I currently have a caramel like substance on my oil filler cap but nothing I'm my expansion bottle. What is it?!
It is an emulsion of oil and water. Whilst some people will say that you will get this due to condensation and short runs in cold weather (when the engine supposedly doesn't get hot enough to evaporate it); I no longer subscribe to that view.
Since replacing the cylinder head gasket on my ZR four and a half years ago, I have never had any sign of condensation or emulsified oil under the oil filler cap at all, regardless of how cold the weather has been and the number of short journeys. The cap has remained bone dry.
It is therefore my conclusion that if you have emulsion in the filler cap, you may well have one of the signs of head gasket failure. If you are finding the level of coolant in the expansion tank keeps dropping and needs topping up, this would also support that idea (my ZR has not needed any
topping up of the coolant in the last 4 years).
However, we really do need to know a bit more about the car - we are assuming it is a 25 or ZR with a petrol K series engine.
Inlet manifold gasket failure will not allow coolant into the oil - it will either get burned off while the engine is running, or will cause the engine to hydrolock. If coolant leaking into the cylinders from the IMG is able to get past the piston rings and into the sump, you have badly worn piston rings and/or worn/cracked cylinder liners, and a much more serious problem with the engine than an IMG failure!