Low oil pressure @ idle - fresh rebuilt 3.6 - GREAT pressure @ all other rpms -ideas?

[DW SD]

Gents-
Mysterious situation is going on with my newly rebuilt 3.6 and the idle oil pressure. Ollie's did the machine work and they seem stymied, too, as is the builder. This rebuild has about 300 miles, now. I'm running 20W-50 Castrol dyno oil and have changed it along with the filter after the cam break in and after a dyno session.

Here's some information about the pressure and data I've collected:

I have great oil pressure from off idle throughout the rev range. The pressure is at least 1 bar per 1k rpm. @ 3k rpm the pressure reads 3.2 to 3.3 bar and oil temp is verified ~ 215 degrees F. (I used a very accurate digital laboratory thermometer to calibrate the factory temp gauge).

Here's the issue:
The warm idle pressure is 7 to 7.5 psi, where both of my idiot light senders turn on below 12 psi. I checked the sender with a regulator to determine at what point the resistance went to zero in the idiot light sender.

We've taken a number of steps to verify information and remedy the issue:

1. replace the 993 pressure sender (it is both the idiot light and pressure sender.)

Result: No change at the gauge or the idiot light.

2. connect a large-face (4") and very accurate mechanical gauge to verify the factory oil pressure sender / gauge.

Result: The mechanical and factory dash gauges were in synch and calibrated.

3. we removed the oil pressure relief piston assemblies and took a close look. A burr was found in the bore which was causing scratches at the piston radius along the vertical axis. The burr was carefully removed with emery cloth.

Result: There was a small noticeable difference in that the pressure seemed to build up smoother as the engine reved (the gauge was less jumpy). I think the piston was sticking a bit. But the problem with low pressure at idle remained.

4. Finally, this morning, we changed all of the springs, both pistons and the shim in the pressure relief assembly.

Result: no change. pressure is great everywhere, but idle.

During the rebuild process, Ollie's polished and cleaned the crank and R&R'd the plugs. They also cleaned the case and R&R'ed all plugs and shuffle-pinned the case because they believed that the case was "walking" a bit. They also checked the align bore and cleaned the piston squirters. During the rebuild, the oil pump was inspected and cleaned and determined in excellent condition.

PS - When I drive around, I keep the idle up just a notch with my foot. The light extinguishes at 1200 rpm.

Here's my guess: maybe a pinched o-ring between the case and oil pump? OR maybe the main bearings are the wrong size?

The builder is more than willing to disassemble and troubleshoot. I'm just trying to get any other viewpoints / ideas before the motor is dropped and stripped again.

Any other thoughts? This engine runs great and dyno'd very well, too.

Thank you in advance!

Doug

[Eagledriver]

Sounds perfectly normal to me. I see about 10 PSI at idle hot and every other 911 I've seen is about the same.

-Andy

[DW SD]

Originally posted by Eagledriver
Sounds perfectly normal to me. I see about 10 PSI at idle hot and every other 911 I've seen is about the same.

-Andy

Andy,
Does your idiot light at normal operating temperature at idle? If mine flickered occassionally, especially when extremely hot, I'd agree it is normal.

My light comes on and stays on at normal temp ~ 200 to 215 and idle speed.

Ollie's and my builder agree it is not normal.

I did find the factory's oil pressure specs for 964:
2.0 to 2.5 Bars @ idle. 5.5 bars at 2500 rpm. My engine is far from that amount.

Thank you for your consideration,

Doug

[ischmitz]

Interesting discussion. I believe it's got to be some sort of pretty major leak in the pressure circuit that's bleeding off the oil during idle. You do need to tear into the motor again and find the root cause. If your oil flow is not sufficient you will damage lobes, cams etc. during idle. Did you check the bearing clearances during assembly? Maybe the line bore required oversized mains. I also like the assumption of a bad/pinched O-ring on the oil pump to case. How about plugs of the crank oil galleys. Can they come loose and fall out?

BTW: I once tried to battle the opposite issue with my 3.6: The oil pressure is great but will never go higher than 4.5 bar on the 964 gauge unless the car engine is stone cold. It gets up there but pegs sligtly under 5 bar. I changed the two springs in the oil pressure reliev valve and the saftey valve believing this was my issue. But it still acts the same. Of course I have not used calibrated pressure reading equipment to verify whether my gauge and sender are reading accurately. However, my idle oil pressure never makes the idiot light so much a even flicker. Not even when it gets really hot under the hood.

Keep us posted on what you find.
Ingo

[DW SD]

Originally posted by ischmitz
Maybe the line bore required oversized mains. I also like the assumption of a bad/pinched O-ring on the oil pump to case. How about plugs of the crank oil galleys. Can they come loose and fall out?

Keep us posted on what you find.
Ingo

Ingo-
Thank you for your comments!

The plugs in the case and crank were removed and replaced. My builder suggested if any came out, there would be ZERO pressure at all rpms.

I'd have to defer to the builder to see if he checked bearing clearances. I don't know. I'm very comfortable that he follows standard practices.

I also wondered if the there was maybe a problem at the cam oil lines / reducers - I don't know how those work - in the later engines. Maybe they are letting too much oil through (like the old, early style cam oil lines)?

Will keep you posted.

thanks for your interest and assistance,

Doug

[Grady Clay]

Doug,

“The plugs in the case and crank were removed and replaced. My builder suggested if any came out, there would be ZERO pressure at all rpms.”

I don’t think so. The crank is fed from #1 and #8 main bearings. The oil system pressure is measured in the main oil galley in the case. If a plug came out of the crankshaft, particularly in the middle, it would only affect the main oil pressure at idle. Once the engine is at revs, the extra loss of oil is compensated for by the pressure regulating relief valve not opening as much.

The dangerous part is the nearby rod bearings aren’t seeing as much pressure as they should or consequently passing as much oil. There is some oil but not the normal quantity or pressure (the bearings themselves operate at very low pressure, the edges being zero, but flow quite a bit of oil.)

If this is the situation a bearing haven’t failed – yet.

I understand that if a piston squirter falls out, the symptoms are the same. I just got off the phone with a friend who had that happen about six weeks ago. No indication of a problem on track. When he came off track (200F Mobil 1) the standard Porsche oil light flickered and came on full at about 1000 rpm. He revved it up and the light was full off by 2000-2500 rpm. Not his normal situation so he shut it off.

Upon disassembly a piston squirter was in the bottom of the engine. I saw the rod bearings. There was some discoloration on the crank from too hot oil film (probably from the low pressure and flow) but no damage. He had the crank polished and inspected, and replaced all the bearings. Needless to say the piston squirters are firmly in place.

This clearly indicates to me that even a missing oil squirter doesn’t divert enough oil to have the pressure be zero and will even turn off the oil light at modest revs.



While I’m not a fan of the restrictor fittings in the cam oil circuit, would this engine have come with them? If so, did they get changed out and replaced with the full-flow earlier version?

I have both center oiling and spray bar oiling for the cams in my race engines.

Best,
Grady

[DW SD]

Grady,
I SURE appeciate your consideration and explanation....thank you!

The cam oil circuit was backdated. The factory 964 style cam bridges were used. They were installed new with the rebuild.

Would either the crank oil plugs / piston squirters come out of the crank case with an oil change? I have changed the oil several times since initial fire - up and have not noticed anything unusual. I know I would have seen those parts.

I'll await your further consideration for proposed next steps.

Thanks again,

Doug

[Grady Clay]

Doug,

“Would either the crank oil plugs / piston squirters come out of the crank case with an oil change?”

I suppose that depends on how your pump screen is situated. Does anything have to go through the screen to exit the drain or not. Even if it can come out, doesn’t mean it has. It very well can be setting on the bottom of the case.

Even with these symptoms and feedback, there is some chance you won’t find anything definitive. Discuss this with all the parties. Have a plan.


“In your estimation should the motor come out immediately and be disassembled looking for various issues you pointed out - is this your advice?”

Given these symptoms, yes. Even if there were only a 5% chance this might be the problem, I would still investigate. The reason is the potential consequences are so severe. It might be that everything works great until some event and a bearing fails, a rod breaks, the case, crank, some P&Cs and more are destroyed. Not a desirable possibility.

The cost to find out is minimal in comparison. Even if you don’t find a problem, the peace of mind is worth the effort.


This illustrates the need for some definitive oil pressure vs. rpm data at different temperatures, types of oil, engine configurations and degrees of wear. Using your calibrated gauges, make a graph of your pressure vs. rpm and note the conditions. It will be instructive to see the difference.

Best,
Grady

[DW SD]

Grady,
More excellent advice! I will collect the data you have suggested.

Would you be willing to suggest a plan? I have to admit, I feel stupid for asking, but my limited experience is a downfall, here.

If you are willing, feel free to email me any suggestions off-line, if that is better in some way.

Thank you,

Doug

[DW SD]

After discussion with Grady, seems like somewhere the system is bleeding off pressure and that is only noticeable at idle conditions.

* Can a faulty cam oil restrictor drop the oil pressure only at idle (by letting full oil flow to the cams)? I don't know how these mechanisms work. I'm assuming it is just a restrictive port or "jet" to reduce flow, which probably doesn't break, but that's only my guess.

* Is there another location at which I may test oil pressure to narrow the focus of the pressure drop? (In other words, once this is taken apart, is there an area of the oil system on which to focus our attention)?

*Can this be an issue with the pump itself?

some more data:
* Oil pressure is 2.5 to 3 bar at 1200 rpm @ idle upon cold startup (ambient is about 70 degrees in the morning in San Diego, currently).

* Around 180 to 190 degrees, the pressure reduces to the point of turning on the idiot oil pressure light.

* At >2.5k rpm, the gauge is pegged at 5.5 bar, cold.

[Eagledriver]

I still think you have no problem. Try another idiot light switch. I thought the idiot light goes out at a very low pressure (I'm guessing 5 PSI). My car is a 72 and doesn't have an idiot light. My oil pressure at 210 degrees and idle of 900 RPM is around 10 PSI according to my guage.

In Bruce Andersons 911 performance handbook he mentions a minimum of 10 PSI per 1000 RPM. While this is lower than most cars seem to have I think that gives you some perspective that you don't have a problem.

I spun my car one day on the track and the throttle linkage popped off. When I restarted the engine I shut it back off because I thought I had zero oil pressure. The engine oil temp was 240 degrees and I couldn't rev it to check the pressure because of the linkage being popped off.

Drive another 911 and get it good and hot and see what you get.

-Andy

[Eagledriver]

I do see now that you have tried another switch. I'll also admit that I don't know much about 3.6 engines so the info I gave was about 3.2's and earlier.

-Andy

[MBruns]

I have seen the aluminum plug in the intermediate left out and that will create an internal leak and thats not all that hard to inspect.
Mike Bruns

[Grady Clay]

Mike,

Exactly where are you refering?

Best,
Grady

[Jeff Alton]

I have a 3.6 intermediate shaft in my hand right now and do not see a plug. Are we talking about the same thing here? I am confused. (fairly common though)

Cheers

[MBruns]

Below the crank pulley is an acess cover, behind that is the end of the lay shaft, in the end is an alum. plug and snap ring, it is for clean out of the layshaft, I have seen them left out.
Mike

[DW SD]

Originally posted by MBruns
Below the crank pulley is an acess cover, behind that is the end of the lay shaft, in the end is an alum. plug and snap ring, it is for clean out of the layshaft, I have seen them left out.
Mike

Mike,
Is it accessible from the crank pulley end?

Thank you - sounds like another good idea to explore!

Doug

[Jeff Alton]

Ahhhh, yes, I see it now!

Cheers

[DW SD]

Jeff,
I'm curious, is it accessible without splitting the case?
Can you post a pic?
Thank you,

Doug

[MBruns]

Yes, you can remove the rear engine mount (the console that covers the pulley area) and just below the pulley you will see a round plate with 2 - 6 mm bolts that is sealed with an "o" ring, just remove it and you will see the end, and should see a plug about the size of a quarter held in place with a snap ring. you can do this with the eng. in the car, just support the eng. with a jack.
Mike Bruns