is the oil pressure gauge just an alarm? - PeachParts Mercedes-Benz Forum

bricktron · #1 10-21-2015, 02:51 PM

hi folks. i just got my oil pressure gauge working (thanks to ROLLGUY and DeliveryValve) and, as others have reported, it spends nearly all its time pegged at 3 bar. i know i need to watch it for a sudden drop, though visually that's not so easy. there is an excellent writeup on adding an oil pressure alarm: http://www.peachparts.com/shopforum/diesel-discussion/280490-simple-oil-pressure-alarm-diy.html

does the stock pressure gauge have any utility at all other than warning of a catastrophic loss of pressure?

dude99 · #2 10-21-2015, 03:07 PM

Quote:

does the stock pressure gauge have any utility at all other than warning of a catastrophic loss of pressure?

Not really, most of them spend their time at idle around 1.5bar when hat and peg to 3 as soon as you touch the accelerator.

Ceristimo · #3 10-21-2015, 03:15 PM

And since nobody looks at that gauge while driving, - since you have to watch traffic and the road and whatnot -, it doesn't work that well as an alarm either. There are no lights or beeps or anything associated with the pressure being low.
In the real world that means that as you're driving and you notice a loss of power and a knocking sound you'll glance at the pressure gauge, realize it is at 0 and at least have visual confirmation on that silly gauge that your engine is now, indeed, toast.

Desert Panther · #4 10-21-2015, 03:41 PM

Quote:

Originally Posted by dude99

Not really, most of them spend their time at idle around 1.5bar when hat and peg to 3 as soon as you touch the accelerator.

It is good to hear this from someone else, as I wasn't sure if it was just my own car that did that. Are they (the early cars) all like this?

leathermang · #5 10-21-2015, 03:48 PM

It is sort of an oil at idle warning mostly ....
If your car will not hold one bar at idle..
then you need to be looking at something...
you can check idle pressure regularly....at a stop of course... so in that function it works...
but what is needed when driving is an Audible buzzer warning when you lose pressure...
even a light is not as quick to be noticed as a Sound.... thus important aviation warnings have use a sound in addition to sight or vibration, etc..

Maxbumpo · #6 10-21-2015, 04:02 PM

I use the oil pressure gauge to know when the engine oil has reached full temperature and to gauge the health of the engine. I suppose one could also detect oil thickening if very observant.

When the engine is cold (and also the oil) I drive my cars gently, keeping the RPMs low (below 2k if possible) and allowing the transmission to shift into higher gears early. At each stoplight, I'll check the oil pressure with the engine idling, and when I see that the oil pressure is normal for my car (about 1.5 bar), then I know the oil is close to full temperature and higher RPM and faster acceleration are possible without harm to the engine.

mannys9130 · #7 10-21-2015, 04:02 PM

Heavy 15W-40 oil will show 3 bar until it is very thoroughly warmed up. In summer in Tucson when I used to run 15W-40, I would show 2 bar at idle when the engine was totally warmed up and it was 110 outside. Now with 5W-40 year round, I show 1.5 bar when totally warmed up and 1.25 bar if I have just come off the highway. All situations bring it back to 3 bar as I accelerate away. 1 bar is ~15 psi and you want at least 10 psi per 1000 rpm for a healthy engine. The FSM has some incredible number for the lowest acceptable oil pressure.

I just put 1 bar as a safe minimum.

Pressure is important but so is flow. Pressure is resistance to flow, so when your pressure is very high, the flow rate is low. Pressure keeps the crank bearings suspended within an oil film, but without a high flow rate that film will break down with high loads and metal will contact metal. Pressure isn't the only point to consider.

Also keep in mind that when your oil is extremely thick the oil filter is most likely bypassing and you aren't really filtering any oil. The quicker the oil comes to operating temp and thins to its final viscosity, the quicker the oil filter leaves bypass. It takes me about 10 minutes for the 5W-40 to come down to 2 bar. It takes me about 30 minutes for 15W-40 to do that. The 5W-40 is being filtered much more.

All of this is in the FSM.

strelnik · #8 10-21-2015, 04:10 PM

Quote:

Originally Posted by leathermang

It is sort of an oil at idle warning mostly ....
If your car will not hold one bar at idle..
then you need to be looking at something...
you can check idle pressure regularly....at a stop of course... so in that function it works...
but what is needed when driving is an Audible buzzer warning when you lose pressure...
even a light is not as quick to be noticed as a Sound.... thus important aviation warnings have use a sound in addition to sight or vibration, etc..

I disagree. Even though there is no alarm or bright idiot light on my 350SDL, when the OP faded quicly due to a bad wire, I noticed it immediately going from 45 to 4

leathermang · #9 10-21-2015, 04:34 PM

Quote:

Originally Posted by strelnik

I disagree. Even though there is no alarm or bright idiot light on my 350SDL, when the OP faded quicly due to a bad wire, I noticed it immediately going from 45 to 4

You are bragging...
most people do not do their cross check often enough to catch a loss of oil pressure without an audible warning... if you read the forum you will see lots of examples of ' I heard an engine noise and then looked at the oil pressure gauge' ....

mannys9130 · #10 10-21-2015, 05:02 PM

Most people check their gauges when they check their speed, which is probably every 30 seconds or so. On the highway I personally look at my temp gauge much more often and I see the oil pressure as a result.

dude99 · #11 10-21-2015, 05:52 PM

Quote:

Originally Posted by Desert Panther

It is good to hear this from someone else, as I wasn't sure if it was just my own car that did that. Are they (the early cars) all like this?

All the OM617\OM616 engines follow this pattern. It is totally normal

Jeremy5848 · #12 10-21-2015, 06:33 PM

I don't trust myself to look at the gauge often enough, especially since (except at hot idle) it rarely says anything other than "3 bar." A gauge that never moves is a gauge that doesn't get watched. To remedy my lack of concentration, I added a "real" alarm that blinks a very bright red LED whenever the oil pressure drops below 1 bar (15 psi). This occasionally happens when the engine slows from highway speed to idle, for example, at a stop sign and before ELR has had a chance to get the idle speed back up to where it belongs. Otherwise, it blinks after the engine starts and until the oil pressure comes up ("self-check") but it does not blink during pre-glow ("unnecessary distraction"). One could also add a noise-maker if one chose, to operate in parallel with the light. In daylight the LED is easy to see; at night it cannot be ignored. I posted a thread on this as a DIY project here.

Jeremy

cooljjay · #13 10-21-2015, 08:11 PM

I would suggest......yes....members are now....with my permission....can ready the torches and pitch forks.....

That if you have a non turbo 300 or 240.....That if you are experience a drop at oil pressure at idle.....To check their oil thrust bushings....

You see this bushing has a bronze bushing, after so many miles/years of use....This wears down...when this wears down....It causes the gear that runs the oil pump to back off the oil pump at idle....Thus wearing out more gears and thus hampering oil flow/pressure...

There is also an oil pressure valve in the oil filter housing that can wear....

Both of these can and do attribute to the oil gauge not pegging at idea....

So can a clogged oil filter, thick oil.....

As folks don't seem to change their oil every 3k miles....

Mxfrank · #14 10-21-2015, 08:45 PM

I've owned Benz's from the 123 on, and all of them pegged at 3 bar above 2000 RPM or so. 1.5 at idle.

Mostly what I've used the gauge for is to detect that either the gauge or sender are bad. Never seen a problem otherwise.

Diesel911 · #15 10-21-2015, 10:25 PM

I have a low Oil Pressure Alam with a buzzer (if you look in the thread that the OP posted it is their). But, as other pointed out it only functions properly at idle speeds.

If you are going 70mph and your Oil Pressure droped down to lests say 2 bar my Alarm is not going to buzz and due to the Engine speed and load the Engine is still going to be starved for Oil because 2 bar is not enough.

So the Low Oil Pressure Alarm that I have is OK for around the City where you make frequent stops and the Engine retirms to idles speed.

There is at least 3 longish Oil Pressure Alarm threads. In one of them some of our Members were trying to hook up the Alarm to the Tach Amp so that the Alarm would work also at higher pressures and speeds.

Someone was close to achieveing that but I don't know if they did.