Low oil pressure (or not?)

[misterdimwiddy]

Hopefully not going over old ground here; I have used the search facility and found some comment about the TRX having low oil pressure when revs are low. Mincehead and AlexTRX850 appear to have commented to this effect.

My rebuilt motor with new 878JE kit now has a VW T2 type oil pressure switch on the cylinder head oil feed pipe as detailed elsewhere on the forum. When cold, the light goes out and stays out. When the motor is hot the lamp glows dimly at anything below 2000rpm.

Annoying bacause I now don't know whether my motor has developed low pressure since the rebuild or was like this for the last 40000 miles. Oil pump condition was good when stripped and new genuine O-ring seals and gaskets used when they went back in. Cams and head bearings appeared good so no evidence of poor oil supply. Oil flows from the gallery bolt hole when removed as it should.

My gut feeling is that all is OK and that I should not have bothered putting the lamp on in the first place but wondered whether others have experienced similar symptoms to reassure me that my rebuilt motor is not going to self destruct. I have done 200 miles so far running in and it sounds mechanically sweet. Can't hear that much though with PHUK's old Akro headers and Devil can on there. Lovely.

I would also question the switching tolerances of a pressure switch that cost just under 4 quid. Who knows how wide the hysteresis is between the open and closed position? Not sure what the actual switched pressure of these units is?

Any reassuring comment would be welcome. I'll let you know if it goes bang in the mean time.

[M.V.]

...Not sure what the actual switched pressure of these units is?

Any reassuring comment would be welcome. I'll let you know if it goes bang in the mean time.

I expect this is the problem, the activation pressure is unknown.

Check the 10mm nut on the head (undo it & see if oil comes out), if so, you're probably ok.

The oil pump is driven from the crank, so lower RPM = lower oil pressure.

The other moral of the story is, don't set your idle too low! It sounds great at 800rpm ticking over, but it's probably not a good idea!

[dandywarhol]

I don't think there's much pressure in there anyway, as there's a scavenge pump in there too. Certainly loosening the 6mm nut in the head doesn't give the pressure you'd expect from a wet sump pumped system

[lenny]

The scavenge pump has nothing to do with oil pressure. The value of an oil pressure light is that it DOES come on at a low idle when hot. That way you get to know your oil pressure behavior and can detect changes in your big end/bearing health by the behavior of the oil pressure light. This was an old trick of switched on British bike owners. By memory the vw oil pressure switch activates at about 5 psi.

[dandywarhol]

Ok, I'll retract the scavenge pump statement - does it pick up the return oil from a well in the bottom of the motor?

The oil relief valve opens between 3.5 - 4.5 bar so why is there only a trickle when you open the bolt on the side of the head?

[misterdimwiddy]

An interesting question re: the comparatively high oil pressure relief valve opening pressure.

I have found a link detailing the following comment regarding fitment of an Aprilia RSV oil pressure switch to TDM/TRX/XTZ motors. The RSV/Tuono/Falco also have a dry sump and the switching pressure is very low at about 5psi. Might try one of these as an alternative to the VW item I currently use.

The modification comment reads as follows; 'The oil pressure sensor switch is normally open when oil pressure is at about 0.6 bars during normal operation. It closes when oil pressure drops below 0.3 bars, and the warning indicator lights up'.

Quite a contrast to a CBR600 for example which has a stated oil pressure at 80°C of 71 psi (5.0 kg/cm^2) @ 6,000 rpm.

My motor is still running happily and I will delve deeper into the lubrication differences between dry and wet sump engines more out of interest than anything else.

[fung]

hi mr d
Would you please post the link up, so we may take a look as well .
Cheers
Fung

[sadmansteve]

Try this link.

http://jbx9.free.fr/tdm/index.php?page=OPSW

[lenny]

VW oil pressure switch, works just how I like it.

[misterdimwiddy]

Many thanks Sadmansteve; that was the same link as the one I was referring to

Lenny - I am unclear from your first image what kind of VW switch you are using. Is that the sensor screwed into the M6 oil gallery bolt hole on the LHS of the head? Do you have a part number or VW vehicle fitment spec so that I have another option to try? The item I have used bolts into a double banjo adapter on the cylinder head top RHS oil pipe feed.

I have since read that the blue VW switch activates at between 0.15 and 0.35 Bar. The same order of magnitude as the Aprilia switch detailed in the above link. There is also a grey coloured VW version that switches at higher pressure (about 1Bar).

I did wonder whether the M6 oil gallery bolt in the head is simply open to crankcase pressure rather than pump pressure; hence the low rate of flow/pressure when you do crack it open. This process may simply demonstrate that there is oil reaching the head as opposed to being an indication of satisfactory pumped pressure level? This thought does not however fit with Lenny having his pressure switch at that location...........??

All quite puzzling but interesting stuff.

I am doing my best to supress my oil pressure light paranoia; revving gently to 5k now at 300 miles since the 878 kit went in and all seems good. The over bored motor feels great. Lots more torque and revving cleanly so far on stock carb jetting (K&N filter). I have changed gearing at the same time to 17/43 so it's gonna feel quite different to stock you would think. Will see how it revs out in top in a few weeks time and then go for a dyno run.

As a quick aside Lenny; what clocks are you using? I currently run around with no speedo and the old TRX tacho mounted in an SRX600 clock face (retro look). Need another clock of some sort when my YZF USD's go on and I like the look of your dash.

[lenny]

I am using the brown vw switch. I fitted it there because I figured it was the lowest pressure connection into the main gallery. On a hot day after a hard run I can adjust the idle down until the light blinks.Changes to this behaviour can give me an idea of big end/main bearing health. An old British bike trick. The instruments are ex Suzuki sv650 ($40 off evilbay). Required rewiring a new plug to suit and a home made speed sensor made from a hall effect sensor for a home burglar alarm ($5) mounted to the bottom surface of the fork leg and 4 neo magnets epoxied to the wheel bearing hub. Works great, but still trying to get temperature readout working.

[dandywarhol]

So, if the oil relief valve blows between 3.5 and 4.5 BAR and oil pressure is partly controlled by bearing clearances, why is the pressure so low at the cam bearings? Especially as the cam runs directly in the head!

[dixonj]

Hi misterdimwiddy did you sort out your oil pressure as i have just put an oil pressure switch on my bike today and it does exactly the same as your bike. The engine was fully rebuilt 1000 miles ago uses no oil and goes very well, I used a vw blue top switch.

[misterdimwiddy]

An update for anyone following this thread.

I have just moved the switch from the top end of the oil feed pipe to the crankcase end following a message exchange with DixonJ and my oil light problem has now gone. At full operating temperature the lamp does not even blink even at a very low tick over. Seems that oil pressure from the feed close to the rear balance shaft is a more realistic measure of system pressure. Still a bit puzzling that head pressure would be so much lower but hey ho.

Switch will at least give me an indication of total loss of oil pressure and I don't have to stare at the thing at anything below 3000rpm thinking, is my motor about to explode.

Thanks John. Hope the fix also works for you.

[Killerwhale]

I am using the brown vw switch. I fitted it there because I figured it was the lowest pressure connection into the main gallery. On a hot day after a hard run I can adjust the idle down until the light blinks.Changes to this behaviour can give me an idea of big end/main bearing health. An old British bike trick. The instruments are ex Suzuki sv650 ($40 off evilbay). Required rewiring a new plug to suit and a home made speed sensor made from a hall effect sensor for a home burglar alarm ($5) mounted to the bottom surface of the fork leg and 4 neo magnets epoxied to the wheel bearing hub. Works great, but still trying to get temperature readout working.

Clever boy!!