The oil system in a VW engine is, like any vehicle’s engine, crucial to proper operation. The oil system of the air-cooled VW flat-four engine is even more important than one might realize since along with the primary lubrication function, the oil also serves a critical cooling purpose, with some of the oil being directed through the oil cooler located in the fan shroud.
It basically follows the following routing, with almost all activity occurring on the cylinder 3-4 side side of the case:
- The oil is picked up at the sump pick-up on the cylinder 1-2 side of the case.
- It travels through the oil pump (which of course caused it to be picked up in the first place) and heads to the top of the case
- It passes the oil pressure sender (connected to the idiot light on the speedometer)
- The oil then passes through the Oil Relief Pressure valve, and depending on the temperature and viscosity of the oil, goes through 2 possible routes. 1) The main route for low/normal temperature oil is the most needed route through the gallery feeding the crankshaft bearings, camshaft bearings, and the lifters (which then take it up through the push rod tubes to feed the rocker arms which then can flood back down the pushrod tubes back to the sump. If the oil temperature is too high, the Oil Relief Pressure valve can 2) sent up up to the top of the case in which it hits…
- The oil cooler and is routed up through the cooler with the fanshroud dropping the temperature, and hence the viscosity, of the oil.
- Both the gallery from the oil cooler and the main gallery feeding the crank/cam shafts join up before hitting the Oil Control Valve which ultimately deposits that oil path down through a port where,
- The spent/used oil is accumulated again where it then gets picked up by the sump siphon restarting the process.
One of the best pictures I could find… well… the only picture I could find of the case cut-away to actually show the oil gallery passages along the main axis is from The Samba member Jimmy111 posted way back on Jun 04, 2008 (image below)
You can clearly see the front to rear and top to bottom oil galleries clearly as well as the “tubes” for the front Oil Relief Valve and the rear Oil Control Valve. Within the galleries you can see the holes leading to the pathers that feed the camshaft and crank.
Once you see how these tunnels snake through the alloy of the case, it becomes apparent that without pulling the oil plugs on the external part of the case you just cannot ensure that all of these passages are free of debris completely clean of sediment or other deposits.
Sure, the solvents sort of washed through these. And yes, you can try to force high temp/high pressure water through these as well as compressed air, but there’s no real access to any of these without truly prepping the case prior to build to allow for access.
Here’s a video I found posted on YouTube by guidoguitar that helps to explain the paths too:
Sources:
Article – Gene Berg – About Oil Coolers » About Oil Coolers – Technical Info
Video – guidoguitar – Oil Pathways and Flows in VW Dual Pressure Relief Case. July 6, 2015
Article Thread – The Samba – Member Jimmy111 – Dual relief oiling system. How it works. February 17, 2008
Article – Bob Hoover – HVX MODS, May 29, 2007