Did u say only 40 Psi I would look at that for sure might just be rust build up on rings let it run and see if it goes away 40 is way low and can cause a mis fire code... I have seen one where the litter took a **** and rounded the cam causing a mis fire code I would put oil in there and crank it 3 times and see what it tops out at maybe it has bad valve springs or something easy