Recently replaced coolant tank and AOS on a '07 C4S. Coolant tank was cracked, but not leaking, and AOS was preventative at 43 K mile mark. Oil pressure on fully warmed engine is showing 3 at idle where it previously was around 1.5. Pressure at 3500 RPM is 5, and when clutch is disengaged at speed pressure only drops to around 3. Was careful to insure that all connections to the AOS were fully engaged on the new AOS after install but concerned something is a miss on the install. No other changes where made during the install and engine appears to be running normally. Should I be concerned about the oil pressure numbers ?


UPDATE : Drove up to store, oil pressure gauge pegged on 5 on or off the gas for the whole 5 minute trip. On the return trip all of a sudden gauge seems to reading more normally. At 3 K RPM and above it's reading 5. At 2500 RPM it reads 4 and at idle it's reading 2. Off the gas and clutch disengaged pressure starts to drop. While slightly higher than pre-AOS install the gauge seems to be reacting normally. Faulty gauge ? Something dislodged during the AOS install ? Any thoughts and/or potential solutions would be appreciated.

Could be the oil was just not as warm as you believed. It can take some time for the engine oil to get fully up to temperature. Oil temperature lags coolant temperature so just because the coolant temp gage needle indicated the engine was warmed up doesn't mean the oil was too.


You can also check the oil pressure sensor connector.


Gently unplug the oil pressure sensor connector from the wiring harness connector then connect the two again. Be sure the sensor connector fully engages with the wiring harness connector. If there is a spring latch verify this latches securely to help keep the connectors connected to each other.


While you are there check the wiring looking for some sign the wire got bent sharply. If it did this can result in a crack a wire in the sensor lead which can affect the reading.


If this doesn't help and you find nothing about the wiring to worry you, I could suspect the oil pressure sensor is failing.


If you didn't do something to the oil pressure sensor or its wiring, or that of the wiring harness, I can't imagine anything arising from the AOS directly affecting engine oil pressure.

At your suggestion I checked the wiring at the sensor and the connections are tight and the wires are straight and appear undisturbed. I went so far as to remove the rubber insulator on the inner most connection to make sure all was well underneath it. As for the harness connection I traced the wires from the sensor inside the wrapped wiring loom that lays inside the plastic tray attached to the face of the intake manifold and connects to one of the primary harness plugs on the passenger side inner fender wall. Both of those connections appear solid. Is there a secondary harness connection for the sending unit that exits that loom before it connects at the fender well wall ? Is so where does it terminate ?

Do not have a wiring diagram for the model of car you have so I can't say with any certainty.

My WAG is the sensor reading goes to the DME which converts the voltage level signal to a digital reading and this is packaged in a CAN bus message which is sent out on the CAN bus which is then received by the instrument cluster controller and converted back in to a voltage which is used to drive the oil pressure gage needle.

If you are confident the sensor is intact -- was not say damaged during any engine work -- and the connections are good and the wiring is good then I think you've done about all you can do.

The oil pressure gage of my Turbo displays inconsistent and jumpy pressure readings sometimes -- I can detect no pattern to when it does this -- and has been this way ever since I bought it back in 2009. I sort of suspect it is a marginal connection between the instrument cluster and the oil pressure gage as the DME has never flashed a low oil pressure warning -- but the oil pressure has never dipped to scary low levels either about as low as it dips when it dips is to the 1 bar level -- so I ignore the behavior.

While I'd like to have it corrected, I'm not willing to fund a loose or marginal connector expedition in the guts of the dash with the likely course of action to just replace the instrument cluster for a Turbo with no warranty. I've probably bought cars for what that would cost...

I'm watching the gauge to see how it's responding. Seems to be better when oil is at full op temp, but when the issue started oil was at full temp. Could be I never paid that much attention to it before but readings still seem to be inconsistent or more variable than in the past. Typically would check occasionally to insure I was getting good pressure at highway speeds. As there is no evidence of any other engine related issues I can live with the sending unit as is for awhile. Will replace it if it gets any worse. Just strikes me as odd that it should surface immediately after the coolant tank and AOS R/R. Primary concern was that the engine drop required to remove the coolant tank resulted in wiring issues but nothing I have checked exhibits any evidence of damage and there are no other electrical gremlins present at the cluster.

Have to admit I don't pay that much attention the oil pressure gage. While I scan the dash while driving I'm mainly looking for warning lights. Primarily my focus is on the road and traffic conditions and how the car feels, how the engine sounds.

The jumping pressure gage needle has caught me off guard more than once simply because once in a great while I happen to glance at it when it is acting erratically and there is an instant while one fears the worse then remembers the jumpy gage.

The same is true of the voltage gage. I was way out in NV on I-80 and having just filled up the fuel tank and was driving away from the station when I noticed the voltage level reading lower than I remembered. It was above 12V close to if not over 13V, but this seemed low to me. I was alarmed enough I hate to admit that I called the SM at my dealer and one of the senior techs happened to be handy and we talked and he convinced me there was nothing amiss. That the reading was what it was because that is what the electrical requirement of the car was at that time. I think I had turned on the A/C after fueling as the weather was getting warm. Since then I have noticed that after a cold start with no other electrical loads the voltage reads nearly 14V but after some driving drops a bit probably because the battery has been topped up. Turning on the A/C causes the reading to drop even more as the power is consumed by other electrical loads.

Everyone clamors for more gages, but there's a downside to gages. They require one to know or at least be familiar with the idiosyncrasies of systems the gages are connected to.