Low oil pressure at high temps

[sethrobot]

Hello, I’ve been working on bringing a 1975 911s back to life. It has been sitting for 20+ years and I’ve got it working pretty well but have an interesting oil pressure issue. After driving the car for 20 minutes or so and getting it up to operating temps (215f oil temps based on an IR temp gun) the oil pressure drops from around normal to about 0.5 - 1 bar when driving at around 3k rpm.

The temps don’t seem to spike but this seems way too low. Revving the motor doesn’t seem to make the oil pressure move much. Could this be due to blockage at the oil sump pickup that only comes into play once the oil is hot and less viscous? I’m a bit perplexed… any ideas or ways to troubleshoot this problem?

[GH85Carrera]

What oil are you using? 20-50 is standard for the air cooled 911s.

[HarryD]

What does the gauge on the dashboard say?

[Schulisco]

215 is not too high temperature, but the oil pressure at this temp is way too low!

Rule of thumb is 1bar oilpressure per 1000rpms up to 4,5-5bar at 5000rpm and above. These numbers are from the latter 3,0l / 930 / SC engines. I suspect your car has the older oil pressure gauge up to 10 bars, right?

Does the (at your car too) green oil pressure warning light comes up? Porsche manual says it may flicker when idling, but must not when car drives. If this happens and the gauge suddenly shows reasonable low pressures than before then stop immediately and check it out.

A common failure on these cars, esp. when sitting too long, is that either the oil pressure sender, the wiring or the gauge itself going bad. You can cross check this by loosening the sender cable at the sender and pulling it to ground when ignition is on (engine stands). Then the pressure gauge must show max. pressure. Also mostly the sensors get stuck or may leak internally and showing incorrect values. Cross check in any case your oil pressure with a mechanical oil pressure gauge! This is the only and most reliable way to check the pressure. All the gauges in the dashboard are not that reliable.
Does the car already has the hydraulic chain tensioners? A common upgrade on every aircooled 911 up to 1983. If yes, they also reduce the oil pressure in general a bit, but not that much as you report. If the pressure remains that low with the mech. pressure gauge at high rpm, then you might found the reason why it sat that long time...

How is the pressure when engine is cold?

[Schulisco]

Addendum: Porsche Workshop manual tells the pressure for the 1976 911S as follows:

[proporsche]

I would add little washer to you oil pressure spring on the right side of the case..Same time look what kind of piston you have there.There was an update back in 1977 ..
Here is some reading for you....

Ultimate Oil Pressure Relief Valve Thread

Ivan

[PeteKz]

Does the oil pressure indication drop suddenly when it warms up, or gradually? Like the others, I suspect a sender problem. But, you should test it with a mechanical gauge too.

[sethrobot]

Thanks for the ideas and apologies for the slow response

Originally Posted by GH85Carrera

What oil are you using?

--> I'm running 20w50

Originally Posted by HarryD

What does the gauge on the dashboard say?

--> Gauge reads just under 1 bar per 1k RPM when cold, after running for a while and getting up to temp it drops to between .5 and 1 bar when ar 3-4k rpm. Revving engine moves the dash oil pressure gauge slightly but not more than .25-.5 bar. It rarely gets above 1 bar when running at temp.

Originally Posted by Schulisco

Does the (at your car too) green oil pressure warning light comes up?

A common failure on these cars, esp. when sitting too long, is that either the oil pressure sender, the wiring or the gauge itself going bad. You can cross check this by loosening the sender cable at the sender and pulling it to ground when ignition is on (engine stands). Then the pressure gauge must show max. pressure. Also mostly the sensors get stuck or may leak internally and showing incorrect values. Cross check in any case your oil pressure with a mechanical oil pressure gauge! This is the only and most reliable way to check the pressure. All the gauges in the dashboard are not that reliable.
Does the car already has the hydraulic chain tensioners? A common upgrade on every aircooled 911 up to 1983. If yes, they also reduce the oil pressure in general a bit, but not that much as you report. If the pressure remains that low with the mech. pressure gauge at high rpm, then you might found the reason why it sat that long time...
How is the pressure when engine is cold?

--> I do have hydraulic chain tensioners. Pressure when cold is within normal range. Good call on testing the gauge and the sender, I picked up a manual gauge to swap out for the sender. I have one other issue that popped up (parking brake lever snapped) so I'm working on fixing that before I can take it out for a drive but will report back.

Originally Posted by proporsche

I would add little washer to you oil pressure spring on the right side of the case..Same time look what kind of piston you have there.There was an update back in 1977 ..
Ivan

--> Thanks for the tip Ivan. I'm going to verify the sender/dash are reporting correctly and if so, will try this as a fix.

There are two other possible oil system components I've been noodling on while trying to fix my other issue (broken parking brake lever)...

I've read that the oil drain plate orientation can cause issues with the sump. My drain plug is facing the rear, I've read it should be on the left/driver's side of the engine. I'll drop the plate and check for the pickup tube location but it seems odd to me that this would only be an issue that surfaces when the oil is hot.

It may be worth noting that I only have the cooler on the engine (no front wheel well oil cooler), I assume the thermostat for the engine mounted cooler opens at a certain temp, would there be a cause for oil pressure drop when that occurs?

Cheers,
Seth

[HarryD]

What year is your car. On my 1973, I see 60 psi at cold start. It falls as the car warms. Something is not right.

[Schulisco]

Don't dismantle the pressure sender without unscrewing the block/adaptor. It may break the engine case...

https://forums.pelicanparts.com/porsche-911-technical-forum/1012332-proper-method-safely-remove-oil-pressure-sender.html

[HarryD]

This is a diagram of the oil flow in your 911 up to 1989:



Note there are two circulation loops.

They are as follows when at operating temperatures:

Scavenge Side: From the engine sump, through the scavenge side of the pump (ignore the loop through the external cooler), through the filter and into the tank.

Pressure side: From the tank, into the pump, through the engine mounted cooler, to the cams, crankshaft, Carrera tensioners and into the engine sump.

You will notice two relief valves. One for high pressure and one safety valve. I suspect one is stuck open. Someone here should be able to post a picture of their locations.

[sethrobot]

Originally Posted by PeteKz

Does the oil pressure indication drop suddenly when it warms up, or gradually? Like the others, I suspect a sender problem. But, you should test it with a mechanical gauge too.

--> It seems to be fairly abrupt, pressure is slightly lower than spec when started and ran for the first ~20 minutes and then it drops under 1 bar and stays low regardless of wether I'm idling or driving.

Originally Posted by HarryD

What year is your car. On my 1973, I see 60 psi at cold start. It falls as the car warms. Something is not right.

--> It's a 75 car but it has a 74 case, S heads and an E cam with CIS, no heat exchangers.

Originally Posted by Schulisco

Don't dismantle the pressure sender without unscrewing the block/adaptor. It may break the engine case...

--> THANK YOU, Schulisco, you've saved me from wrenching it off, breaking the case, kicking things and then cursing a lot.

Adding a gratuitous photo. I'm hoping the mid-80's Mercedes color paint isn't the source of all my troubles...

[Schulisco]

Welcome. Relally nice car! I see wider fenders and deep fuchs wheels from the 930...great! In Germany it's not legal to go only with the front spoiler. On the autobahn the car gets too light in the rear on higher speeds above 110mls/h / 200km/h...so I recommend to add a rear spoiler too...

The behaviour of the pressure gauge sound to me that this is the issue. I recommend to purchase an original VDO/Porsche pressure sender. Others doesn't seem to be the quality as desired and they even showing lower values than real. But in general the gauges in the dashboard are not that precise anyway. Also crosscheck with mechanical gauge is mandatory.

VDO 29-25 bzw. VDO 360 081 029 025C, 0-5bar M18x1,5 10-184Ohm, single contact:
https://vdo-webshop.nl/en/pressure-senders/639-vdo-pressure-sender-0-5-bar-m18-4103590949523.html

When we have the real pressures from the mechanical gauge we can think about the next steps if necessary. There are some options to increase a too low the oil pressure on a 911 without overhaul - but this also depends on the engine condition. If it's too bad an overhaul is the only option.

Thomas

[PeteKz]

If the pressure indication drops abruptly and stays there, then almost certainly the sender is the problem. Nothing inside the engine will cause oil pressure to drop abruptly. The oil pressure relief valve and safety valve are spring-loaded and will not relieve pressure differently after warmup. Even the thermostat opens gradually (and it would not affect pressure anyway)

[HarryD]

Originally Posted by PeteKz

If the pressure indication drops abruptly and stays there, then almost certainly the sender is the problem. Nothing inside the engine will cause oil pressure to drop abruptly. The oil pressure relief valve and safety valve are spring-loaded and will not relieve pressure differently after warmup. Even the thermostat opens gradually (and it would not affect pressure anyway)

My concern is that at cold startup, the gauge does not show consistent high pressure. Per the OP "Gauge reads just under 1 bar per 1k RPM when cold". Mine shows 50-60 psi at cold start (2000 rpm) and gradually falls at the oil temp rises.

[PeteKz]

Agree. Mine behaves like yours. Let's see what he finds. Just for fun, I'll bet money he has a faulty sender, at least. Anyone game?

[sethrobot]

Originally Posted by Schulisco

I recommend to add a rear spoiler too...

--> Thanks Thomas for the link and the recommendation. You've got a good eye! I drove the car at 90mph without the front splitter on and it felt like it was going to take off - not recommended. Below is a photo from where I found it sitting for years, you can see it had a whale tail at some point but that was lost long before I started working on it. I'm thinking about a duck tail and some other '74/'75 period modifications to the body work once the mechanical is done.

I have most everything to build a 2nd motor with my son, 2.7 9.5:1 compression, E cams and likely EFI, but that project will take me many months to complete so I'm hoping to keep this motor alive until then.

Originally Posted by PeteKz

If the pressure indication drops abruptly and stays there, then almost certainly the sender is the problem. Nothing inside the engine will cause oil pressure to drop abruptly. The oil pressure relief valve and safety valve are spring-loaded and will not relieve pressure differently after warmup. Even the thermostat opens gradually (and it would not affect pressure anyway)

--> This is great info, Pete. I haven't taken one of these apart yet and I've been scratching my head about what mechanically could be causing the drop. I'm still going to take a look at the oil relief & safety valves to see if everything is moving smoothly and to see if I can tell if the oil bypass mod was done, and drop the oil drain plate to make sure the oil pickup isn't loose or blocked, and to get to know the motor a little better.

Originally Posted by HarryD

My concern is that at cold startup, the gauge does not show consistent high pressure. Per the OP "Gauge reads just under 1 bar per 1k RPM when cold". Mine shows 50-60 psi at cold start (2000 rpm) and gradually falls at the oil temp rises.

Originally Posted by PeteKz

Agree. Mine behaves like yours. Let's see what he finds. Just for fun, I'll bet money he has a faulty sender, at least. Anyone game?

--> I've never had my oil light come on while driving (only comes on briefly when first starting the car), so I'm optimistic this is a sender or gauge issue. I'll feel a lot less pressure when I'm seeing higher oil pressures. So I like that wager, I'm hoping that's it.

Thanks for all the info, I've got a plan and will report back on what I find for anyone else on here who runs into these symptoms - I'm awaiting shipment on a few random rivets that are needed to rebuild my parking brake lever so I can go drive it around with the mechanical gauge. One of the downsides of living in the hills is that without a parking brake I can't start or park the car without an extra set of hands to move wheel chucks around!

[Schulisco]

When you rebuild a 2.7 engine with magnesium case, always strengthen the threads of the head studs!! Either heli coils, case savers or similar. They you have worked out a main concern of these engine cases. Also the head studs material is worth a closer look. If using the dilavar head studs, take the latest version of them being used on the 993 too. They're powder coated to prevent their corrosion of hydrogen which let them broke again sooner or later. Opinions differ, others may add some info here, also have a look in the engine rebuild forum aside this forum. But all in all the dilavar seems to take load from the magnesium case that it will last longer.

Kurt from KlassiKATS.com recently made a nice video about oil pressure on 911 2.7:
https://www.youtube.com/watch?v=y7AArJzG4uw&list=PLkWFofpgz2OgF9FdliKnlzuY0pygqRkMj&index=13
The whole playlist about the 2.7 is worth a closer look for all CIS 911 owners!

Unfortunately he mounts the new oil pressure sender with the "dangerous method" without unscrewing the adaptor block...if the sender is not overthighten previously this may work for pro's like him, but not to recommend for amateurs like most of us...and he also recommends to use the idiots light sender switch to connect the mechanical pressure gauge instead of unscrewing the oil pressure sender...

Thomas

[sethrobot]

Originally Posted by Schulisco

When you rebuild a 2.7 engine...

--> Cheers Thomas, my case has inserts and has been line bored, with machine work done at a reputable shop, but I'll be the first to admit l have lots of learning and planning to do before I start that project (with a rebuild thread here on pelican).

I pulled the oil pressure sender today and heated up the union to the sender block before removing the sender and the sender started leaking, so it's likely the sender is compromised. I have an adapter for my manual oil pressure gauge arriving at the local parts store tomorrow and I'll be able to confirm if my pressures are within spec soon.

[sethrobot]

I installed a manual oil pressure gauge and confirmed the oil pressure drop at hot temps (200f).

Here are the readings as the oil warms up:
178f : 32psi at idle (900 rpm)
200f : 4-6psi at idle (900 rpm), 20psi at 2k rpm
212f : 0-3psi at idle (900 rpm), 10psi at 2k rpm

The oil pressure warning light does not come on.

Here are two videos of the 200f and 212f oil pressure gauge
https://www.youtube.com/shorts/eunL9ayZMGg
https://www.youtube.com/shorts/vad46IFko5Y

My gauge and sender were mismatched, (10 bar sender from '73 with a leak and was likely bad) and 5 bar gauge. I can correct that situation but the manual oil pressure gauge seems to indicate there is very low oil pressure when the engine is at 212f.

I do have some blow by, but the cylinders look pretty good (no significant bore scoring, can still see honing marks) but one cylinder looks like rings are on the way out since there is oil seeping through into the combustion area of the cylinder after sitting for a few weeks. I don't believe this would cause low oil pressure, but curious to hear what the experts think.

The only other three areas I can think of checking next based on the oil circulation diagram that Harry posted are the sump pickup/screen/plate interfering with oil pickup, the pressure relief valve being stuck open, or the oil cooler (new) somehow causing issues once the thermostat opens.

Again, I have no overheating issues, the temp maxes out around 220f even when running for a long time or driving hard (verified with a temperature gun at the temp gauge and at oil filter), which would indicate that oil is effectively moving through the system at a sufficient volume.

Is there something else I may be missing?