Oil Light comes on at low revs

[GT6Craig]

It makes sense that the oil light comes on at low revs - seeing as low revs equates to a lower pressure from the oil pump, but, surely it shouldnt happen at 600-700 rpm? I have my idle (finally - after 7 months of ownership) set at a happy 800. But it occasionally dips below this and the oil light begins to glow. there is plenty of oil in according to the dips stick (about mid point on the marker)

Is it just a case of top up the oil until full then see? or is this the reason why the previous owner set the idle so high - to disguise something? I guess its doing its job and telling me to fill it back up, but im always cautious these days

cheers

[Velocita Rosso]

It may be just a case of the wrong sender in the block. For the sake of a couple of quid fit a new one to see if it still occurs........bet it doesnt

[GreenV8Machine]

hook up an oil prssure gauge and check whether the pumps worn out. Or if using cheap oil try some millers 20/50 semi synthetic.

[Pete Lewis]

unless theres nothing showing on the dip stick any level between low and high wont afect oil presure apart from a lower volume gets hotter quicker.
go for the suggested pressure switch and check that the oil light and charge lamp have not been swapped over   in their bulb holders    Pete

[rotoflex]

This is always something to get to the bottom of quick.  It could be something superficial & easy, or it could be The Bad Thing.

GT6U&A has the correct first step.  Get an oil pressure gauge for testing & find out what the oil pressure really is.

After you've got that, you can then make decisions.
Here are the specs for the GT6's in-car oil sending unit:
http://www.geocities.com/rotoflex/gt6-maint.htm#Oil_Pressure_warning_sensor
As you see, the unit is SUPPOSED to make the light come on when the oil pressure drops to 3.5 psi (unless you've still got the earlier sending unit on the car which lights it at 4.5+ psi).

So if you've got 4+ psi at idle reading from the oil pressure gauge, you're OK & just need a new sender or something.
If you've got less than 3.5 psi at idle, it is time for rebuilding the engine.  Or else it will be badly damaged and/or make a bang-ish type noise in the middle of nowhere & in a very expensive way stop running.

At any rate, find out quick what the oil pressure really is quick.

Another thought is oil:  I remember a million years ago when Mobil 1 synthetic oil first came out tring it in the GT6, & getting lower oil pressure plus oil sneaking out everywhere that dinosaur oil did not.  So if you're running synthetic oil, put 20W50 dino oil in it.

Good luck.  My 1st engine rebuild came under similar circumstances to yours - at college, about your age.
If it must be done, don't get upset.  It's a lot of fairly easy, large operations to just do one at a time.  I did mine at the time without a socket wrench set at first (just end wrenches to take it apart) and a borrowed torque wrench & engine stand.

[TR7th Heaven]

184 wrote:

...but, surely it shouldnt happen at 600-700 rpm?

Forgive me if I'm way off the mark here, but isn't this exactly what's supposed to happen when the revs drop too low? I'd be worried if the light came on at tickover or above, but with a mechanically driven oil pump surely you should expect the oil light to glow as the revs drop below where they're supposed to be?
If it were me I'd change the oil & filter, pop the correct sensor in but hold off the engine rebuild until you're sure there's a problem. I'm not convinced there is.
Fingers crossed for you
Rich  ;)

[GT6Craig]

Quote:

Forgive me if I'm way off the mark here, but isn't this exactly what's supposed to happen when the revs drop too low?

no, you are right, i think im just reading too much into it. The light doesnt come on when it idles nicely at 800, but it does occasionly seem to skip a beat and drop below this, i think its probably more of a problem with my idle mixture than anything else causing the revs to dip below - hence causing the light to come on.
I do already have an oil gauge fitted, but since Rotoflexs reply i havent had chance to warm it up and take it for a spin to get some values. Ill do that today and let you all know what pressure im getting at what RPM. The previous owner supplied some receipts that seem to verify an engine rebuild quite some years ago, apart from this little niggle (and my attempts at tuning) the engine seems to run excellently.

[JohnD]

Whatever you do, even if the oil pressure is OK, get a non-original sensor.
As stated above the OE sensor only lights up when the pressure falls below about 3psi.
Although at idle it may be OK, damage will have already occured if you are driving it when the pressure has fallen that far, and you can't watch the oil pressure gauge all the time.
Get a sensor that triggers are 20-30psi.
Fit a much larger additional light - an indicator light is good! - so you don't miss that tiny little 'idiot' light.

John

[Pete Lewis]

or fit a buzzer  audibles are always better than wot warning lamp,,  how longs that been on??
oops to late   ......... Pete

[JohnD]

If you can miss a 21Watt indicator light going off on your dashboard, maybe you should retake the "can you read the numberplate on that car?" part of the driving test.

[GT6Craig]

ha ha! I notice it as sooon as it comes on, i drive by my ears and eyes. Every single noise, vibration and gauge needle are hard wired into my brain!
Ive bumped up my idle today and the light hasnt come on at all, so it idles at 900, then if it does dip below it dips to 800 rather than 600. Forgot to look at the specific oil pressure though. Anybody got any ideas of what it should be at idle?

[tiggrr1]

Don't know what its supposed to be but mines about 20 psi after a couple of hundred miles on the motorway :)

(and about 55 psi at 70mph in 4th overdrive)

[GT6boy]

The issue with oil pressure is observing a change in it.
It's question of monitoring the pressure and getting used to the values under different conditions.
Pressure needs to be there when the engine is under load- not necessarily ticking over.
Some people build engines for a nice low friction 'wide tolerance' with mineral oil, some go for nice and tight tolerances and semi or actual synthetic oil.
Fit an oil pressure gauge or connect a test gauge to the oil gallery and see what you get. Mine idles at 850-900rpm, when properly warmed up- not just engine coolant warmed up, I get 15psi at idle. At speed on the motorway- like Steve's, 50psi.

  

[Royboy66]

Craig,
pretty much what Bruce just said, its when under load  above tick over you need the pressure more.

mine too can sit at around 20psi tick over after a jolly good thrashing! but more importantly I think, it shoots nicely to 50-60psi when the throttle blipped. ;)

from cold it can sit at 80psi 8), but its not usually colf for long!  :B

An oil gauge is handy to have there, but once fitted, you WILL always be watching it!!  ??)

Roy.

[GT6Craig]

okay, heres what i get from my oil gauge after a good half hour run
My gauge reads from 0-7 Kg/cm[sup]2[/sup]
At
800-900RPM = 3Kg/cm[sup]2[/sup] or 42 PSI
2000 = 4.5Kg/cm[sup]2[/sup] or 64 PSI
3000 = 5Kg/cm[sup]2[/sup] or 71 PSI
3.5 = 5.5Kg/cm[sup]2[/sup] or 78 PSI

That said the guage was under halfway as it always is, and it wasnt thrashed either. This translate to anything to anyone? Compared to yours Roy at idle it seems quite high. Dont forget i mentioned it was only when it dipped below the idle that the light came on, im now thinking it was dying due to weak mixture, causing pressure to drop.

and now i come to think of it, im currently using a lighter oil (which ill replace asap) and there is air in the gauge hose - which i suppose will render them readings....useless!?

[Royboy66]

if those readings are when fully warmed up, then i wouldnt be worrying Craig ! ;)

[rotoflex]

Jeez, those are so good for an older engine I'd suspect something's wrong with the gauge!

[tiggrr1]

Either that or the previous owner has replaced the oil pressure relief valve spring with something stronger.

[Slowdown]

My 1972 GT6 used to get those sort of pressures, which worried me a bit.  On investigation, I found my oil pressure relief valve had the longer spring that tiggrr mentioned.  Swapped it for the standard spring and I now get hot pressures of 30 psi at tickover (6-700 rpm), 60 at 2000, and 70 at 3000.  Mine also has air in the pipe to the gauge, but if anything you would expect that to lower the pressure figures, wouldn't you?

[GT6Craig]

Quote:

but if anything you would expect that to lower the pressure figures, wouldn't you?

Well thats what i thought, but thinking about fluid compressibility my guess is that the air would do sod all....
Read this somewhere

Quote:

Oil will simply flow up the line to compress whatever air is there to the same pressure as the oil, and the gauge works anyway

Well ill be happy with them figures then, as mentioned there is evidence of a rebuild in the docs so maybe its just a good engine, but i will check the oil pressure relief spring soon to check.