Oil Pressure jump while idling

[Alatriste]

Hello Guys,

I bought a 1987 911 Targa last February with 45,000 miles and I did several service/repairs this year:
- Valve adjustment, distributor rotor and cover, spark plugs, oils, filters, adjusted idling speed bypassing ECU and recently oil return tubes. I love the car and is running great (I haven't driven another one so I cannot compare though). One thing my car is doing from the first day is that oil pressure has a little jump on the gouge when is warm at idling and rpm hesitate at the same time. Does anyone know what could be the cause?
Please see the video in the link below:

https://photos.app.goo.gl/N3mHZ53MiLVwV3KK9

Thank you very much and Happy Holidays!!!

Ernesto

[7783911]

if idle was jumping this would make sense but OP is moving alot (usually rule of thumb is 1 bar per 1000 RMP). Have you checked your OP sender connection (cleaned contacts etc) or have access to someone with w known good OP sender to test with..5 minute job. I cheated and added a separate OP gauge off the same block as the OP sender to get more accurate (direct) reading while working in the engine bay (since i cant see gauge)

[gomezoneill]

Your idle is hunting causing the fluctuation. Could be mixture and a assortment of other things.

[7783911]

Cant say I agree..the fluctuation in the rpm is not as significant as the fluctuation in the OP?

[john walker's workshop]

Not a 5 minute job if the a/c is there as in most carreras. A/C unbolted and out of the way, muffler off to get at the bracket bolts, bracket removed. The sender is tight on the block it screws into and the block needs to be clamped in a vise in order to crack the sender loose.

[7783911]

True John, I have a 77 without AC so this is easy for me :-)

[Alatriste]

Thank you guys. The variation on rpm is correlated in timing with the jump on OP. GomezOneill if we could assume RPM is causing this jump in OP, you mentioned a sort of things that could be the reason. Could you please list the ones that come to your mind?
It will be very useful for me. Thank you again for your patience,

Ernesto

[gomezoneill]

Does this happen while cold or warm or both? What weight oil? How many miles on the car?

[arbita1]

I had a similar issue. Maybe worse. It turned out to be a loose electrical contact on the sensor. Once I cleaned and tightened the contact the gauge worked fine.

I would start there before going into mechanical troubleshooting.


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[Quicksilver]

The wire at the sender could be breaking down. Old copper gets hard. Mine got to the point where I went to pull it off and it just snapped off at the edge of the connector.

If that is the issue I would loop the wire with a "service loop" at the connector to take up vibration.

[Alatriste]

It happen when warm/hot. Oil is 20W50, 45,000 Miles

[mo-mon]

Hi Ernesto. I believe that they are not related. They are not moving in unison. And that oil pressure is too high for idle on a warm/hot engine. Your sender is on its way out. It has to be replaced. There are threads here that can guide you with the correct procedure. Be very careful changing the sender as it is attached to the case.

The hunting idle will require you to redo your idle adjustment sometime.

[Alatriste]

Thank you all for your input.
Happy Holidays!!!!!!!!!!!!
Ernesto

[KTL]

I agree with the guys that you should first check your wire connection at the sender.
Play with the connection by wiggling it or removing it and reinstall it. If the behavior of the gauge changes after you've fiddled with the wire connection? There's your answer. But don't overlook the wire itself, like Quicksilver mentions.

Regarding the sending unit itself, right before I sold my '87 Carrera in 2014, the oil pressure gauge stopped working with no indication of progressive decline in operation. One day it just stopped working. Of course that was a bit concerning but I knew I had oil pressure since the "OH @#$%" red warning light in the oil pressure/temperature combination gauge was not lit. So it had to be either the gauge itself or the sending unit. You can test the gauge operation by grounding it to ensure the needle pegs at maximum. Obviously you know your gauge works because it's displaying a pressure reading.

The oil pressure sending unit is a combination of mechanical and electrical signal. It converts mechanical movement to an electrical "signal" in order to avoid the use of a fully mechanical gauge, with an oil feed directly to the gauge, which is of course is hazardous if there's a leak. Reason I share the mechanical vs. electrical relationship is because I suspect your fluctuation is the degrading of the mechanical-to-electrical translation. The sender varies the electrical output and there is likely a "hiccup" in the electrical translation at that pressure range you're showing in your picture. Here's a simple video that generally explains how these typically work

https://www.youtube.com/watch?v=UZLiLRlJzbU

If you do choose to replace the sender, take John Walker's recommendation and absolutely remove the gauge block from the engine case.

Not a 5 minute job if the a/c is there as in most carreras. A/C unbolted and out of the way, muffler off to get at the bracket bolts, bracket removed. The sender is tight on the block it screws into and the block needs to be clamped in a vise in order to crack the sender loose.

With the block still screwed into the engine case, you don't want to lean on the sending unit with a lot of force. The case is not very strong here and there's a high chance you can break it. Don't risk it. John's advice isn't "ah you could possibly break the case" sort of talk. I'm sure he's done a number of sending unit replacements and the sender is pretty much always tight in the block. The guy flash951 in the thread link below learned the hard way that the case can indeed break (his was magnesium, so the chance was even higher) from the force you must apply to loosen the sending unit. Ignore the debate in the thread link over what is a proper fix.

Broke of piece of metal, how to fix?

[Alatriste]

Thank you Kevin!!!!!
Much appreciate it.

Ernesto