Oil Presure
 

80 911 SC

Has anyone experienced a malfunctioning oil pressure sending unit?

I had my 3.0 rebuilt back in 2002 by Motormeister in Calif. Have not put that many miles on it since the rebuild . Anyway > when I first crank it up > oil pressure reads normal [1 bar per 1k rpm] when it gets to operating temp though > oil pressure drops. to the point where it reads a tad above 0 at idle. [my right ankle is getting tired rolling over to the throttle and brake in keeping a high idle]..

I was reading on a Mercedes site that oil pressure sending units do go bad - read correctly at low temps, and then fail at higher temps.

I was wondering if this can also be the case with P-car senser units

Bentlys says to check it by removing the sender unit > screwing in a mechanical gauge 22mm threads and using that to determine the oil pressure > I don't have that tool > ergo can not check via the mechanical means >

Should I just buy another sender and check it that way [$56 or so]??

suggestions from the wise-men???


kgl

The first bit of info from this board will be: don't use motor meister!! WIsh I could tell you more about the switch.

"The first bit of info: Don't use MM." WRONG. If you've never had any work done from them, you can't be so definitive. I had them build me a race engine (out of a 2.7) SEVEN years ago, and it's still running strong. There's a lot of BS on the web about them.

Now, on to the issue at hand. What you may be experiencing is a short in the oil sender lead (first place to check) or a malfunctioning oil sender unit (NOT related to MM, mechanical equipment do have failures that are not related to the rebuilder/manufacturer). So, those are the two places to check, as a starting point. Ensure the wiring is in good condition. From your write-up, it seems that as heat builds, so does resistance in your wiring. . .Could be wrong. I only know what I know and don't know what I don't know. :P

I have replaced my oil pressure sender. It's not an expensive part and you can do it with the engine in the car. I have no A/C on my car, so I don't know if that's an issue. It takes a big crows foot wrench (I forget the size but I found one at Sears, maybe 19mm) and a pretty decent length extension. You have to be real careful not to rotate the piece that the sender screws into as it is itself screwed on to stuff, like a T and those joints can then leak.

My "pressure switch" failed in a similar manner. When warm and at idle, the red light was on with ~ 1.5 bar on the guage. At around 3.5 bar the light would go out. As I understand it, they are a similar type of gizmo, one controls a light, one controls a guage.

You'll probably be buying a new one anyhow, so you could start there. Then take it to sears and figure out what size wrench to buy. It's really like 22 or 24mm, which is why I didn't remove it when my engine was out.

There is an oil pressure switch in the very front of the engine (firewall side) that will alarm of low pressure (red light in gauge). If you dont get this warning during warm idle, then I suspect your pressures are fine, but the sender may be bad.

Heed Bob's warning about using the large crows foot on the sender as the sender sits in a block thats attached to the case. Too much pressure with the crows foot and its very easy to crack the case (easier to see than explain). Its much easier and safer to disconnect the oil line from the block and remove sender/block as a unit. Mount the block/sender in a vise and now you can safely R&R the sender w/o worrying about cracking the case.

I found out the hard way that the sending unit must be matched with the gauge. The gauges work on electrical resistance with the sending unit, which I believe can be affected by heat and ground. I’m sure there are plenty of EEs out there that will set me straight if I’m wrong. I chased a similar problem for a couple of months until I stumbled on the real cause. I should have used a mechanical gauge to diagnose the correctness of the pressure. Another ‘possible’ cause could be a stuck thermostat (but unlikely). Cheers, Rich

I rooted around and found the crow's foot wrench. I'm embarassed to say it's 15/16". (That's close to 24mm) :oops:

Charlie, do you know the switch point fot the idiot light?
Might also consider the cam oil restrictors.
This is a simple swop (especially with engine out).
Best is that it raises oil pressure to the lower end while at idle.
Waynes' engine book and maybe thier site has a good explanation for the upgrade.
Can't imagine a mechanical gauge would cost much. Just wonder what threads are on the sender and if they would match up with a cheap mechanicle unit that you could get at a discount store like autozone etc.

Where's its location? It sits next to the breather cover and above the internal t-stat. Hope this is what you were asking.