Oil level question (for 79 300sd)
 

here's a question...

my oil gauge is measured in lbs. per quare inch from 45 (at the top) to 0 (at the bottom). when i'm in idle or stopped, the gauge rests at the top line (45lbs) but as soon as i start driving, it slowly shifts downward and rests just around 30 lbs (the second line). when i took the car in for an oil change and filter change [thanks MB Motors!], it was hanging around 40 -45 for about a week and then is shifted to around 30. does this signify a loss of oil or is this standard? why does it shoot up to the top line (45) when i'm in idle or at a stop sign? :confused:

also... i've yet to take the car out on a long freeway drive (longer than 25 minutes) but i'm feeling the transmission thump when it shifts up. granted, i've been doing mostly street driving but the transmission fluid levels are fine. what does anyone out there think may be causing this gentler thump into a higher gear? anyone think it's time to try a TRANS-X deal? or the "italian tune-up" i hear about alot on these boards? or is this something to be expected with stop and start L.A. traffic? :confused:

thanks

All my gauges are in metric bar. The typical range is 1.5 bar at idle rising immediately to the "peg" of 3 bar at anything above fast idle. Hopefully someone who has gauges in psi can tell you what the nominal is for yours. Is your oil level correct- between the min and max marks on the stick?

The transmission, if it otherwise shifts normally, sounds like needs a modulator adjustment. MBZ automatics tend to shift a little harder than American cars. Check the archives and you will get tons of info.

An Italian tuneup won't help your transmission, but will blow the carbon out of your engine.

Chetbaker,

Not sure about your transmission issue, but I think vacuum is used to control shifting, and it is possible you may have some leaks. Do a search on the subject as much has been written in the past.

The oil pressure issue is peculiar. Sounds more like the pressure gage is failing as higher rpm should mean higher oil pressure. The pump is a "positive displacement pump" and as the rpm speeds up the pump speeds up. This means the pumped volume increases linearly with engine speed. The flow resistance forces the pressure to rise. There is a relief valve in the oil circuit, and it is feasible that this valve is off kilter, openning too soon (too low a pressure) and then hanging open until your flow rate drops at idle. In either case, this would drive me nuts and I would have to fix it as low oil pressure under load is a death sentence for your engine.

Hope this helps. Jim

Chet,

The oil pressure problem you have is very unusual.

You might connect another oil pressure gage to see if it reads the same your dash gage. If they read the same, I would look at the relief valve as being the culprit.

P E H