Fluctuating Oil Pressure at Idle

Hi guys, I did an oil change the other day with Pentrite HPR10 (10w 50) in my 88 944s. The following weekend I was driving around in traffic, and at idle my oil pressure and engine rpm fluctuate in unison. This was only happening when the engine got warm. It was not doing this prior to the oil change. Although, I am confused since this oil is to spec and quality. Any idea what could be going wrong? Thanks in advance

A lot of people use Penrite HPR30, What oil was in there before the oil change?

@944s2cab Can’t be sure. Was done by previous owner and they only recorded the compatible weights 10w 50 and 15w 40. I believe it was thicker than what I’ve got now.

Have you accidently disturbed a inlet manifold vacuum hose while doing the oil change to cause the uneven warm idle? The oil change itself won't effect hot idle quality.

Its the fluctuating idle speed that is making the engine oil pressure gauge to fluctuate in unison , meaning the oil pressure pump / sender unit & gauge are doing their job , if the idle was at a constant speed ( as it should be ) the engine oil pressure gauge would also be constant at idle Note } The main reason why you notice the oil pressure gauge fluctuating ( hot ) is the oil pressure gauge will be reading lower oil pressures because the oil pressure gauge only reads part of the actual oil pressure the engine makes , meaning the 944 , 944S  , 944S2 , 951 ,968 have an oil pressure relief valve that opens above 7 Bar , but the oil pressure gauge on the dash only reads up to a max of 5 Bar ,so when the oil is cold or warm only the oil pressure gauge is Maxed out  ,where as the earlier Porsche 931( 924 turbo ) and earlier 911 series had a more accurate  0 - 10 Bar oil pressure gauge & the oil pressure gauge was never maxed out and was always moving , meaning it never maxed out and hence always moving in unison with engine RPM hot or cold Remember , the engine oil pump is driven mechanically by the rotating crankshaft ( pump mounted at the front of the crankshaft ) and as you rev the engine the oil pressure increases , its basic physics What you have is a changing air fuel ratio at the ( engine core temp ) , meaning at a engine temperature higher that just driving around the block , so if the air fuel ratio changes when hot ( even a tiny amount ) the idle quality will fluctuate & if a tiny bit worse , then the engine will stall , everything has to be perfect for idle quality to be perfect at "all" engine temperatures This could be a lot of things , like } TPS idle contact ( ohms delivered to the ECU ) NCTII sensor , these can go out of spec at higher core temps in old age Quality of the old engine harness ( injection harness ) Quality of the idle motor ( engine idle stabilizer motor ) Quality of the engine earths Qualitty of the old air flow meter Quality of the O2 sensor Fuel pressure regulator & fuel pressure dampener All of the above must be perfect to obtain perfect idle at all engine temps and all ambient temps , particularly on a 944S

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Wow! There's $25 worth. What great resources we have at our disposal here, take that one to the bank.

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A huge thankyou for all of this information. I was always intrigued as to why the gauge was pegged at 5 bar on cold starts... Just about the only things on this list that I haven't checked / replaced are the TPS (the connection to DME I've tested at plug) pressure dampener and o2 sensor (which is definitely whats going next). I had the Adrian at Hartech Automotive advice that my AFM is 'lazy'. However, I've tested the resistance throughout the opening of it and it is to spec. I have also reindexed the wiper arm to a new part of the ceramic plate since there were white wear lines on it. Particularly around the idling area. This said, I'd love to swap mine out for one in good condition and take it for a drive. It could be the culprit of my off and on throttle shuddering but i'm not spending $1500+ to find out.

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No I dont believe so. The car cold starts better after the oil change.

@finnystew , how did you end up with this issue, did you dig any deeper into the AFM performance?

“All of the above must be perfect to obtain perfect idle at all engine temps and all ambient temps , particularly on a 9 44S” Thank you Bruce for another detailed answer, that I haven’t even asked - yet.