oil warning light intermitant when driving

[288NNM]

Hi all,
Could do with some advice please. Change oil and filter a fortnight ago. Put Classic 20/40 motor oil in as 20/50 seemed to be a little thin and my engine has a few oil leaks. Changed the filter for a new paper one. The camister is the old type with a thin rubber ring to give you the seal as you tighten the long bolt at the bottom of the metal canister. I did run an STP oil flush treatment for around 10 -15 mins at a fast idle to assist in removing sludge or stuck componets as it said on the bottle!
Anyway driven the car about 125 miles since a trip to the Liecester Branch rally and back about 90 round trip and knocking about for the rest.
Tonight special trip to see sun set from local pub with awesome view about 5 miles oil warning light comes on as I pull into car park! Engines had no catastrophic oil seal failure externally. Oil in the sump, dipstick showing half full. No oil in the radiator. Visibly no excessive oil on the outer faces of the cylinder head or block apart from the usual oil leaks.
Oil light comes on when engine at tickover or just at low speed ie under 25mph and almost coasting. Oil light seems to ficker and then goes out when speed is increased. However, as soon as you slow to turn off a main road to go on an estate road it flickers then will come on if sedate driving style is employed.
Additionally slight very low hissing sound from around the block area when youve just turned off ignition. No over run when turning ignition off. Engine sounded as it normally does drove it back from the pub about 5 miles with this oil warning light problem.
Any theories on this worrying issue would be much appreciated. The engine is old and does burn oil as I have been informed a sort of greyish smoke out of the exhaust indicates oil is getting into the cylinders. Spark plugs did indicate an oily/ sooty deposite at the bottom of the threaded section of the plug nearest the ceramic sparking end.
Many thanks

[biomed32uk]

20/40 is actually thinner than 20/50, so this may well be whats just tipped it into the light coming on, on an engine thats not worn you would get away with it, but with worn bearings the thinner oil will escape quicker with the resultant lower pressure.

It's looking likely that the bottom end is showng signs of wear, it may be that the oil pressure switch is failing though.

The best course of action would be to measure the oil pressure, then you know exactly whats happening. Normally you can hear the bottom end complaining once you get to 50mph or so, so you may be lucky with the switch, and put some 20/50 in.

I have a feeling though that an engine overhaul and rebuild will be required. If you don't use the car over the winter I would put some 20/50 in for now, run round in it as is and plan a rebuild over winter.

Keep us posted what you find,

[palacebear]

First thing. You say dipstick shows 'half full'. Oil level should be maintained at 'maximum' (top) mark on dipstick. If level is too low the light may well come on at low revs and possibly when cornering. Top the oil up to the right level and see if light behaves as it should, before trying anything else. Also check wire to oil pressure switch hasn't been disturbed or damaged.
Hissing noise... depends if you can pinpoint where its coming from. Leakage from heater valve perhaps...?

[philthehill]

Unfortunately your engine is showing all the classic signs of being worn out.
Before doing anything you must check and record the oil pressures at idle, medium revs and at normal running revs.
Post those readings on here to get further advice on the way forward.
At a minimum you may get away with just replacing the big end shells and at worst a full bearing set and crankshaft regrind required.

It is not always a good idea to flush an old engine as that bit of crud may be what is holding it together
Regular oil and filter changes and use will keep the inside of the engine clean without having to resort to the use of flushing oil/additives.

[288NNM]

Thanks for the comments chaps keep them coming please. Firstly may I opologise I made an error I refilled with 20/40 not 20/50 the latter was already in. I got some 20/40 as it was intimated this viscosity is good for the older leakier engine. Again sorry for error. I will fill engine to the full level as advised as this is the easiest and cheapest first step. I will check the wire to the oil pressure switch in daylight and I do have a used spare engine will transfer the switch if issue continues. Hopefully that switch is good if the oil light continues to intermmitently come on during running I should possibly assume there is no fault with the original pressure switch.
When it has been mentioned if the big end bearings are failing and it will be apparent at around 50mph will it sound like a bag of spanners or a vibration or what do I need to be listening out or feeling for?
I have purchased a used Smiths oil pressure guage and will have a go at fitting it. I additionally purchased an old cylinder compression pressure gauge although I assume the readings on this will have no bearing on the state of the big ends condition.
Thanks again for those who have responded so far your feedback is really useful. If it is big end bearings then so be it the car has covered 148K with only a decoke so its elderly former keeper advised me. Just one last point do said bearings make a hell of a racket before failing and when ive fitted the pressure guage will a low reading indicate bearing wear?
Cheers,
Paul

[oliver90owner]

The dipstick has marks on it for a good reason. This is provided the correct dipstick is used, of course.

There is no reason whatsoever to maintain the level at the naximum mark. That mark is there to avoid problems which may become evident if that level is exceede - leaking seal, crank splashing in oil under some conditions, etc. The minimum level is the safety level to ensure oil pick up at steep angles. As long as the oil level is maintained between these two limts there should be no operating problems with an engine within service specification.

Your postings still leave doubt as to whether you filled with a higher viscosity oil or not. 50 viscosity is thicker than 40 grade. Not all oils are of premium grade either. Some advertising may not be what is actually interpreted by the caual reader.

As PTH, using a flushing oil should not be necessary (particularly if the engine has used detergent oils for a long period since non-detergent lubricant). If it has loosened crud, there is the possibility of the pressure relief valve being stuck part way open, thus reducing the provision of excess pump capacity at lower revs.

I suspect, as others, that your engine requires early intervention before the crank bearing surfaces are destroyed. The crank may already be worn oval and the bearings badly worn (beyond sevice specification). You may be lucky and find the oil pressure warning switch is faulty, but real pressure readings are the way to go.

RAB

[palacebear]

As suggested by others, try and obtain oil pressure readings from a gauge if possible and post on here. By all means do a compression test. It won't give any indication to crank/bearing wear but may give you answers regarding the exhaust smoke you mentioned in your original post.

Worn main bearings usually produce low oil pressure symptoms and are normally identifiable by a low 'rumbling' sound all the time the engine is running (more audible at warm idle as can be drowned out by road noise when on the move) and often accompanied by a flickering oil light at idle/low revs. Depending on the severity of the bearing wear, the noise may be less audible and the oil light may behave normally when the engine is cold and the oil is thicker.

Worn big-ends usually don't produce symptoms of low oil pressure but are easily identifiably by a pronounced 'knocking' sound, whenever the engine is running, as the con-rods will no longer be a close fit to the crankshaft.

[biomed32uk]

Perfectly described in the above post.

My engine was tired, not to the point of bringing the warning light on at tick over but not far away. Think it would manage something like 15PSI hot at tickover. Getting up to 50mph plus it would just sound awful, something I can only describe as a growling noise.

You'll know when it has really gone as the knocking and rumbling when really shot is obvious, like throwing a few bricks in a cement mixer, just faster.

The oil pressure gauge will reveal all, a really good diagnostic tool for bottom end health.

As a guide I get around 45psi on my gauge after a good hard run ticking over, and it wlll get up to 65ish when running along hot. The engines only got about 3000 miles on it so a recent rebuild.

[Mark Wilson]

Slight deviation from subject, but if my memory is correct, Vizard or some other Mini tuning expert recommended limiting oil pressure by adjusting the pressure relief valve spring, as excessive pressure was counterproductive in terms of bhp and economy. Too much pressure is rarely going to be a problem for Minor engines, but as I will be installing my rebuilt 1275 within the next couple of months I'd be interested to learn if anyone thinks this theory has any merit.

Mark

[philthehill]

What you want out of a oil pump is flow not necessarily pressure.

High pressure does two things:-

1. It produces heat which can heat the oil to a temp where the oil starts to break down.

2. Pumping at high pressure takes power.

Here is a good link to oil pumps:-

http://www.calverst.com/technical-info/ ... oil-pumps/

Fitting the 5/8" ball bearing and oil pressure relief valve spring is a good move.

http://www.minispares.com/product/Class ... lassic/oil pressure relief valve.aspx|Back to search

If you want to be more precise in your oil pressure max setting use one of these:-
http://www.minispares.com/product/Class ... .aspx|Back to

[Mark Wilson]

Thanks Phil - the Calver article was the one I'd seen before. So anything above 50 psi when warmed up isn't really that good a thing, but obviously better than low pressure.

[philthehill]

If you have over 50lb/sq in pressure when warm I would not worry too much about it - it is a good indicator that all is well with the engine and that clearances are within tolerances.
It should be remembered that with an engine in good order the relief valve at standard setting operates at 60lb/sq in allowing surplus oil to spill back to the sump.
The standard relief valve spring should have a free length of 72.63 mm.

[Declan_Burns]

Paul,
What size engine is in the car?
Regards
Declan

[philthehill]

Declan
I think that you will find that the engine size is either 918cc, 948cc, 1098cc or 1275cc as Paul refers to the long bolt that secures the oil filter bowl to the filter head in his original post.
Phil

[Declan_Burns]

Phil,
I think you are right. I had just thought it might have been the old 803cc oil filter problem cropping up again.
Regards
Declan

[288NNM]

Thanks all,
My plan is when possible to swop the oil pressure switch and establish if thats where the problem is. Fit the Smiths oil pressure gauge I have and post readings. Get feedback from the senate and take appropriate action. Gut feeling is engine replacement or prefered rebuild of original engine as my wish is to keep Stan as a two owner unmodified old boy with the patina that only comes from years of respectful use.
Will keep you all posted,
Paul

[dalebrignall]

if you have 50 psi on the oil while driving you should be ok i think the book says it can go as low as 8psi on tickover when warm try putting some stp ,or whynns oil charge if your worried might help a little .

[philthehill]

The BMC Minor wksp manual states that oil pressure at idling should 15 lb / sq in minimum.

[288NNM]

Hi all,
Well some excellent news bought a new oil pressure switch from Minimine in Stoke £4.00 fitted it yesterday topped the oil to the full level on the dipstick and yes. After running Stan so he's at operating temperature no oil light flickering. Took him out for a 20 minute drive .... No oil light on at tick over. Oil light only comes on when it should, at start up. Ran him back from my 20 minute drive in town traffic again no oil light issues. I will be fitting an oil pressure gauge as also purchased the two way connector which will allow the plastic pipe for the gauge to connect with the pressure switch into the block. A great little job done. I know the engine is tired but the gauge when fitted should indicate how tired. Thanks for all info folks I will post the pressures when I've fitted afore mentioned gauge.

[dalebrignall]

let us know how things are going after a bit of running around as long as there are no bottem end knocking sounds you should be ok for a while .