The car is a 92 190E 2.3. I run 20W50 oil in the summer (70F-90F average temps here). Lately, I noticed it sometimes takes a whole lot more time to build oil pressure at startup. When I start the car with a cold engine but cold ambient temp, it will take about 5-10 seconds to register oil pressure on the gauge. It always did that and I think it is normal. The problem is when I come back from work at around 5 PM when the ambient air temp is close to 85F and the car has been sitting under the sun all day. Even if the engine is cold, it can take up to 45 seconds to register oil pressure on the gauge. I don't hear no abnormal sounds from the engine. Is this normal or should I be worried? Also, the car is dark grey (almost black), which I guess makes underhood temperatures a bit higher when the sun is out...

Extra info: Last November I was driving the car and noticed very low oil pressure at idle with a hot engine. Sometimes the gauge would drop to 0 (FSM says 0.3 is fine I think). This was with 10W30 oil because of colder temps (40F). I then parked the car for the winter and put a new oil sending unit in May. But I haven't got the chance to re-test the car in the same climatic conditions yet.

This engine only needs to last until this november, after that it gets thrown away.

Does the oil pressure gauge dip below 1 when idling?

Sixto
98 E320s sedan and wagon

No, it'll get close to 1 when you are sitting in traffic for a while, but other than that it stays around 1.5-2.

Likely a bad oil pressure sending unit. Replace it at the next oil change when the oil filter is off. I like to use 15W-40 weight almost all year. Sometime 10W-30 in the winter. Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

It sounds like your oil filter has emptied out and it takes awhile to fill it up before pressure can build. Once the pressure has built up, if you turn it off, wait a few minutes, and start it again does the pressure build quickly? If so, replace all o-rings associated with the oil filter to try to stop it's draining down.

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'97 E 300 D

Re-read your post and see that you changed the sending unit...I hope it is fixed! Nelbur has an interesting theory and may be on to something. If it persists try a Mann, Mahle or Knecht filter if you haven't.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Does the car in question use a cartridge style filter like the 80s cars did, or does it use a spin-on filter? If it uses a spin-on filter, there are no O-rings to replace, you likely have a worn oil pump or something leaking down in the engine.

If you have a cartridge style filter, there are a couple of O-rings on the end of the central rod as mentioned above that will cause loss of oil pressure and leak-down of the oil filter housing.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

The oil filter stem o'rings are almost universally ignored. Over time they get hard or collapsed and must be periodically replaced or the filter will drain back. Pull your oil filter lid and have a look. O'rings are available at the dealer or any big-box auto supply such as AutoZone or O'Rielly will have a metric o'ring assortment that will have many of the size you need.

Does the M102 have a filter step? I think it uses spin on cartridges like the M103.

Sixto
98 E320s sedan and wagon

It has a regular spin-on filter which is also new (Mann). Interesting theory about the filter draining back... Started the car after work today, took 36 seconds to register pressure on the gauge.

That is correct.

As a rule of thumb, you need 10 PSI for every 1000 RPM. So at 5000, it should be pinned at 3 bar. If that isn't happening, the engine won't last.

Past 2000, it's pinned at 3 bar already. I think it's still good

But today I started the car and both times I got like 6-7 seconds like it was before. I don't think it's anything to worry about anyways since I always get good pressure after a while.