Oil pressure gauge reads high when cold
02-07-2005, 03:46 PM
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Oil pressure gauge reads high when cold
So, I just noticed that my oil pressure gauge stays pegged at 5psi upon starting and then over the period of 15 minutes as the engine warms up, it begins to operate normally, fluxuating between 2-3 at idle and increasing to 5psi around 3000 rpm. Clearly I need to get under the car and check the sensor with an ohm meter, but I was wondering if anyone had any thoughts. Mark
02-07-2005, 03:49 PM
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It's supposed to do that.. The higher pressure at start up is to lube the internals while the motor warms up.. Most of your engine wear occurs at start-up when no oil is on the parts, but down in the sump
HTH,
HTH,
02-07-2005, 04:01 PM
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Perfectly normal. If it stays pegged at 5 bar ALL the time, then the sender has failed. Once warm, it should be 4.5 to pegged above 2k RPM and 1.5-3 at idle.
02-07-2005, 04:30 PM
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I'll pass along a great rule of thumb told to me a few years ago by a 928 friend who's initials are Jim Bailey (my apology Jim, but you should be recognized): you should see at least 1 bar oil pressure for each 1k rpm increment. Anything less and something is wrong.
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02-07-2005, 06:14 PM
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Oil pressure drop as the enign warms up due to expansion of all aluminum surfaces as compared to steel....crank journals, cam journals, pump surfaces. Bigger "gaps" less oil pressure.
Marc
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02-07-2005, 06:25 PM
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just noticed that my oil pressure gauge stays pegged at 5psi upon starting and then over the period of 15 minutes as the engine warms up
02-08-2005, 10:10 AM
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Hi,
Mark - Bill hit the nail on the head and you may find this site of some assistance too;
http://www.landsharkoz.com/tt/ttlubricant.htm
If you were to use a 25w-60 or 70 mineral oil it is possible that you could get around 5bar at idle - what a worry!!!
Marc - you said this:
"Oil pressure drop as the enign warms up due to expansion of all aluminum surfaces as compared to steel....crank journals, cam journals, pump surfaces. Bigger "gaps" less oil pressure."
Marc - Really? - ALL engines usually lose pressure as the oil (and engine) warms to normal core operating temperature regardless of their metallurgy! That's why we have a by-pass in the lubrication system and a differential by-pass valve in the oil filter. The pressure loss has more to do with the oil's viscosity, structure and formulation than the metallurgy of the engine!
In particular, a Viscosity Calculator can somewhat predict this loss using a key ingredient determined from the oil's structure, its Viscosity Index! This will provide a viscosity outcome at any point in a range from 40c to 100c and can be trended at either side of those temperatures too.
The actual pressure is not as important as the flow which is of course constant per engine rev and these factors are integral at the engine's preliminary design stage and well factored in. It takes on board then the metallurgy used along with all the other components within the lubrication system and other "hardware". The lubrication system may be modified in production as happened to the 928. Losses at idle in the early engines were caused by a number of things and modified accordingly in production
Due to their superior FLOW characteristics most synthetic oils will show less pressure at idle than their mineral cousins - when comparing similar viscosities of course. Oil flow per engine revolution will be the same - oil flow is what assists journal bearing lubrication the most (after their design of course)
Too high a viscosity (and especially so with mineral oils) will cause the differential pressure by-pass valve in the oil filter (MANN filters "crack" at 2.2bar) to allow some unfiltered oil to circulate for a longer period than desirable
The oil temperature in a 928 used "normally" is in a range around 87c-94c which is ideal. It trails the rising engine temperature at warmup by about 10c-15C and appears to take about 30minutes to reach
Regards
Mark - Bill hit the nail on the head and you may find this site of some assistance too;
http://www.landsharkoz.com/tt/ttlubricant.htm
If you were to use a 25w-60 or 70 mineral oil it is possible that you could get around 5bar at idle - what a worry!!!
Marc - you said this:
"Oil pressure drop as the enign warms up due to expansion of all aluminum surfaces as compared to steel....crank journals, cam journals, pump surfaces. Bigger "gaps" less oil pressure."
Marc - Really? - ALL engines usually lose pressure as the oil (and engine) warms to normal core operating temperature regardless of their metallurgy! That's why we have a by-pass in the lubrication system and a differential by-pass valve in the oil filter. The pressure loss has more to do with the oil's viscosity, structure and formulation than the metallurgy of the engine!
In particular, a Viscosity Calculator can somewhat predict this loss using a key ingredient determined from the oil's structure, its Viscosity Index! This will provide a viscosity outcome at any point in a range from 40c to 100c and can be trended at either side of those temperatures too.
The actual pressure is not as important as the flow which is of course constant per engine rev and these factors are integral at the engine's preliminary design stage and well factored in. It takes on board then the metallurgy used along with all the other components within the lubrication system and other "hardware". The lubrication system may be modified in production as happened to the 928. Losses at idle in the early engines were caused by a number of things and modified accordingly in production
Due to their superior FLOW characteristics most synthetic oils will show less pressure at idle than their mineral cousins - when comparing similar viscosities of course. Oil flow per engine revolution will be the same - oil flow is what assists journal bearing lubrication the most (after their design of course)
Too high a viscosity (and especially so with mineral oils) will cause the differential pressure by-pass valve in the oil filter (MANN filters "crack" at 2.2bar) to allow some unfiltered oil to circulate for a longer period than desirable
The oil temperature in a 928 used "normally" is in a range around 87c-94c which is ideal. It trails the rising engine temperature at warmup by about 10c-15C and appears to take about 30minutes to reach
Regards
