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I know what you mean, some ISX owners have been plagued with problems and have cost them a lot of $. Us being one of them, had an 2011 ISX with 350,000 on it and had the head crack around the injector on #3 and melt it down, when comparing it to one of our 6nz's that went almost 800k without any major problems, makes the ISX look bad.

We had an opportunity to run a MX-13 against our ISX, same trans, rears, rubber. The MX was a 500/1850, our ISX is a 550/1850. It ran ok, but couldn't quite cut it, just couldn't handle the long hard pulls at 105,500. In the 250 mile one way trip 45% is 5-6%. Fuel mileage was low 4's.
 

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Apparently the new 2013 emission ISX engines haven't been plagued with as many problems as it's predecessor. We bought a 2014 388 with an ISX 550 and it has been problem free thus far. That being said, in a year and a half I've only accumulated 17,000 km....so not much of a test. I'm averaging around 60L/100km pulling Super B's.....lot's of stops and starts on a 125km round trip. Power is OK, but 600 would be better.....:p.
 

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Apparently the new 2013 emission ISX engines haven't been plagued with as many problems as it's predecessor. We bought a 2014 388 with an ISX 550 and it has been problem free thus far. That being said, in a year and a half I've only accumulated 17,000 km....so not much of a test. I'm averaging around 60L/100km pulling Super B's.....lot's of stops and starts on a 125km round trip. Power is OK, but 600 would be better.....:p.
You are right, our 2014 W900 has 75,000ish on it and it hasn't had any problems, that's not very many miles to make a good judgment, but other guys I know have well over 500k on there 2013 emission ISX and have near zero problems.
 

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we have a 2014 Pete 579 with a MX 455 HP , 1700 ft / lb / 13spd ultrashift running a 4 axle livestock trailer , 1 way loaded and back empty on a 772 km round trip .
Has 170,368 km as of last night , been decent truck / engine combination.
Average fuel economy so far from new is at 6.84 mpg cdn .
We have it choked down to 105 km/h as per provincial rules here .
it grosses 58600 kg or 129,100 lbs every time it leaves .
Also have a 2012 ISX 485 , 18 speed pulling a 3 axle pot mostly and some 5 axle end dump , it averages just shy of 5.5 mpg cdn .
Both drivers complain a bit about holding power on a hill , but i don't mind the the extra fuel economy .
Both have been decent , we found most of the the emission issues with the ISX's were from 2008 to 2011 years , which a delete kit cures .
The hwy tractor we have on order has a MX 500 with ultrashift in it .
 

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I have customers with 2010 and newer ISX, no major complaints from them, yet. Minor emission related issues (DEF tanks/pumps/heaters, there have been updates galore and sometimes back-ordered parts) and minor coolant leaks from time to time. Any customers or otherwise that have feedback on the Paccar engines have expressed mild contentment with the engine, but all have complained of poor power. Come to think of it, any customer with sub-500HP rated ISX have complained of poor power, too. Remember to compare apples to apples, ISX12 or ISX15? Power and torque ratings? Even the ISX15 rated 550 or 600 has or had different torque ratings, either 1850 or 2050 lbs/ft.
The usual complaint is staying power on the hills, or throttle response. In order to keep smoke under control from transient load (think rack control or aneroids), the injector signal is ramped up gradually, instead of a square-wave on/off signal. This results in what feels like sluggish performance, even though the engine might dyno out at 500 or so HP. I'm not trying to hijack the thread, but I sell and install Steinbauers that have proven quite adept at improving the overall feel and performance of these later-model "soft power" engines.
 
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