oil pressure drop after oil change - PeachParts Mercedes-Benz Forum

joeblack · #1 10-09-2008, 03:28 PM

Panic here,

I have a 240 D with 275000 miles. Oil pressure between 2.1 and 2.4 warm at idle.
Today I changed filter and oil and the pressure when warm is now 1.5 to 1.9

What has happened ???
Old filter purolator and oil Rotella 15W 40
New filter Mahle and oil Rotella 15W 40

Any thoughts?

Thanks because this is very concerning.

Graplr · #2 10-09-2008, 03:31 PM

Not enough oil put in?

bobodaclown · #3 10-09-2008, 03:36 PM

I'm pretty sure your still in specs....
according to my manual it states .5 bar at idle and 3 bar at 3000 rpm.

When you increase RPM you should have an increase in pressure. Does it (pressure) fluctuate at idle or on speed. If at 30mph it jumps all over I'd be more concerned.

Overall all I think your ok.

After a little more thought "O" Rings in the oil filter canister getting worn? I think I've read something in the forums about that and oil pressure.

Bob

winmutt · #4 10-09-2008, 03:55 PM

1.5 - 1.9 is what it *should* be when warm and at idle.

joeblack · #5 10-09-2008, 03:58 PM

Yes , there is enough oil, I always let it run for a couple of minutes and then wait a couple and check the level.

The oil pressure goes up right away above idle but definetely slower the it was before the oil change.

The 2 O-rings on the bottom were there but kind of flat I did not replace them.

With the old puralator the pressure was always repeatable. I do not get that directly after this oil change with a "better brand " filter the pressure dropped so much , an dthat is not even driving highway.

I will try to find the situation on these O-rings and exchange this Mahle filter for a purlator to see the difference.

Thanks for the help.

winmutt · #6 10-09-2008, 04:28 PM

You have this completely backwards. The oil pressure you had before at idle was ok per FSM but abnormal. The oil pressure you have now is normal.

rrgrassi · #7 10-09-2008, 04:45 PM

If your old filter had flow resistance, it can cause a higher pressure reading, but a flow resistance also means oil is not reaching the moving parts very well. Purolator is not the best name in oil filters.

Your oil pressure is fine.

patbob · #8 10-09-2008, 05:32 PM

Quote:

Originally Posted by rrgrassi

If your old filter had flow resistance, it can cause a higher pressure reading, but a flow resistance also means oil is not reaching the moving parts very well. Purolator is not the best name in oil filters.

Your oil pressure is fine.

Could you have that backwards? Its been a while, but whether the oil pressure reading was pre or post filter was one of the things I explicitly looked up when I first got my car. As I recall, it was post filter, which means a plugged (or more efficient) filter would restrict flow and produce a lower oil pressure. Or is my memory playing tricks on me again?

joeblack · #9 10-09-2008, 07:15 PM

Ok, I am back from the car,

Thanks for all the response and piece of mind about the "remaining oil pressure"

May the pressure was to high original according to the specification?
I assume that the oil pressure is taken somewhere in the oil gallery so that should be after the filter.
Please correct if I am wrong.

In the mean time I have left the Mahle filter in and replaced the completely hardened O-rings with new ones.
It did not make a difference.

In the oil stem pipe ( the one through the filter ) are a hole in top and bottom. This should be to allow the air out. I can only blow through it in one direction and that is towards the bottom of the filter.
That would make sense, it would allow the air out and not allow the air back in and drain the filter out.
So I assume this is good.

I bought another purolator filter and changed it with the Mahle. No difference.

So it only can be the viscosity of the old oil versus the new oil.

I have the car for about 14000 miles and the pressure has always been 2.1 to 2.4 at idle depending on wheather and driving situation.

I regret not bringing up one more variable:
WVO

Since the last oil change the car has driven 3500 miles on veggie oil, centrifuge filtered soybean. Two tank system. Heated to 160-170 before the ip and up to 220 through injector line heaters electronic induction style.

Can veggie oil pollution have increased the viscosity of the old motor oil?
How does that relate to my oil pressure before the conversion.

Thanks again for al your inputs.

.

joeblack · #10 10-09-2008, 10:05 PM

patbob and qwerty,

Thanks I learned something here , as an engineer I would have designed it the other way around , interesting to realize that the mercedes engineer were more interested in pressure of the pump then the pressure at the bearings????

Thanks again

Diesel911 · #11 10-09-2008, 10:11 PM

Quote:

Originally Posted by joeblack

Ok, I am back from the car,

Thanks for all the response and piece of mind about the "remaining oil pressure"

May the pressure was to high original according to the specification?
I assume that the oil pressure is taken somewhere in the oil gallery so that should be after the filter.
Please correct if I am wrong.

In the mean time I have left the Mahle filter in and replaced the completely hardened O-rings with new ones.
It did not make a difference.

In the oil stem pipe ( the one through the filter ) are a hole in top and bottom. This should be to allow the air out. I can only blow through it in one direction and that is towards the bottom of the filter.
That would make sense, it would allow the air out and not allow the air back in and drain the filter out.
So I assume this is good.

I bought another purolator filter and changed it with the Mahle. No difference.

So it only can be the viscosity of the old oil versus the new oil.

I have the car for about 14000 miles and the pressure has always been 2.1 to 2.4 at idle depending on wheather and driving situation.

I regret not bringing up one more variable:
WVO

Since the last oil change the car has driven 3500 miles on veggie oil, centrifuge filtered soybean. Two tank system. Heated to 160-170 before the ip and up to 220 through injector line heaters electronic induction style.

Can veggie oil pollution have increased the viscosity of the old motor oil?
How does that relate to my oil pressure before the conversion.

Thanks again for al your inputs.

.

The hole you are speaking of is part of the Oil Bypass Filtration part of the oil system. The Oil filtered by the larger upper part of the Oil Filter goes through that little hole, down the hollow stem and past a Check Valve in the stem (why the air will only blow 1 way throught the stem). The Oil from this part of the system gets dumped back into the pan.

The smaller lower part of the Oil Filter is the Full Flow part of the oil system and that Oil goes into the Oil Gallery.

joeblack · #12 10-09-2008, 10:35 PM

That is a nice sketch,

Yeah I was wondering about the max pressure, Thought about taking the peg of the meter and see where it would go. It would be nice to talk with one of the old desiners of the car and read in their notebooks why they decided to pick the oil pressure at the pump and not at the bearing.
So may roads to Rome.

patbob · #13 10-12-2008, 02:09 PM

Quote:

Originally Posted by joeblack

That is a nice sketch,

Yeah I was wondering about the max pressure, Thought about taking the peg of the meter and see where it would go. It would be nice to talk with one of the old desiners of the car and read in their notebooks why they decided to pick the oil pressure at the pump and not at the bearing.
So may roads to Rome.

Here's the very next picture in the FSM, which exists solely to describe the oil flow through the filter:

oil pressure drop after oil change-oilflowthroughfilter.jpg

It shows that the unfiltered oil (a) goes through the oil filter and then to the turbo bearings (e), oil pressure gauge (e) and bearings (b). The pressure is measured from the filter housing, but there will be only a small pressure drop between where the gauge taps off and where oil is dispensed at the bearings.

Sooo.. the designers were not stupid. They plumbed the oil pressure to read the pressure of filtered oil being sent to the bearings, which is the single most important measurement of the system to have. A restricted oil flow through the filter, would cause a drop in indicated pressure. That drop in flow can be from a plugged filter, one designed to restrict flow more (fewer pores in the filter media), or even thicker oil.

Veg oils have been known to polymerize, thicking the crankcase oil (there's a small amount of fuel oil in the blowby gasses).. but I'd think that would be more likely to cause a rise in oil pressure by raising its viscosity. Sounds to me like you don't want to wait quite so long before the next oil change if you're still running veg oil.

Diesel911 · #14 10-12-2008, 03:13 PM

This is my oil pressure at idle with the engine cold. When I rev the engine cold or hot it goes up to 98 psi. Notice that I tapped into the bypass part of the oil filter at thetop of the tubular stem. The oil pressure on the out side of the filter element was the same.