oil pressure issue - PeachParts Mercedes-Benz Forum

cydonia51 · #1 06-10-2009, 12:29 AM

I have had my '80 300D for about 2 1/2 years now. The oil pressure has always ran at 3 no matter what, if it was cold or hot. Sunday I drove out to Disney World, about 30 miles from my house. I noticed that while on the interstate doing 70mph the pressure was just over 2 but would go almost to 3 if I accelerated. Once I got off the interstate I checked the oil level and it was fine. I had just checked it a few days before and it was fine. The rest of the day it would run just over 2 and at idle would be at between .5 to just over 1. From other posts it seems that this is normal but is it for my car since it had always ran at 3? Is the oil pump getting ready to go out? The car runs fine and doesn't leak a drop of oil, never has since I have owned it.

Eric

Jeremy5848 · #2 06-10-2009, 01:17 AM

Temporarily install an external mechanical oil pressure gauge replacing the one in the car. Consider running new tubing from the attachment point on the engine (at the oil filter?) and make sure everything is clean and tight. Your factory 3-bar gauge is equivalent to 45 psi. Drive the car with the temporary gauge for a few days and see what it says. You could have only a tired gauge, much less expensive than a tired oil pump.

t walgamuth · #3 06-10-2009, 05:45 AM

I'd say its more likely you just never noticed it before. STaying at three all the time is not possible and the oil pressure guage is mechanical so there is no chance for a loose electrical connection causing it. I would not worry about it.

nhdoc · #4 06-10-2009, 06:39 AM

It is abnormal for the pressure gauge to not be pegged while you are driving. It is normal for it to drop to 1.5 Bar or so when idling hot. So, if you are seeing 2 Bar while driving I would say you do have a problem...it could be the gauge or a problem inside the engine. The only way to know for sure is to try another gauge.

vtmbz · #5 06-10-2009, 07:03 AM

Try changing your oil and filter-- cheapest things first.

mplafleur · #6 06-10-2009, 07:26 AM

Mine does the same. At operating temperature, it can run at 2.5 while driving and .7-.8 at idle.

Sometimes it just makes it to 3 during driving. If it's at 2.5 or so, I found I can back off on the throttle and the needle dips a little and then picks back up. If I apply throttle again it will continue up to the 3 mark. Has anybody seen this before?

I've tried three sensors and a mechanical gauge. They all verify. I've also replaced the oil pump.

I think there is a mechanical reason for this somewhere. Maybe the bypass valve in the oil filter housing is faulty and beginning to open?

nhdoc · #7 06-10-2009, 09:35 AM

On the gas models there are plastic connectors which hold the cam oiler in place and those are known to break which can cause low oil pressure. Not sure if the diesels use the same connection at the cam oiler but it would be easy enough to pull the VC off and see. Otherwise you could have diluted oil (fuel contamination) or worn rod bearings. Not sure about a bypass that would cause low OP though.

tbomachines · #8 06-10-2009, 09:40 AM

Hmm... my OP is constantly pegged at about 3.5 (maxxed out gauge) with normal driving, which is normal from feedback on the forum. Warm idle will dip to about 0.8. Maybe the torque converter engaging/disengaging and bringing the RPMs up slightly after you lay off the power, does it happen at idle?

Diesel911 · #9 06-10-2009, 10:11 AM

I would try changing the 2 O-rings down at the end of the Filter Cap Stem (size 6x10x2 mm). If the O-rings leak there some oil goes back to the Oil Pan (#20 and #21 on the diagram).
Also there is a Check Valve inside of that stem. Blow compressed air through the hole at the top of the Stem incalse something is trapped and keeping the valve from seating.

If this is a non-turbo Engine I think I have read of some Oil Pump Pressure Relief Valve issures somewhere on this forum. It has a different Oil Pump than the turboed engines.

mplafleur · #10 06-10-2009, 10:53 AM

It has a brand new oil pump. I'm hoping it's not rod or main bearings as I just rebuilt this motor. I had the crankshaft ground to 1st oversizze and bought the appropriate bearings, but the crank may have been ground a little too much. I didn't measure it. I've never measured a machined crank. I had always assumed they were machined correctly.

As for the torque converter disengaging, I don't see any rise in RPM at the same time as the oil pressure increase.

The O-rings are new, but maybe one has become dislodged. I'll also make sure the stem is clear.

Diesel911 · #11 06-10-2009, 08:45 PM

Quote:

Originally Posted by mplafleur

It has a brand new oil pump. I'm hoping it's not rod or main bearings as I just rebuilt this motor. I had the crankshaft ground to 1st oversizze and bought the appropriate bearings, but the crank may have been ground a little too much. I didn't measure it. I've never measured a machined crank. I had always assumed they were machined correctly.

As for the torque converter disengaging, I don't see any rise in RPM at the same time as the oil pressure increase.

The O-rings are new, but maybe one has become dislodged. I'll also make sure the stem is clear.

Where I used to work they put a Crank in Detroit Diesel 71 series V-6. My Friend was under the Engine tightening the Bearing Caps and wanted a break.
I took over for him. After I torqued the one of the Main Bearing Cap I tried to rotate the Crankshaft . The Crank would not rotate.

The main Bearing Bolts are tightend up in several stages increasing the torque till you reach the desired torque. I was taught in Trade School to rotate the Crankshaft after each torque stage. You want to know as soon as possible if anything is binding the turning of the Crankshaft.

The reason for the Crank not turning was the parts place had sent us the wrong oversized Main Bearings.
You cannot assume things without taking a risk.

If you do not have Micromicrometers and Telescopic gauges to measure you can use the inexpensive Plastigauge to check the bearing clearances (they must have a website with general instructions).

Even in a New Oil Pump the Pressure Relief Valve could be sticking due to some foreign object getting inside of it. Sometimes something can get inside while you are assembling it sometimes there is still stuff inside from when the pump was assembled or bits of the shipping packageing gets inside.
You can pull the Valve and look at the sides of it. If the Valve is scored up on the side something has gone throug it.

79Mercy · #12 06-10-2009, 09:59 PM

if the gauge was always at 3 bars then i'd expect a bad gauge. While driving the gauge should be at 3 bars all the time. At an idle it should be at about 1-2 bars.

Don't drive the car until you get a external OP gauge and test the oil pressure. Just remove the factory oil pressure line and then screw in the gauge to test it.

Also replace those little o-rings that another forum member mention.

mplafleur · #13 06-10-2009, 11:10 PM

I know where it should be, from the 5 other Benz's that I've owned.

The SD I've been driving is always pegged. Idle is 2.5.

The SDL, I've tried 3 sensors.
I've verified the pressure with a mechanical gauge.
There was little change when I replaced the oil pump.

Maybe there is an obstruction in the port for the gauge?
Is there another place I can take the pressure from?

the heap · #14 06-11-2009, 01:55 AM

The 617 engine operates at ~5 bar at highway cruise speed, and should idle at 1.5-2 bar, when hot. If you're not pegging the gauge at 3+ bar at cruise, replace oil filter, check for cracked hose to oil cooler. There's stories of Fram oil filters collapsing and shutting off oil flow to engine. Buy good filters and oil.

Gerry
'77 300D, 360K mi
'95 C220, 180k mi
Dodge B150, 330k mi
Norton Interstate, 170k mi
Moto Guzzi LeMans, 80k mi

WD8CDH · #15 06-11-2009, 10:22 AM

Quote:

Originally Posted by the heap

There's stories of Fram oil filters collapsing and shutting off oil flow to engine. Buy good filters and oil.

Gerry
'77 300D, 360K mi
'95 C220, 180k mi
Dodge B150, 330k mi
Norton Interstate, 170k mi
Moto Guzzi LeMans, 80k mi

That's not true for the Fram filter for the 617 (at least not the one sold at Pep Boys). The oil filter that Fram sells for the 617 is about the only thing that I trust from Fram.

All of my W123s have been full scale at anything above about 1200 rpm and about mid scale or higher at idle. Idle speed seems to be the main thing effecting pressure on a warm 617 at idle.

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