Oil gauge peaks at only 2.75 when pegged - is this bad?
 

91 300E, 129K

I just noticed on my drive home today that my oil gauge peaks at 2.75 when pegged. I could have sworn that it used to go to a full 3.0, but I really don't remember. For all I know, maybe it always did this? Anyway, at idle in D, it is at about 1.0 or just slightly below and goes all the way to 2.75 pretty quickly when gas is applied, perhaps around 2500 RPM or thereabouts (I'll have to check again to be sure). Is this bad? Shouldn't it go all the way to 3.0? What damage could be caused by this lower pressure? It troubles me that there isn't a red zone either at the top or bottom of the gauge to indicate that the needle is sitting somewhere it shouldn't.

I just replaced my water pump last night, and got to thinking: could I have disturbed a wire or sensor in the process that might be contributing to this lower pressure reading? Could there be a higher resistance somewhere as a result of me jostling a connection or maybe spilling coolant on it? Where exactly is the oil pressure sending unit located?

I am hyper paranoid about damaging something by starving the engine - is this an overreaction? For the record, I am running 20W50 and don't burn a drop of oil. The engine is in perfect running condition. I just resealed the upper cam timing cover a few days ago to fix chronic seeping. I also just checked my oil level, and it is just under the high mark on the dipstick.

Any opinions please?

Thanks in advance!

I would suspect that its more likely that you have a problem with either the gauge, the sender, or the associated wires, rather than an engine problem.

A quick, and easy test you can do is to unplug the wire at the oil pressure sender unit and look at you oil pressure gauge. With the sender unit unplugged, the oil gauge should now peg at 3. If it does not, than your problem is in one of the items that I mentioned.

If it does peg at 3, than you either have a bad oil pressure sender unit, or you may indeed have a problem with the engine. I would bet on a defective sender unit however.

Phil

Thanks for the quick response! Now regarding my water pump replacement last night, is the oil pressure sender located somewhere in that vicinity (front driver's side of engine)? Could I have bumped it loose or spilled oil on the connections? Where exactly is the sender located? Thanks again in advance!

I watched the oil gauge the whole way to work today. Still pegged at about 2.75. Now here's the kicker - if I put the car in neutral and rev it really high, the pressure slowly moves up, hitting almost 3 right as the tach hits redline. Does this indicate a blockage somewhere?

I've been thinking about what I said before; I'm positive the gauge used to hit 3 before I did the water pump. I'm psycho about my gauges and always watch them as I drive around. I'm absolutely certain that I would have noticed the gauge at 2.75. So the question remains - could I have bumped something while doing the water pump that could have caused this to happen? Why would the gauge read a perfect 3 before the water pump replacement, and then only 2.75 afterwards? Could I have dripped antifreeze or oil onto an electrical connector somewhere, forming a resistance in a circuit? I don't really know how the gauge works - I'm just throwing a wild theory out there.

As a reminder, the gauge always reads steady at just below 1 when the car is hot and put in drive. Slightly higher when it's in neutral or park. Aside from the new maximum of 2.75, it seems to behave properly in every other way. I run dino 20W50 and change the oil every 3000 miles. It is always clean when it comes out.

It's going to be raining today when I get off work, and the big spring above our garage door broke last night, so the door is stuck in the down position (any suggestions on that problem are also welcome). As a result, I won't be able to troubleshoot this problem for at least the next day or so.

Help! Anyone??? I don't want to damage my engine!

It is possible you have a problem in the oiling system that won't let it build full pressure, but chances are good that it is an indicater problem. If you are really low on pressure, you have enough to not harm the engine. You didn't say how the oil gets changed. Do you do it yourself, take it to an Indie, take it to the dealer, or take it to Goofy Lube? Some of those cheap filters can cause low pressure. The problem of cheap filters and low pressure has come up in other threads. Good quality filters are still cheaper than engines.

Peter

I change the oil myself and use good oil and a good filter. But again, the part that bugs me about this problem is that the gauge was reading 3.0 before I replaced the water pump, then only 2.75 afterwards. Is it just coincidental? Could I have broken something under there? On the other hand, if my pressure really is only 2.75, will that kill my engine? Doesn't the fact that the gauge pegs pretty quickly upon accelerating mean that the oil system is healthy overall?

What brand filters and where do you buy them?

I don't see how you could have broken anything. It may have been coincidental. Check the connection at the sending unit.

As I said you appear to have enough pressure so as to not damage the engine, but you do want to correct the problem. 2.75 atmospheres is a safe enough pressure.

Peter

Fram filters from Autozone. Wait a minute, those aren't exactly great filters. Should I try changing the filter with an OEM and see if that corrects the problem? There's a dealer on the way home and I can just swing in there and pick one up. Where exactly is the sending unit located, and what is the pressure it measures? Does it measure before or after the filter?

That is exactly what I would do. Pick up a dealer filter and get that POS FRAM filter out of there. That is exactly what caused the low oil pressure on the thread I mentioned earlier. You lied to me when you said quality filter. You are new here so you missed some of the threads where people have cut these cheap filters open to find hay and all kinds of junk in them. We still don't know if this is your problem, but it can't hurt to get that filter out of there. You can buy a stock of good filters from Fast Lane on this site. It also helps support the site.

Good luck,
Peter

Actually, I read that thread you're talking about, the one where the filters had the cotton gin material and insects in them. But I recall that the OEM filters also had the same thing inside when somebody cut one of them open. It seemed the consensus from that thread was that even the OEM suppliers are manufacturing on the cheap in India or Turkey or wherever.

I just placed an order on Fastlane a few minutes ago, for hoses and intake tube I cracked under there while changing the water pump. I would have ordered the OEM filter if not for having to wait on shipping time. It might be quicker to swing by the dealer on the way home. I know they're a ripoff though; the bypass hose for the water pump cost me about $16 there; it's only $5 on Fastlane. Of course, after shipping they might be close in price and at least if I pick up from the dealer I won't have to wait several days for shipping while the engine is ripped apart...

Where is the oil sender unit located on the engine? I'd like to follow a previous poster's suggestion that I unplug the sender and see if the gauge goes to 3. Do you concur with this suggestion?

I don't remember the thread exactly, but I seem to recall that these were like Bosch filters supplied only to the cheap DIY stores. I believe the sender is on the back of the filter housing. I know being a professional and owning several of these M103 cars, I should know, but it is something I haven't dealt with. I'll look at the 91 300E when I get to the shop today.

Peter

If you disconnect the sender, it will go to ZERO!

When my sender went bad, it pegged to 3 once the ignition key was twisted to the II position.

You have a different engine configuration than mine, and on mine, it's by the rear of the oil sump on the driver's side. You have to access it from underneath the car.

I respectfully disagree. If you disconnet the sender wire it will, (or should) peg at three. If you ground the wire it will, (or should) go to Zero.

Phil

Interesting...I recall just the opposite happening when I had to deal with mine, until I replaced the offending sender...

...then again, I also had to rebuild the harness in the process as it seemed to suffer the same fate as the main wiring harness...

Contractor is scheduled to come to the house tonight - the coil spring that helps the garage door up snapped and the door is stuck shut. Somehow our door has only 1 spring, so the door can't be lifted. We are having a 2nd backup spring added to provide a limp mode in the case of another spring failure; that way at least the door can be opened manually. Hopefully he'll finish in enough time to allow me to use the garage tonight (it's raining outside).

I will try unplugging the oil pressure sender. So are we in agreement that it should go to 3 when unplugged and 0 when the wire is grounded to the chassis? And that I have to access it from underneath the car?