Oil Pressure Specs for m130 engine

[toms]

There have been some posts on this and other boards regarding catastrophic oil pressure failures. I believe our moderator Aaron had this occur on his w113. This prompted me to figure out how to rig a warning buzzer for my 71 280sl.

I was able to connect an electronic oil pressure gauge with an alarm while preserving the original pressure gauge.

I am now trying to calibrate the new oil pressure gauge. Can anyone provide me with the statistics (in bars and PSI) on the correct factory spec oil pressure for the 71 model at a given RPM?

Thanks in advance,

[Aaron]

Tom,

Exactly how did you integrate the gauge? If it's electronic it must have a pressure sensor which has to be installed somewhere. If you are in need of specs, I will do some research and see what I come up with. One other option is to connect a known accurate oil pressure gauge which reads higher than the factory one and run the engine at different RPMs and make appropriate recordings. Find a Smiths or Jaeger gauge and hook it up to the oil line from the engine (those gauges read in psi and go higher than the MB gauge).

My W113 did have a catastrophic loss of oil pressure while returning from a rallye in Lexington, Kentucky last year. No warning, just immediate engine melt down (no noises) and coast to the side of the road. The next day when I rolled it into the workshop to find out what happened, I got the dreaded "squeaking" when I tried to turn the engine over with the starter. Right then I knew it was toast. Well, at least we finished the rallye before she blew!

One interesting note: Midway through the rallye, she stalled for no apparent reason. We coasted over to the side of the road and she fired right back up. Maybe that was a sign from the M130 engine Gods! Shoulda listened!!

[toms]

Aaron,

Thanks for the quick response. Your regrettable experience is what prompted me to do the research that would allow an audible warning. I did attach a VDO electric sender to a tee fitting on the existing line that feeds the mechanical pressure gauge in the dash, enabling me to preserve my existing gauge.

I do have a VDO gauge that came with the sender, but does not have a buzzer. Did not even occur to me to connect that gauge and do test reading--a sure sign I am way to close to the trees on this one! Thanks for the thought. If you have a source of specs, I would still be interested in the factory specs. The data book for the 71 model year suggest 5 kp/cm2, but I need a reading in bars to calibrate to psi. (I don't think the kp/cm2 is the equivalent measure to bars).

Attached is a picture of the sender attached via a tee fitting to the oil filter housing. Also, I am writing an article for the Pagoda World newsletter that details the enhancement.

[vince]

5 kp/cm2 (kilopascals) = .05 bar = .73 lbs/inch2. Seems awful low. I would think an alarm setting of 15-20 psi would be adequate for warning purposes in the case of a rapid failure. That should be enough oil pressure to protect the engine under all but the highest load conditions. Watching the dash guage regularly and carefully should be adequate for long term deterioration in the engine.

[psfred]

Minimum idle pressure is 8 psi, but I'd get alarmed at anything less than 45 psi at speed, since that will surely indicate lack of oil flow, either from excess viscosity loss due to heat or lack of oil.

You won't get much warning at any rate -- if a cooler line lets go, the mains will be gone by the time you get the engine shut off anyway, but at least it will only be the bearings and not the cylinders, too.

Peter

[toms]

Peter,

Suppose I develop a pinpoint leak in the cooler hose that goes to the oil cooler. Seems like I would still register lower oil pressure at the filter housing. What am I missing?

[ctaylor738]

I would want an alarm that went off at like 80% of the lowest pressure you have ever seen - like idling in gear at a stop after coming off the freeway on a hot day. But I see Peter's point. I wonder if you could wire it so that it was proportionate to engine RPM by taking a feed off an electrical device that was driven by engine RPM?

Remember the 113 guy who hooked up the interval wipers and cruise control on his car? He would know how to do this.

[toms]

Chuck,

Good idea-John could figure that rpm connection out. Regrettably, I am past my technical competence on this project as it stands today!!!

[psfred]

A pinhole leak will result in excess oil consumption, but the highway symptoms will be very similar -- when the oil gets low enough, the pressure will drop when the pump runs dry. Just very low will give you enough time to get off the road, and oil will run down off the head and you will get foamy oil with variable pressure for a while. I know, I've run the 220D out once or twice (it usually uses 4 qts to a tank of fuel, but sometimes gets thirsty).

I'd set it for 30 psi or so -- on cars with idiot lights, it's usually 10 psi or less, not much warning.

The pump bypass opens at around 65 psi, I believe.

Turbodiesels are different -- the oil sprays open at 2 bar, so you almost never have less oil pressure.

Peter

[toms]

Peter, Chuck, Vince and Aaron,

Thanks for your posts. I guess the only protection from damage related to the oil cooler lines is to make sure those lines are replaced over time. I have new oil cooler rubber hoses so I feel like that is addressed as best I can.

The catastrophic failures I have heard about have generally been related to oil pump failure.

Aaron, in your case, the bolts came off, causing the pump to disengage.

So if I set the alarm at 30 psi, would that limit or eliminate damage (assuming I have the good sense to stop the car upon alarm) from oil pump failure?

[Mark DiSilvestro]

Tom,
One other point - if you set the alarm to go off at 30 PSI, but your engine normally idles, hot, at 20 PSI, then your alarm will go off everytime you come to a stop. Seems to me the answer is to have it set for just a hair below your hottest idle reading, while idling in gear. Buy the way, There are auxiliary oil pressure and temperature warning systems availible from racing suppliers. Also, my 87 Isuzu pickup came factory equipped with an oil pressure guage AND oil pressure warning light.

Happy Motoring, Mark

[Aaron]

Yes, I agree. I would set the alarm to trigger when oil pressure is maybe .8 bar or about 11.7psi of pressure. Say for some reason your oil pressure were to drop while at speed. The engine won't lock up as long as there is SOME pressure. My Triumph TR6's bottom end bearings are worn and it runs as follows (this is with Castrol 20W-50 oil):

COLD IDLE: 50psi

HOT IDLE: 5psi

HOT @ 3,000 RPM: 25psi



Not good by any means. The engine is coming out soon for a rebuild. BUT it still runs fine but oil consumption is HEAVY, about 1 quart every 200 miles.

[Mark DiSilvestro]

Tom,
Here's something that could simplify your oil pressure alarm design. BMW and Volkswagen use oil pressure switches for warning lights. Usually these switches have the working pressure marked on them. You might find an oil pressure switch of a suitable rating to match your minimum oil pressure and work your alarm directly from it instead of rigging something through an extra pressure guage.

Happy Motoring, Mark

[vince]

Prevention against a catastrophic failure such as Aaron experienced may not be possible. There are two types of lubrication, hydrodynamic and boundary. The rotating parts, such as the crank, rods, etc. are protected by hydrodynamic lubrication where the oil actually wedges the surfaces apart and when running there is no metal to metal contact. The cam lobes and other parts with sliding friction are protected by boundary lubrication where the oil basically makes things slide easier. In both types the oil also provides the critical cooling of the parts. In a total, instantaneous failure everything is dependent on whatever boundary lubrication remains from the oil on the parts at the time of the failure. Cooling goes to zero. Metal to metal contact begins rapidly, high local temperatures at the site of the contact occur rapidly, total failure ensues. The higher the load, the faster the meltdown. In my opinion, the alarm is fine, but unless you are able to reduce the load FAST and shut down within a few seconds of the alarm, a total, instantaneous loss of oil pressure is going to fry the engine. That said, set the alarm slightly below the lowest pressure you normally see at idle, in gear if it's an automatic, on a hot day. Use the best oil you can find and change it every 3,000 miles or once a year if you don't use the car enough. Keep a close eye on the guages and enjoy the car. I think Aaron's failure was more of an aberation than a common problem.

[toms]

Mark, Vince

Thanks for the additional thoughts. Seems like any additional protection other than staring at my oil pressure gauge without pause is an improvement. I will set the pressure alarm as suggested and hope that I never need it.