If you do a lot of miles, and want to spend as little as possible on fuel, you can't beat a diesel car. Something Escort-sized should give you around 50-55 mpg and still be able to cruise all day at a steady 80 mph. Diesel cars really took off in the UK in the mid 1980s, and so plenty of diesels have now fallen well and truly into banger territory.
Before you rush out and spend all your pennies on a smart little diesel car, there are one or two things you need to know, if you don't want to end up with an immobile vehicle cluttering up your drive. Firstly, whatever you might have heard, small diesels are not indestructible. Sure, all the engine internals are much stronger than in a petrol engine, but the manufacturers did that for a reason, not just because they had lots of steel and aluminium to use up. A diesel runs a compression ratio of about 22:1, against 9:1 for a typical petrol engine, and that puts a lot of strain on the revolving parts inside. A diesel engine will not magically keep going for ever - eventually they die, so you need to check very carefully for signs of impending disaster.
Most older diesels need much more regular oil changes than their petrol equivalents - some manufacturers recommend as little as 3,000 miles between services. The oil becomes rapidly contaminated with soot particles, and if not changed regularly, all the oilways inside the engine will slowly block up with soot. This problem usually attacks the camshaft and valve gear first, so listen for unusual top end rattling or tapping sounds. The trouble is that diesels, especially older ones, tend to be a bit clattery anyway, so it is hard to distinguish normal sounds from abnormal ones. So if the car lacks any service history, tread carefully.
A badly worn engine will be reluctant to start from cold due to lack of compression, so when you go to view the car, open the bonnet and check the engine is stone cold before you try and start it. If it is warm, the owner may have just used it to pop out to the shops - or he may have spent most of the 30 minutes before you turned up trying to get it to start.
Cambelts - most small diesels have them. Diesels have only a very small clearance between valves and pistons, so a broken cambelt always leads to mechanical carnage. Unless you have proof of a recent cambelt change, make this the first job you do after buying the vehicle.
Emissions can be a major problem with older diesels. The MOT requires that the engine is run up to maximum revs several times, and the smoke emissions are measured (unless the engine blows up during the test, which is not unknown). Worn injectors, incorrect injection pump timing or a clogged air filter will all lead to a fail. A marginal case can usually be coaxed through by changing (or removing) the air filter, and then taking the car for a long run on the way to the MOT, thrashing it hard through the gears. Anti-smoke additives can also help, as can screwing down the maximum speed setting on the injection pump (but only mess about with pump settings if you know what you are doing). If none of the above help, you are probably going to end up spending a fair bit of money on new fuel system components.
As well as emissions problems, incorrect pump timing or other fuel system problems can also lead to a cracked cylinder head. This problem is distressingly common on old diesels - the head is under a lot of stress anyway, and can fail even when everything else is perfectly in order. Usual symptoms are similar to a blown head gasket - lots of water vapour in the exhaust, and pressure will build up in the cooling system right from a cold start-up. Sometimes the cracks are between the combustion chamber and the inlet or exhaust ports, in which case you get a loss of compression, leading to a reluctance to fire up when cold. Minor cracks can sometimes be welded, but in most cases you are looking at a replacement head, which won't be cheap.
Electrics - diesels need a lot more effort to turn over than petrols, so they have big, expensive batteries, starters and alternators. Check that the starter turns the engine over at a decent speed and doesn't make any nasty grating noises. Most diesels rely on heater plugs to aid cold starting. These often have quite complex circuitry, with timers and relays which can fail and make the car a pig to start. Don't be tempted to use Easy-Start spray to get round the problem, as it will eventually take the tops off the pistons. If an engine suddenly becomes difficult to start, 9 times out of 10 it is a heater plug circuit failure. And if the engine starts, but runs very roughly for the first few seconds from cold, probably one or more of the heater plugs has failed - there is one for each cylinder. Cheap and easy to replace, and failed heater plugs are another potential cause of cylinder head cracking, so don't leave it too long.
And finally, a word about 'runaway diesels'. This is a nasty little problem which can affect any diesel where the crankcase breather pipe feeds direct into the air intake. Land Rover turbodiesels are notorious for it, and I have seen it on 1.9 Peugeot/Citroen diesels as well. What happens is that on a worn engine, gases blow past the sides of the pistons and into the crankcase. They emerge from the crankcase breather laden with oil mist from the crankcase, which feeds into the air intake. Now a diesel will run quite happily on oil mist, and so the revs will increase as this extra 'fuel' is taken in. The higher revs result in greater crankcase pressure, more oil mist is forced out of the crankcase and sucked in to the engine, and a vicious circle is created.
Eventually the point is reached where the engine is generating enough oil mist that shutting off the supply of diesel by switching off the ignition will not stop it. The engine runs faster and faster, generating huge clouds of grey or black smoke, until it blows up. If your engine starts to run away, you can sometimes stop it by selecting top gear, jamming your right foot hard on the brake and letting the clutch out to try and stall the engine. But diesels generate a lot of torque, and if the clutch is weak, it will just slip. In this case there is nothing you can do other than abandon the car, retire to a safe distance and wait for the engine to explode. Do not under any circumstances open the bonnet - it isn't worth the risk.
This is not a common problem, but it is worth knowing about, so you can be prepared if it happens to you. It is only likely on a badly worn engine (which will probably be using a lot of oil, lack power and be difficult to start, so a runaway shouldn't come as a total surprise). A temporary fix is to disconnect the crankcase breather hose from the air intake and redirect it into a plastic bottle to catch the oil, but if you have to do this, your engine has not long to live, so time to start looking for a replacement.