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  1. #1
    Medium Load Member
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    Cummins ISX15 VS Paccar MX

    Anyone have any experience with either or both of these engines?

    Looking into a new T660 and I want to know what's the best way to go.

    Thank you.


  2. #2
    Road Train Member
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    News and views on the MX engine seem to be hard to come by, but the Cummins engine would give you a bigger service network if you had problems.

  3. #3
    Medium Load Member
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    DOH! Sorry, posted in the wrong section!

    Could someone please move to Trucks for me, thank you!

  4. #4
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    I have driven both in the same speced truck. Both were 450hp with 10 speed trans. The MX engine is loud. The turbo was so loud I got out to check for leaks around the turbo. The MX had decent power. Better than a detriot. The fuel mileage was worse than the cummins by half a mile to the gallon. It was harder to shift. With the engine brake on the rpms would fall to fast and with it off it would fall real slow. If you listen to a 2007 emmisons spec Cummins the MX has the same sound and loudness while idling. It wasn't all that bad, but I will stick with cummins.

  5. #5
    Road Train Member Tazz's Avatar
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    Coming out of a 387 with the ISX into a 587 with the MX.

    Higher RPM's in the MX. Kinda flat in low rpm's like a Mack E7. Mine is quieter than the ISX and I do not hear turbo noise at all. I do hear the rotary air compressor though.

    Lower MPG now but brand new so maybe will be better after break in?


    So far my MX is growing on me.

  6. #6
    Light Load Member
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    While I don't own either, a friend of mine had the ISX in a 2008 Pete 386 and now has the Paccar in a 2011 386. His findings so far:

    Paccar much better fuel mileage. Lifetime 6.4 on the Cummins, so far (about 45,000 miles) 7.15 with the MX, running midwest to southern Cali. An O/O at the company I'm leased to has a brand new KW T660 with the MX, running southern Minnesota to western New York he's at 7.5 right out of the box.

    No shop time yet with the MX. By 50,000 miles he had the 08 in the shop at least 10 times. Sensors kept going bad and derating his engine, giving out false failure codes, etc. The DPF sensors were a constant headache.

    It was the continual shop time with the Cummins that steered him to the MX. His salesman sells all the Petes to Decker in Ft. Dodge, Ia. They had ordered 50 of the MX, the salesman talked them into 25 because of his concerns with getting repairs and parts in some areas. When they ordered their next batch it was almost all MX. They had virtually no problems with the Pete engine in comparison to the Cummins.

    All in all, my friend is very happy with the new MX. SpeedCo even has filters for the thing, and they almost never have a fuel filter for my 07 Shaker.

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  8. #7
    Bobtail Member
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    I Have a 2011 Pete 389 with an ISX 15, 500 HP Cummins. Have operated this truck for 17 months now, and am totally dissapointed (that's putting it mildly) with this engine.

    So far I have gone through 5 sets of valve seals, two EGR coolers, a fuel injector, and now a new DP filter. That from the excessive amount of oil consumpsion because of the valve seal failures.

    The engine at first had a high DEF consumpsion rate, over 6%, but has come down to about 2% after the break in peroid, and several software updates.

    What is most upsetting is the amount of down time this truck has had so far. Well over a months worth of time spent in the shop. I estimate at least $25,000.00 worth of lost revenue.

    Cummins so far has not impresed me with their ability to either solve this valve seal issue (which is the root of the problem), or listen to my complaints.

    Any one considering purchasing one of these engines would be well advised to not use it for anything other than 5 axle work. Several other owner operaters I work with, have the same issues, so I know that I'm not in a class by myself.

    Cummins if you read this, please get your s--- together, and fix this problem! If you don't, I'll end up going broke. There isn't enough money in trucking anymore to offset what I'm losing to downtime.

  9. #8
    Bobtail Member kind's Avatar
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    I have a 2012 389 with a MX485. I can't complain about the motor, besides it's not overly impressive when empty or pulling 61,300kg's. To be expected. As long as you aren't stuck behind me accelerating or climbing a hill, it's not that bad haha.... It is boring as hell to drive tho lol.

    However I have had issues with the SCR system. It has caused a month and a half of downtime.

    Fuel economy is comparable to a 2007 379L, exact same rig other then it having a ISX 565. Our fuel economy is very similar. I average 4.7mpg(imperial).

    Overal I'm happy with the motor. I am beyond frustrated in the SCR system tho. Cummins wouldn't be any better there.

  10. #9
    Road Train Member
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    Wait till the Irish guys get here.

  11. #10
    Bobtail Member nakoBilly's Avatar
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    Norwalk CA
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    2012 Kw t700
    Cummins isx 400hp
    9speed
    3.55 rears
    So far I've put 85k miles on it and no problems
    Average gvw 73k I run midwest to south california
    Average mpg so far 7
    I pull doubles and 53' vans
    I'm really happy with this...coming from an 03 freightliner with detroit egr 500hp 13speed,
    My average mpg was 5.75 pulling same type of loads and running same area.

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