N54 Oil Pressure Switch/Oil Pressure Questions..

[jon dee 500]

Search function was not helpful, so apologies if these questions have been answered elsewhere (2008 335i N54).

1. The OEM oil pressure sensor (switch) sets a warning light (but does not latch) if pressure drops approx below 8psi. Correct ?

2. Operation of the oil pressure switch does not cause a "Reduce power" warning or limp mode, but may set other codes. Correct ?

3. Oil pressure is not monitored by the DME and cannot be logged. Correct ?

If the above is true, then I am not entirely happy that in the event of suddenly losing oil pressure at high rpm I have to wait until it drops to 8psi before I get any warning. Even having the engine running at reduced oil pressure (but still above 8psi) could be fatal for the engine. So I am looking at ways of monitoring oil pressure and getting some early warning if the pressure drops to a dangerous level. First thoughts are for a gauge with an adjustable alarm output, or even a gauge sensor that has a fixed alarm output at 30 or 35psi.

The gauge can be concealed as I am not that paranoid But I would like a nice bright LED out somewhere visible so that I can pick it up in my peripheral vision without taking my eyes off the road. The gauge is more for general condition monitoring and diagnostic purposes, as I find watching gauges while driving rather distracting.

If anyone has got any suggestions on how to set this up/what to use I'd be happy to hear about it. Better still if you have figured a way to implement/trigger the automatic DME "Reduce Power"/Limp Mode engine protection function in the event of the loss of oil pressure.... that would be awesome

Cheers...

[smiddy 76]

Jon, Vehicular DIY has a vid on his channel with a digital pressure gauge with alarm which addresses your concerns, was aftermarket but fitted neatly behind dash vent from memory.

Have a look on You tube.

[jon dee 500]

That's a coincidence... I was atually watching that video a couple of days ago. It was what started me off on this quest. That guy is the best source of 335i DIY information on the web, and makes a lot of really helpful videos. That's a great gauge that he used, only way over the top for what I need. Thinking it thru I don't even really need a gauge. I could get by with a light that turns ON when the pressure drops using a basic adjustable pressure switch like the one in the pic below. Just do the same thing as the OEM switch but at a higher pressure.

If I set it a little above hot idle pressure when the engine is fully warmed it should not be too annoying. Probably get an adapter and give that a trial. Easy enough to add a gauge later on if I decide that I need one. Funny how BMW decided they didn't need an oil pressure gauge in the 335i. They must have a lot of faith in the quality of their oil pumps if they figure the oil pump will be the last thing to wear out in the engine  

Cheers...

 

 

Edited June 27, 2021 by jon dee

[jon dee 500]

Started looking to see what hardware I would need to make this all work. The idea is to keep the installation as simple as possible... up where it is easy to access to check for leaks and make any adjustments. The plan is to introduce a fitting with a 1/8"NPT female thread between the OEM oil pressure switch and whatever it currently screws into. This will move the OEM switch directly rearward a short distance and not require any changes to the factory wiring. The new adjustable pressure switch would screw straight into the new 1/8"NPT fitting and other than the wiring...  job done. That was the plan

Pic of the OEM switch location to come.

A couple of potential problems have revealed themselves. 1. There is not a lot of space !! 2. Suitable adapters are thin on the ground. 3. the new switch will need to be a slimline design to fit in the available space. There is one really nice application specific swivel adapter available in Europe but a bit spendy at GBP188 plus shipping. However, I am on the track of an Aeroflow adapter with a hexagon barrel and three 1/8"NPT tappings at 120 degrees apart. So if it fits, it should be possible to clock the output where it has to be with a bit of finessing

If it becomes difficult to fit a suitable switch beside the oil filter housing, I may have to resort to using a pressure sensor and gauge with adjustable alarm output.

Cheers...

[NZ00Z3 187]

You have an interesting control systems problem.

To stop the alarm/light/limp-mode coming in at idle/low rev's, you need to be able to either:

- Arm the alarm based on a certain rev level, say 25 PSI when over 2,000 rpm

- Vary the alarm with a model, say 5 PSI below normal oil pressure.  Needs an oil pressure gage so you can do the modelling.

 

For flicking the DME into limp mode, all you need to do is find a sensor signal that when lost, put the DME into limp mode, then put a relay contact into the signal that is driven from your oil pressure monitor.  I'm not up to date with the N54's, but on the later automatic E46's, the loss of the MAF signal put the EGS into limp mode.  The DME runs on Alpha-N's for air control, which don't harm the engine in any way

[jon dee 500]

Thanks for the response

As far as I can ascertain, oil pressure at idle for the n54 at operating temperature is 22-30psi rising to a maximum between 60 and 90psi at higher loads and rpm's. The pump output volume varies with engine demand to avoid pumping more oil than is required. This will cause the pressure to also vary in a non-linear way, but it could be expected that oil pressure in everyday driving conditions would be somewhere between 55-75psi. Cold engine pressures will be higher.

Therefore I am considering the use of an adjustable pressure switch that can be set around 45psi. The actual pressure setting can be reached by a bit of trial and error to find a pressure where the switch does not trigger a light under normal driving, and only lights up at idle. Having the light turn on at idle is unnecessary, and the easiest way to inhibit that would be to tap a pressure switch into the charge pipe. If the pressure switch changes state at 100kPa(abs) the "engine protection" system would only be active when the engine was actually operating in boost.

And that's a good suggestion about interrupting the MAP signal to trigger limp mode. These functions can easily be set up with a couple of miniature relays and do not require any interfacing with the DME itself. Sounds like a plan With an aftermarket ECU the engine protection settings have access to all operating parameters and can be set up to cut fuel, ignition or boost in any combination of parameters in the event of losing oil pressure. I don't want or need that level of sophistication. All I want is some basic engine protection in the event of a pump failure or sudden oil loss while driving.

Let me know if you see any problems with what I am suggesting.

Cheers...

EDIT: Pic shows OEM switch. This will move 25-30mm rearwards. There is room to fit a sensor providing the adapter is clocked correctly. But clearance for the OEM switch over the #1 intake runner will be minimal. I will pull the clip and do a bit of measuring.

 

Edited June 30, 2021 by jon dee
Added pic...

[Ninjaspartan 23]

I have experienced this exact issue, low oil pressure when tracking the car caused a spun rod bearing. I did not have any warning that it was sufferrng from low oil pressure. 

Kind of silly how it is just a switch rather than a proper gauge to read pressure at all times.  

What I ended up doing as a solution to that. 

[jon dee 500]

Thanks for chipping in That setup of yours for the sensors would be like a Plan B for me if I can't get a suitable adjustable pressure switch. I'm hoping that Plan A will actually do away with the need for a gauge, as I'm not really planning to hotrod my car or track it (unless someone twists my arm). Given that when I am concentrating on my driving I don't like to take my eyes off the road, I am actually thinking that a little piezo "buzzer" might be better than a light. Nothing gets my attention faster than a strange/unexpected noise in the car !!!

Having your gauges up where you can see them without looking down is the only way to go for racing. I put some down in front of the shifter on my last car and they were not particularly useful. I had to get a passenger to tell me what they were reading

Cheers...

[jon dee 500]

Got this little adpater that looks as so it will work. M12 x 1.5 either end and 3 x 1/8"NPT tappings. Two plugs provided to block off the tappings that are not used. Got to thinking while waiting for this, that a water temperature warning might also be a good idea. The N54 has a reputation for water pump failures and blown head gaskets / warped head if you don't pick up the overheating quickly enough. So I will be taking a look at either a gauge with alarm function or a temperature switch for the cooling system while collecting parts.

Cheers...

[jon dee 500]

Having to do a major re-think on this little project. The pressure vs engine load characteristics of the N54 combined with the huge hysteresis of mechanical pressure switches make setting a 45 ~50psi alarm point problematic. Therefore I am thinking that I shall have to do some old fashioned trial and error experimentation with an electric pressure sensor and a gauge with an adjustable low alarm set-point.

@Ninjaspartan How easy is it to access 12V constant and 12V key switched around the engine bay ? Virtually no current involved so vampire clips would work for the purposes of experimentation.

Cheers...

Edited July 12, 2021 by jon dee

[Ninjaspartan 23]

1 hour ago, jon dee said:

Having to do a major re-think on this little project. The pressure vs engine load characteristics of the N54 combined with the huge hysteresis of mechanical pressure switches make setting a 45 ~50psi alarm point problematic. Therefore I am thinking that I shall have to do some old fashioned trial and error experimentation with an electric pressure sensor and a gauge with an adjustable low alarm set-point.

@Ninjaspartan How easy is it to access 12V constant and 12V key switched around the engine bay ? Virtually no current involved so vampire clips would work for the purposes of experimentation.

Cheers...

Yes, I agree it would be rather tricky just using pressure alone. Once warm (100c) I read about 1.5 bar at 700rpm. under load it goes to about 6bar above 2500rpm

Not too sure for the 12v around the engine bay sorry. I already had a boost gauge which I wired into the 12v cigarette lighter. So when installing the oil pressure gauge I could just connect off the existing gauge and run the lead through firewall to the sensor. It is very easy to run a wire through the firewall though.