More newbie questions: Crankcase breather and oil pressure gauge flickering

[SteveH1960]

Apologies in advance as I have mentioned already this is a new, to me, car/experience it was 30 plus years ago when I last picked up a spanner in anger.

Last night I managed to take the car out on a reasonable run and towards the end I noticed at idle the oil pressure gauge was fluctuating, somewhere between 20 and 50 as soon as I gently increased the revs, and when driving, it was well over 50 solid and steady, I assume not too much to worry about?

This prompted me to check the oil this morning and it has dropped about half way down the dipstick level from max to min, but no obvious oil leaks so I will refill, I see the recommended oil is Castrol 20/50, is the Halfords Classic 20w50 OK as an alternative?

However I also googled the oil pressure fluctuating issue and one recommendation was to check the crankcase breather. Now for the life of me I can't find the breather filter. I assume it has one? 1972 Roadster 18V581F engine.

I am slowly getting a bit hacked off finding things that aren't quite right on the car. I do realise this is all part of the fun but I was hoping the fun could wait a bit.

[Peter Cresswell]

I'm not sure why but many (if not all) seem to do this - mine included! It might be down to the rotor design of the oil pump causing pulsing in the oil system at low revs. If the pressure comes up as soon as the revs are raised, I don't think there is much to worry about. They seem to do it more when the oil is hot which the present weather will help to achieve!

[Ian F]

Steve, a couple of points from me:

There should be on your car ( same year as mine) a couple of black rubber pipes leading into each carb. They come from a "Y" piece which in turn comes from a cover on the tappet side of the engine. This is the crankcase breathing system. No maintenance required. The cover on the side of the engine has a gauze filter but this should require no maintenance. The system feeds all of the crankcase fumes into the combustion chambers for burning.

Like Peter's B, my car also displays this flickering gauge when hot. Your car sound perfectly good and typical. My only slight concern may be the apparent loss/consumption of oil? Check for blue smoke (burning oil) or oil leaks underneath.

You can most easily get good 20/50 oil from suppliers such as Opie oils. I buy 20 litre containers of Duckhams or Castrol, whichever is best priced. Almost certainly cheaper than Halfords and probably better quality.

Enjoy the tinkering!! All part of classic car ownership.

Ian F

[Bumpa]

Oh, you can spend many a happy hour researching oils for your MGB. The crucial thing, it seems, is to have some ZDDP (a sort of zinc phosphate) in the oil. The MGB's old fashioned engine has a couple of sliding components, mainly the flat topped tappets and the rocker pads that shove the valves down. These so-called "flat tappet" engines need ZDDP to prevent premature wear on the sliding components. That's why "classic oils" exist. I don't know too much about it, but you can read lots of interesting stuff on Penrite's PDF sheet tikobv.nl/m/ZDDP.pdf. Of course Penrite are trying to sell you their oil, and I'm sure it is very good, but there are plenty of alternatives.

Definitely stick to 20/50 is my advice and steer away from modern synthetic or semi-synthetic oils. I expect, but don't know for sure, that most classic oils will be fine. I use Valvoline VR1 Racing 20/50. Don't be put off by the racing tag - it isn't anything special, just a good classic oil.

As for pressure, my experience of MGB engines would suggest that your oil pressure is a tad low - not desperately low, but lower than I have seen on my own cars. I would expect around 40 psi idling unless very hot, and above 60 psi running. As for the fluttering needle, this is quite common and Paul Hunt mentions it on his page about oil gauges http://mgb-stuff.org.uk/enginetext.htm#gauge2. Incidentally, Paul's website mgb-stuff.org.uk is a mine of information about the MGB and well worth looking at when you need to know about some aspect of the car.

Ian has given you the necessary description of the breather system. He didn't mention the rocker box oil cap which is an integral part. It contains a wire gauze which needs to be clean enough to allow air to pass through. I think air enters the engine through this route and is blown out the way Ian described.

[SteveH1960]

Great stuff thanks for the replies, Bumpa, that link is excellent but has now got me worried about the coolant, the stuff in the rad looks very brown !

[Bumpa]

It's no great hassle to drain the radiator buy pulling off the bottom hose (unless you nhave a rad drain tap), flush with a hose until it runs clear and refill with a fresh mix of glycol antifreeze. Make sure the heater is on HOT so that can be flushed too. Be aware that if you have the radiator with the central cap, it will never appear to be full up. There is meant to be an air gap at the top and the system will eject the excess coolant until it reaches the normal level, which to the eye appears about half full.

[Paul Heffernan]

Just my 2p worth and hopefully to give you some further comfort. Ref the flickering oil pressure gauge, mine does this too. It's pretty steady at cruise (between 60 and 70) and between 70 to 80 under hard acceleration and around 50 at tickover. However once the engine is properly hot, the needle flickers at tickover, ie at traffic lights etc.

[JohnH]

Hello Steve. Mine flickers too when hot and has done for 47 years and over 3 engines!
I think its caused by the feed to the rocker box which is only fed when the camshaft hole through which the oil flows, is in line causing an on off effect.
Do check those rubber pipes and Y to the carbs for blockage and also clean the filler cap with petrol (or replace if blocked). The breathing route if from the cap to the crankcase and then to the carbs with the 18V engine.

[Paul Hollingworth]

My oil pressure fluctuates but only at idle and only when hot. I had imagined that it was caused by the pressure relief valve shuttling. Your oil consumption and low ish oil pressure (mine is 65 psi) lead me to believe you engine is a bit tired. No need for immediate alarm, the B series will plod on for many thousands of miles like this. The question is is the oil getting down the valve guides or past the rings. The test is to go down a steep hill on zero throttle then boot it at the bottom. If there's an immediate puff of smoke then its guides which is good news because its a cheaper repair. I would try cleaning the gauze filter in the tappet chest (although this is difficult to get off and on in situ without removing the carbs and perhaps the manifolds) and refilling with a decent oil like Duckhams. You could try adding STP too, it will improve your oil pressure and reduce your oil consumption. Oil is cheap, keep topping it up for now. I find that when the car has stood for days some oil drains down out of the filter making you think your oil level in the sump is higher than it actually is. When you test it within an hour or two of a run it wont have had time to drain down so can appear lower thus fooling you you consumption is worse than it is. My car leaks a bit rather than burns it so I tend to keep topping up. I can usually get 6000 miles after changing the oil out of what's left in a 5 litre tin.