12 replies to this topic

[Minidude7]

Hello everyone,

This is my first post. I was looking at a mini today that was running fine until you remove the oil filler cap whilst running and the engine seems to slow down as if revs are reduced.

This also happens when the dip stick is removed also. I can't work it out!! There seems to be pressure build up in the crank case, more than expected but its the leaning off that's the issue. I can't seem to work out where it is getting its air intake from as the air filter doesn't seem to be sucking!!

Please help

[KernowCooper]

Excessive crankcase pressure is caused by blow by of compression gas of the piston rings to bore clearance indicating a worn bore/s, or blocked crankcase breathers, what mileage has the engine covered.

[jakejakejake1]

On my car there is a breather on the back of the block, which is connected to the carb. When all the caps/ dispstick etc are on the car this breather cant actually suck in air, as there is no air (well no continuous supply) in the engine it just pulls a vacuum, whereas when the oil cap or dip stick is off, it allows air in which in turn goes through the breather to the carb.

[Minidude7]

I have never noticed this issue with any other Minis I've owned... Is it a drastic costly issue or can I easily get it fixed?? The engine has done around 65000 but I don't know if its true mileage as it looks like a reconditioned engine...

[KernowCooper]

To actually answer your questions, this is how the oil breather/vacuum system works. By removing the dipstick or cap your effectively creating a air leak...........

 

Oil vapour sucked into the combustion chamber lowers the octane rating of the fuel and dilutes the mixture 

 

Which is actually what jake said actually!

[Dan]

I have never noticed this issue with any other Minis I've owned... Is it a drastic costly issue or can I easily get it fixed??

 

  Fix what?  It's not a problem, it's what you should expect it to do.

 

  There is an air supply through the block though, a metered inlet through the oil cap which has a filter inside.  The PCV system usually draws a tiny amount of air through here to keep the crankcase clean but as above if you open the crankcase up anywhere it will lean the engine out.

[Earwax]

As above - fix what?   - i actually believe it is a sign of a system operating well. stick your palm over the oil filler hole and feel the engine trying to suck in that air- when they blow air you have breather problems

[Minidude7]

Ok well it doesn't suck it blows as I said it was pressure build up in the crank case... So there is breather problems... And something to fix right? Any ideas at a cost estimate as I don't want to go ahead and buy this car if its going to cost a fortune to rectify...

[Dan]

  Can you explain again then?  Because it seems that everyone who has replied so far has failed to understand what you have found.

[Minidude7]

Ok...
I went to see a car today...
When the car was idling I took off the oil filler cap and the revs dropped away to stalling... This happened when I pulled the dipstick too... When the filler cap was off and engine running I put my hand over the whole in the rocker cover and it pushes as in a build up of pressure and brings the revs back up but when I take my hand off the revs drop again as it leans out...

I also noticed that no air seems to be flowing through the air filter as I took the hose off and put my hand over the end and it made no effect of the engine at all...

I am confused that's probably why I may not be explaining it right... Thanks

[Earwax]

Probably need to give a bit more info on the carbie, filters, connections to the carbie etc  or a picture (although we know this is not your car so could be problematic

[Minidude7]

Yeah pictures aren't possible... I have done more research and think its either a worn bore or cracked/worn ringland so I am going to pass up on it knowing it will be a heafty job to rectify...

[Dan]

  Yes that would explain the crankcase pressure, but I would expect the car to run better with crankcase pressure released not slower.  Unless more than 1 cylinder is leaking so badly that the only way the engine can develop any sort of cylinder pressure is through having crankcase pressure like that to slow the leakage.  The breathers would have to be badly blocked up too.  I can't imagine it would drive well at all, did you manage to get out in it?  I wouldn't be concerned about the apparent lack of flow through the filter, the filter case could be leaking just about anywhere and it only seals to the carb elbow with a strip of foam.