I was driving today on a 60km/h road with traffic lights etc. for about 45 minutes, then when I pulled up at a set of lights the engine was idling very rough and stumbling as if it were going to cut out. I kept going and it got worse, even to the point of doing it a bit while cruising on a light throttle. So I pulled into the shops for about 45 minutes. When I drove it after that it was good for the rest of the day.

The rear spark plugs have <5k on them and the fronts have 25k. The leads are 110k old. The cap and rotor are original at 205k but still looks ok (seen far worse). I was thinking oxygen sensor but the fuel consumption is still good at 9.7L/100km around the south east suburbs of Melbourne. The only 2 things I'm thinking it could be are the airflow meter or more likely a vacuum leak (possibly from the EGR valve)

In theory the MAF, working or otherwise, shouldn't affect idle speed. At idle the ISC essentially governs the idle speed taking coolant temperature and AC load into account, the MAF reading isn't a factor as far as I know. The 'here now, gone later' aspect of your fault is odd and doesn't suggest a dirty TB, maybe a bit of bad fuel? Could have been a stuck EGR valve or solenoid, worth checking out in any case. Did you notice if the coolant temperature was abnormal in any way while it was playing up?

Off fuel sounds like.....lots of problems with getting clean fuel lately

I'd start by looking at the front plugs which I assume are regular plugs that have done 25k. This would be a quick and easy thing to check/replace, before digging any deeper.

Fuel filter changed recently? also if you are in a southern state - throw a bottle of metho in the tank - could be water from condensation

Today I checked the EGR valve. It seems to open and close ok when applying vacuum to it, so I don't think it is getting stuck (at least not when cold). I was going to unbolt it to have a look, but given the problem has gone away I shall leave it. I still think it may be the culprit as the road I was traveling on required only very light throttle which is usally when EGR is in operation.

I have a new fuel filter ready to go on at the 210k service, the current one would have over 100k on it (still within the service recomendations). Given the mysterious appearance and then disappearance of the problem I'm not sure it could be the spark plugs. I'm putting this down to bad fuel for now, possibly water in the tank. I will run this tank down right down then fill up with some E10 to get rid of any water. Temp guage was normal when this happened. Going to keep driving it and if it happens again I will do some troubleshotting while it is happening.

very light throttle which is usally when EGR is in operation.
Not necessarily correct depending on what you call very light throttle... - I can't post a screenshot of the EGR solenoid duty cycle map (it is a proportial control via PWM) but in H & later ECUs EGR is active from 1000 to 4000rpm; from 2000rpm upwards it is active from just off the idle switch to WOT and below that as revs drop the activation point moves up the throttle range to about normal cruise throttle (about 30-35%).

Drove the car yesterday for the first time in 2 weeks. It was doing it again, only starts doing it when the engine is fully warmed up (20 mins at least). Today I took the MAF out and cleaned it. I checked the brake booster and fuel pressure regulator for vacuum leaks - nothing obivous found. Looked at the distributor cap & rotor button, they are showing signs of wear with over 200km on them but I have seen worse. I cleaned the throttle body (done this recently when I changed the rear plugs). Pulled out the EGR valve and found it to be dirty with oil like the throttle body but still appeared functional (not leaking etc). Pulled out all the front plugs and found nothing unusal. Took it for a drive and after 20 mins it was stumbling at idle again. On the return trip it was good as gold. The idle has improved slightly after cleaning the throttle body/EGR. I plugged the computer in and found no codes. Fuel trims are around 4%, but it wasn't really stumbling when I checked this. I'm still thinking this is fuel related and may get a bottle of fuel injector cleaner to put through.

Hey jimbo,off the topic but what scan tool do you use for the magna?

Hey jimbo,off the topic but what scan tool do you use for the magna?

I've been using EvoScan.

I put a bottle of fuel system cleaner in 2 weeks ago and have driven it on several short trips since then where it has been idling really well. Yesterday I took it for a longer drive and after 20 mins the idle was rough again with a bit of stumbling as well. I managed to get a log on EvoScan the other week which shows the RPM dropping very sharply then picking back up.

Download link for EvoScan file http://s000.tinyupload.com/index.php?file_id=47369956367230139407

This has now gotten a lot worse. The misfiring started to occur when accelerating as well, to the point where it became almost undrivable. I disconnected the MAF which made it driveable again. I replaced the MAF with one from the wreckers to no avail. Now it is missfiring even with it disconnected. I had a good look for vacuum leaks and replaced the hose from the PCV valve but found no others. I replaced the fuel filter as it was due. Dirty looking fuel came out of the old filter, altough the fuel from the hose was clean.

When driving if I am very light on the throttle it is fine, but go a bit harder and it starts missfiring. If I put it in 1st then accelerate slowly for a bit it will rev to redline with good power. Push the accelerator too much too early it has no power and misfires a lot. I'm going to get another fuel pump and pressure regulator today from the wreckers as this seems like a lack of fuel and I can't feel much pressure in the fuel lines.

Ignition coil.

The symptoms sound more like a weak spark than any fuel problem.

If you are going to the wreckers, pick up a spare distributor?
New coil - available on eBay.

Ignition coil.

The symptoms sound more like a weak spark than any fuel problem.

If you are going to the wreckers, pick up a spare distributor?
New coil - available on eBay.

Thanks. Have pulled the old coil out and will goto the wreckers to find a 2nd hand one.

It make sense now since pulling the plug on the MAF reverts the ignition timing back to a basic map allowing it to run with a bad coil.

One simple test you can do in future to see if it is spark or fuel, is to induce the problem to the point the engine cuts out, then pull a front plug. If the plug is wet with fuel, or very dark, you know you have a spark problem. If it is a fuel problem the plug will be dry and show signs of lean burning, ie be pale in colour. (That's the theory anyway)

EDIT: I have a spare new coil here for when it happens to me. Not too hard to swap them over, I assume?

I ended up getting another distributor from the wreckers. They wouldn't sell the coil on its own as its part of an assembly. I put it in with my cap and rotor, reset the computer and it fired up fine. Bursons wanted $107 for a new coil and $22 for a rotor.


EDIT: I have a spare new coil here for when it happens to me. Not too hard to swap them over, I assume?

The 2 nuts that hold the distributor on are very hard to undo as there is limited room. You can't get a socket on them as the coil is in the way. Best thing would be a long 12mm spanner. Much easier to just swap the coil in the car although it is hard to get it all apart as they are stuck together with a sealant. Once the airflow meter is out of the way there is plently of room to work.

Thanks for the diagnosis Max. Much appreciated.

If anyone is interested in the spare distributor or airflow meter contact me.

Well I spoke too soon. After changing it I plugged the MAF back in and let it idle for 20mins to reset itself. Left it for a few hours then when I went to drive it the idle was very rough when put into gear. Same problems with missfiring when driving unless very very light on the throttle. Seems to be a bit better with the newer coil as at least it will run somewhat with the MAF plugged in.

You may have to revisit the ignition side of things, leads, cap, rotor. probably can't assume the leads are ok even with only 110 000k. How old and what brand were they? My old galant just recently had a breakdown. Different setup though, has 3 coil packs firing 2 cyls at once, 1 being a waste spark. when it went it was almost undrivable , felt like 4 cyl.

Ended getting plugs, 2 coils and a lead set. Plugs were platinum with 50 000k . parts arrived at different times but pulling the plugs on the front bank gave me a clue, 2 good, 1 looking oily and unburnt fuel, sooty.

Plugs did not sort it, neither the coilpack, leads arrived and so started with the easy set at the front, on removal I found the problem which went unspotted. lead 3, the long plastic insulator had a white powdery residue, looked closer and could see tracking where the spark had managed to break through and was arcing on the spark plug tube so took out 2 cyls since all the energy was taking path of least resistance.

I know this setup is different but are any of the plugs looking different to the others or all just as good/bad?

This is the problem with a unknown part from u-pull-it or similar.

Getting something like a coil, I'd be expecting some sort of confirmation that it's operative - like it's been put on a running car and tested.

It's quite possible the coil has been sitting in a wreck for a really long time. Several AMC members have had problems recently with parts they've pulled from a wreckers being faulty.

The fact that it only happens when hot is a really good indication it's ignition related.

Perhaps post some pictures of your distributor cap and rotor to the forum?

You may have to revisit the ignition side of things, leads, cap, rotor. probably can't assume the leads are ok even with only 110 000k. How old and what brand were they?

The leads are NGK from 2008. I tested the front ones, they have around 5K ohm resistance. There is some oil from around where they go through the rocker cover, this could be causing a short (gaskets and tube seals were replaced 4yrs ago). I might get some spare leads from the wreckers just to try.

If the engine spins up cleanly with very light throttle but staggers with a bit more welly it's definitely coil. Unfortunately it's fairly common, best to grab one from a wrecker as it will have been taken from a runner... it's pot luck from pick-a-part type places.

Maybe I should of gotten the coil from a wreck. The car it came off looked complete and intact to the point of wondering why it was there. Is it possible that ECU is faulty or damaged by the bad coil? Or is the coil controlled via the transisitor near the cam sensor?

Maybe I should of gotten the coil from a wreck. The car it came off looked complete and intact to the point of wondering why it was there. Is it possible that ECU is faulty or damaged by the bad coil? Or is the coil controlled via the transisitor near the cam sensor?

In my opinion, Unlikely , ECU would drive the transistor which switches the high current, really a go / no-go situation.

As mentioned heat causing the fault to occur , coil would be my suspect, resistance check of the lead not really going to give a good indication, high voltage can break through insulation that's deteriorated

Is it possible that poor leads caused the coil to fail originally?

Drove it this morning and it is still missing. Gets worse as the car warms up. What are the odds of the replacement coil being faulty as well. I'm thinking it may be the leads that are at fault.

Had a similar problem when I replaced a leaking o ring on distributor.Did have some backfire also when accelerating though!
I just removed the distributor in parts and re-installed.All fixed.maybe the rotor was not seated properly..who knows?
Another possibility would be a faulty lead.Check to see they are plugged into the distributor correctly (sequence).
Best of luck

Checked inside the passenger footwell for coolant leak onto the ECU?

You would be unlucky to get a coil from the wreckers that is as bad as the one you started with, but you never know.
I had a TS with a dead alternator, went to the wreckers and got a S/H one. Same, dead. Exchanged, same - dead too.
Eventually got my original reconditioned - yep, that failed soon after, but at least it got fixed under warranty.
As a consequence, there are some bits I just wouldn't buy from a wrecker.

Took a trip to a different wreckers today. They have over 40 third gen Magna's in stock. Found a TL that had just come in yesterday and had suffered a decent hit on the passenger side. I thought this would definitely be a runner. It had relativley new leads and a clean looking distributor (most have oil stains on them). So I got the leads and the coil out of the distributor. Put the leads on my car and it now runs normal again. The idle is probably better than it has ever been with only a little bit of roughness, however I understand not many of them idle that well. I think the bad leads caused the original coil to fail, although it may have always had a fault as the idle used to get really rough after long hi-way runs in the heat.

It looks like most of these cars goto the wreckers with the original leads, dizzy cap and rotor. Pull the spark plug leads on most of them and they are covered with oil but I'm guessing they still ran.

Good to hear it is sorted.