BMW 3 Series Engine Problems

The PCV valve and the vacuum pump are the most likely causes of the high oil consumption, and although you say they have been done I would strongly suggest you recheck them. The pump in particular is a well-documented cause of oil leaks. I doubt changing oil would fix your problem.

Yes, it is important to change the coolant, particularly with a BMW with its alloy head. When you do it it’s vital you use the correct coolant as specified in the owner’s manual. Older BMWs, particularly the 3-Series, can be troublesome with high mileage. There are a number of plastic fittings used in the cooling system, including the radiator, and they are known trouble spots. The power steering can also give trouble, the suspension bushes often need replacing, and there are many others. Only buy one if it’s a low mileage car with a known history and a full service record, and only if you’ve got deep pockets.

It's unlikely to be the battery, as you've proved when the fully charged one didn't fix the problem. BMWs of that model had problems with the ignition switch and I would suspect that that's your problem. Next time it happens rapidly turn the switch on and off several times and see if you get any response. If you do it's the switch that is your problem.

If you've checked the battery and that's ok, check the starter to make sure there's no problem with that, and then pressure check for a blown head gasket. It's not unknown for a head gasket to blow on the 318 and you can get an hydraulic lock in a cylinder that will make it difficult to start if that cylinder is the one that's on compression when you go to start it. It doesn't always show up on the temperature gauge as overheating, but it is sometimes evident with white smoke from the tailpipe when the engine is cold.

The official combined cycle fuel economy of BMW's 318i is 5.4L/100km.

Our experience is that older BMWs suffer problems with things like the plastic fittings on the engine, the plastic radiator tanks etc. that can add up to a sizeable sum of money when you have to replace them. One car, an older 3-Series that had done around 250,000km developed a problem with the power steering that had mechanics completely stumped. After four visits to the workshop, having replaced the steering rack, the power steering pump and many other steering parts, it was finally found that the fault was in the steering column. That was replaced and the problem fixed, but it cost more than $2000, and that's using second hand parts. That said the engines and gearboxes, etc., are bulletproof. We don't have the same feedback on the C-Class, but remember all cars have problems; no car is completely free of trouble.

I would only replace the faulty one, unless the others are suspect in which case it’s probably best to replace all of them.

Take it to an independent mechanic who specializes in BMWs or European brands and have them assess the problem for you. The quote from the BMW dealer would suggest that they want to replace the gearbox, but an independent mechanic might be able to repair it at a much-reduced cost, and even if the gearbox does have to be replaced it will be cheaper.

There are two likely suspects here. The first is a blown head gasket which is allowing the oil and coolant to mix. Sometimes this shows up as oil in the coolant, sometimes as coolant in the oil. Sometimes both.

But before tearing the engine to pieces to change the gasket, it’s worth checking if the car is fitted with either an engine or transmission oil cooler. These often use the engine’s coolant to keep the oil or transmission fluid cool, and a failed one can allow the two fluids to mix, showing up as oil in the radiator or header tank.

BMW's F30-series 318i is claimed to average 5.4-litres of fuel consumed per-100km, measured on a combined cycle of city and highway driving. This is quite efficient for a large four-door car.