help to diagnose a sick r model in Mack Truck Q & A Posted February 15, 2017 I think before i started taking the pump off I would try what Dirtymilkman suggested. Check the puff limiter rack limiting cylinder on the front of the pump. Before you do this …. If you mash the throttle with the brakes on , does it blow black smoke ? From what you said it won't , I just wanted to make sure you did this simple test to see if it is pump related or puff limiter related. My 350 4V seemed lazy . It would only get boost around 1600 - 1700 rpm . It turned out to be the puff limiter relay valve on the manifold.