Sudden low oil pressure

[antijam]

After an unexpected snowfall yesterday we had brilliant sunshine this morning so I seized the opportunity to take the '71 for a short blast after weeks of inactivity. Car is running well and I'm enjoying the (very!) fresh air when a casual glance to check the gauges shows the oil pressure is around 5 psi. Although I hadn't checked it this time before setting off, on startup oil pressure normally rises - albeit very slowly - to around 25 psi where it pretty much stays constant regardless of engine speed. . I quickly curtail my trip and drive home.
Apart from the low pressure reading the engine sounds fine, it revs freely with plenty of power and no unusual noises. For some time there has been a hint of blue smoke from the exhaust on startup, particularly the drivers side bank but it disappears after warming up and I had decided that this was most probably due to worn valve stem seals, the replacement of which is a job on the list.
Now 5 psi is the pressure on startup. The fact that the gauge lifts off the pin suggests that the problem probably doesn't reside in the gauge, although I'll do an independent check with a test gauge, so I'm wondering what other possible causes there could be. Clogged filter seems unlikely since it been changed very recently. Clogged oil pickup could be a possibility i suppose but I'm wondering if anyone has experienced anything similar or has any ideas as to the likely cause?

 

[Chuffer]

Maybe the sender unit is faulty and is not expensive and is an easy fix . If you have or can borrow an independent test gauge that will prove one way or the other if it`s the sender or the gauge . I had the same trouble with my C3 when I first had it . Turned out be the sensor . 25psi at any engine speed doesn`t sound right to me . I thought 10psi per 1000 rpm was the general rule of thumb . Mine runs at about 40psi when warmed up at normal cruising speeds and climbs ,all be it a bit `notchy ` to 60 plus if you get up to 5000rpm . What oil do you use ?

 

[Roscobbc]

Presumably being an 'early' C3 the oil pressure gauge is capillery with a small metal or nylon tube.....that will rule out sender. Low oil pressure is generally indicative of minimal restriction of oil flow through the oil galleries and bearings......worn bearings, perhaps mains can be a culprit....although there should be other 'clues'.
I remember the Daimler 2 1/2 litre V8 I had years ago.....ultra low oil pressure (like about 7 psi max - although they were a low pressure engine as I recall) - it eventiually 'threw' a rod and cracked a main bearing shell............Opposite extreme on my BB stroker lump when first installed. Pressure on start-up was 'off the gauge' coming down to about 85 psi normal running and 50 psi minimum on idle.........turns out that big end tolerances on no. 1 and 2 big ends (and probably others) were far too 'tight' and the bearing shells on no. big-end broke up requiring a couple of new 'rods a and a crank grind! ........just mentioning extremes!

 

[teamzr1]

I assume you checked the oil level
Is this the sensor type that then runs a hose to the pressure gauge in the dash ?
If so, maybe the oil has gone stiff and needs to be cleaned out of the hose and sensor ?

If electric

The Oil Pressure Gauge Needle Should Point to:
Left Line (Low, 0 PSI) when sender resistance = ~0 Ω
Middle Line (30, 40, or 50 PSI) when sender resistance = 30 Ω
Right Line (High, 60, 80, or 100 PSI) when sender resistance = 60 Ω

 

[johng]

I'd want to check that the gauge is reading correctly first. If it's the capillary type, can you find a way to connect it to a known pressure, air compressor, tyre pump, etc?

 

[Chuffer]

Presumably being an 'early' C3 the oil pressure gauge is capillery with a small metal or nylon tube.....that will rule out sender. Low oil pressure is generally indicative of minimal restriction of oil flow through the oil galleries and bearings......worn bearings, perhaps mains can be a culprit....although there should be other 'clues'.
I remember the Daimler 2 1/2 litre V8 I had years ago.....ultra low oil pressure (like about 7 psi max - although they were a low pressure engine as I recall) - it eventiually 'threw' a rod and cracked a main bearing shell............Opposite extreme on my BB stroker lump when first installed. Pressure on start-up was 'off the gauge' coming down to about 85 psi normal running and 50 psi minimum on idle.........turns out that big end tolerances on no. 1 and 2 big ends (and probably others) were far too 'tight' and the bearing shells on no. big-end broke up requiring a couple of new 'rods a and a crank grind! ........just mentioning extremes!

Good point Ross , I wasn`t aware it as an early C3 with caterpillary tube !!!

 

[antijam]

Thanks for the input guys. The gauge is the mechanical type type as Rosco points out so it's not a sender problem. It could still be a problem with the gauge itself - there's something suspicious about its previous constant 25 psi - as Chuffer states a good rule of thumb is around 10 psi per 1000 rpm, and as teamZR suggests there is the possibility of the capillary tube becoming partially or completely blocked. As soon as I can find my oil pressure test kit I'll take the pressure at the block take-off point. If there's good pressure there then it's either the gauge or the capillary. If that's the problem I can remove the gauge and pressurise that with air as johng suggests to check its functionality.
Down in the bowels of the engine another possibility is that the pump pressure relief valve is stuck partially or now fully open. I'm obviously hoping that one or other of these possibilities is the cause. The suddenness of the pressure drop with no other indication of more catastrophic component failure seems incompatible with wear or bearing failure.
Still, one step at a time - I'm going to have plenty of time to diagnose the problem. After eight decades of wear and tear my knees have finally thrown in the towel and said 'no more!' I'm getting one replaced next week and if all goes well, the other later in the year. Since my 'Vette, and my other classics, are manual transmission I'm going to be unable to drive them for a few weeks. My Mustang and my daily driver are automatic so I should be back on the road in them rather sooner.
Any more suggestions are welcome and I'll keep you updated on what I find.

 

[antijam]

Finally managed to check the oil pressure at the block today. It was a bit of a fiddle since the gauge tapping in the casting is in a small casting well and wouldn't allow clearance for the large hex on my test adaptors, stopping the fittings from screwing in. Anyway after cobbling up a flex grease gun line which would screw in and adapting the other end to accept the test adaptor I was finally ready to take a reading. Fired her up and after a tense wait to see if pressure would rise, the test gauge finally lifted off....



...and stabilised at 2 bar at cold tickover. Phew! I'll live with that - at least I now know I have oil pressure and the problem resides either in the gauge or the capillary line. My instrument panel is still waiting to come out to replace my duff ammeter so I can now add the oil pressure gauge to the job list. Hopefully this is a job that my new knee won't prevent me tackling in the near future!

 

[Roscobbc]

Finally managed to check the oil pressure at the block today. It was a bit of a fiddle since the gauge tapping in the casting is in a small casting well and wouldn't allow clearance for the large hex on my test adaptors, stopping the fittings from screwing in. Anyway after cobbling up a flex grease gun line which would screw in and adapting the other end to accept the test adaptor I was finally ready to take a reading. Fired her up and after a tense wait to see if pressure would rise, the test gauge finally lifted off....

View attachment 25579

...and stabilised at 2 bar at cold tickover. Phew! I'll live with that - at least I now know I have oil pressure and the problem resides either in the gauge or the capillary line. My instrument panel is still waiting to come out to replace my duff ammeter so I can now add the oil pressure gauge to the job list. Hopefully this is a job that my new knee won't prevent me tackling in the near future!

Pleased for you Antijam - at least its not on deaths door - still perhaps a little lower than ideal when cold? - are you going to take it for a run and see what it is when hot?

 

[antijam]

Pleased for you Antijam - at least its not on deaths door - still perhaps a little lower than ideal when cold? - are you going to take it for a run and see what it is when hot?

Only after I've repaired or replaced the gauge Rosco. I don't think my engine is in the first flush of youth any more and to be honest as long as there's pressure commensurate with age I'll be happy with that. If the engine needs a rebuild it won't be getting one from me - I've too much to do and not much time in which to do it.

 

[Roscobbc]

Only after I've repaired or replaced the gauge Rosco. I don't think my engine is in the first flush of youth any more and to be honest as long as there's pressure commensurate with age I'll be happy with that. If the engine needs a rebuild it won't be getting one from me - I've too much to do and not much time in which to do it.

Understand where you are coming from Antijam!

 

[antijam]

Finally managed to check the oil pressure at the block today. It was a bit of a fiddle since the gauge tapping in the casting is in a small casting well and wouldn't allow clearance for the large hex on my test adaptors, stopping the fittings from screwing in. Anyway after cobbling up a flex grease gun line which would screw in and adapting the other end to accept the test adaptor I was finally ready to take a reading. Fired her up and after a tense wait to see if pressure would rise, the test gauge finally lifted off....

View attachment 25579

...and stabilised at 2 bar at cold tickover. Phew! I'll live with that - at least I now know I have oil pressure and the problem resides either in the gauge or the capillary line. My instrument panel is still waiting to come out to replace my duff ammeter so I can now add the oil pressure gauge to the job list. Hopefully this is a job that my new knee won't prevent me tackling in the near future!

Got a new oil pressure gauge, oil line and end fittings ready to install, but before stripping out the centre gauge console (yet again!) I thought it would be wise to check that everything worked and that there were no leaks. So, disconnected the old gauge at the engine block, removed the old fitting and installed the new gauge and fittings at the engine bay. Fired up and the gauge pressure rose satisfyingly, stabilising at around 40 psi as the auto choke came off.



I'm happy with that and everything seemed tight with no leaks, so all I've got to do now is summon up the energy to strip down the console and fit the new gauge, together with a replacement ammeter. My new knee is rather hampering my flexibility in the cockpit at the moment but hopefully it - and the weather - will improve enough to get the job underway soon.

 

[Chuffer]

Got a new oil pressure gauge, oil line and end fittings ready to install, but before stripping out the centre gauge console (yet again!) I thought it would be wise to check that everything worked and that there were no leaks. So, disconnected the old gauge at the engine block, removed the old fitting and installed the new gauge and fittings at the engine bay. Fired up and the gauge pressure rose satisfyingly, stabilising at around 40 psi as the auto choke came off.

View attachment 25792

I'm happy with that and everything seemed tight with no leaks, so all I've got to do now is summon up the energy to strip down the console and fit the new gauge, together with a replacement ammeter. My new knee is rather hampering my flexibility in the cockpit at the moment but hopefully it - and the weather - will improve enough to get the job underway soon.

Good to hear you seem to have overcome your oil pressure issues and are champing at the bit to get it all finished . You will soon be leaping about on your new knee joint . I was driving a 166 ton steam locomotive just six weeks after having my right knee done and that was 6 months after having my left one done !!! I wasn`t even driving my car at that point .

 

[antijam]

Good to hear you seem to have overcome your oil pressure issues and are champing at the bit to get it all finished . You will soon be leaping about on your new knee joint . I was driving a 166 ton steam locomotive just six weeks after having my right knee done and that was 6 months after having my left one done !!! I wasn`t even driving my car at that point .

Thanks for the encouragement Chuffer. My new left knee is now three weeks old and already feels better than the original. Being the left meant I could get back in my automatics pretty much straight away - the manual 'Vette will probably be a couple more weeks. Like you I need the other doing too - hopefully later this year, but although less painful than the left, it's considerably more distorted - to the extent that it has to be scanned and a custom prosthesis made in the States - and I'm guessing that's not going to be cheap! Ho hum - it's only money......

 

[Chuffer]

Thanks for the encouragement Chuffer. My new left knee is now three weeks old and already feels better than the original. Being the left meant I could get back in my automatics pretty much straight away - the manual 'Vette will probably be a couple more weeks. Like you I need the other doing too - hopefully later this year, but although less painful than the left, it's considerably more distorted - to the extent that it has to be scanned and a custom prosthesis made in the States - and I'm guessing that's not going to be cheap! Ho hum - it's only money......

A fully custom built knee , now that must be good for bragging rights !! Will it have a custom paint job too ?
Both of mine were done in 2009 so will probably be worn out soon .

 

[antijam]

A fully custom built knee , now that must be good for bragging rights !! Will it have a custom paint job too ?

I'm thinking of having it lit up with 'Knee-on' lights......

 

[Emc]

I am looking for a capillary oil gauge to do a video of the problems they have