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2011 Mack pinnacle MP7 RPM problems


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We have a MP7 that has no foot pedal when it’s cold. If it drops below 1200 RPM it won’t run right or accelerate. After the truck gets going and is warmed up it runs great. We’ve had it in and out of shops and no one can seem to figure out what the problem is. One shop said ECM and another said. ABS module. If anyone else has experienced this problem and got it fixed I’d love to hear what you found. Thanks!

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Hi Bdschafer.

How did the one shop come to the conclusion that the engine ECM was defective and another shop said the ABS module was defective? Did they explain a diagnostic process to come to that conclusion?

There is an old saying in our industry, keep it simple stupid. I am a firm believer that the simple things should be ruled out first. Even though people here did not experience the same situation, others may guide you to help in a diagnostic process. 

Was the fuel pressure checked during the problem and after it cleared up? Was the cooling system checked for combustion gases? When you say it wont run right, does is sputter and miss while trying to accelerate? That could be an egr valve stuck open. The engine ECU will not command the egr valve to turn on until the coolant temp reaches 150 degf. Can you hook up a service tool and check for egr mass flow and egr temp? Any codes? Is it pushing coolant out of the coolant tank? If you can report back with some clearer information this will help. I'd hate to fire a parts cannon at the truck. There are a lot of smart people here that will collaborate and help you out.

V

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Could be.

It would be interesting if he/she can take a snapshot of some live data and we can see whats happening. EGR data, injector data, and codes.

But I think do the easy checks first would rule a lot out.

V

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We have had this truck laid up for so long It’s hard to keep track of everything that has been done. As far as I know it only had ABS codes. There’s no sputter or miss. when the driver pulls out and starts shifting and the RPM drops below 1200 into the next gear  there’s no foot pedal or accelerator at all if it stays above 1200 there’s no issue usually. One shop said the ABS module wasn’t communicating with the ECM correctly causing the truck to derate. The other shop just said the ECM wasn’t working correctly. (neither shop was very descriptive). Also I don’t have the necessary tools to send the live data sorry. 

Edited by Bdschafer
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Is there any ABS or or V-Mac(Lightning bolt) lights on if there is an ABS fault one of those lights will be on, most likely the ABS light. Another trick to try is if the truck is a manual, unplug the transmission output speedo sensor from the harness on top of the trans. An electrical malfunction light(V-Mac) will come on. Then drive the truck when it's cold and see if it continues to have issues.

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