My 1929 Plymouth Model U

May 27, 2008. 

I have been having a lot of trouble with burning an excessive amount of oil. When at idle the oil pressure reads just under 40 psi and when driving my gauge is maxed out. The motor runs strong and was rebuilt not too many miles ago by the car’s former owners. Anyway since I was draining the coolant to replace the radiator with an original one I decided to remove the head. My two purposes are one to inspect the pistons & cylinder walls, and the other reason is to repaint the head silver.

With the head removed I found pristine cylinder walls with hardly noticeable edge ring. The cylinders had oil puddled in the cylinders with a lot of black carbon on the pistons. I cleaned up the area and decided to trouble shoot the oil pressure.

Thinking that my car’s gauge was inaccurate I installed a new modern oil gauge and cranked the engine with the starter. The gauge read 38 psi after about a half a minute of cranking. This is the same value as my interior gauge so I guess my old gauge is correct.

I then decided to remove the pressure relief valve assembly. After inspection I could not push the valve in far enough to allow the port to open. I disassembled the valve and cleaned the spring, plunger, and cylinder. After reassembling the parts I could now push the plunger in to allow the valve to open. I think this was my problem. I reassembled the valve into the engine and set the pressure one turn from minimum pressure. I cranked the engine with the starter and the pressure pumped up to 20 psi and with continued cranking would go up to 35 psi. The pressure looks much better now then when I started.

Once I get the motor back together I will adjust for 35 - 40 psi maximum when driving at a normal speed. I think this should help the excess oil burning problem by allowing less oil to blow by the cylinder rings.

The motor was freshly rebuilt and the pistons and valves are all new. The cylinders show no appreciable wear and they look like they just came back from the shop.  I wonder if the rings and pistons were correctly spec. out or installed properly. Just because things look good that doesn’t necessarily mean that the machine shop or the home rebuilder did everything correctly.

Judging by everything I just said I would not have expected the pressure relief valve to be in such a sorry condition. The engine might have been hot-tanked with the valve in place so that it might not have been properly cleaned which leads me to believe that the rebuilder was not familiar with this vintage engine or the car was unused for a long time after the engine rebuild.???

I will put everything back together and hope for the best. If no improvement then I will most likely pull the engine out and start all over again. I have a bad feeling that the pistons might be slightly undersized with oversized rings improperly installed. Before I reinstall the head I will mic. the cylinders and piston gap. The cylinders are stamped on the top with a part number beginning with X and the valves are marked with manufacture & part number, none of which are Chrysler.