I have a nissan x trail 2.2 diesel and oil is mixing with water and overflowing into the radiator and water bottle

Go to the radiator and look for a hose either on top going to the overflow to the bottle or at the bottom hose look for a tee fitting with the hose going to the bottle it's a white plastic bottle search your van for 2002 ford transit overflow tank and you'll see it ok

No just the coolant surge tank.

Some time the float or censor gets stuck most of the time you can tap on the float in the bottle and it will un stick

sounds like a head gasket in the first stages

It has to be a gasket related problem or radiator. Check your oil cooler lines, they maybe allowing oil to get into radiator, therefore prob bad radiator but sounds like intake or worse yet Head gasket problem.

could be 2 things... the oil cooler or head gaskets. most likely an oil cooler

A 91 should have regular green coolant. Make sure to read the label on the bottle. Sometimes it is premixed and some times you need to mix it yourself. If you have to mix it your self you will use 1/2 coolant and 1/2 water.

You can add it directly into the radiator through the radiator cap. Make sure the engine is cold before you remove the radiator cap!. Once the radiator is full you can top off the overflow container. It should be on the passenger side of the car near the battery. It will be the white bottle with a black hose going into it.

Do not mistake this with the washer bottle. The washer bottle will have the blue cap.

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Removal

CAUTION: Do not mix Standard (green) Coolant with Extended Life Coolant (orange). If mixing occurs, drain engine cooling system and refill with originally equipped coolant type. If this contamination occurs, the service change interval on Extended Life Coolant will be reduced from 6 years/150,000 miles to 3 years/30,000 miles.

1. Drain radiator until engine coolant is out of degas bottle. Disconnect radiator overflow hose and engine coolant vent hose at degas bottle.
2. Remove power steering oil reservoir retaining screws and position power steering oil reservoir out of the way.
3. Remove degas bottle return hose and clamp. Remove degas bottle retainers and remove degas bottle.

Installation

1. Position degas bottle in vehicle and install retaining screw and nut. Tighten retaining bolt and nut to 9-12 Nm (80-106 inch lbs.)
2. Connect degas bottle hose and clamp to degas bottle.
3. Position power steering oil reservoir onto degas bottle and install retaining screws securely.
4. Connect radiator overflow hose and engine coolant vent hose to degas bottle.

the transmission oil cooler is on the lower part of the radiator. it might have a hole inside. you need to replace the radiator and flush the transmission fluid several times

If the gauges read normal and the fans are working properly and the vehicle dosen't overheat. I would suggest that it's the normal operation of the vehicle.

The overflow bottle is there because when the vehicle get's hot it draws in the extra coolant from the overflow. When the engine cools down it will push the extra coolant back into the overflow, that's what the radiator cap is for, it's not just a cover for the radiator. At a certian pressure the radiator cap will release the coolant to the overflow bottle. Otherwise the super heated coolant would have nowhere to expand to and blow apart hoses and such.

One other thing to check is if the coolant is a proper mix of antifreeze and water. You can buy a coolant check tool from any auto store, they don't cost too much.

Tip: Use distilled water instead of tap water (or use pre-mixed antifreeze). In some cars regular tap water will speed up corrosion in the cooling system.