hot weather low oil pressure

[bill420e]

It has been 90F and hotter and with AC on full and stuck in traffic idling and crawling, the temp for the car stays OK and only slightly higher than 185F, but oil pressure is dropping below one bar on these occasions. I believe the oil is now 10-40 and am thinking to go 20-50. I am in Northern CA just above San Francisco. The car does not have tappet noise and drives very nicely with what seems to me proper power. I had a 500sl in the past with the larger m119 and lower gearing. I had switched that car to 20-50 with the same thought and I think that I had an oil backup draining that caused a pressure burst in the crank case that blew into the intake manifold and blew a few air lines open. Once I had put it back together it was fine, but I did switch back to 10-40, not sure if that was the cause, but I am seeking expect advice before I proceed.

It is my understanding that being below 1 bar anytime is not really OK.

Thoughts?

Also, I do not think that the vacuum valves to set the AC on recycle are operational as these is no change or action when the switch for recycle is pressed. I guess that those vacuum valves can fail and are hard to get to in order to service or replace.

Maybe you have a link on this procedure.

 

[gsxr]

Low oil pressure readings are often caused by an old / original sending unit. If yours is original, I'd start there... new OE/OEM Hella/VDO only, they're not that expensive. That alone may fix the "low" pressure, which may be fine.
You could use 20W-50 in the mild NorCal climate, but I wouldn't expect a significant difference in pressure. Official MB minimum spec is something like 0.5 or 0.3 bar, but in general you would rarely see 1.0 bar or less except in very high ambient temperatures (think Sacramento, not SanFran).

The AC recirculation function is supposed to fully close the main air inlet flap, which is normally only 80% closed in cooling mode. You can test this with a MityVac by pulling the passenger airbag and checking all 7 vacuum tubes from the manifold, but if one or both of the vacuum pods for the main air flap is bad, you have to pull the entire dash to fix it. Search the forum for details on the testing and replacement procedures, this has been discussed multiple times. Keep in mind that turning on the recirc function does not make a large, obvious difference in AC performance - if you were expecting to hear or feel something right away, you won't, even with the system working perfectly.

 

[darek_u]

I have pretty much the same symptoms as Bill. After rebuild (bottom end untouched) engine was filled in with 10W40 Motul oil (after number of flushes). Now in bumper to bumper traffic and 28C ambient temp (83F) engine temp stays at approx. 90 deg C with no problem (AC on) but oil pressure drops to 0.3, sometimes it touches 0... As soon as I touch the throttle it climbs up. Before oil change engine was filled in with 10W50 oil and pressure never dropped below 0.8. My mechanic says that this is normal in this conditions considering the oil I'm using. Engine runs perfectly fine. I don't know what to think about it...

:?

 

[gsxr]

Darek, same question... how old is your oil pressure sender, and is it OE/OEM?

 

[darek_u]

OE, 25 years old. I'm already looking at replacement options. FEBI and SWAG available off the shelf, VDO on request. Is it easily accessible from below?

 

[gsxr]

OE, 25 years old. I'm already looking at replacement options. FEBI and SWAG available off the shelf, VDO on request. Is it easily accessible from below?

Yep, relatively easy replacement from below, but space is tight... small fingers will help.

Febi and SWAG are no-go. VDO or Hella are the only ones you want, even if they cost triple and take forever to arrive.

 

[darek_u]

OK, understood. I keep searching for VDO/Hella then.

Update. OE p/n A006 542 9417, VDO p/n 360-081-052-003C

Febi and SWAG are no-go. VDO or Hella are the only ones you want, even if they cost triple and take forever to arrive.

VDO is approx. 50% more expensive than FEBI, that's way better than gsxr's 3x prediction... Part ordered.

 

[LWB250]

Absolutely go with VDO or OE part - the aftermarket versions are really awful and often bad out of the box, or so far off they're worthless. Easy job, takes all of about 10 minutes to do. Be sure to use the new aluminum crush washer that is supplied with the VDO sender. Use this opportunity to check the condition of your lower wiring harness, too, if you have a car built from 1990-95 or thereabouts.

As a point of reference, with at least four M119 powered cars in Florida in the middle of summer with 95F ambients, dropping to 1.0 bar at idle or even slightly below that isn't unusual with 0W-40 Mobil 1. On a couple of the older engines (+250k miles) I often went with 15W-50 to get oil pressures up a bit.

Dan