I_Like_Waffles Posted December 14, 2015 Share Posted December 14, 2015 Hello there!Currently have 2 2008 Mack trucks with the MP8 motors in them. I'm having issues with both of them getting up to speed. There is a intermittent issue going on with these trucks. Ive changed the foot pedals, speed sensor, and speed sensor pigtails. These issues happen randomly. They just cant get up to speed. Sometimes they wont go past 45 mph sometimes 55mph. Does anybody have any feedback on what to check on these trucks. I'm at a loss. Both are equipped with a 13 speed and 1 has a check engine light (lighting bolt) for a crank case breather code. The other shows nothing. Quote Link to comment https://www.bigmacktrucks.com/topic/43101-issues-getting-to-top-speed/ Share on other sites More sharing options...
220cummins Posted December 14, 2015 Share Posted December 14, 2015 Are they building any boost? Quote Link to comment https://www.bigmacktrucks.com/topic/43101-issues-getting-to-top-speed/#findComment-315696 Share on other sites More sharing options...
I_Like_Waffles Posted December 14, 2015 Author Share Posted December 14, 2015 yes they are. the speedometer sometimes drops out and comes back on. if that is any helpful to know then there u go. Quote Link to comment https://www.bigmacktrucks.com/topic/43101-issues-getting-to-top-speed/#findComment-315698 Share on other sites More sharing options...
bbigrig Posted December 15, 2015 Share Posted December 15, 2015 Speedo sensor issue at transmission output shaft. That will cause speedo drop out and intermid. Top speed issues. Do you have any loss of prime issues or miss/vibration from engine? Quote Link to comment https://www.bigmacktrucks.com/topic/43101-issues-getting-to-top-speed/#findComment-315799 Share on other sites More sharing options...
I_Like_Waffles Posted December 15, 2015 Author Share Posted December 15, 2015 motor runs fine. no miss or vibrations from the engine at all. its usually when its running. Quote Link to comment https://www.bigmacktrucks.com/topic/43101-issues-getting-to-top-speed/#findComment-315858 Share on other sites More sharing options...
bbigrig Posted December 15, 2015 Share Posted December 15, 2015 I was just checking to see if you had other reasons for your power loss of a mechanical nature. Try replacing the output speed sensor on the trans. You could have active and or inactive codes for it. That will cause your speedo drop and sometimes power/speed loss. Quote Link to comment https://www.bigmacktrucks.com/topic/43101-issues-getting-to-top-speed/#findComment-315867 Share on other sites More sharing options...
theakerstwo Posted December 15, 2015 Share Posted December 15, 2015 Before you start replacing parts you need to read the ecm for codes. Quote glenn akers Link to comment https://www.bigmacktrucks.com/topic/43101-issues-getting-to-top-speed/#findComment-315912 Share on other sites More sharing options...
220cummins Posted December 16, 2015 Share Posted December 16, 2015 Does throttle response cut out intermittently as this is happening? Quote Link to comment https://www.bigmacktrucks.com/topic/43101-issues-getting-to-top-speed/#findComment-315972 Share on other sites More sharing options...
I_Like_Waffles Posted December 16, 2015 Author Share Posted December 16, 2015 new speed sensor. new pigtail. no codes logged at all. like nothing. yes we lose throttle. Quote Link to comment https://www.bigmacktrucks.com/topic/43101-issues-getting-to-top-speed/#findComment-316030 Share on other sites More sharing options...
220cummins Posted December 17, 2015 Share Posted December 17, 2015 Ok watch your boost and if the boost drops out when you step on it and then lags up it could be the egr valve sticking open. I recently seen that happen on a 2009 and a 2011 GU. A lot of trouble shooting to determine that was happening. Other than checking with PTT you'd have to tear it apart to check to see if the valve sticks or hangs up. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/43101-issues-getting-to-top-speed/#findComment-316116 Share on other sites More sharing options...
I_Like_Waffles Posted December 18, 2015 Author Share Posted December 18, 2015 sounds good. I can definitely see boost tapering off when the egr opening. its weird how it wont log codes. Quote Link to comment https://www.bigmacktrucks.com/topic/43101-issues-getting-to-top-speed/#findComment-316219 Share on other sites More sharing options...
220cummins Posted December 18, 2015 Share Posted December 18, 2015 Yeah you might bet a sound of a boost leak when this is happening intermittently. That's what these two trucks were doing as well. No codes were present. Also check the stalk stick wiring harness in the steering column for chaffing. I've seen one other that was dropping out like that on a CXU and that was an issue. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/43101-issues-getting-to-top-speed/#findComment-316221 Share on other sites More sharing options...
I_Like_Waffles Posted December 18, 2015 Author Share Posted December 18, 2015 yup. checked over everything. Found the CAC had exploded up top and somehow with the sound/heat pad on the hood it pushed down on the cooler and would seal it off and would not notice the boost lose. since it was intermittent I'm guessing a bump or road conditions had an effect where the engine bay would shake and boom boost leak galore. Quote Link to comment https://www.bigmacktrucks.com/topic/43101-issues-getting-to-top-speed/#findComment-316324 Share on other sites More sharing options...
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