[Lynn]

Hi wondered of someone had any ideas of what could be causing an oil leak? Oil seems to be in water tank and some light yellow stuff on the cap. Not much power when taking off at roundabouts and difficult getting into 1st and reverse gear.
Im taking car to a garage and would like some idea of what may need fixed and if it is costly before taking car in?
Thanks very much, lynn

 

[Goudy]

If engine oil is getting into the coolant it's likely the head gasket has gone.

Petrol cars tend to weep oil into the coolant and a little water into the engine oil, hence the mayonnaise on the inside of the filler cap when the H/G goes.

Diesels will quite often blow fuel and engine oil into the coolant system quite violently, popping coolant hoses off, popping off the cap off the expansion bottle and splattering it everywhere as the run much higher compression than a pertol.

 

[Walksall]

Is this reply for real???
If there is oil on the coolant, the head gasket COULD have failed, but you could also have a leak from the coolant pump gasket into the oil, but the fact that the engine is down on power as well COULD be an indication of this type of problem.

Whenever you find that the oil is low or that the engine 'rattles' (due to low oil level) stop the engine IMMEDIATELY as continued running can ruin it within seconds.

The 'mayonnaise' that you get on the inside of the oil filler cap is normally due to condensation and can be worse if the car is used only for short journeys, particularly in cold weather.
If the engine is in good condition, there should be NO migration of oil into coolant or coolant into oil.

There is more to diagnosing a failed HG than looking for 'mayo' in the oil cap. There have been many engines needlessly stripped down because people (professional and otherwise) have misdiagnosed the problem.
Symptoms COULD include: Misfire, Oil in coolant', Coolant in oil, 'Mayo' in oil filler cap, Water from the exhaust (again, this could be condensation), Low compression on one or more cylinders. Lack of power, In extreme conditions, the engine may fail to turn on the starter, due to coolant in a cylinder.
Both petrol and Diesel engines have a pressure relief valve in the cooling system, which should relieve any excess pressure to atmosphere.

Your gear selection problem probably is not related and may well be down to the linkage being slightly out of adjustment.

 

[Goudy]

Is this reply for real???

Yes it is.
As you point out yourself symptoms of head failure could be:

Misfire, Oil in coolant', Coolant in oil, 'Mayo' in oil filler cap, Water from the exhaust (again, this could be condensation), Low compression on one or more cylinders. Lack of power, In extreme conditions, the engine may fail to turn on the starter, due to coolant in a cylinder.

The OP has reported oil in coolant, coolant in oil and also a lack of power.

There is more to diagnosing a failed HG than looking for 'mayo' in the oil cap.

I wasn't attempting to diagnose anything, I wrote it was "likely".
To Diagnose over the internet would be extremely difficult, but again the OP had described symptoms that could indicate a possible gasket problem was likely.

I suspect the OP already knows the problem as they are in contact with a garage and their post was an attempt at judging what their garage may have told them and to gain an idea of cost.
The garage should be able to perform the required tests which can't be done on a forum, then a proper diagnosis can be made.

Whenever you find that the oil is low or that the engine 'rattles' (due to low oil level) stop the engine IMMEDIATELY as continued running can ruin it within seconds.

You seem to have presumed more than the OP described.
There was no mention of low oil or rattles.

Both petrol and Diesel engines have a pressure relief valve in the cooling system, which should relieve any excess pressure to atmosphere.

Yes they have, but nothing to deal with the release of compression a diesel engine will output into the coolant system when they go.
This is a typical diesel head gasket failure
http://www.vauxhallownersnetwork.co.uk/index.php?threads/04-dti-cooling-disaster-help.364900/
As petrol cars are not compression ignition engines, they run much less compression and do not tend to blow the coolant system so violently, but will still tend to pressurize the coolant and it's expansion bottle.