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Confusing Oil Pressure Problem


New member
29 Nov 2023
I'm after a little help with a sight problematic 996 4S. I bought this as a project as the engine was suffering with scored bore. It's the 3rd 996 engine I have rebuilt but this one has been extremely problematic.

The car is a 2005 C4S in Atlas grey, Tiptronic with a lot of factory options and just 98k.
The engine was stripped and the block was re-sleeved by a well known company in Bolton. The engine was re-assembled by myself with no issues but when it got to hot idle after the engine was re-installed, it indicated an oil pressure issue at 0.5 bar which tripped the lights. A mechanical gauge was fitted directly to bank 2 which showed the same reading. Some oil was removed and it had a distinct shimmer to it with flakes in the oil filter.

I nearly cried as it had only 30 mins of idle on it!

Engine was removed again and partially stripped. I found that bank 1 intake cam had picked up on the bearing seat and worn. It looks like like the oil way had become partially blocked with RTV during rebuild.
The engine was completely stripped down again to the crank, extensively cleans and rebuilt with a replacement cylinder head, refitted and it started up. Oil pressure is very good until its at hot idle and the pressure dropped back down to 0.5 bar, 6-7 psi and rises with revs. :cry:

I'm running through a list of areas that could be of concern but I could do with some help as its becoming quite frustrating.
The engine sounds great, even when at 0.5bar, no ticks, knocks or anything and no CEL (oil is clean)
During the rebuild, it had new main/big bearings, pistons, chains, guides, IMS, gaskets, AOS, oil cooler, oil pressure sensor, oil filter housing, oil pump relief valve was replaced with the upgraded parts and bolts. The oil pump shows no wear inside and the crank was within specification during re-build. Current oil is a quality 5W-40.

I have found that bank 1 chain tensioner is faulty but not effecting tension and no apparent oil pressure loss but I have a new one on order.
Does anyone have any ideas on this? - next on the list of areas to check/replace are the sump oil returns/seperator and change the oil pump (incase there is an issue that I can't see with the current one)

I feels like I'm just throwing parts at this now!
You are using a good quality filter(with the plastic reinforcement inside?), I've seen a couple of cases where cheap filters collapse causing low oil pressure problems. Doesn't totally fit your symptoms though.

This also sounds like it could be an oil pressure relief valve problem, seeing as you've fitted different parts can you refit the original parts, that's what I'd do anyway. I'd also want to be checking out the inside of the bore where the relief valve plunger fits and also the plunger itself for any scratches or signs of damage.

Years ago I fitted an uprated oil relief valve plunger to a mini engine which stuck open and killed the engine. The original was a very close fit in the bore, the uprated one had a small amount of clearance which turned out to be just enough for a tiny bit of swarf to get jammed in between holding it open!
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The engine was stripped and the block was re-sleeved by a well known company in Bolton.

I guess you mean Hartech, have you spoken to them already about the issues? They probably know more about m96 issues than most of us on the forums... (no disrespect intended to anyone!)
yes, the car currently has a genuine Porsche oil filter fitted which I removed a few days to check on, its in perfect condition. I'll try fitting the old oil pressure valve to see if that makes a difference and put a borescope up where the plunger/spring goes. I suppose now you mention it, it could be sticking if the plunger if its is even slightly damaged.

Ive spoke to them and they've been very helpful but they've recommended to drop the engine off with them for more diagnosis. I'll probably end up doing that if I can't find the fault. For clarity, they only re-sleeved it, I rebuilt it myself as part of a project!

I'll check the relief valve and come back with what I find. Thanks
If you change out the oil pressure relief valve for brand new parts you could use 997 parts which include a longer spring and a bevelled piston which prevents cavitation at low RPM.

The only problem is that you can't get the bevelled piston at the moment since it's made from Russian steel.

To be fair though, such a low hot oil pressure reading looks like pressure could also be being wasted by a tensioner or loose bearing surface. I would hope for up to 2 bar at hot idle with a fresh engine and 5w40.

The fact that the engine sounds nice though is encouraging. Hopefully it's only a minor issue.
I've already changed out the spring and piston for the 997 parts. It has been suggested and it did cross mind that there is an issue with the/a piston oil squirter being stuck open.

I have a new tensioner on order which will arrive in a few days. Does anyone know how much oil should drain out of bank 1 tensioner when its removed? - mind drained 2-2.5 litres which seems a lot.

here is a picture of the tensioner which shows no obvious oil leak when tested and holds tension. The plunger comes all the way out:
I changed out my oil pressure spring and piston last month for the updated 997 version, my oil pressure dropped below 1 bar when extremely hot, I won't know if it has cured it until the weather get warmer.

It does sound like a collapsed filter but again you have checked this. Could the oil pick up or the two oil separator's in the sump be clogged?

The tensioner, plunger should't come out at all, the only time that has happened to me was when my chain broke and damaged the plunger, when it gave it a whack, but this shouldn't cause the low oil pressure?

Sealant can be a killer if you use too much, did you definitely get it out from all the oil ways, I use it very sparingly when I do a rebuild?
Im planning to pull the sump this weekend and check the pickup and the 2 oil separators. I was surprised so much oil dropped from the tensioner hole when I removed it. is it possible too much oil is holding in the heads? - would this suggest a blocked oil separator or an issue with the scavenge pump for bank 1?

I was very careful with the sealant but if the tensioner makes no difference and the sump is all good, I'm going to pull the covers off and check if the oil ways are clear in the heads. I will also fit the mechanical gauge to bank 1 and see if there is any difference in reading.
Changed the tensioner, no obvious difference apart from the pressure seems to be up slightly by 0.1 bar - it needed replacing anyway.
Oil separators are clear and the sump is clean as well as the pickup.

I fitted a very accurate digital pressure gauge to bank 1. At start up, oil pressure was 7.2 bar and gradually dropped during warm up to 1.4bar. Once it hit 1.4, it stayed there for 5-6 mins and then started dropping to a steady 0.9bar.

Not sure what to do now. My company car arrives in 2 weeks, when it arrives, I can get the Porsche road legal (insured/tax) and give it a road test which may help point out any issues.
This is a real poser. You mentioned above that you felt that oil was pooling in the head, as evidenced by a lot of oil coming out when you removed the chain tensioner plunger. I'm not sure what normal looks like but 2.5l seems a lot. Most oil should be back in the sump in fairly short order after switching the engine off.

Could it be that as time progresses with the engine running, this problem exacerbates itself and the sump level eventually goes below a critical point - where it then starts to negatively affect the pump's ability to build pressure?

In other words, a form of starvation. Are the scavenge pumps working as expected on both heads? Another small blockage perhaps?
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Not sure how to test the pump but they are on the correct heads and look ok. The bolts holding them together are corroded so I can't dismantle them.
I think I will order new scavenge pumps and new oil pump tomorrow. I get a very good discount from my local Porsche centre so its not a huge cost and even if it doesn't help, I will be safe in the knowledge that they are in perfect condition
Not sure how to test the pump but they are on the correct heads and look ok. The bolts holding them together are corroded so I can't dismantle them.
I think I will order new scavenge pumps and new oil pump tomorrow. I get a very good discount from my local Porsche centre so its not a huge cost and even if it doesn't help, I will be safe in the knowledge that they are in perfect condition
any updated tony?
i faced the same issue
Sorry for the lack of updates. The pumps arrived a few weeks ago and I've installed 2 of the 3, I'm doing the main oil pump later today.

I had to take the bank2 tensioner off in preparation to fit the main oil pump and I had to remove the inlet manifolds. I found a lot of oil in the inlet manifolds.
I w curious to know. But now we lost the reference because you are changing the main oil pump too

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