Hi everyone

 

A few weeks ago the engine on my vrs was running lumpy, turns out the oil filler cap came off, so bought one from eBay for £7 screwed it back on and it was back to normal.

 

it has just happened again and the filler cap has come off, I’m definitely screwing it in right, but only the top half of the filler cap has come off, and the other is still screwed in.

 

do any of you guys have any idea what might be the reason? Have I got a serious problem or is it just a cheap oil filler cap?

 

thanks 

If you're sure both caps were secured properly and not faulty then there is the possibility that the oil pressure relief valve in the oil pump is faulty resulting in very high oil pressure when the engine is cold. This excess pressure could be blowing the cap off. The fix will likely require a new oil pump.

 

More seriously it could be 'blow-by', a consequence of piston ring wear, allowing fuel and gases into the crankcase. If this were the case you'd begin to notice a reduction in power and an increase in fuel consumption.

 

The fix would likely involve an engine rebuild, but this can be delayed by increasing the regularity of the oil changes, although not for ever, things will continue to get progressively worse, albeit more slowly.

 

I'd be looking to source a genuine oil cap to rule out just being unlucky with the two previous caps.

 

Edited 25 December, 20196 yr by silver1011

Engine oil is only under pressure within the oil gallerys and the centre of the crankshaft, camshaft etc, once it rejoins the crankcase its at normal crankcase pressure.

 

Normal crankcase pressure should not be sufficient to inflate a condom let alone blow an oil filler cap apart.

 

Sounds like cheapo self destructing pattern part oil filler cap or caps.

Or your PCV system needs attention.

 

edit: although I’ve not known many cases of blowing the oil cap apart.

Edited 26 December, 20196 yr by Vrsburnzy

16 hours ago, J.R. said:

Normal crankcase pressure should not be sufficient to inflate a condom let alone blow an oil filler cap apart.

 

That's my point, the crank case pressure may not be normal, in fact if it isn't due to a weakness in the cap then it is likely the pressure is anything but normal.

No, your point was that a stuck oil pressure relief valve could blow the oil filler cap off, it cannot but it could do far worse things like blow the oil filter cartridge seal out and dump all the oil.

 

Luckily for me the start line marshalls at Lydden Hill had lots of cement and several brooms.

Edited 26 December, 20196 yr by J.R.

I offered two possibilities, the oil pump OR blow-by. They aren't mutually exclusive.

 

Either can result in a blown oil cap, granted it's rare, the dipstick would likely eject first, but there are plenty of examples out there of blocked breathers resulting in the same issues experienced by the OP.

 

Edited 27 December, 20196 yr by silver1011

The first could never pressurise the crankcase let alone blow an oil filler cap off except within a fertile imagination.

 

I completely agree regarding blow by but all the OP would have to do is run the engine with the cap removed to know instantly and visually if that were the cause, I would hope they have already done so.

I'll bow down to your expertise, but you don't have to look very far to find plenty of examples of defective parts, modifications, blocked breathers / PCV's all resulting in missing oil caps.

I said that I agreed with you on that point.

 

Regarding the scenario of excess oil pressure, I will explain why it cannot change the normally very slight positive crankcase pressure. For the sake of the example we will ignore the PCV system and say that the crankcase is at atmospheric pressure and effectively sealed.

 

The oil pump draws oil from the sump increasing the volume of air within the crankcase and theoretically reducing its pressure very slightly, berely a measurable quantity, a couple of millibars, it then pressurises the oil within the pressurised galleries to say 65psi, with a faulty PRV then say double that but this pressure is confined to the oil galleries, once it has flowed through the bearings it drains back into the sump reducing the air volume and the crankcase pressure equalises.

 

Were there an external oil reservoir like a dry sump system (for the example a very large one) but without a scavenge then the  pumped oil would gradually fill the sump and every airspace in the engine, again for the example were there no engine breathers then every internal volume of the engine would be compressed to 130 psi (PRV valve blocked) the oil having filled 9/10 of the volume and a 1/10 volume of compressed air at the top acting against the oil filler cap.

 

This would exert a force in the region of 400 lbs on the oil filler cap, the above is the only scenario outside of Hollywood where excess oil pressure could blow the oil filler cap off or apart, indeed normal oil pressure would do it.