My Dodge Ram, that I bought used recently, wont go over 40MPH and has vacuum in the crankcase. It runs rough unless I take off the oil fill cap, then it runs close to “normal” but wont go much over 40MPH. Vacuum line hooked wrong? Cracked head? Did research but can’t find squat. Where to begin?
The crankcase vacuum is caused by a leaking intake manifold belly pan gasket, that can result in oil loss and fouled spark plugs.
The reason the truck won’t go over 40 MPH, you will need to assess the operation of the vehicle, is it stuck in first gear? Does the tachometer move? Accelerator cable fell off? Cylinder misfires?