Diesel Engine Care

Most people are not aware that the main reason a diesel engine wears out is improper combustion, usually caused by worn out injectors, improper timing, or simply lack of air (influenced by several factors, one of which may simply be a dirty air filter). Anything which produces soot (eg. particulates), including the "black smoke" effect, results in particulates ending up in the engine oil. This soot then accumulates in areas where oil flow is low, like behind the piston rings. As the deposits grow, the ring is being forced outwards. The user knows nothing of this until the very end--the constant tension actually improves compression, but wears out the cylinder and the rings, reducing the life of the engine at least ten times.

This is the reason why oil has to be changed more often in diesels. The oil itself would wear out slower than in a gasoline engines because the temperatures are generally lower than in those engines. For newer engines, oil change intervals have been increased because electronic regulation keeps the fuel/air ratio at full power accurately below the smoke limit, hence less particulates are created. In addition, the new common rail, higher pressure injection systems are less sensitive to injector wear because the injection timing is not derived from the injector opening pressure any more, and the very high pressures involved produce good atomization of the fuel even when the injectors are worn, again resulting in less particulate matter being created.

But even if the oil could remain operative for a longer time, black soot accumulating in the liquid will damage many parts of the engine, including the automatic valve adjusters. What's worse, this damage is actually hidden--at least initially--from the owner by the fact that the deposits cure the problems they have just caused: by filling and sealing the worn-out gaps, the problems remain unobserved until the next oil change. The new, fresh oil then washes out the soot, and due to the missing sealing effect, the valves end up having huge "clearances", which can be heard as a clicking noise at lower engine revolutions. As soon as the engine speed increases, the increased oil flow easily accounts for the leakage and the adjusters start working again. Non-working adjusters also influence airflow because the valves do not open completely--resulting in black exhaust smoke once more.

However, if there is a greater amount of soot deposits within the engine, after a few days the clicking might disappear as the new oil dissolves and distribute the deposits, which will again end up sealing up the adjusters. If, due to the symptoms described, you suspect that one or more of your adjusters fail, you should change all of them at the same time. It ouwld be possible to find out exactly which one failed, however, due to the nature of this failure, the rest have already had their share of the oil deposits, and they will also fail, usually in a short time (20-30,000 kms at most).

Before you replace the adjusters, it might be a good idea to change the oil and put an engine cleaning additive in it, as well as changing the oil filter (it's even better to change the filter twice while the cleaner is still in the engine; the cleaner can dissolve huge amounts of soot that can end up clogging the filter far earlier than normal, maybe only in 1,000 km or so). Then, after about 500-1000 km (sometimes even sooner), the adjuster(s) will start clicking again. At this point, drain the oil, change the adjusters, put in new oil and a new filter. As complicated as it may sound, you will doing the engine (and yourself, too) a great favor. The usual life expectancy of the adjusters is around 150,000 km; if they die prematurely, this is a sign of insufficient engine care.

The injectors also deserve some attention; if they are bad, you may end up changing oil very frequently and still end up with dead adjusters. When renewing the injectors, a typical and possibly fatal mistake is to forget to put new sealing rings between the injectors and the head. These are usually usually copper or brass profiled rings. When the injector is mounted, it squashes the profiled ring flat, which provides the seal. This means that whenever you take the injector out, you have to put in a new seal ring. If this is ignored, and either the old ring is put back or there is no ring at all, the hot combustion gases can leak past the injector, damaging its thread. The injector itself may overheat causing it to seize, which in turn may result in all sorts of bad things as the pump tries to compress uncompressable liquid into a closed volume: pump failure, intermittent engine runaway, and of course, injector destruction, leading to glow plug and even prechamber damage.

These seal rings not only seal but their thickness is accounted for in the positioning of the injector face. If they are missing, the injector face gets in the way of the direct gas path inside the prechamber. The prechamber is roughly spherical, and if you drill a hole in it to inject fuel inside, you would in theory have to fill out the hole by a piece of the sphere you have just drilled out--this means that the face of the injector, which is what fills the hole whould be concave. As you know, this face is actually flat. If you don't put the sealing rings in, the gases in the prechamber will actually wear out the flat face of the injector into a roughly concave space to restore proper gas flow and turbulence. While this is done, the turbulence will be inadequate, resulting in less than perfect combustion. Unfortunately, by wearing out the injector face, the action of the injector is compromised, so this is a lose-lose situation. With the seal in place, the injector face is slightly retracted from the wall of the prechamber. This makes the gases actually bypass the injector face, it's really in a 'wake' of the gases. Wear is far less (about 10%) and the geometry of the prechamber is actually less compromised this way.

Injectors that didn't have the seal ring fitted can be easily identified by being sooty and having a depression worn out of the injector face exactly the size of the opening towards the prechamber.

Glow plug failure is a sign that something is wrong with the injector inserts. The injectors themselves consist of an insert which is really a sort of a needle valve, and a body which holds the insert, plus a spring that determines the opening pressure. Unless there has been some sort of catastrophic failure (or a terrible case of rust because of abominable fuel quality) the body is never changed. The inserts should be checked at regular interval (about 50,000 kms, sometimes even more often if fuel quality is bad) and at the very least cleaned if not renewed. The more regulairly they are cleaned, the longer they will last. With good injectors, glow plugs last a very long time--at the very least 100,000 to 150,000 km.

We mentioned the injection pumps as well. Those manufactured by CAV will wear and eventually fail in due time, their construction is less than perfect. They have a sleeve bearing at the front which will wear out, then the shaft starts to roll around in the sleeve bearing, opening up too big a hole gradually, producing a beat. As the bearing wears out, it becomes roughly oval (it wears towards the force of the cam belt). This results in the shaft not being parallel with the axis of the pump. Bosch pumps are less sensitive to this but CAV ones start to wear their rollers and the cam ring out. The first symptom is a strange noise around 1,200 rpm, a ciclycal change in the noise of the engine that happens about once every three to four engine revolutions. It can be misleading but by using a stethoscope (or a wooden stick) you can find out that it actually comes from the pump. The pump itself will work for thousands or tens of thousand of kilometers after this problem develops, but sooner or later this uneven running will wear it out internally, making it more and more imprecise about fuel metering and injection timing.

Bosch pumps, on the other hand, seem to last forever, although they need a timing test (and possible adjustment) every 150,000 to 200,000 km. There are no problems as long as the fuel filter is changed and possible water drained regulairly. The CAV ones usually need a rebuild around 150,000 to 200,000 km which will change the cam ring, the rollers, the two radial pistons, the front bearing and a part from the uptake pump. CAV may also develop leaks on the manifold pressure sensor (for turbo) and the timing advance solenoid. Besides, CAV pumps are incredibly sensitive to air in the fuel uptake: this is because they use hydraulics to transfer certain forces within the pump, most notably to the speed regulator assembly and the turbo boost membrane. It uses the fuel for this, relying on its uncompressibility. Of course, if the fuel has air bubbles in it, they together become compressible which completely screws up the hydraulic stuff. The results are very annoying such as sudden shutouts, getting stuck at 2000 rpm, oscillations (sometimes really bad—from 1000 to 5000 rpm and back, once every second or so), even engine runaway. Very nasty, indeed.