transient low oil pressure

[Banjobolt]

I'm not sure, but this may be my first post; I've been lurking around for abour 1.5 years. I've just gotten serious about the rehab of my basketcase that I picked up 1.5 years ago. This is my first porsche project after several previous cars. Anyway, did a brief search of old posts and found nothing like this described; I'd appreciate anyone's suggestions.

I've gotten my 1969 911T running (which it did not do upon purchase). At last check compressions were uniform and decent although she smokes pretty well (so I figure a rebuild or at least a top end is in my near future). A tuned up ignition system and rebuilding the carbs has finally resulted in sufficiently even running (once it's warm) that I could attempt properly setting the dwell & timing. As y'all know, while setting the timing you rev to 6K and shoot for 35deg BTDC. Well, two things occurred:

1. RPMs hung at 5K per the tach: part of my ignition tune up included the rev limiting rotor: could this be an inaccurate tach or an innappropriately early limiting rotor? I don't want to toss the old rotor back on and rev to what I think is 6K when it's really 8...

2. After the rather loud experience of setting the timing (as best I could) at 5000rpm, I noticed the oil pressure dropped to ~30 where it had been at 60 or higher. Even once it was at idle the pressure remained low. I hurriedly put a couple of quarts in, only to realize that there was plenty of oil already. I shut it down, let it cool, and then restarted it. Hey presto, normal oil pressure.

If this weren't the oil system, I would think vapor lock (suggesting a heat issue - did I mention my oil temp sender is on the fritz?) but I've never heard of that in an oil system.

I'd love to solve all the minor issues before I take it out of the car.

[Banjobolt]

Addendum: I failed to mention this happened the last time I tried to set the timing as well: when I first realized the carbs needed rebuilding. At that time it didn't run well enough to be sure what I was seeing was real. Now with roughly set carbs and smoother running it looks real. All the oil is new as is the filter.

[Jim Sims]

Oil pressure varies with temperature and oil viscosity; engine should be warm when assessing oil pressure (approximately 10 to 15 psi per 1000 rpm). What oil viscosity are you running?

[ischmitz]

Some comments to the rev-limiter:

1. I would get a new cap and a new rotor anyways as part of the tune-up work. It ensures good and stable sparks.

2. If you stab the throttle the tach needle can not quite track the fast change of RPMs. It trails and tends to overshoot. This happens especially with older gauges where the damping oil has dried up. If you raise the RPM slowly and hold them steady the tachs usually are pretty accurate. If you experienced cutting out at 5K that might be due to a weakened spring in the rev-limiting rotor.

If you want to be absolutely sure you can always get a cheapo aftermarket tach at PepBoys or a Digital Multimeter with tachometer function.

Have fun,
Ingo

[Eagledriver]

One possible explaination would be that when you rev the motor and the oil pressure relief valve opens maybe it is sticking open after that. It sounds like this car has been sitting alot and not driven much. I'd put in a new rotor and drive it for a couple of months. Change the oil and maybe the problem will go away. As far as setting the ignition just set the dwell or points gap and set the timing as specified at idle and don't worry about the 6000 rpm numbers for now.

One other thing. The oil pressure senders can go bad and act funny. The one on my car jumps around and reads different things on different days even with the engine off!

-Andy

[ischmitz]

Check the connectors at the pressure sender. When they are dirty or oxidized it causes "jumpy" readings. Once you clean that up and crimp a new spade connector on it will most likely be fine.

[Banjobolt]

10W-40, and pressure is generally fine when the engine is warm, plus the pressure appropriately goes up when it's revved.

[Banjobolt]

Ah, a few more responses, thanks!
Re: the rev limiter - brand new rotor, cap & leads. Yes it's been sitting. As for the tach, I think I'll pick up a cheapo tach (my tach/dwell meter only goes to 2500). The oil pressure gauge seems pretty steady, not jumpy. Like the idea on the pressure relief valve - I'll look into that.

My other thought, re: vapor lock, is that I hear the #6 cyl is notoriously a bit hotter, being next to the oil cooler. Perhaps, if I have a mouse nest or a bunch of leaves under there it could be block air flow & be excessively hot & cause vapor lock in the oil lines? Is this within the realm of possibility?

Re timing set: my haynes manual says the 69T is only set at 6K, what would the advance be at idle? 5 deg BTDC?

[2.7RACER]

The advance is going to be very close to maximum at 5000 rpm.
I wouldn't break a sweat over this.
The idea is to verify the fact that the disributor does advance the timing as the rpm's increase.
If you suspect leaves or other debris, I would remove the Alternator/fan assembly and look with a strong light.
Any debris and I would drop the engine and clean it out.
Fix the oil temp too. This is your only indication of engine temp.
You have the makings for an engine failure if the engine or oil cooler are blocked. Especially without oil temp.

[Banjobolt]

"Fix the oil temp too. This is your only indication of engine temp.
You have the makings for an engine failure if the engine or oil cooler are blocked. Especially without oil temp."

Yep - have the temp sender and it's the next thing to go in.

[Banjobolt]

For all who assisted me with my April post, thanks! (especially Eagledriver).

Replacing the pressure relief and safety oil valve springs solved the oil pressure problem. Incidentally, the side effect was that engine runs FAR more smoothly than before suggesting that the system was relatively starved for pressure, regardless of what the pressure guage said.

Now to finish up tuning it and then to remove the engine from the vehicle and get serious about the frame rot. I'm certain y'all will hear from me occasionally.

Any suggestions on

1. A decent review from someone who's done it on cutting out and then welding in new rockers? I've seen an online narrative done on a 912.

2. source for a quality front pan (since they're unavailable right now from Pelican)?

BB

[rvanderpyl]

Try www.restoration-design.com for a floor pan

[ChrisBennet]

Originally posted by Banjobolt
My other thought, re: vapor lock, is that I hear the #6 cyl is notoriously a bit hotter, being next to the oil cooler. Perhaps, if I have a mouse nest or a bunch of leaves under there it could be block air flow & be excessively hot & cause vapor lock in the oil lines? Is this within the realm of possibility?

I doubt it has anything to do with "vapor lock" but it is quite possible that you have a mouse nest in your motor. As a fellow New Englander, I'd say that you have a 25% chance of having a mouse nest in any 911 motor that has sat a while. It can easily happen just sitting over the winter.

Definately clean the top of the motor out. Otherwise it's a bit like not having (or checking for) coolant in a water cooled motor.
-Chris

[Banjobolt]

Checked the top of the cylinders, no mice. Some were living in the carpet when I first bought it. Subsequently evicted.
Appreciate the link for the sheet metal.

[ChrisBennet]

Originally posted by Banjobolt
Checked the top of the cylinders, no mice.

Hey, that's just what my friend said about his motor. I told him to humor me and take off the shroud. We found about 1 cup of acorn shells on top of the oil coated oil cooler alone...
-Chris