Misfire under load when in gear, low rpm, after a bunch of work done

[e32lover]

Hi,

We did a bunch of work to my 02 NM V6 petrol, including timing belt, rocker cover gaskets, water pump, spark plugs, and now it is misfiring only when in drive and under load, at low RPMS.

The misfire manifests as an intermittent brief cutout, on the highway it causes a thud as the engine quickly cuts back in. It was barely noticeable at first, but seems to have gotten worse. When stopped, it is fine in park but as soon as I change to drive, it starts cutting out again.

We have replaced the MAF, re checked the timing, replaced the spark plug leads and ignition coils, but the problem persists.

Can anyone recommend what else I should check/replace?

Thanks

[goosetrousers]

Funny you mention it, I've had the same thing for a little while now. Rather than fix it, I've just adjusted my driving style a little so that I'm rarely pushing hard in the lower rev range.
At the last service (Auto Masters in SA) the bloke suggested I might like to have the intake manifold cleaned and reteach the ECU. I'm still sceptical, but I might stump up for it next time around and see if it makes a difference.

[erad]

This sounds like your high tension leads or the spark plugs. What type of HT leads did you use? Did you check the spark plug gaps? How did you run the new HT Leads?

You can easily check the ignition system by getting into a very dark place at night and lifting the bonnet whilst the engine is running. If there is a weakness in the HT system, you will see it arcing to earth. If you ran the new HT leads such that a lead crossed and touches another lead, cross firing can occur. If the leads get too close to an earth point (the rocker cover for instance), you can get shorting to earth. Misfires at low revs are more noticeable than at higher revs.

As a temporary measure, try closing the spark plug gaps down a little. This reduces the voltage required to make the spark jump the plug gap. If you run on LPG, it is recommended by Champion to reduce the gap from 1.1 mm down to 0.9 mm. LPG requires a higher voltage to get the spark at the plug, not somewhere else in the leads etc. Even if you don't use LPG, closing the gap will help.

What brand of lead did you buy? I found over the years with my NL that NGK was the only really satisfactory lead to use. Firstly, they are numbered so that you use the correct lead for the appropriate plug. This means that you can run the leads without them crossing each other. At one stage, I used Top Gun leads and they were absolute shite. They caused massive backfires and when I looked at them in the dark, it was like a fireworks display. Bosch leads were too long and they sagged between the saddles and contacted the rocker covers. They also melted onto the spark plug insulators.

You could even have a coil which is dying. Again, checking in the darkness will show if it is breaking down.

If all else fails, a lean mixture requires a higher voltage at the spark plug. Lean mixtures will be worst at low throttle opening (the vacuum in the manifold is highest then). You could have an air leak or a hose which is not connected. As you increase the throttle opening more air comes in and the fuel is increased accordingly and the mixtures will become richer and override the air leak.

[goosetrousers]

Bosch leads were too long and they sagged between the saddles and contacted the rocker covers. They also melted onto the spark plug insulators.

Thanks for the in depth post, hopefully it helps OP like it did me.

New leads were sagging, now they aren't and so far, I can't force the issue again.

Fingers crossed.

[erad]

goosetrousers:
Remember that when a HT lead breaks down, there will be a weak spot in the insulation where the spark can leak again. DO your best to run the leads as they originally were laid ie not touching anything. it rakes a lot of fiddling but it is worth doing.

Further, if you are replacing the spark plugs, naturally you have to remove the leads. If those leads have been there for 100000 km, 100% guaranteed they will be brittle and you are likely to have HT problems later. It is best (although expensive) to replace the leads as well as the plugs at the same time because you will only have to pull it down to access the leads later.

[goosetrousers]

Good advice.

Plugs and leads were done ~15,000 km ago before I started the big lap. Took the vehicle to a mechanic I've used for years (coincidentally, he was working at Mitsubishi and did the first warranty service on it 20 years ago).

Long story short, I had the 200,000 km service done 20,000 km early, plus a few extras. Kind of frustrated that the leads were just tossed in and were hanging out of the comb.

Pays to do your own work, or at least to check things. I wouldn't have even considered the spark without your post though, what with the leads being insulated and all.

[erad]

The HT leads on the Pajero have insulation which is 5 mm diameter. They also have very high voltages inside them. The equivalent Magnas of the time had 7 mm diameter HT leads and rarely had problems in this area. The 80 series Toyota Landbruiser also had the 5 mm leads and they had problems too. Sadly, I don't think you can go to a large diameter lead because the saddles which support the leads are only suited to 5 mm leads. Further, the coils won't take the larger leads.

[Having Fun]

Hi,

We did a bunch of work to my 02 NM V6 petrol, including timing belt, rocker cover gaskets, water pump, spark plugs, and now it is misfiring only when in drive and under load, at low RPMS.

The misfire manifests as an intermittent brief cutout, on the highway it causes a thud as the engine quickly cuts back in. It was barely noticeable at first, but seems to have gotten worse. When stopped, it is fine in park but as soon as I change to drive, it starts cutting out again.

We have replaced the MAF, re checked the timing, replaced the spark plug leads and ignition coils, but the problem persists.

Can anyone recommend what else I should check/replace?

Thanks

Has a vacuum hose come off the intake side somewhere?

[Having Fun]

Funny you mention it, I've had the same thing for a little while now. Rather than fix it, I've just adjusted my driving style a little so that I'm rarely pushing hard in the lower rev range.
At the last service (Auto Masters in SA) the bloke suggested I might like to have the intake manifold cleaned and reteach the ECU. I'm still sceptical, but I might stump up for it next time around and see if it makes a difference.

Has the throttle body been cleaned? If not, I'd give it a try. Probably the MAF too.

[e32lover]

Turned out to be leads. Bought some new leads, and changed a whole lot of stuff and that's when the problem started. Thought it could have been leads so returned them for a replacement. Problem still persisted. Went on to change heaps of other parts chasing the problem. Eventually put the old original leads back on and the problem was solved.

I recommend if you change leads use only genuine Mitsubishi.

[erad]

What HT leads did you use? At one stage, I used Top Gun leads and they were absolute crap. Caused massive backfires (I used to run on LPG mainly) which blew the guts out of the air flow meter.I also tried Bosch leads and they were not too good. I found the best leads to use were NGK because they were numbered (so you put the correct lead on the correct plug) and they were the exact length to fit in the lead carriers. The Bosch leads were all too long and sagged, causing either cross firing onto other cylinders (if the leads touched each other) or misfires (if the leads touched the rocker covers etc).

As far as I could determine, the original leads were NGK brand and they certainly worked for me. BUying them was another matter, but if you go to Rockauto, and buy a heap of other stuff FROM THE SAME SUPPLIERyou can save a considerable amount of money. Rockauto is only a clearance house and they source their products from all over the place, so try to be selective as to where you get your components from and bundle all of them into one purchase and you will save heaps on the freight