I've got the cylinder head off, and have turned over the block a few times now. There is a mark on the crank pulley to indicate TDC, and obviously the pistons can be seen. As it is a 4 stroke (Audi 1.8 20v), I've now got a worry in my head that it'll be on the wrong stroke when it is put back together (i.e., instead of being at compression TDC, it'll be at exhaust TDC).
Put my mind at ease and tell me where my thinking has gone wrong. Thanks Alan.
The issue is not what stroke is it on now, but what stroke is it on when you have connected all the valve gear back on. The cam drive will determine (and define) what stroke it's on. If you connect the drive up according to the timing marks then it will automatically be OK.
Rob Graham
I wouldn't worry, that is what the cam position sensor is for, its the only way the engine knows which cylinder ison compression stroke etc.
alan@darkroom.+.com (A.Lee) gurgled happily, sounding much like they were saying:
Simple. You're forgetting that with the head off, there's no difference whatsoever between compression and exhaust TDC, because there IS currently no compression or exhaust - that's all governed by the cams, which are on the head...
As long as the relationship between crank and cams is correct, it'll be fine.
Assuming an overhead cam it doesn't matter. It does matter if the cam (or distributor) is in the block as those run at half crank speed, and you haven't disconnected the drive to those.
That's not true. The camshaft sensor is responsible for initiating the fuel injectors at the right time. Its the position of the cams in relation to the crank which determines what stroke the cylinder is on.
No, you are wrong. The crank position sensor is responsible for initiating fuel injectors, the cam sensor is to tell the ECU which of the possible 2 conditions exist at start up, i.e cylinder 1 on compression at TDC or exhaust at TDC. The ECU cannot tell this by looking at the crank position sensoe alone, it must look at the Cam sensor. Some engines only use this sensor at start up, after that it is no longer used, all timing comes from the crank sensor. The cam sensor on non variable cam timing engines gets one on and one off signal per 2 revolutions of the crank shaft, this is not enough to accurratly control injection and spark timing.
Yes I appreciate that the cam sensor works in conjunction with the crank sensor. And I also know that in many cases the cam sensor is only read at start up (and can often be disconnected after start up), but the point I was making is that its the cam sensor which initiates the injection sequence on the correct stroke. Yes I know that the crank sensor fires the injectors (through the ECU) when the engine is running but its the cam sensor which provides the initial timing.
Which one of these two electronic sensors is it that tells the valves whether to mash into the pistons or not?
The OP has the head off, thus has disturbed the relative timing of cranks and camshafts. That's what he need to re-align. Once that's set, the ECU can read the sensors without further intervention.
Neither. With variable valve timing the valves can't hit the pistons with any fault on the 'variable' mechanism. Only if the cam drive fails.
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