oil pressure mystery: new sender or new oil?

Quick summary : Peak oil pressure is now ~4bar instead of 4.5bar+ after installing new pressure sender and changing oil from Penzoil ultra platinum euro 5W-40 to DT-40 5W-40 and trying to figure out why before changing more parts. Car is 2004 C4S 6MT with 104k miles Details ; Previous oil pressure behavior : Car has run both Mobil 1 0W-40 and Penzoil ultra platinum euro 5W-40 in the past and oil pressure behavior was basically identical for both oils: cold engine showed 4.5bars at idle and 3k revs, while after warm-up, idle showed 1.5-2bars and peak was still 4.5 bars. Oil pressure peak after extended driving was normally 4.5bars, not 5, although occasionally I noticed the gauge at the max value of 5 bars. (I have seen other people on RL say their peak oil pressure is 5 bars at 3krpm+ but mine has been 4.5 bars for as long as I can recall and I've owned the car since 2005). Oil pressure sender has gotten flaky in the past 2yrs, twice showing a heart-stopping 0 bar reading while driving, along with the dash alarm, only to return to normal pressure a moment later. Perhaps the occasional 5bar readings could have been malfunctions also. So clearly time for a new pressure sender that would hopefully read 5 bars peak and not have the heart-stopping dropouts. For this year's oil change I changed 3 things : installed a new oil pressure sender and went with the LN engineering oil change kit that with the Napa Gold spin-on filter +adapter (in place of normal Porsche cartridge filter) and Driven DT-40 5W-40 oil . New oil pressure behavior : Cold engine is the same as before: 4.5 bars at idle and 3k revs, but after warm-up, now the peak oil pressure is now only ~4bars (or just slightly over 4 bars) at 3-4k rpm, while still 1.5bars at idle. Peak pressure behavior is same in both hot ambient temps (100degF) and cooler temps (65degF), while idle pressure is 1.5bars in hot temps and between 1.5-2bars in cooler temps (depending on how long I've been driving the car). The new warm peak pressure of 4bars is troubling. It's also of interest that the cold peak is still 4.5 bars when I expected to get 5 bar full scale reading. My first suspicion was that the new spin-on oil filter was the cause of the lower peak pressure so I installed a new WIX 57211 filter cartridge in the original cartrdige holder but oil pressure behavior remains unchanged. So not the filter. My next suspect is the oil pressure sender but cold peak and warm idle behavior is same as before, indicating same range of electrical response. Only hot peak is different, which I suppose could be the sender, but before I throw another new part at the problem I thought I'd check with you folks for ideas. Last suspect is the Driven DT-40 oil. Everyone here speaks highly of this oil. Has anyone else seen peak 4 bars pressure running this oil? I've got 104k on my engine, so it's no spring chicken, but peak pressure with both the Mobil 1 0W-40 and Penzoil 5W-40 was identical and higher. Thoughts?

First off check the oil pressure with a "real" gauge on the bank 1 head port provided by the factory...It should have 100+psi ( 7bar )cold, if not, you could have a weak/broken oil pump by-pass spring, it a 20min fix, easy peasy... If you have 100+psi cold using a "real" gauge, yet your dash gauge will not go past 4.5bar, then you either have a whacked dash gauge, or another bad sender...

Thanks for the advice! I found your post describing how to do this check and will tackle this in a future weekend!

Quotes source post: 18989566

+1 In addition, I would not be surprised if your engine is just fine with the readings you are getting. Most of the time, low oil pressure is a problem if it�s too low, and yours looks good if it�s 1-2bar at idle.

Quotes source post: 18989635

Agree! I just found in my tool box my old oil pressure gauge I used 30yrs ago to diagnose issues on my '75 240D. Now just to match the fitting and buy a new crush washer.

I had the exact same problem. Replaced failed oil pressure sending unit and it would not read above 4 Bar. Replaced it again and all is well..4.76 Bar at startup.

Quotes source post: 18991082

Thanks for letting me know! It does make sense that the new sender could be bad.

VDO quality is far from what it once was...

Quotes source post: 18991611

Yep....all their old engineers have retired/died...

Yea there are 60 year old bugs running around with original untouched VDO Speedometers that still work..

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Yep, the constant push for lower emissions, better economy, lighter weight, coupled with younger engineers and new technologies = some less durable products, but most of the time a higher performance/lighter product...

Quotes source post: 18993479

Tangent coming... Apologies. I'm not sure what younger engineers has to do with it. The only time I've seen young Engineers struggle is when old Engineers are threatened and defensive, or just too cantankerous to teach. Some old engineers rely on "experience" but don't have data and are actually mistaking "just always done it that way" for "the best way". Some of the best engineers I've worked with are less than 5 year out of school, and some of the worst have been coasting for 30 years. Of course the opposite is true too. I don't think age actually has anything to do with it. Engineers also design with the constraints they are given. So you are completely right about lower cost and higher performance driving shorter life.

Quotes source post: 18993502

No apology necessary, I appreciate your point of view and the manner in which you present it.. I was just giving my anecdotal point of view ( and venting a bit) having been tasked with fixing the problems on the cars for 50 years, problems from the engineering design to the manufacturing faults... But at about the 30 year mark,I started noticing that the problems/mistakes being made ( that had to be fixed/repaired/and or redesigned) were strikingly similar to the ones I had to fix 30 years prior....( albeit some involved a different material and/or technology) ...and in trying to make since of the de-je-vu, I assumed the 30 year cycle of these recurrences was due to a turnover of the engineering work force....it was the only thing to make since to me, and is totally anecdotal conjecture, but my thinking was that if the same people were doing the work, they would have known not to do it that way... you know the old saying, hindsight is 20-20 ..

Quotes source post: 18993624

Okay, we're in complete agreement actually. I'm at the point of starting to notice the repetition as well, even though I'm more in research. However, I'm still young enough to get defensive about old-guys ranting "kids these days". Too many times, the old-timers just say "We tried that; it doesn't work!", but can't offer any reasons or data to say why. Often, it didn't work for a very specific reason, but that there may be new solutions to that previous issue, so without offering the reason, it just shoots down good ideas that might have a chance. Hence my comments defending the young engineers. Cheers.

If history isn�t taught, studied & learned, we�re bound to repeat the same mistakes over and over again�This is strictly on an engineering level of course�😏

Ok, an update on the original topic. I installed an oil pressure gauge into the test port on bank 1 (as suggested by @Porschetech3 , thanks!) and got the following: cold start idle 800-1k rpm, dash 4.5 bar, aux gauge 95psi (not pegged at 100) warm idle 800 rpm, dash 1.75 bar, aux gauge 20psi warm 2k rpm, dash 3 bar, aux gauge 45psi warm 3k rpm, dash 4 bar, aux gauge 59psi warm 4k rpm, dash 4.1 bar, aux gauge 75psi warm 5k rpm, dash 4.2 bar, aux gauge 75 psi Observations: - cold max pressure is 95psi, not 100+. Could be my cheapo gauge but it definitely didn�t peg at 100pai - dash seems accurate up to 4 bar and then won�t go higher. So either bad sender or bad dash gauge. Suspect sender. - warm max pressure is 75psi reached at 4k rpm and never goes above this. My next step is replace the sender (again). Is 75psi absolute max oil pressure at 4k+ rpm normal or should I be chasing a relief valve issue? Does 95psi cold offer a hint here is that irrelevant? Regardless of all gauge readings the engine feels happy and normal. Inputs welcome!

Your dash readings seem oddly similar (to not say identical) to mine, after replacing the sender last year as it was showing ghost readings with engine off. Given the rather low resolution of reading above 3k rpm anyway, I don't think it's worth the time and effort to replace the sender again

Quotes source post: 19052009

Here is some pressure data from a couple different engines at elevated track temps. If you dig around in the oil pressure thread you can probably find some data from when the engines aren't so toasty that would be more representative of street driving.

Looking at your "actual pressures" using a 5w-40 oil your engine is fine and the pressures are above the Factory min specs...so nothing "serious" to worry about..... But the discrepancies from the dash gauge to the actual gauge are mostly in the upper pressures.. The actual cold pressure of 6.5 Bar ( 95 psi) when the dash is showing 4.5 Bar, and won't go any higher...( this is probably in the dash unit) .. The 75 psi max at warm temp is fine and shows the oil pump by-pass is working fine( and should be pegging the dash gauge at 5 Bar if the dash gauge was working correctly)....also Remember that the oil pressure at the oil filter and main/rod bearings will be 95psi... 20 psi higher...than what is shown on the Porsche recommended test port..

Thanks, @zbomb for sending along @golock911 's post with great data and for @Porschetech3 for the additional data and comments. It's reassuring that the low reading of ~20psi and high reading 75psi I'm seeing on the test port are normal. And it was fun to measure learn about the test port! To get my old cheapo gauge to work, I had to buy an adapter set at amazon that fits the M14x1.5 test port and the 1/8-27 thread on the gauge. FTR: Equus 9848 Oil Pressure and Electronic Temperature Metric Adapter Kit Equus 9848 Oil Pressure and Electronic Temperature Metric Adapter Kit I've owned the car for 18 years and I am sure in the beginningn the gauge used to peg at startup and at 4kprm-ish, so I'm optimistic it's the sender and not the dash, but who knows. I'll see if Porsche Marin will give me a warranty exchange on the sender and take one last stab at it. This forum is great, thanks for the help!

Update: I installed a new oil pressure sender and voila! the dash guage readings all now match the auxiliary gauge installed into the test port in the opposite cylinder head. Fun: it took me less than 15min to install the oil pressure sender. I was able to reach it with a 19mm crowsfoot wrench on a 6" extension and 3/8" ratchet. Loosened easily and was able turn out by hand reaching my left arm in there. Some days things just go well and I'm taking the win this time! Savings and Service: I reached out to Porsche Marin for help with the faulty oil pressure sender. Even though I bought the unit almost 1.5yrs ago in June'22, Sherman covered the part under warranty for me and shipped me a new part quickly. Great service!