Faulty Oil Pressure Reading

[Newbe]

I have a 997.1 3.8s and I appear to have a faulty oil pressure reading.

The car was at Hartech in August and they pointed out that with the ignition on and the car not running the gauge is reading 2.5 bar. When you start the car it goes up to 5 bar and then drops to 3.5/3 bar once warmed up.

As a test Hartech connected a working oil pressure sensor and it done the same thing. They said it's not the sensor and the issue lays elsewhere. As I only had a limited amount of time I took the car away and driven it fine ever since. Hartech said there's not an issue with the engine as it does it with just the ignition on and there's no problem driving it.

It's starting to annoy me now.

If it's not the sender unit where else do I start looking?

Thanks

 

[deMort]

The sender is the cause in 9.5 cases out of ten to be honest .

If it's definitely not the sender then i can only think of two other items ..

You have a short circuit in the wiring from the sender ..this will be a partial short or a resistance .

You have a fault with the gauge which will be an instrument cluster replacement unless you can find anyone to repair your old one .

Ignition on and reading 2.5 bar .. when running at temp its reading 3.5 bar .. that is the correct oil pressure for a hot engine .. take one from the other and you have 1.5 bar ..

In a sense you have a working oil pressure indicator .

 

[Alex]

Sounds like it's shorting slightly as demort says.

 

[Newbe]

Cheers guys

Hartech said the same about taking one from the other and come to the conclusion it is actually working. In a roundabout way.

They also said the same regarding a partial short or instrument cluster fault.

Do you know the route of the wiring? I would like to bell the wiring out to eliminate it, would you happen to know the first place I could unplug it in the engine bay?

Just trying to think of the quickest way to eliminate either? Any pointers?

I have all the gear, just no idea :grin:

Ok, so I have a multi meter and ok with electronics, just some pointers would be good?

 

[Alex]

This thread may give you some pointers:

http://911uk.com/viewtopic.php?t=140085

 

[Newbe]

That's really helpful, I'll give it a go this week at some point.

I'll also try disconnecting it first to see what happens just in case it is the sender. Worth double checking

 

[deMort]

I'm not sure if im agreeing with Hartech or they are agreeing with me at this point .. i think i'm agreeing with them as they got there first

Ok in the world of diag we prove everything inside out and upside down before we move on .. 100% a wire is ok basically ..

First off ..

Unplug both wires from the pressure sender unit .. ignition on .. what reading do you get .. for a wiring short or a gauge issue then you should have a reading .. if nothing then logically we are back at the sensor .

The sensor btw has a pressure wire and the oil light for no pressure so a 2 in one sensor .

Next .. the engine needs a good earth .. it can't possibly have a bad earth as the starter motor needs a good one to spin the engine over ..

Nonetheless .. a jumper cable connected to the engine and the chassis to prove its ok .. the 100% check there .

After studying a wiring diagram .. i only have 2005/2006 and the sensor wires go direct to the Dme unlike a 996 ..

This is a problem in that its a can bus message from the dme to the instrument cluster ..

This now adds another possible control unit into the mix .. a faulty Dme .

We can't with the equipment we have test Can Bus messages .. MC probably has this kit but you and me certainly don't have it .

We can check the wires from the sensor to the Dme though .

You would need to unplug the Dme and the sensor .. we are not unplugging the round engine plugs at this point .

You would .. with a meter test the wire for a short to battery , to negative and for a resistance on the wire .. if an issue then from the engine plug to sensor and other side to Dme to break down where the short is .

The actual wire should be from pin D35 on the dme to the sensor wire which is green / red .


EDIT ....

Annnddddd Alex,s link shows i've already said this .. sigh .. putting the wiring diagrams away now and wandering off to a different site .

 

[Newbe]

Cheers Demort

I know there's a couple of control modules under each seat but I believe the DME is behind the rear bulk head? I have a Cab 997. 2005/2006

Am I right in saying put the roof in service mode and lift up the back of the glass. Remove the carpet and the DME is there?

 

[deMort]

Yup !

Screwed to the roof frame... the one with the roll over bars in it .

 

[Max65]

Sorry to hijack this post ,so on my second drive ( 40 min ) my oil pressure ranged between 4 bar and 5 reading through previous posts 5 bar seems to be healthy, my question is why do Porsche not fit a gauge that shows more than 5 bar as it doesn't look healthy running it at maximum pressure.

 

[deMort]

Cold engine is between 4.5 and 5 bar .. hot engine and 1.1 - 1.5 bar at idle .. faulty sensor will give different readings and a 0.5 over read is not uncommon .

5 Bar afaik is the max pressure .