Tuesday, October 9, 2012

Volkswagen (VW) Beetle Basics: Additional Gauge- Oil Pressure Gauge

The simplicity of the Beetle in form and function can be easily seen in the instrument cluster offering only the very basic gauges. As much as I wanted to maintain the pristine workmanship of the classic VW, as a practical car newbie not completely confident with basic car engine knowledge and maintenance in general, any additional information I can get from my engine to hopefully anticipate road disasters would help take the edge off my deficiencies. In addition to the Oil Temperature gauge I mentioned in my previous post (VW Beetle Basics: Additional Gauges- Oil Temperature Gauge) here is another  gauge I added to my VW beetle over the years:

Oil Pressure Gauges (Electrical)
An additional oil pressure gauge runs a very close second to an oil temperature gauge in terms of importance. With the classic setup without an additional oil pressure gauge, when the "Oil" light (right light green in earlier models, red in later) on the classic speedometer flickers at speed, one is advised to STOP as this may indicate low oil condition. The thing is this green "idiot" light is supposed to come on at around 7 psi and are more often than not NOT that reliable. Knowing what your oil pressure is at different engine conditions (hot, cold, idling, running at different rpms, etc.) can give you an excellent indication of the health of various engine systems. The key is to establish baseline pressure readings when your engine is healthy, and then be aware of any changes you see over time.

In general, one should be aware also on what determines oil pressure. At lower engine speeds, oil pressure is limited by the clearances between the various bearings and journals.
checking for crankshaft journal wear
As the space between the crankshaft bearing journals and its bearings increases through wear, oil pressure will be lower because oil can flow out of the space more easily. The same is true for the journals on the big ends of the connecting rods. Thus, everything else being equal, low oil pressure can indicate worn bearings. 

There are other factors that affect oil pressure. When the oil is colder, it has a higher viscosity (it's thicker), which means it cannot slip through the bearing clearances as easily. Oil pressure at idle is usually a bit higher when the engine is first started up. On some cars, low oil pressure at idle when the engine is hot is quite normal. Oil flow may be perfectly adequate, even though the pressure is low. 

It stands to reason that thinner (lower-weight) oil will indicate lower oil pressure than a thicker oil, at least at idle and moderate engine speeds. Lower pressure caused by changing to a lower-viscosity oil may not indicate a problem, provided it is not being overheated. If the oil is thinner because it is breaking down, too hot, or diluted with gasoline from an over-rich mixture or worn rings, you should change oil at once and correct the problem. 

Potential Causes of Low Oil Pressure:

  • Low Oil Level - oil pressure dip during a hard corner or under sharp braking. Stop at once and top up the oil, or you can seriously damage your engine.
  • Diluted or Worn-Out Oil - (see above).
  • Damaged Oil Pan or Pickup Tube - may be due to scraped or banged oil pan.
  • High Oil Temperature - generally not a big factor, but if you're pulling a trailer or running flat out in really hot weather, your oil can run well over 250 degrees F., and oil pressure will be lower.
  • Worn Engine Bearings - (see above). A further indication can be a heavy knocking under engine load (main bearings) or a lighter knocking (connecting rod bearings).
  • Worn Oil Pump - This could be anything from a slight reduction all the way to catastrophic failure.
  • Dropped Crankshaft Plug(s) - These metal plugs fill the holes where the factory drilled oil passages in the crankshaft. If one falls out, oil pressure will suddenly drop across the board. You can still drive (slowly) to get home, but the plug(s) will need to be replaced.

High Oil Pressure:

High oil pressure is not generally a concern but if pressure suddenly increases, there may be a problem with the pressure relief valve. Switching to a higher-viscosity oil will also show higher readings. In choosing oil weight, it's best to follow the manufacturer's recommendations for the season and type of driving.

Types of oil pressure gauges.

Mechanical gauge
Oil Pressure Gauge and Sender

  1. Mechanical gauge- oil pressure signal is sent via a small tube to the gauge. Inside the gauge is a hollow curved spring called a Bourdon tube. As pressure increases, it tries to straighten the spring attached to a linkage and arm which in turn is connected to a needle which sweeps a calibrated face giving you your reading.
  2. Electrical gauge is comprised of an electro mechanical sending unit, an electrical circuit and an electrically operated gauge. With this system, a voltage source is sent to the gauge through a coil which interfaces with a needle arrangement that moves according to the current flowing through the coil. The current is controlled by the sending unit which is screwed into an oil galley in the engine block. There are different sensing arrangements but the most common is a variable resistor which allows a calibrated amount of current and is calibrated to the gauge face which gives the readings.
These are the additional gauges in my 1973 VW Beetle
The upper two are the Oil Temperature and the Oil Pressure Gauges while the bottom one is the Voltmeter


Volkswagen (VW) Beetle Basics: Additional Gauges- Voltmeter and Ammeter


Volkswagen (VW) Beetle Basics: The Speedometer and Warning lights for Idiots…

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