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Why do cars have low oil pressure

BenB - 14/1/08 at 04:03 PM

Assuming it's not something simple like not having enough oil or the oil being too hot what causes an engine to be low on oil pressure? On old engines is it just knackered oil pumps or generally worn components that cause it?

And before anyone asks my oil pressure is fine

I only ask because I'm currently brewing up a dry-sump installation for the ST1100. On the pace pump you can adjust the oil pressure. Surely isn't it possible through this to cover up for worn components ie if you buy a car with a dry sump having oil pressure is no guarantees of anything in particular???

JAG - 14/1/08 at 04:11 PM

The classic reason is worn bearings.

The oil is pumped through holes and into the gap between the crankshaft journal and the crankshaft bearing. Also the camshaft journal and camshaft bearings. If the bearing and journal are worn (after high mileage for instance) then the gap is bigger and the oil escapes back to the sump quicker hence reducing the overall oil pressure available.

[Edited on 14/1/08 by JAG]

bonzoronnie - 14/1/08 at 04:14 PM

quote:
Originally posted by JAG
The classic reason is worn bearings.

The oil is pumped through holes and into the gap between the crankshaft journal and the crankshaft bearing. Also the camshaft journal and camshaft bearings. If the bearing and journal are worn (after high mileage for instance) then the gap is bigger and the oil escapes back to the sump quicker hence reducing the overall oil pressure available.

[Edited on 14/1/08 by JAG]

I agree.

Main bearing wear is by far the most common reason for low oil pressure readins.

Ronnie

02GF74 - 14/1/08 at 04:17 PM

oil pressure is dependent on two things, maybe more:

1. pressure the pump can develop
2. resistance to flow.

think of it as pimping up a bicycle inner tube.

is you have a crappy pump, the max pressure it can produce is much lower than a high pressure pump.

in car terms, low pressure would be down to worn pump.

the other part is the resistance.

again think of bike pump. if you just start pumping with nothing connected to the pump, the pressure will be low but cover your finger over the pump end, not only will you proiduce comedy fart noises but you find the pressure in the pump is much higher. translatated to engine terms, this is resistance to oil flow, namely by viscority of oil and clearances of engine parts, the major one being the mains (rememember the crank floats on the oil cushion). as these wear and oil gets hotter, the oil finds it easier to get round the block so oil pressure decreases. the oil valve is there to limit the max. pressure.

if is a common misconception that upping oil pressure above the manufacturiers spec. is the clever thing to do; no so as this causes wear to pump and the driving mechanism, usually off the camshaft. Tt is better to have an increased flow instead.

BenB - 14/1/08 at 04:34 PM

Okay, looks like the consensus is the main culprit being bearings and shells.... Sounds very logical!!

snapper - 14/1/08 at 04:58 PM

A cause of low or lowering oil pressure in a perfectly good engine is often the oil getting hotter than its design limits and breaking down.
Usually seen by the oil temp going up high and the oil pressure dropping, time to stop and cool off then fit oil cooler

MikeRJ - 14/1/08 at 05:35 PM

quote:
Originally posted by 02GF74
if is a common misconception that upping oil pressure above the manufacturiers spec. is the clever thing to do; no so as this causes wear to pump and the driving mechanism, usually off the camshaft. Tt is better to have an increased flow instead.

Very true, the actual protection comes from the hydrostatic pressure created by the bearing itself, as the journal rotates within it, not the oil pressure created by the pump.

However, increasing flow without increasing pressure is not possible without opening up clearances or using a lower viscosity oil.

i.e. a high capacity oil pump running at the same pressure as a standard one will be pumping the same amount of oil around the bearings, just more oil will be going through the bypass valve with the bigger pump.

rusty nuts - 14/1/08 at 05:57 PM

Another thing that causes low oil pressure is a blocked pick up. More common than you may think especially CVH and Vauxhall 8 valve engines although improvements in oils have reduced the problem

Avoneer - 14/1/08 at 06:00 PM

quote:
Originally posted by 02GF74
think of it as pimping up a bicycle inner tube.

Is that something Tim Westwood does?

Pat...

thomas4age - 16/1/08 at 10:43 AM

Also need mentioning I think.

wrongly plumed in remote oil filters, often there's a one way valve in the filter. the overpresure regulator that is in the filterhousing then open letting exes presure into the gallery, so you won't really notice something is at fault from the beginning. (seems stupid but I know of at least three that destroyed perfectly good toyota engines this way)

wrongly mounted sump tubes ( the oil suck up tube that's in the sump) the gaskets need to be replaced always, otherwise the pump will suck up oil and air, this reads as a lower than normal presure on the gauge, this fault cost me one 16v 4age engine.

for the rest main and big end bearings. and cloged oil coolers due to some routing oil through the cooler first and then trhough the filter, needs to be the other way round.

funny thing is that these reasons all take care of the bearing's and shells, which brings us back to the afformentioned reasons for low presure.....

Grtz Thomas

britishtrident - 16/1/08 at 03:39 PM

quote:
Originally posted by rusty nuts
Another thing that causes low oil pressure is a blocked pick up. More common than you may think especially CVH and Vauxhall 8 valve engines although improvements in oils have reduced the problem

Honda V6 engines are prone to this.

britishtrident - 16/1/08 at 03:59 PM

Most common reason is worn big end bearings, main bearing tend to get less wear because they get a better oil supply.

Back in the old days pre 1970 or so it was a big problem, oils weren't as good as they were now, they were thicker when cold, suffered from sludge build up and as a result suffered from big end and crankshaft wear.

The worst offender by a country mile was the Mk1 Cortina, the original 3 bearing Kent engine in 1500 pre-xflo form would run its big ends at 30,000 miles. Older Morris (ie pre BMC) engines were also notirious.

The oil flow through the big end bearing is the only coolant it gets, if it dosen get enough oil the bearing material will melt, in the old pre-ww2 days the bearing material was white metal which was cast into the connecting rod and would literally melt "run" out if over heated -- hence the expression a "run its bearings". Shell bearings were introduced just before WW2 and by about 1955 bearing material had improved enormously (Vanderwell "Thin wall" tin-alumium bearing shells --- which funded Valwall F1 team).

About 1960 modern multigrade oils such as GTX, Esso Uniflo and Duckhams 20w/50 started to appear with detergent additives which made a further improvement.

Screwing up the spring tension on the oil pump pressure relife valve won't hide a worn bearing when the oil is up to working temperature.

Oil pressure is a symptom not an objective what matters is that the bearings are getting enough oil flow -- only if the oil pressure is lower than normal for the engine and conditions something is wrong.