Disappearing Oil Pressure!

[alanp]

Ok, have a think about this.
Engine - SRM build with SRM oil pump, Wiseco pistons, 3000miles, oil pressure gauge, return oil filter, oil cooler, 20W50 Silkolene Comp 4 oil, runs well.
Problem - A week or so back I had just got about a 1/4 mile from home when I noticed that the oil pressure, which is normally about 65/70psi cold, was falling rapidly to the 'oh my god' level, i.e. maybe 5/10psi. If I let it tick over it comes back up again which led me to think that the pump was being starved of oil at its inlet. Normally, when fully hot during a high speed run , it would run at 25/30psi and hold 65/70psi for maybe 10 miles or so depending on how fast I was going and ambient temp.
Action so far -
Oil level in tank - ok
There is no valve in the oil line to have left closed (before someone suggests I open the valve)
Drained oil from tank
Checked the gauze filter in the tank - ok
Removed the oil line to the engine and confirmed it is clear and not blocked.
Blew through the fixed oil line inside the tank - ok
Removed the timing side cover
Removed the SRM pump
Dismantled the SRM pump and all inside is in mint condition.
Checked oil pump drive - ok
Blew through the oilway from the outside of the engine casing to the pump inlet port - ok
Passed a wire into the inlet oil way until I could see its end at the oil pump inlet port - ok
Removed and dismantled the SRM PRV - ok
Reassembled it all and test rode it again - same result !!!!
Fitted a standard PRV in place of the SRM PRV and same result only 50psi with this PRV.
Refitted the SRM PRV.
Any suggestions gentlemen?  

[bsa- bill]

Pretty comprehensive test of everything Alan - bar the pressure gauge! - just a thought

Wayhay

bar the pressure gauge!

just reread that, there's a pun in there, unintended

[kiwipom]

hi guys,sounds like a faulty pressure gauge to me Alan,cheers, bob

[A10Boy]

Al
A few questions.

You say SRM build, is that with an end feed conversion or std bush? Did SRM assemble the engine, I'm assuming yes.

Have you done 3000 miles since the build without problem?

Where in the system is the oil cooler?

Are all the pipes new? I've seen pipes colapse internally when hot and close up.

Did you do anything at all to the bike prior to noticing this, like change oil?

Have you tried disconnecting the filter and cooler so you just have a basic oil supply to the engine? [I think I would try that]

Cheers

[alanp]

I owe you chaps an update.
Yes, a10boy, it's fully SRM and had been going well and the filter and cooler are on the return side of the system.
After the pretty comprehensive check of pipes, pump and oilways and refitting the SRM PRV I have had to wait for a dry'ish spell to give it a run. I managed a short run of a few miles and the oil pressure seems stable. If (still an 'if') it is now ok I'm baffled. Before this run I took the plugs out and kicked it over until I saw around 25psi on the gauge before starting it up to make sure that I had primed the oil system since the pressure tapping is after the pump. But, then again, the gauge showed normal pressure before it dropped on the first occurence run so I had assumed it was primed.
The problem of course started after I had changed the pipe from the oil tank to the engine and, yes, you need to get oil primed thru' to the PRV before starting after this but I didn't. However, don't hang me yet... that doesn't explain how, after the pressure had dropped on the run, that it came back up again at tickover when I stopped and dropped again right after I restarted. It all seemed like oil starvation to the pump to me which is why I concentrated on the oil feed to the pump.
So, did I get an airlock in the line to the pressure gauge? I have some difficulty in relating this to what happened but I don't really know.
I need to give it a longer run but I have been preoccupied with dealing with a carburation problem on my Honda XR 250. The weather is promised to clear up in a few days so I'll run the BSA again and let you know how it turns out. I will be peering nervously at the pressure gauge though I assure you.
Cheers
Alan

[A10Boy]

Yes, I thought it could be related, do you need to "bleed" the gauge pipe assuming its a pipe not a sender?

[manosound]

I think it is correct to say that the return being faster than the supply puts bubbles in the oil. Bubbles at the top of the oil tank when viewing the return seem to verify this. I have wondered if the little hole in the left connecting rod was not actually put there to prevent bubbles at the end of the sludge trap from forming an air chamber that dams oil flow.  Back to the point,  bubbly oil won't show the same pressure on the guage as viscous liquid oil.  It would seem bubbles are no(t present in start-up, early-run oil,  but build up over time.

Richard L.

P. S.  Notice my many attempts to avoid sounding sure of myself.  That's for a reason

[bsa- bill]

Hi Richard
Bubbles are indeed a result of the return have more flow than the feed, that is once the pump has cleared any backlog of oil in the sump.
At start up there will nearly always be some oil in the sump - drain down from after the engine stops - any wet sumping that may have occurred.
so at start up there may be no bubbles for a little while, and I'm not sure but perhaps this can happen after returning to idle

[manosound]

Bill,

I think even if the sump were dry at startup it would take some time to churn bubbles into the oil that might build up in a dead-end passage like a pressure guage.

Richard L.

[bsa- bill]

Richard your into what for me is one of life's great mysteries, working on a farm most of my life I was always amazed at how oil filled a hydraulic ram so quickly and completely even after it had been removed or replaced, I'm talking about single acting rams where oil is fed into one end (no breather or means to bleed them ), mechanics I've asked just say " well it sorts itself out with the pressure" and it seems to be true so possibly it's the same for a pressure gauge, a bit of air in there would give a low reading and in a short time get squeezed out - I'm no hydraulic engineer though.

As a paramedic said to a man giving artificial respiration to a half drowned man on the side of a pond " you'll get on faster if you pull his ass out of the water"

[trevinoz]

Could I say that this is the reason oil pressure gauges were discontinued from being fitted to bikes?

  Trev.

[chaterlea25]

Hi All,
oil or air the gauge doesnt care 
the same gauge works for all media, if it didnt it would mean hundreds of different gauges would be required   

Alan, the BSA engine should self prime, Your problem started with changing the feed pipe, maybe there was something inside the length of pipe??, probably lost when you drained blew everything out
when you say the gauge tapping is after the pump,I'm assuming its before the PRV?

The oil tank acts as a settling vessel the air in the oil should rise away from the feed supply at the bottom,

Cheers
John O R

[manosound]

Darn it!  The one sentence I didn't equivocate and I was wrong.  Apparently.  So,  John,  the mix of densities (it is densities,  right?) between liquid oil and bubbly oil makes no difference on the pressure guage's diaphram or tube or whatever? Answering my own question, I think, the air pressue inside the bubbles is the same as the oil pressure.

Give me a break,  the degree is in electrical.

Richard L.

[chaterlea25]

Hi Richard,
Dont beat yourself up over it   

Regards
John O R

[bsa- bill]

oil or air the gauge doesn't care

I can see that John but air is compressible oil is not, if there was sufficient air  would it not make some difference to the gauges reading

[alanp]

when you say the gauge tapping is after the pump,I'm assuming its before the PRV?

Yes John, it's tapped into the oilway in the crankcase leading to the PRV.
Thinking about it further, maybe, just maybe, the oil seal in the timing case cover which allows oil pressure to feed into the end of the crank journal (SRM conversion) was somehow misplaced (?) and did a pressure relieving job of its own?? The seal looked ok when I did my check. I'm thinking about getting a new one from SRM just in case.
Ref. the pressure gauge, it's virtually impossible to bleed all of the air out of the instrument anyhow but this is of no concern because pressure is pressure if it's measuring air or oil, but air pockets will soften the gauges reaction to rapid pressure changes a tad methinks.
Cheers
Alan

[kommando]

Make sure the seal in branded 'pioneer', they seem to be the only garter seal you can rely on to stand the pressure on startup. They are UK made and available from Eriks.

http://www.eriks.co.uk/

[chaterlea25]

Hi Bill and all,
its a hard one to wrap your brain around 
if oil or any liquid is displaced by a positive displacement pump against a closed circuit the result is the same as a mechanical block of the pump, a dead stop 
in our engines oil pressure leaks off through the bearing clearances and of course the pressure relief valve
if it didnt the pump would lock and probably the drive fail !!
Take Bills hydraulic ram, the ram will move under hydraulic pressure, if theres air in the cylinder this will be pressurised up to whats needed to lift or move whats loading the ram, Air operated rams are also used in industry
On most hydraulic control systems a pressure vessel is included in the circuit to smoothen the actuation.
There are a couple of ways to dampen the movement of a pressure gauge, some are filled with oil or glycerine
or a needle valve is put before the gauge, restricting the flow, this dampens out the fluctuations in readings
(carb balancing gauges or gauges on piston compressor heads)

HTH
John O R

[bsa- bill]

Cheers John, I live and learn thanks to you lot.
Hadn't thought about air rams but right enough we've had stuff with them, most of our Jonh Deere's came fitted with compressors, probably not cost effective in the long run but saved the day more than a few times (we carried a line and tyre adaptor in the cab)

[beezermacc]

Sounds like a partially blocked tank breather to me, or a collapsing feed pipe. On tickover there isn't sufficient suction to cause a problem. As the engine revs increase the suction on the feed pipe can be so great it causes the pipe to collapse in on itself. A similar effect would be caused if the tank breather is blocked then the tank won't allow oil down the feed pipe. Try taking the oil tank cap off and/or temporarily replace the feed pipe with an original old one.