Conflicting engine diagnostic results. What say you?

Ok, the other day my benz (1992 300D, 2.5L TD, 303,000 miles on it) showed zero oil pressure at idle. I revved it up and at about 1500 rpm it came to about 1.5 BAR. This only happens when the oil is warm, seemed like only after freeway driving but I'm not sure if that was the case. So, on the way to work and back I'd two-foot it to keep the revs (and oil pressure up) while stopped till the weekend so I can diag it.
I didn't really think it was the sending unit or the gauge because the movement was perfect, not erratic like you'd expect. So, I tapped a hole in the filter housing lid and installed a mechanical gauge for temporary purposes. Drove it for less than 2 miles and the pressure would read 0 at idle again. Resting on the peg. The under-hood gauge showed about 17 psi.
Some sources say 1 bar is normal, some say 1.5 is normal so, 17 psi is right in the ball park. That would say everything is ok.
But....... When it was idling in the garage, still cold, I pulled the oil fill cap and saw no oil at all, just residual on the bottom of the head. Nothing! Then when I got home, with the dash showing 0 bar and the under-hood gauge showing 17 psi I removed the cap and there was oil splashing out. I'm not sure if it was as much as I'd expect but I'm not sure. One other note. The engine seemed quite loud when I was standing next to it, loud enough that it actually hurt my ears just a bit. But the hood was open and I was in a garage. Usually I hear it outside, with the hood closed so it may be normal. You know, 80's and 90's diesels weren't known for being quiet and refined.

One more note. About 12,000 miles ago I did the head gasket and oil pump. While doing the head gasket I found scoring on the rear cam bearing surface (not the lobe). A Fram oil filter came apart and a chunk of plastic worked it's way to that oil feed and buggard up the bearing surface. I didn't want to buy a head so I put it back together. After the repair, oil pressure was a bit higher and all's been well for a year.

So, am I gonna throw a rod through the block this week?

Sounds like the relief spring in the pump has a problem or a plugged screen to me. Pour some ATF or kerosene in and flush it?

I might try a flush, but I think the engine is tired. I suspected one of the owners before me didn't take care of it very well, so the engine might just be done.

Recently had a car showing odd oil pressure readings. Performed an oil change, replaced the filter, and all returned to normal. Probably a good idea to replace the oil filter and see what happens. Cut the old filter open and see if anything nasty is in there.

My vote would be an issue with the sending unit/gauge. I don't see how you could be showing pressure like that at the filter housing using a mechanical gauge and not have something close at sending unit port. There are some posts on Peach Parts reflecting that:

From the PP Forum: "for some reason its also gone lazy, it reads 0.5 at 650 rpm idle and pegs to 3 at 1500 rpm."

So, it's not unheard of for gauge/sending unit to have odd issues on these cars. If it weren't for the semi-normal reading with the mechanical gauge, I might think that an oil squirter for one of the pistons had come loose leaking oil back into the sump. But the readings you are getting with the mechanical gauge makes me think it's more likely an electrical issue.

You should be T-ing into where the sending unit is... That way you can see the real world pressure and confirm if its bad or not..

I am pretty sure the sending unit is bad since it doesn't jive with the mech gauge. The thing that has me concnerned though is that I didn't see ANY oil at the front if the head with I know there was 80 psi oil pressure. That's odd.
And to my Canadian friend, yeah I would have liked to but I didn't for a few reasons:
1) the sending unit is a PITA to get to
2) I wanted to see when the dash gauge said it was at 0 BAR so I was sure I was looking at the pressure WHEN I needed to
3) The factory sending unit is at the base of the filter housing, I tapped it into the top. From everything I read, there's full pressure in the housing at all times.
Even having said all that, if I could have gotten to the sending unit, I'd have done that for sure.

Oil pressure readings should be taken at a point farthest from the pump. this shows that oil is being delivered under pressure to all points. First thought, as suggested, is to do a flush and oil change. The relief valve spring or just stuck is another possibility.

If the relief valve spring is accessible, I would check and clean that ASAP. Not having oil in the filter housing at idle sounds bad.

DrBoost wrote: I am pretty sure the sending unit is bad since it doesn't jive with the mech gauge. The thing that has me concnerned though is that I didn't see ANY oil at the front if the head with I know there was 80 psi oil pressure. That's odd. And to my Canadian friend, yeah I would have liked to but I didn't for a few reasons: 1) the sending unit is a PITA to get to 2) I wanted to see when the dash gauge said it was at 0 BAR so I was sure I was looking at the pressure WHEN I needed to 3) The factory sending unit is at the base of the filter housing, I tapped it into the top. From everything I read, there's full pressure in the housing at all times. Even having said all that, if I could have gotten to the sending unit, I'd have done that for sure.

Exactly. The sending unit is at the base of the filter housing where the oil comes IN. The hole you tapped is at the top covering the area where the oil EXITS. If anything, the oil pressure should read HIGHER where the factory sending unit is located than where you tapped in. It would seem impossible that the reading at the base could be lower since that is where the oil is coming from. I don't know if there is a place to tap into the head to check how good it is there, but your pressure from the pump would seem to be ok.

carzan wrote:

DrBoost wrote: I am pretty sure the sending unit is bad since it doesn't jive with the mech gauge. The thing that has me concnerned though is that I didn't see ANY oil at the front if the head with I know there was 80 psi oil pressure. That's odd. And to my Canadian friend, yeah I would have liked to but I didn't for a few reasons: 1) the sending unit is a PITA to get to 2) I wanted to see when the dash gauge said it was at 0 BAR so I was sure I was looking at the pressure WHEN I needed to 3) The factory sending unit is at the base of the filter housing, I tapped it into the top. From everything I read, there's full pressure in the housing at all times. Even having said all that, if I could have gotten to the sending unit, I'd have done that for sure.

Exactly. The sending unit is at the base of the filter housing where the oil comes IN. The hole you tapped is at the top covering the area where the oil EXITS. If anything, the oil pressure should read HIGHER where the factory sending unit is located than where you tapped in. It would seem impossible that the reading at the base could be lower since that is where the oil is coming from. I don't know if there is a place to tap into the head to check how good it is there, but your pressure from the pump would seem to be ok.

I'm not sure, but I'm not arguing by any means. I did some scouring at PeachTree parts and the consensus is that the filter lid is under the same pressure, that it's effectively before the filter. I don't want to drill any holes in the head though. I agree, it appears the pump is putting out adequate pressure.
Now, why the heck would I see NO oil at the front of the cam (through the oil fill hole) when the gauge is showing 80 psi, but I do see some when it's showing 17? I'm all corn-fused.

Where exactly is the oil pressure sender on the car? I think I'd want to mechanically test the pressure exactly where the sender is. If you are measuring the system pressure at two different points it doesn't surprise me that they disagree. Are you sure the canister housing is under pressure and not just prepump? If the canister is on the feed side to the pump there wouldn't be any pressure over the mass of the oil. Did you prime the oil pump when you put the new one on? Just a few thoughts!

benzbaronDaryn wrote: Where exactly is the oil pressure sender on the car? I think I'd want to mechanically test the pressure exactly where the sender is. If you are measuring the system pressure at two different points it doesn't surprise me that they disagree. Are you sure the canister housing is under pressure and not just prepump? If the canister is on the feed side to the pump there wouldn't be any pressure over the mass of the oil. Did you prime the oil pump when you put the new one on? Just a few thoughts!

The sender is on the bottom of the filter housing. My understanding is that the oil enters the canister at the bottom, then flows up the center. from there it goes down the sides and through the filter, but I'm not sure.
I did prime the pump when I put it in. It's been in for about 12,000 miles now.

DrBoost wrote:

benzbaronDaryn wrote: Where exactly is the oil pressure sender on the car? I think I'd want to mechanically test the pressure exactly where the sender is. If you are measuring the system pressure at two different points it doesn't surprise me that they disagree. Are you sure the canister housing is under pressure and not just prepump? If the canister is on the feed side to the pump there wouldn't be any pressure over the mass of the oil. Did you prime the oil pump when you put the new one on? Just a few thoughts!

The sender is on the bottom of the filter housing. My understanding is that the oil enters the canister at the bottom, then flows up the center. from there it goes down the sides and through the filter, but I'm not sure. I did prime the pump when I put it in. It's been in for about 12,000 miles now.

Actually, after scanning and blowing up the image from my manual where my crappy eyes can see it, it looks like the flow is upward to the outside passing through the filter to the center, then back down. That would make the factory sending unit port and the port that you drilled exactly the same, pressure-wise:

Hopefully, this is legible and will help clarify.

Thanks Carzan. It looks like I understood that they were seeing the same pressure, but had it backwards.
Thanks for reassuring me.

Yeah, I originally was thinking the top was separated from the lower canister body by the element, but after looking at the diagram again, it appears to be all the same chamber. I blame it on CSS (can't see s*** ) Doesn't seem to matter where on the canister oil pressure is measured from and seems odd MB chose to put the sending unit in a difficult spot. It would be nice to see a reading from the head, but I feel fairly confident the sending unit and/or gauge and/or related electrical circuitry is the culprit in the original question.

carzan wrote: ....seems odd MB chose to put the sending unit in a difficult spot.

Haha! You've never owned a 90's or later Benz have you? It never ceases to amaze me how they are willing to go the extra mile just to do something stupid. I can just hear the engineering meetings now "yes Geunter, the thermostat normally goes right at the end of the upper radiator hose, but we are going to run coolant pipes under the car, through the seats (heated seats!!), then out back, through a set of quick-disconnects to the trailer they now have to tow with them. Due to the routing of the coolant pipes, we're going to install the thermostat in the headliner."

DrBoost wrote:

carzan wrote: ....seems odd MB chose to put the sending unit in a difficult spot.

Haha! You've never owned a 90's or later Benz have you? It never ceases to amaze me how they are willing to go the extra mile just to do something stupid. I can just hear the engineering meetings now "yes Geunter, the thermostat normally goes right at the end of the upper radiator hose, but we are going to run coolant pipes under the car, through the seats (heated seats!!), then out back, through a set of quick-disconnects to the trailer they now have to tow with them. Due to the routing of the coolant pipes, we're going to install the thermostat in the headliner."

LOL Here's mine. I feel your pain

Ah. I knew there were a few others with 124's but I just couldn't remember if you had one. How long have you had it, and how much work have you had to do to it? It looks nice.

I've had it since early '09. The PO had given up on it. When I saw the ad for it on CL, I was thinking BABE car. But when I went to see it, I kinda fell in love with it. Right away, had to replace the radiator (due to a broken upper hose neck) and the fan clutch. Then, I had to repair the leak that the PO caused by attempting to replace the thermostat that didn't need replacing. Then, I had to sort out the induction system because there was no boost (mostly leaking out of a broken EGR valve. The OVP relay was bad so the tach and the ABS didn't work ($20 on ebay because I was too lazy to go to a boneyard).

Didn't really do anything to it mechanically after that until last year when I replaced glow plugs, the vacuum pump (because it was original and it can destroy the engine if it fails) and I changed the transmission fluid and filter. Right now, the climate control doesn't work properly (I have another unit I haven't installed yet), and the ABS comes and goes, but other than that, it's been pretty trouble free. I'm really having a great time with it.