How it Works: The Oil Gauge

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Smiths Oil Gauge
One thing that always seems to be missing from your instrument cluster is the oil pressure gauge - we can think of only a handful of current cars that have one fitted, such as the Mazda MX-5. And thats a shame, because the oil-pressure gauge can prove to be very valuable.

Most people know what the function of an oil pressure gauge is ... to display the pressure of the oil being supplied to the engine lubrication system. Older type gauges if fitted to your classic car are most likely to be mechanical: oil pressure from the engine is conducted to the gauge, the pressure determining the position of a pointer. Modern types are electrical, these first being used on marine engines.

The mechanical gauge mechanism consists of a flat tube which is bent to form a semi-circle. One end of the tube is fixed and is connected to the pressurised oil supply. The free end of the tube is connected to a pointer by a linkage which is so arranged as to produce a large movement of the pointer for a correspondingly small movement of the tube. As the oil pressure increases, the tube tends to straighten and the movement is transmitted to the pointer, which indicates the pressure on an appropriate scale.

The oil pressure warning light, which IS fitted to most cars, is extinguished when the oil pressure exceeds a few pounds per square inch. It provides no indication of variations in oil pressure which may occur, above the minimum pressure at which it operates; because it operates at a low pressure of about seven pounds per square inch it may not provide an indication of a fault before damage has occurred.

Conversely, an oil pressure gauge provides a constant indication, instantly showing the reduction in oil pressure if anything goes wrong with the oil supply, so enabling a driver to stop before any damage occurs. Aspects .of the condition and state of wear of an engine can be discerned from intelligent reading of an oil pressure gauge. Over an extended period of driving, for example up to 100,000 miles, it is likely that a gradual reduction in oil pressure will be apparent. This will be caused by general wear in the engine bearings.

It should not normally be a cause for concern until the oil pressure of the engine at idling speed, with the engine hot, is less than 5-15lb sq in, depending on the particular engine. The normal oil pressure for an engine at running temperature is in the region of 40-65Ib sq in, although it may fall considerably below this before the supply becomes inadequate.

Over a period of 5,000 - 10,000 kilometers a small but significant reduction in oil pressure may be observed. Such a variation is likely to be due to the molecules of a modern multigrade oil breaking down over this period of use, indicating the necessity for an oil change. In very cold conditions engine oil becomes more viscous, that is, thicker. It is then difficult to force between the small bearing clearances and the oil pump, operating constantly, builds up a pressure which is higher than desirable. Too high an oil pressure can 'blow' gaskets (causing oil leaks), cause frothing of the oil (inhibiting its lubricating function), and accelerate the deterioration of the oil, necessitating more frequent oil changes.

Worn Big End Bearings or Worn Main Bearings

To avoid such faults, when the oil pressure exceeds a pre-determined value, an oil pressure relief valve will open, returning some of the oil to the sump and so relieving the excess pressure. If, in cold conditions, the oil pressure gauge shows an increase to an unusually high figure, it may be advisable to refer to the service manual, to check that the relief valve is operating at the correct pressure. A low indicated oil pressure can be caused by a number of factors. Wear of the engine bearings will reduce the oil pressure. If bearing wear is suspected, run the engine at a fast speed, quickly release the throttle and listen. A deep clatter indicates worn big end bearings; a rumble indicates worn main bearings; a light tapping is an indication of worn small end bearings.

If any of these noises are heard, in conjunction with a low oil pressure gauge reading, it will be necessary to overhaul the engine, examining all the bearings and renewing them as necessary. If the engine is not noisy, and appears to be in good condition, in spite of a low oil pressure indication, it is possible that the oil pressure relief valve is stuck in the 'open' position or that there is a partial blockage in the oil supply. The latter can be caused by a choked oil filter, which must be appropriately renewed or cleaned. This should be done as a routine item at the regular servicing intervals. If the oil pressure drops suddenly while the engine is running, the engine should be stopped without delay and the cause investigated.

Large losses of oil from the engine are normally easy to locate, and the cause is often evident. Occasionally, particularly if the oil level in the sump is low, hard cornering or severe braking may fling the oil to the side or to the front of the sump, starving the pump of oil, which will show on the gauge as a momentary drop in oil pressure. Baffles were often fitted to the sump of cars built from the late 1960's onwards, so this effect should not occur unless the oil level is well below the minimum mark on the dipstick. Very hard cornering or braking could overcome the standard baffles, even with an adequate oil level ; if this is a frequent occurrence, the standard sump can often be replaced with a sump suited to 'competition' motoring, which will incorporate additional baffles.

The fitting of an oil pressure gauge as an accessory is a straightforward operation. Kits are usually available from accessory shops, ensuring the compatibility and correct sizing of components. It is good practice to retain the low oil pressure warning light, as this provides an indication which is more likely to catch the eye of the driver than the more subtle movement of a gauge needle. First locate the low oil pressure warning light switch (usually on the side of the engine block). If there is any possibility of confusing this with the water temperature gauge sensor refer to the service manual for the location. Alternatively, the two can be distinguished by noting the colour coding of the wiring, and cross-checking this to the colour-coded wiring diagram normally included in the car handbook.

Unscrew the switch-be prepared for some oil spillage when the switch is removed - and in its place fit aT-connection. Fit the switch to one branch of the T-connection and the oil pressure gauge adaptor to the other branch of the T-connection. Fit the gauge in a suitable position on the dashboard, connect the tubing to the gauge and run this from the gauge to the adaptor on the T-connection. Before connecting, coil the excess length of tubing in a loop of about three inches diameter, a few inches from the T-connection; the tubing can be retained in a loop by tape. Then connect the free end of the tubing to the adaptor. Secure the capillary tubing a few inches away from the loop, on the gauge side, so that the loop can absorb the engine vibration and movement without damage. Start the engine, ensure that all the connections are tight and leak-free, and check that the gauge and the warning light operate correctly.
Oil Gauge Schematic
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