Someone at work asked me to check out their truck, apparently its misfiring at speed. Wont go over 60 mph, etc.

Its a 2000 Mitsubishi Montero.. I've never dealt with misfiring. I'm guessing replace spark plugs, then if that doesnt do it then either timing or o2 sensor?

1 plugs
2 HT leads (plug leads) if it has them.
3 Clean all electrical contacts to do with spark system.

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Grumpy Old Diesel Owners Club group

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1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
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1968 Ford F750 truck. 6-354 diesel conversion.
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ht leads? what what? Its a coil ignition system.

Coil ignition system as in coil-on-plug with individual coils?

Anyway, part throttle misfiring usually ends up being worn out plugs (that is if the engine seems to perform OK at idle). I've lost count of how many vehicles I have gotten that were written up as a slipping transmission only to test drive and find a part throttle misfire that is cured by a set of spark plugs. Sometimes the plugs just don't make it to the 100K mile service interval.

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Erich Loepke
2010 Ford Focus
Currently Benz-less

ah, yeah the engine idles fine. Its just at speed it acts up.

looks like I'll have to read up on how to gap spark plugs.. lol

I dont know if its wasted spark or not. I didnt see a dizzy so its coil lol

the offending cylinder will have a plug with much different characteristics, this should help with the diagnosis.

don't forget egr if nothing else makes sense.

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First pull the codes, it will probably tell you what cylinder is missfiring.

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2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200

Check the intake tube between the MAF and throttle body. I have seen a cracked tube combined with a dirty air filter cause these same symptoms.
The Mitsu 2.6 also used to have a horrible problem of running lean and burning valves or the head gasket, but that typically showed a misfire or shake at idle.