oil pressure through the roof - PeachParts Mercedes-Benz Forum

surveyguy · #1 11-17-2010, 01:13 AM

1979 240D 123 616

my oil pressure is pegged. when off, it is 0. as soon as it starts, it goes right up to the top (during idle). Perhaps this is because it's cold, i changed the oil 2 days ago, always was lower during idle, maybe because the oil was so old.

i cleaned the contacts at the two senders, one i know was the temp, but then i looked in the dash and it's a mechanical guage, looked like an oil fitting on the back of the guage.

any ideas? could the guage be broken? is there some way that something is plugged doing this? it was pretty darn hot when i was doing the checking after i got home from work, still pegged at idle.

thanks!

jt20 · #2 11-17-2010, 01:22 AM

does it shoot up before you start the engine?

surveyguy · #3 11-17-2010, 01:28 AM

no, it is all the way at the bottom, no movement until it starts up.

bustedbenz · #4 11-17-2010, 01:42 AM

That's exactly what it's supposed to do.

At road speed, these things have a lot more than 3 bar oil pressure. The designers simply assumed (For some unpleasant reason) that the only "useful" information was the bottom end of the gauge. in other words, finding a close focus on the "low" end of the spectrum was all that mattered, because there's not really any such thing as oil pressure being too high, so the only MEANINGFUL message the gauge could provide is if it's dangerously low.

As such, the 3 is just an artificial limit of the gauge; it can actually climb significantly higher than that. Every cold start should see it peg at idle because the oil is cooler and thicker; this is a phenomenon in ALL engines. It's just more visible on the Benz because the gauge is so narrow-ranged at the low end. Ambient temperature and engine temperature as well as oil viscosity (at the immediate moment; not just the number on the bottle) all play a part in dictating whether the idle pressure will drop from 3 or not. Sometimes it does, sometimes not. USUALLY on a warm engine, it's more like 1.0-2.0 bar. However, I wouldn't assume it's a PROBLEM for the thing to stay at 3 even at idle.

Probably perfectly normal, in other words.

barry123400 · #5 11-17-2010, 01:57 AM

Three at cold idle is normal. A real good engine will indicate two on hot idle with decent viscosity oil.

The third bar on the gauge above the bottom peg on the oil gauge is three bars or about 45 pounds pressure.

vstech · #6 11-17-2010, 08:40 AM

it's likely, that you had plugged passages before, and now that you are putting fresh oil in, the motor is getting better flow and showing you the 3 bar now.
when you changed the oil, did you replace the two o-rings on the bottom of the shaft?
they will change the behavior in a positive way to how your oil pressure gauge responds to the oil pressure you have.

Alastair · #7 11-17-2010, 08:47 AM

Those little 'O' rings also allow oil to drain back if they are worn/squashed, and after starting it can take a couple of seconds More to get full pressure as the pump first has to fill the filter up...

They seem to be seldom replaced by your average service-place...

I tend to change 'em every other filter change....

funola · #8 11-17-2010, 09:08 AM

I changed those 2 o-rings when I bought the car. Time to change them again.
Can someone verify these are the correct size?

6 mm i.d. x 10 mm o.d.
7 mm i.d. x 11 mm o.d.

vstech · #9 11-17-2010, 09:10 AM

verified. I use 5.8/1.9 and 6.8/1.9 o-rings. id/wall thickness...

funola · #10 11-17-2010, 09:23 AM

Quote:

Originally Posted by vstech

verified. I use 5.8/1.9 and 6.8/1.9 o-rings. id/wall thickness...

Strange to have a decimal point in a metric o-ring size??? Usually they are whole numbers???

Oldwolf · #11 11-17-2010, 09:41 AM

Surveygy, what does your pressure read once the engine is warmed?

Mine will start at 3 and once warm, at idle, it will be about 1.8.

vstech · #12 11-17-2010, 09:45 AM

Quote:

Originally Posted by funola

Strange to have a decimal point in a metric o-ring size??? Usually they are whole numbers???

yup, strange.
it's in the kit for 4.99 from harbor freight.
hard to find o-rings by size anymore. ya get kits, and you can order by picture. annoying.

Biodiesel300TD · #13 11-17-2010, 11:59 AM

Sounds to me like your gauge and pressure are doing what they are supposed to. The needle should be pegged at 3 anytime the car is cold or moving. Once the car is warm and idling the pressure should drop to 1.5 to 2 bar. But as soon as you touch the go pedal it should peg again. If you EVER see the oil pressure bobble or drop from 3 while your driving pull over and check your oil imeadiately.

Jeremy5848 · #14 11-17-2010, 12:06 PM

With a "real" gauge tee'd into an oil line, you will find the cold oil pressure to be on the order of 70 psi and even hot the oil pressure will be 50-60 psi with the engine above idle. The factory gauge is more of an "indicator" than a measuring gauge. As long as it pins high at cold idle and stays there while driving you're OK.

surveyguy · #15 11-17-2010, 12:55 PM

thanks for all the advice, i don't know what is meant by "bar" because mine goes from 0-45 psi.

I guess the previous oil was bad, because i just changed it, and didn't notice i guess right away. before at idle it would be about 15psi. yesterday when it was warm, it did come off of 45 during idle.

hey, to change the subject, i see a small vacuum tube coming out of the firewall just below the battery. it is broken at about 18" (could have been burned by exhaust). any idea where it goes? perhaps under the air cleaner?

thanks again.

Guy