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  HomeArticles/VideosTechnical BulletinsFiat/LanciaTips on Reading Gauges >

Tips on Reading Gauges: Oil Pressure Gauges


Oil Pressure Gauge
Oil Pressure Gauge

An oil pressure gauge can give you an excellent indication of the health of various systems in your engine. The key is to establish baseline readings when your engine is healthy, and then be aware of any changes you see over time.

The Lubrication System:
The oil pump takes in oil from the sump (oil pan), and forces it through the engine under pressure. Since the oil pump is driven (indirectly) by the crankshaft, oil pressure is to some extent determined by engine RPM. Pressure is kept from rising too high by a relief valve. Typically, this is a spring-loaded ball, where a predetermined amount of pressure lifts the ball off its seat to allow oil to return to the oil pan without circulating through the engine.

The oil is pumped through drillings in the block and head, lubricating the bearings and also helping cool the engine. After it emerges from the drillings in the crankshaft and other parts, it drains back (with virtually no pressure) into the sump to be re-circulated. A filter in the circuit keeps dirt and metal filings from scratching the bearings or damaging the oil pump.

What Determines Oil Pressure?
At lower engine speeds, oil pressure is limited by the clearances between the various bearings and journals. For example, 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, though. When the oil is colder, it has a higher viscosity (it's thicker), which means it cannot slip through the bearing clearances as easily. You'll notice that oil pressure at idle is quite a bit higher when the engine is first started up. On some cars (Fiats, for example), 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 - You may first notice your 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 - Have you scraped or banged your oil pan? Stop at once!

  • 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 (which is rare unless the pump has ingested bits of metal from some other failure).

  • Dropped Crankshaft Plug(s) - This is not terribly uncommon in 4-cylinder Alfa Romeo engines. 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.

What About 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.

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All Technical Bulletins and Tech Tips contained on this website are based on our own experience, and are not necessarily "factory approved." While we make a reasonable effort to verify the information contained in them, we assume no liability for the use or misuse of the information offered.

*Overseas customers please note that our entire website is based on US-spec cars, as sold before the manufacturer left the US market: up to 1985 (Fiat/Lancia) and 1995 (Alfa Romeo). We do not carry parts for current models.