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Generation 2 Pajero NH - NL Models 1991 - 2000

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Old 21-06-19
Reece. Reece. is offline
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Join Date: Jun 2019
Location: lilydale
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Exclamation NK 3.5 dohc duel fuel miss-firing after backfire

So my Pajero decide that it wants to backfire, thats fine it is on gas. After starting it again i have a miss at idle and low rpm but when accelerating its fine. i havent been able to drive it as it is not registered yet. Am i looking at timing out a tooth for some reason or possibly plug or lead or maybe the backfire popped some gasket out.
Any positive suggestions would be fantastic.thank you
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Old 21-06-19
erad erad is offline
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Join Date: Mar 2015
Location: Cooma NSW
Posts: 2,623

You can easily check the timing by removing the little cover plates from the plastic timing cover. Rotate the engine until the crank is at TDC, and you should see the timing marks on the camshaft pulleys lined up.

First thing to check is the airflow meter, located at the inboard end of the air filter. I had a big backfire on my NL, and it blew the aluminium honeycomb foil bits loose from their mountings. The air filter box cover got lifted off its seating, and came down on the loose aluminium bits, crushing the lot. The purpose of these bits is to straighten the flow out after it leaves the filter area, and also to generate turbulence at low flows so that the sensor (a hot wire element) can detect the airflow better. I finished up 'panel beating' the honeycomb back into shape with a fine 2" nail.

Backfires, particularly on LPG are caused by the fact that it is harder for the spark to jump the gap at the spark plug with LPG compared to petrol. Thus, the spark looks for the easiest path to earth. If the park only goes to earth (eg the rocker cover), then no damage other than a misfire. But if it cross fires eg from No 2 to No 4 spark plug lead, and No 4 inlet valve is open, then you get a very large BANG. For this reason it is critical to have good HT leads, to close the gap in the spark plugs down by 0.1 to 0.2 mm (making the gap slightly less than 1.0 mm) and finally to ensure that the leads do not run close to an earth source (eg rocker cover, or the thermostat housing), and that they do not cross and touch adjacent leads eg No 4 touches No 6 lead etc). It takes a bit of fiddling to get it right, but you can make the leads go where the designers intended, using the little insulating saddles.

I experimented a lot with HT leads and plugs. Eventually I think I got it sorted, but it took a lot of changes to get there. I used iridium plugs (they are better than Platinum plugs) and NGK leads. NGK were the only ones which worked OK for me. The main feature with them (apart from the fact that they work well) was that they were numbered, so you could get the correct length lead to the correct plug. This enabled much easier routing of the leads so that they did not sag onto the rocker cover, r cross each other and thus crossfire. I used Bosch leads, bit they were about 15 mm or more longer and I had troubles in routing them correctly. I also used Top Gun leads but they were absolute crap, and caused the big backfire which blew the air cleaner apart. After that, I had a close look at them and I could see the spark running down the core of the leads, and arcing to earth at various places. Things would have been a lot easier if Mitsubishi had used 7 mm diameter leads instead of the 5 mm they do use. We had a Magna, also running on gas, and it never gave any troubles with its HT leads except on one occasion, and that was a poor batch of Bosch leads.

You can easily check if your HT leads are shorting to earth, or crossfiring by setting the car in a totally dark place, start the engine, lift the bonnet and look. If there is any electrical arcing going on, you will easily see it then.
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