The most rare and/or difficult cases in motor expertise
For a number of years we have been collecting our collection of the most difficult cases in the expertise of engine malfunctions. Only a few of them are presented here, which are especially characteristic of their complexity and intricacy. And also — of the actions of “experts” in their study and the most fantastic conclusions.
CASE 1
A crack along the piston pin is a typical sign of excessive gas pressure in the cylinder. But for a real “expert” it is always a sign of a “factory” defect
Crack in the piston crown along the axis of the piston pin. Usually, “experts” see in this breakdown signs of a manufacturing defect of a poorly manufactured piston. This happens only on the basis that they do not read any literature, and therefore they are not able to see any other reasons. In fact, such destruction is quite typical for turbo engines, gasoline and diesel. As a rule, this will be the case if the owner has performed an abnormal change in the design of the engine by “tuning screwing” — increasing power by the way of increasing boost pressure.
CASE 2
Counterbores from exceeding the permissible speed (below) have the form of a “ladder”. This is usually accompanied by brittle failure or deformation of the valve, while incorrect valve timing leads to even counterbores and fatigue failure of the valve. But for a real “expert” these are all unnecessary trifles …
“Factory” valve counterbores on a smooth piston bottom. In a completely obvious way, any “expert” knows (wake him up at night — he will say) that the reason for the formation of abnormal counterbores lies only in the inertial collision of the valve head with the piston and only at exceeding of permissible speed. However, the excess gives a completely different profile of counterbores from valve strikes — a characteristic “ladder”. This is due to the deformation of the stem during the collision of the valve head with the piston. If there is no “ladder”, then the reason is in a completely different place — in turning the chain sprocket on the crankshaft. Then, at the valve break, there will be characteristic signs of fatigue failure — rest lines, a break zone. However, for a “real” expert, all this is too complicated — why bother with it, if everything is clear anyway?
CASE 3
Complete destroying of a multi-cylinder engine due to fatigue failure of the piston pin, which occurred as a result of an abnormal double fillet in the hole. What a reason the real “experts” would find in this case is even hard to imagine …
The complete destruction of the powerful V-8 engine of the new car — 5 out of 8 connecting rods were destroyed, a huge amount of debris, and the cylinder block received 8 (!) holes from both sides. Oil, hitting the catalyst, caused a fire in which the engine compartment was completely burned out. What can be found in such a pile of melted debris? It turns out you can. Moreover, you can find everything if you know what to look for. And you can only know when you imagine all the causes of the breakdown and their signs. When this was done, they found an abnormal double fillet in the hole of the piston pins, one of which could not stand such a “mockery” and collapsed.
CASE 4
“Burn-out” of the piston due to the “wrong” fuel. And how could it be otherwise, if the burn-out exists in everywhere too! This is exactly what 2 “real” experts decided at once and without blinking a eye wrote it down in their conclusions. But we doubted — because everything is very simple. And not in vain. After cleaning the piston from soot, it turned out that this was not a burnout with melting, but mechanical wear — from vibrations of fragments of the piston ring in the piston groove. And the fragment oscillated due to the fact that it was broken off … at the factory when assembling the engine. And why exactly at the factory, and not in operation on bad gasoline? Because there was not a single trace on the piston ring, that it had worked at least a little normally before. But is this a reason for a “real” expert to think about something? Especially when you need to “please your own” dealer …
The “burnout” of the piston? Yes, of course! But after cleaning from carbon deposits gradually turned into a mechanical failure caused by vibrations of the ring fragment in the groove. Moreover, the absence of traces of running-in of the ring to the groove indicates a breakage of the ring even during assembly at the factory. But for a real “expert” all these signs are too complicated — it is much easier to declare a “bad” fuel as the cause, even if, according to the results of the analysis, it is good.
CASE 5
12,000 km after repair (due to scaffing in the cylinders – installation of repair liners in the cylinder block was performed), the 3.0 V6 engine of the Audi S4 completely collapsed. The investigation revealed: heavily worn connecting rod bearings, 2 completely destroyed pistons and connecting rods, as well as a punctured cylinder block. And it would have been possible to announce the cause – for example, insufficient oil supply due to untimely monitoring of its level by the driver or some overheating (and from there it is already not far to hydraulic lock), but…
The attentive eye of the expert paid attention to one of the surviving cylinder liners, and then to the head gasket. It turned out that one of the liners had fallen from the the block surface by a good 0.05-0.07 mm, and the edging of the gasket in this place was hanging in the air with characteristic black traces of a gas breakthrough. There was also confirmation – an entry in the work order during maintenance about topping up the coolant. As a result, all the holes in the swiss cheese matched up – a defect in the mechanical processing of the cylinder block during repair led to a loss of tightness of the head gasket and the ingress of coolant into the oil. Which further led to a deterioration in the lubricating properties of the oil, wear of the bearings, their destruction, followed by the destruction of one of the pistons from an impact on the cylinder head.
Result: the service station that performed the repair is responsible for the repair defect, even if it is hidden – it is obliged to perform a repeat repair under warranty (and then it can make claims to the workshop that performed the poor-quality installation of the liners). But it is no longer possible to perform the repair due to a completely unrepairable engine. And the driver is responsible for such consequences of the failure – he did not stop in time when the engine knock was already obvious and very hard…
Complete breakdown of the engine – destruction of the cylinder block, connecting rods, pistons, bearings. The reason – the subsidence of the liner and the leakage of the head gasket caused the flow of coolant into the oil. However, the driver himself completely destroyed the engine…
