[T3] Engine oil temp?

Jim Adney jadney at vwtype3.org
Sat Jul 18 09:39:56 PDT 2020


On 18 Jul 2020 at 7:44, William Jahn wrote:

> I replaced the oil pressure  and temp gauges and so far the oil pressure
> sender. It's mounted on a square profile steel elbow to the cooler with a
> banjo fitting , don't recall if that came on my engine.

That banjo adaptor came from earlier dual carb engines. It was necessary to 
clear the dual carb air cleaner. That location is fine for oil pressure, but it's 
no good for temp, because there won't be any flow past the sensor in that 
space.

Your other post indicates that you're measuring oil temp either at the sump 
or at one of the relief valves. Either location will be fine.

>  Here's the deal . The oil pressure readings are a little higher than the
> 10 pSI per 1,000 rpm rule. I don't know who made the rule but I do know VDO
> gauges are not 100% accurate at least to say they may not be.
> 
>  7 mile drive 81 * idle PSI 25 . Oil temp read 180* . Shut it down for an
> hour temp dropped to 140 oil pressure back up to 35 pSI 1,000 RPM the
> highest it gets @180 * 40 PSI. Drove home 7 miles ambient 85*. read about
> the same. Shut it down for 10 minutes oil temp was 190* .
> 
>  The temp will rise yet it almost seems as if the cooler always has oil
> flow.

The cooler will always have oil in it, regardless of whether there is significant 
flow or not. There may be an air bubble at the top, where the pressure 
switch/gauge is, but that won't affect the pressure reading. It would affect a 
temp reading if your temp sensor was there.

If you're running heavier oil than VW specified, the pressure relief passages 
can be overwhelmed. This may be why you see pressures above 28 psi, 
which is where the relief valves are supposed to regulate. It may seem 
counterintuitive, but high oil pressures are not necessarily good. What you 
need are sufficient oil volume delivery to the bearings.

Journal bearing lubrication is not an obvious thing. The rotation of the shaft 
forces oil into areas where the shaft has moved closer to the bearing, 
creating high pressure areas where high pressure is needed. It's important 
for oil to be introduced only into low pressure areas, which is why the oil 
entry points in a bearing may seem odd. They are carefully planned.

The oil pressure in the high pressure areas of the bearing will be MUCH 
higher than what's available at the entry point. This is called hydrodynamic 
lubrication. To understand this, draw yourself a sketch of an off-center shaft 
rotating in a bearing. Note how the rotation will push oil into the wedge area 
where the shaft is starting to get close to the bearing. The closer the shaft 
gets to the bearing, the higher the oil pressure will be at that point, and it has 
little to do with the oil pressure at the entry point.

So, it all gets back to the fact that what you need from your oil pump and 
relief valve is high oil delivery volume, not necessarily high oil pressure. In 
their late engines, VW increased flow by enlarging the oil pump gears and 
reduced the need for high pressure at the pump by increasing the size of the 
oil passages, thus reducing pressure drop in the oil passages. The larger oil 
passages were especially useful in cold weather, when the oil was much 
thicker.

I don't see any problem with the temps and pressures you posted, but be 
aware that all commercially available gauges tend to be pretty inaccurate.

-- 
*******************************
Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
*******************************



More information about the type3-vwtype3.org mailing list