug lead out of the tube. Tubes need to be taken off to replace the seals.

Can you point me in the direction of the engine breathers please?
And I take it the seals are at the bottom of the tube - in the head?
Can't find mention of them in the manual supplied on site.

The sparkplug tube seals sit at the underside of the rocker cover, sealing the top of the sparkplug tube against the rocker cover (and thus the outside world).

Plugs screw in the top of the head, there is also a tin tube surrounding the plug, this tube also attached to the head (and to be truthfull, I dunno how they do that aspect).

In order to allow the rocker cover to be lifted off, the plug tube is not attached to the rocker cover in any way. In order to keep oil and gases within the engine, its necessary to seal the tube against the rocker cover.

There is a "ledge" near the top of each plug tube, and a donut-shaped rubber seal is sat on that ledge...... the seal flares out so there is a reasonably wide area of rubber seal which presses against the underside on the rocker cover, surrounding each of the holes in the rocker cover.

Over time, these rubbery "donuts" lose their elasticity and harden-up (Thanks, Chopper!). They begin to lose their effectiveness of sealing against the rocker cover, and thus begin to let oil seep from inside the rocker cover - and thus into the INSIDE of the tube, which is OUTSIDE the engine (hope you know what I mean).

OIL doesn't react kindly with the long rubber plug-lead insulators. If they get oil on them, the insulators will swell and eventually weaken. Maybe the swollen rubber is causing the insulator to pop up off the plug, or maybe some engine gasses are leaking between underside of rocker cover and top of plug tube, and "blowing" the insulator off.

If you have significant amounts of oil on any of your spark plug insulators, its time to treat your car to new plug-tube seals, and rocker cover gaskets while you are at it. Both front and rear engine banks.

Thanks Elwyn, that's a clear explaination :)

As Elwyn's not sure, anyone know how them tubes are attached? Screwed in, or simply pushed in?
How to remove?

Cheers :)

just pushed in

Thanks Dave :)

Went to Mitsi today & bought the seals (3 x $20 ea), may as well do all 3 at once ;)
Mitsi says you can't by the tubes, they're supplied with the head! No doubt can by separately elsewhere, but will see how the seals go. Interestingly, he sugessted I buy the plug leads elsewhere *LOL*
Coming up for 135000k service shortly (timing & front plugs included), so will get them to do the rear seals & plugs at the same time. Son has only had the car about 2mths, so no telling when the plugs were last done. Previous owner claimed to have been a Mitsi mechanic, but log book leaves a bit to be desired :doubt:

Even though I haven't done the new seals yet, I'm confident the problem will be solved :)
So thanks everyone for your patience & time :thumbsup: :thumbsup: :thumbsup:

I'm having a similar issue with a plug lead. Seems to be the rears only, mainly the one beside the TB & sometimes the one on the other end. I only seem to be able to drive about 10-15 minutes when it'll pop up. (I've had to pop it in twice today over a 15km trip.)

Plug leads are less than 1,000kms old; the engine, spark plugs, dissy cap/ button are less than 10,000kms old. (In fact the car's really only been driven from Melbourne to Geelong, GOR day trip & once back up to Melb since the leads & heads went on - so the leads probably haven't even done 1,000kms.)

Any ideas?

Hi alscall

As mentioned early in this thread, if the lead(s) get oil on 'em, it buggers them.
I replaced the seals, but not the leads to begin with. Went 2 days before a lead popped off, a big + since like you, they'd pop off after not driving too far.
Have now replaced all (3 front) leads, but won't know for a couple of more days if the problem is fixed for sure.

Have come to the conclusion that if the rocker cover ever needs to come off, because of the way they're meant to work, it's probably better to fit the seals to the spark plug tubes 1st (leaving them slightly proud/raised) before re-fitting cover - making a much better seal, & less chance of the seal being pinched against the tube, thus creating less of a seal...if you follow *LOL*

I doubt mine have oil on them already.

As said earlier, the heads are newly installed. Within the last 1,000kms or so. Everything is new, even the engine is less than 10,000kms old so I'm thinking the problem may lie elsewhere.