Low oil pressure
 

Hope someone can help me..i have a 1991 300 E 4 matic with 207,000 miles on it-the car has been lovingly-and expensively-maintained...I recently took it on a 1500 mile road trip-on the way back, my oil pressure gauge was acting up-at low revs-below 2300 or so-I would have close to no oil pressure-at higher revolutions I would have the normal 3 reading on the gauge (all of this occurs AFTER the car is warm) . With judicious downshifting, and driving home after midnight, I managed to keep the oil pressure at 3 the whole way home-and there he sits....I have an appointment for Oct 3rd at the local dealership...however, if i had a better idea of WHAT the problem is, I might be able to get it fixed sooner by going to an after-market mechanic ( I trust the dealership to accurately diagnosis) I am not certain that i trust this after market place to diagnosis correctly (afraid they may attempt to find non-existent problems-although they DO have a good reputation)....So far
my suspicions are 1) oil pump problem-or 2) -main bearing...
There is a test I think that can determine this-a trickle-down test? How would that work?
Thanks for your help
Laura
p.s. I swear I am gonna HIT the next person who tells me to "get rid of that car"-which so far is everyone i know.....

Let me know if you need someone to hold that next person while you hit them!

How low is what you call low? What is the reading? Is the needle resting on the lower peg?

No oil pressure is most often associated with worn main, rod and camshaft bearings. Unless you had a massive loss of oil, it should not indicate that these parts are worn. They typically wear out slowly over time giving the loss of hot idle oil pressure over time. For the oil pressure to suddenly drop is quite unusual. MB's minimum hot oil pressure spec. is .7 Bar, or about 10 PSI. This is about 2/3 of the way from zero to one on the guage.

Getting rid of this car is not necessarily the answer. These cars are built to be rebuilt. You are doing the right thing; investigate, learn and then decide. Just getting rid of it without analysis is the ignorant persons decision.

Good luck,

Laura,
I'd try to rule out the easy thing first. Change the oil and filter and see what happens. Maybe the filter is partially clogged...

Hmm.... change oil and filter-oddly enough-I changed the oil-found a ripped axle boot while I was doing it-took it RIGHT to the dealer-where it sat for a week-they fixed it-and then I left DIRECTLY for my trip-all highway till we got to the Outer Banks of North Carolina-which is where my Brother-in law noticed the low pressure....I just LOVE the idea of changing the oil and filter (doing it tomorrow-with my fingers and toes crossed!)
Thanks
Laura
p.s. I will NEVER sell this car-it has driven me thru some horrid snow-including the blizzard of January 1996-got me home that night.....and one awful 16 inches in 12 hour storm this winter(course, he got stuck in the driveway that night-but that was my fault...Not much can go in snow over it's bumper....)

low oil pressure
 

try changing the oil pressure sending unit. they often fail in the way you described.

It is unlikely there is anything wrong internally with the engine.

I love you Guys
 

I am loving all of these ideas...my Brother in law (ass that he can be) had me scared to death..You guys are the best!
Laura
(does this mean i have to RETURN the $3500 I borrowed from my Dad for potential engine work?)

Laura,

I'd agree with the poster who recommended changing the oil press sending unit, although you could have the oil press checked against the gage reading first if you like. I have a '91 300TE 4matic with 212k miles and I know how you feel. There is no safer feeling than knowing when the weather gets nasty my wife and kids are driving around in one of the best snow and slush cars ever built. They have their problems, though. I just spent 3 hours this weekend pulling the heater blower out to fix a loose plug connection. The good news is I didn't have to blow $300 for a new blower motor with fans.:) :)

BTW, I just remembered, I had to replace my oil press sending unit for the same problem! Scared the hell out of me until I replaced it, worrying the engine was going to lock up.

someguyfromMaryland

Laura,

Sounds like it's serious party time with that $3,500 from your Dad if the sender fixes your problem. Of course then you have to repay the $3,500, oh well.

Good luck and don't listen to the naysayers,

with that kind of number on your odometer i'm not suprised that you get low oil pressure readings at low revs. what type oil are you using? if you have 5w-30 or even 10w-30 then switch to something heavier like 15w-50. that'll get your oil pressure up some. in a car with this many miles, it's not going to be any single thing. oil pumps, main bearings etc. all form a delicate balance and unless something is really broken you shouldn't upset the equilibrium. i would even try that oil additive called "restore" which addresses many of the issues and will keep your car going for several thousand more miles. of course, the best way to know is by having the actual oil pressure read off a gauge to determine the real situation and maybe it really is just the sending unit...

Change the oil pressure sending unit, it is located below the oil filter.

jack

You are probably OK if you reach 3 bar at speed, Anything at or about 15 psi at idle is safe. The senders vary quite a bit. Of course temperature and grade (weight) of the oil also come into play. My '89 is just above 1 bar at idle and I am now up to 204,300 mi.

jeffsr,
that's good to know that at 200k+ miles you still get 1 bar. what oil weight do you use? i've had cars with less than half that many miles and the oil pressure is already low...

10W-30 Mobil. The M103 is a sturdy well built engine. If the rods aren't rapping and the pistons ain't slapping then motor on and don't worry..

O.K. Guys-here it is
 

O.K. I changed the oil, filter, AND the oil pressure sending unit (and wasn't THAT in a tight spot! There are times that being a women comes in handy-fitting a wrench into a tight spot is one of them!) I re-filled it with my usual 15-40 weight oil...heated it up to 90 degrees...when actually driving-above 20 mph it reads 3 (yeah!!) when stopped, or at idle, it reads 1.5 or higher..this is good right? The reason I ask is, I swear, the damned gauge NEVER read right-it used to ALWAYS read 3-ALL of the time...( I also had a temp gauge that NEVER read above 80-UNTILL I had the new head gasket installed at 100,000 miles-then it started to work...) So how about it guys-is anything between 3 and 1.5 O.k. (3 at normal driving speed) ??? In other words did I fix this for $75.00 (and your help...)??
Thanks
laura

Good job! Mine actually goes to 0 at idle. I can get it to 1 bar by bumping the RPM by 200. I haven't tried heavier weights yet.

As long as it is above .7 bar at idle and hot, you should be happy. Keep an eye on it for a while.