Imagine a system capable of automatically compensating for an unwanted change in a parameter. Well, at least the injection system. Advantages are well known. But here we need a caveat: everything is good in moderation, extra “compensation” - like a house flooded during a fire. No wonder the aviators change their face at the mere mention of “overcompensation of the rudders”! But let's go down to the ground - to the Audi, which I spoke about in the March issue. Let me remind you that her motor was oddly capricious: after a cold start it was "sausage", but the hot one worked perfectly. The owner turned a blind eye to this, but soon changed his attitude: it was necessary, they say, to bring the motor to mind. And he visited one serious company, where they measured the pressure in the cylinders of a cold and hot engine. The results are shown in Fig. 1. Isn't the picture interesting? It turned out that normal compression in the first cylinder was ensured only after heating to operating temperature. But why? Elementary checks at the time (ЗР, 2006, No. 3) yielded nothing: changing candles, nozzles, sensors, we only lost time. In addition, this motor is complex - many operations take a lot of time, and the owner flies a pretty penny.
This time we connected the Bosch stand, which allows measuring the vacuum in the intake tract behind the throttle, and fixed this parameter at different stages of heating. Our picture (Fig. 2) turned out to be no less interesting. After a cold start (at a temperature of about 0 ° C), when the motor "quintupled", the vacuum at idle was 0.325 kgf / cm2. Up to 40 ° C, the vacuum did not change much, and with further heating it began to increase. Finally, at a temperature of 80 ° C, it reached a maximum of about 0.425 kgf / cm2. And here's what is interesting: if we turned off the first nozzle, then the engine, of course, began to shake, but the vacuum decreased only to 0.365 kgf / cm2. Obviously, some factor depending on the temperature affected. Which one? We checked the compression, as it should be … and it was just perfect, 13 kgf / cm2 in all six cylinders!
You say - it is not clear. We agree that this motor also “loaded” us in full… We checked the valve hydraulic compensators, one was replaced. All gathered, allowed - without improvement. By removing the timing belt, camshafts, etc. (This is a lot of work!), We got to the valves of the first cylinder. We tested the stiffness of the springs with your fingers (you won’t evaluate them without disassembly!) - there are no obvious failures, but the finger is sometimes a very useful “tool”! - in this case, not too reliable. Let's hang up the valves and look at the springs.
And here he is, the answer to the puzzle! The spring of one of the inlet valves is broken, and two of its parts, approaching almost by a turn, reduced the preload - because of this, the closing force of the valve became less. Probably, the elasticity of the debris also decreased (see ЗР, 2006, No. 4). Of course, the valve still closed - on this motor the springs are hidden in narrow wells (five valves are tight), they have nowhere to go - in this sense, everything is fine. But did it close tightly? Do not forget that another player in this field is the hydraulic compensator. It is imperative that it does not turn out to be stronger than the spring, otherwise the valve will not be able to completely close. Here is an example of "overcompensation" in which the cylinder will not work.
What happened after a cold start? With solidified oil, the pressure in the lubrication system is at the upper limit for this design, the maximum compensation force. It is known that with such problems with expansion joints on some motors, the valves did not sit in the saddles very tightly - because of this, the power fell, the valve edges overheated, etc. And the ultimate case is when some valve does not close at all!