Oil pressure

[Baurice]

Hi - an oil pressure question for you!

I have a 1974 2.7 Carrera with mechanical fuel injection. When it is warmed up, the oil pressure gauge shows a maximum of 3.5bar rather than 4.5bar @ 5500rpm as per the manual! Otherwise the car is fine - a few minor oil leaks, minimal oil usage, good leak down test, and correct oil pressure sender unit.

Should I panic? I’ve been told that it must be caused by worn conrod bearings and it needs a rebuild very soon, and, not to rev the engine too much!! That’s proving very difficult. So, how soon should I get the engine rebuilt?

BTW I noticed that the oil cover plate which has the oil sender unit in, is 180 degrees the wrong way round. Would this make a difference to the reading?

Many thanks in advance for your help and advice

Andrew Sharples
NZ

[fred cook]

Andrew,

Have you tried cleaning the connection on the oil pressure sender or replacing the sender with a new one? I recently replaced the pressure sending unit on my car and the max pressure reading went up about 1/2 bar! By the way, if the problem is a bad rod bearing, you will probably see a "flicker" in the oil pressure reading as the engine is idling. This is caused by the rotation of the crankshaft and the relative position of the connecting rod with a bad or worn bearing. If the problem is a worn main bearing, the oil pressure will simply be low. The other problem could be a bad oil pressure release valve spring. Over a period of time, the spring can loose some tension due to exposure to hot oil.

Good Luck!

Fred Cook
'80 911SC coupe

[RoninLB]

Originally posted by Baurice
Hi - an oil pressure question for you!

what oil w are you running ?

[IROC]

Like Fred said, I would check out the sending unit before you get real concerned. I freaked out a few years ago about a "low oil pressure" problem when what I really had was an "oil pressure sending unit" problem.

BTW, you're oil pressure sending unit should be just to the right of the fan and just above the timing chain cover.

Mike

[Superman]

Our oil pressure gauge and sending unit systems are not particularly reliable. the way to get an accurate reading is to replace the sender with an accurate, gauge-type unit that is not electrical. In this mode, you can only check pressure buy opening the hood and reading the dial, but it would settle the question of what is your actual oil pressure.

When an engine starts to have a falling oil pressure problem at higher rpm's, then this does mean the bottom end is wearing out (rod bearings and possibly mains). Particularly if the pressure actually FALLS as you reach the higher rpm's.

But like others have said, before making conclusions or decisions, you should verify with certainty what your OP really is.

[Baurice]

thanks for all your replies.

I'll hunt out the sender unit and see id it need replacing

Ronin - the oil is Mobil XHP 15W40. Is that OK?

Andrew

[RoninLB]

Originally posted by Baurice
Ronin - the oil is Mobil XHP 15W40. Is that OK?

sounds ok.. there is a noticable difference in oil psi among different flavors, so I was curious what you were using. I'd probably try some 15w50 just to see what happens after.
Superman's thread gives hard info. Fred Cook's thread should be followed also.
I don't think anyone likes to see oil psi below 10# per 1000rpm

[Baurice]

Hi again
I've not been able to check the sender yet, but.......
looking underneath the car (again!) the sump plate is the wrong way round and according to Wayne's 101 projects book this could mean that the drain plug is under the oil pickup hole. Could this be the cause of the low oil pressure - or am I clutching at straws??
Thanks again

Andrew

[Westy]

Andrew, I don't know about the plate, but isn't it usually fairly warm down there? I agree with others. I put some 20-50 castrol in all my cars, and the oil pressure went up in every one. 10 viscocity can make a world of difference.

Brian

[johnsjmc]

The position of the sump plate plug can obstruct the oil pick up tube and might cause lower high speed oil pressure. When was it moved? Can you remember a sudden drop in pressure? Worn bearings in my experience will lower idle speed pressure first as the slower pump speed can,t keep up with the bearing demands.I would verify the guage with a mechanical guage. The 911 engine can run a long time with the pressure you have indicated but I would investigate a sudden drop more closely.A change to 20w50 oil and 964 cam tower oil fittings will restore about 1.0 bar pressure. What mileage are we talking , engine temp and is there a front cooler?

[Baurice]

Sump plate always been like this since I had it. likely to have been like this since engine rebiult 50k ago!
Oil pressure gauge moves "normally" with no flickering or drop at higher rpm. It just seems to be lower than expected. Engiine fully rebuilt 50ks ago, engine temp normal, and the car has a front cooler.

Suggestions for a mechanical gauge setup?

I'll replace the oil pressure switch first, then the sump plate if not fixed. Then change the oil - and rebuild of this doesn't work!!!!!

Thanks for your advice
Andrew