Well, over the last few months my wife's third gen manual with about 80,000k had started to develop a shudder - if you let the revs drop below about 800 when taking off it would randomly drop the RPM right down to about 500 and shudder like a pig - sometimes taking 3-5 seconds before recovering. Was obviously dropping a cylinder.

I changed the front plugs two months ago hoping it would be a simple solution, but no. One month ago, I removed the ISC and cleaned it up - but it looked very clean, and this made no difference.

So last week I got my torch and had a good look under the plenum. Sure enough, a very light coating of oil on the top of the rear rocker cover, obviously coming from the plug seals.

So I did a lot of reading and got the parts organised. Gasket kits from Mal (great to deal with), platinum NGK plugs from Repco, and confirmed my local dealer had plug leads in stock in case I needed them. The night before, gave all the bolts and rubber hoses a squirt to make them easier to get off.

Plug leads ranged from $99 to about $150 aftermarket.
Supercheap - Bosch - they don't carry them in stock anymore, have to order in, 3 days for $99
Autobarn - Only carry Topgun, didn't want to go near this cheap rubbish, $135
Repco - Champion lead set, was around $120 from memory, not a fan
NGK have always given me the best results in the past, but from reading Mal's comments, they are not the quality they used to be.
Decided genuine were definitely the way to go for $139.

Couldn't find a torque wrench that goes as low as 3.5nm, but then if Mal can change hundreds of them first time without leaking or overtightening, then surely a little common sense would prevail.

Read Mal's guide about removing the intake for access quite a few times - http://www.aussiemagna.com/forums/showthread.php?t=65188&page=2
I, like others, decided to leave the TB connected, to save removing a ton of vacuum lines and the coolant lines, and just remove the four bolts connecting the plenum to the throttle body instead.

I took a photo of the plenum from different angles and printed in A4. As I removed each bolt, I just placed it on the picture where it goes. Plenum came off in about 45 mins with lots of care. A second time around I could do it in about 15 mins I think.

Plug leads came out, and two of three were wet with oil. The driver's side was fine, but the middle and passenger side were both about 1cm or so deep in oil. So I rolled some paper towel and soaked up as much as I could. We replaced the factory plugs with the new NGK Platinums. Factory rear bank plugs were Champion :nuts:, and had obvious burning of the insulator. One had the beginnings of some carbon tracking.

Headed down to Mitsbushi and got a genuine lead set for $139 - cheaper than some aftermarket, and obviously better quality in all regards. Probably could have got away with re-using the old ones which appeared fine after cleaning the oil off, but not worth the hassle.

I looked carefully at the rocker covers - it was only the back one leaking, and only from the plug seals (not the gasket itself) but was determined to do the lot. The factory installation had a consistent 1mm gap all the way around between the cover and the head. With the bolts off it was a little bit of a challenge to "pop" the covers without bending them, but they both came off fairly easily.

Both original gaskets came out in one piece, but the plug tube seals were very hardened. Cleaned and dried the mating face on the head, the covers themselves on the inside were perfectly clean and no sludge, so left them as is. Replaced the plug tube seals and gasket, then very gently adjusted the inner section of the gasket cover to hold the gasket in place. It wasn't tight at all, just enough so that if I held the cover upside down, the gasket stayed in there.

Replaced both and slowly did the bolts up in an alternating sequence using a mirror until the gap around the cover was exactly the same as before I removed them - 1mm. It's very easy to see how people overtighten them and warp the cover if they don't a) use a torque wrench or b) watch the clearance between the cover and the head very carefully. New plug leads went in on the back, dried and kept the old ones as spares. Front leads look fine, so have left them in there and kept the new front ones for later down the track.

Got the plenum back on, took a while with wiggling to get all the bolts to locate, and then torqued them in accordance with the workshop manual. Had a small puddle of oil on the ground which had run out of the back head when removing the cover.

Started it up and apart from some smoke as the oil which had dripped on the exhaust burnt off for a few mins, she was good as new. Now when we take off with minimum RPM it doesn't bog down and start shuddering.

The job was about 4-5 hours for me, but I'm super anal and clean everything as I go, and put things back together really slowly and carefully. If I had to do it in a hurry, it would take me probably 2 hours from start to finish for all six plugs/leads and both rocker gaskets.

I've checked the gaskets a few times after some 1+ hour drives and both seem fine. There's the tiniest amount of oil there but I assume that's just a little residual stuff. If I'd pinched the gasket or over/under tightened it, I'd expect more than a drop or two after a long drive!

I still have a tiny miss when it's idling hot, but barely noticeable. I think this is just the third gen thing that everyone gets, seems to be a few posts about this on all the 3.5s. I tried cleaning the throttle body (still attached) with a toothbrush and solvent, but it was fairly clean already, and doesn't seem to have made any difference. I only notice it if I turn the radio off and concentrate really carefully while it idles. Cold idle and normal driving it runs smoother than it ever has, so I'm not too worried about it.

It's definitely within the scope of your average DIYer at home, provided you do the reading first, take your time, and don't do anything stupid like drop stuff into the head/intake because you didn't cover them up.

My thanks to Mal for great write ups and advice on common traps (like overtightening), and to the wider AMC community for contributing to such a great resource. Had it not been for AMC, probably wouldn't have done it myself and saved a few hundred in labour.

Next lot of things to do...
1. Get a price on finally fixing the 5cm crease my wife put in the boot
2. Seal the bottom of the front windscreen with this: http://www.supercheapauto.com.au/online-store/products/Selleys-Autofix-Windscreen-Sealant-310g.aspx?pid=219451
3. Rotate the tyres and replace the annoying locking nuts with normal ones so it doesn't take twice as long to do every rotation.

whats your problem with champion plugs, its what my local mits dealer uses, and while i dont know much about their car plugs their mower plugs are fine

It's too late now but some pics would be good, but I thank you for your efforts in documenting this.

I will hopefully soon be buying a TL/TW AWD and will probably find your information useful sometime in the future as I like to undertake such jobs myself.

On some other automotive forums that I have participated in, it is sometimes difficult to get a coherent reply, least of all a step by step explanation.

+1 to you flyboy and MadMagna.

2. Seal the bottom of the front windscreen with this:

I enjoyed your write-up until I came across this bit!

I hope you realise that the leaf guard at the base of the windscreen is not meant to be water proof, and that the bottom of the windscreen is sealed at just below the dashboard level? ie if it leaks there, the whole screen needs to come out - in which case the Selley's stuff won't do the job.

PS Champion makes excellent lawn mower plugs, NGK make good car plugs. End of story.

http://www.rockauto.com/catalog/moreinfo.php?pk=69491&cc=1417182

$2.49 us dollars each .......

+1. I appreciate very much the slow detailed methodology.

re plugs. What's the argument re iridium/platinum plugs versus standard in terms of long term benefit or otherwise. I read someone arguing standard at more frequent change ends up in the long run being more economical from an overall long term engine maintenance perspective. (again someone who swore by NGK). Admittedly this was particularly about a dual fuel system but the argument seems to me to have possible merit in petrol only as well (in that instant (gas / petrol ) ngk standard at .030 gap).

My problem with Champion spark plugs is they have caused my last two cars to run like rubbish. I wasn't going to risk a third time. Either they are just cheap rubbish, or someone at the factory keeps dropping them! If I had a lawnmower I still don't think I'd bother.

Thanks Max. It's just that the rubber is half missing/half cracked and looks like rubbish. If I cut away the rubber gently and seal it with a nice neat bead it will look a ton better.

I saw the rockauto plugs, but couldn't really be bothered waiting 5 days, I needed to get it done.

There's no big peformance difference between iridium, standard copper and platinum. The main difference is in how long they last before they need to be changed. If you are happy to pull the plenum off every 30,000k then copper is fine.

Last update: Pulled the dizzy cap last night and cleaned a lot of white powder and rubbish of the points. Also think the rotor is best to replace. Are they really $80 genuine and $55 from Mal? That seems crazy expensive for a rotor!

It's just that the rubber is half missing/half cracked and looks like rubbish. If I cut away the rubber gently and seal it with a nice neat bead it will look a ton better.

I saw the rockauto plugs, but couldn't really be bothered waiting 5 days, I needed to get it done.

Last update: Pulled the dizzy cap last night and cleaned a lot of white powder and rubbish of the points. Also think the rotor is best to replace. Are they really $80 genuine and $55 from Mal? That seems crazy expensive for a rotor!

The rubber strip can be trimmed off the plastic leaf guards, looks a lot neater and there is no need to seal the joint. Unless you live in an area where a lot of tree material will get blown around and end up going past the leaf guards into the plenum chamber. Some members have replaced the leaf guards with new ones, but I've just trimmed the rubber off.

Rockauto: Price up what the cost of parts you used would have been from rockauto and compare it to what you paid, sometimes it is worth the 5 day wait. lol

Rotors and caps are expensive, for sure. Just a cleanup is fine, but if you want to replace either, check the Rockauto prices.

Thanks Flyboy, nice to see appreciation for what is put into these forums.

Max, IF you get good quality from Rockauto then fine, but Cheap is Cheap, you get no name brands and you will end up with more issues than you started with, have said this many times before and no doubt will say many times more.

No dramas Mal. Hadn't been for your great DIY guides I wouldn't have bothered trying.

Miss at idle is still there. Have now changed all plugs and leads (back leads had oil on them anyway), cleaned distributor cap points, replaced the cap rotor, cleaned ISC and throttle body. Any ideas Mal? Runs fine when you drive it around, no loss of power, cold idle is fine, it's just the hot idle has a really subtle miss - perhaps a small vacuum leak causing a lean idle? Or maybe the EGR is sticking open?

Max - for brand name parts like NGK plugs I would use rockauto no problem, but a lot of their parts are generic no name brands I won't go near. I'd much rather support AMC sponsor Mal and know I'm getting good stuff - and less hassles if something goes wrong.

Mitsu use both NGK and champion in the rear FYI

Mitsu used NGK up to the TJ, they then for reasons unknown to mankind switched to Champion (prob price) and then in the 380 switched to Bosch

I dunno whether production quality dropped or if it's just certain cars prefer certain plugs, but I always swore by Champion in my old Hillman. Same (supposedly) spec Bosch units were sh1te, NGK were so-so. In the TJ, I stick to NGK.

Mitsu always used Champions in the Sigma and first and second gen Magnas, N9YC, and needed regapping often. They were about $1 each, if I remember correctly.

But seeing the third gen plugs live in a deep hole for a long time, and can be time consuming to swap, I will stick to NGKs, thank you.

I always swore by Champion in my old Hillman.

Swore BY or AT you Hillman - you must be an old bastard too, any spark was good then wasn't it?

'69 Hunter Royal was my first car (being a Royal it had carpet and a wireless!), and yes, it got sworn at quite a lot in the 14 years I owned it. Great first car, but a total POS to be honest. You're right about any spark being good, remarkable that any power at all found it's way through the mostly-carbonised wire and mechanical points. Champion N9YC's were fine, if I went to the Bosch W8DC (from memory) I was re-gapping weekly. It ended up with the alloy head, twin CD150 carb engine, and went like a stabbed rat. Ex wife hated it so it was her or the Hunter... reckon I made the wrong decision because from time to time I actually miss the Hillman!

im glad some one else on this forum took the time to research the subject and do it right the 1st time before going at it half arsed and then complaining

great right up mate, listen to MAL , he might come across as a hard bastard but he knows his stuff for sure

Both banks still seem to be trickling a tiny bit of oil, I think probably because the rubber has settled and they were perhaps slightly under torqued.
Might have to borrow a torque wrench that goes as low as 3.5nm from work.

Any 1/4 inch torque wrench will do the job. Need an extension and a 10 mm socket.
Try it on the front bank to see what is happening.

If you have done them to 3.5 Nm initially, and done a few laps to check them all, there is no need for a follow up though.

Oh, and the tiny miss at idle is still there. I've given up - going to take it to get looked at.
Plug leads, distributor cap cleaned, new rotor, new plugs, vacuum leak test, EGR clean, TB clean... None of it has helped, but a lot of it needed doing regardless.

Great writeup flyboy, now you know its done & won't be a problem. Have you considered the fuel or o2 sensors?

Fuel is fine, has a tank of BP98 at the moment. Fuel filter is not likely to cause a miss at idle when the fuel flow is low - more likely to manifest as missing/starvation at open throttle.

Oxygen sensor, I thought about it, but from what I've read they tend to fail lean (causing overly rich mixture). Fuel economy is as good as it has ever been. Unless it has failed rich (causing overly lean mixture) I doubt it is the cause. Does anyone know if they can fail rich causing lean mixtures?

I guess it's fairly easy to unplug to see.

Took it to my local trusted mechanic today.

They had a good look over it, and said they don't know what's wrong. They said everything I've done is exactly what they would have done - TB clean, plugs, leads, distributor cap and rotor, EGR, PCV.

He did however ask about oxygen sensor, and I said that it hasn't been running rich or anything. He said they normally fail lean (and drive the car rich), but sometimes (in rare situations) that's not the case. He said sometimes rather than just fail, they get lazy and have very slow response times, so they still work when cruising (nice and hot) and don't throw any CEL. But what does happen AT IDLE is as the car flip flops back and forward between slightly lean and slightly rich, because the sensor is slow, the ecu leans it out a bit too much before it gets a response, and so it misses a fraction just as the mixture starts richening again. He said this causes a sometimes rhythmical miss at idle, which is what I'm tending to see - about every 2 seconds it wobbles. And it may be why it shudders sometimes - if I release the clutch to takeoff just as it's at the slightly too lean point, two or three cylinders might miss and cause it to shudder. It never stalls though, because the slow sensor eventually responds and richens the mixture, and then it's fine.

I've tested the oxygen sensor heater resistance, meets the MMAL spec. But I'm thinking I might just get a new sensor anyway. He said he'd recommend doing it every 100,000km regardless and the car is almost there so I guess it won't hurt.

It seems to make sense. They didn't do a vacuum leak test with propane, but that's pretty much going to be my last option if the oxygen sensor doesn't fix it.

Mal, does this lazy oxygen sensor scenario sound plausible?

And am I the only one in the "sliced my fingers open on the sharp edges while accessing the oxygen sensor connector" club, or have others done this too? Damn those metal edges are SHARP.

EDIT: Technically the heater circuit isn't meeting the spec. The manual calls for 7-40 ohms at 20 degrees on the heating circuit, and it's testing at 6.5. I'm sure that's close enough, but wish I had an oscilloscope to test the actual oxygen sensor output.

Did this issue get resolved with fitting a new oxygen sensor?

No, didn't fit one. It's only the most minor of idling roughness occasionally so I've decided to live with it for now.

Sorry!