BenB
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| posted on 14/1/08 at 04:03 PM |
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Why do cars have low oil pressure
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???
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JAG
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| posted on 14/1/08 at 04:11 PM |
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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]
Justin
Who is this super hero? Sarge? ...No.
Rosemary, the telephone operator? ...No.
Penry, the mild-mannered janitor? ...Could be!
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bonzoronnie
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 posted on 14/1/08 at 04:14 PM |
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Low oil pressure
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 
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02GF74
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| posted on 14/1/08 at 04:17 PM |
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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.
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BenB
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| posted on 14/1/08 at 04:34 PM |
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Okay, looks like the consensus is the main culprit being bearings and shells.... Sounds very logical!!
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snapper
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| posted on 14/1/08 at 04:58 PM |
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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
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MikeRJ
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| posted on 14/1/08 at 05:35 PM |
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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.
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rusty nuts
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| posted on 14/1/08 at 05:57 PM |
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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
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Avoneer
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| posted on 14/1/08 at 06:00 PM |
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quote:
Originally posted by 02GF74
think of it as pimping up a bicycle inner tube.
Is that something Tim Westwood does?
Pat...
No trees were killed in the sending of this message.
However a large number of electrons were terribly inconvenienced.
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thomas4age
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| posted on 16/1/08 at 10:43 AM |
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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
If Lucas made guns, Wars wouldn't start either.
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britishtrident
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| posted on 16/1/08 at 03:39 PM |
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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.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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britishtrident
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| posted on 16/1/08 at 03:59 PM |
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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.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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