I was driving to work this morning & my engine oil warning light appeared on my dash. I assumed it needed a top up. When Parking up at work I noticed that oily black water was pouring out under neath the car I opened the bonnet & it appears to be overflowing out of a yellow cap located near the coolant chamber, can someone please help as to why this is happening. The car doesn't appear any different when driving sounds & drives as usual? Thanks
You could be really lucky and it could be that the rubber seal within the cap has deteriorated and is no longer holding the pressure in the expansion tank.
** CAUTION ** - Do NOT remove this cap whilst the car is warm, you risk scalding from both super heated water and escaping steam.
Once the car has cooled, remove this cap. If it appears to be loose then it may just need refitted tightly. If it was tight but the inside of the cap looks like it's deteriorated or checked, order a replacement.
Now the bad news; typically this behavior is caused by a failed head gasket where cylinder pressure is escaping into the coolant system.
I suspect it is water / coolant you are seeing under the car and it may be black because the route its taken to get to the ground has seen it wash across various grimy, oily bits under the bonnet.
Your best bet is to get a compression test done. A garage won't change very much for this and it will tell you for certain if any cylinder pressure is escaping. If it is you have three realistic choices; rebuild the engine, replace the engine or change your car.
There are numerous products online which claim you can add them to the coolant and they solve the problem, I have yet to find any which work.