My boss took a Polo in for diagnostics and himself using his scanner advised that a camshaft correlation problem was present. The car was left with us last week and Saturday afternoon I decided to put the scope on it to see what the engine shows!! Before touching the car, the engine starts and runs reasonable smooth at idle except for the odd erractic idle that smooths out after a couple of seconds. The engine management lamp is illuminated.

The crankshaft and camshaft sensors on this vehicle produce square waveforms. I removed all the spark plugs and disconnected each ignition coil. I placed the WPS into cylinder 1, and connected crank to channel A and camshaft to channel B. Channel C was the WPS. A cranking waveform was carried out, but had a couple of problems...

1/ The scope keep loosing connection via USB for some reason, and
2/ After cranking waveform had been recorded, by the time I stopped the engine and got to the scope to stop the trace running, and then using the buffer to play back the waveform, the waveform had not recorded?

This occurred several times and I was ready for giving up until this last attempt when the scope recorded the waveforms. Now it could have been user error because I'm not experienced with the use of the scope software 7. I set a trigger, which then seemed to allow the onscreen waveforms to be recorded (remembered) when I used the buffer to play back the wavforms.

Referring to the waveform I initially used the ruler legend and the rulers from Pico 7. Setting the TDC 0 using ruler 1, and then using ruler 2 moving past TDC 0 to the trigger point of the camshaft sensor, Pico recorded nearly 80 degrees rotation. Now I'm not sure what that result actually means! Looking at where the camshaft sensor is triggering in relation to the WPS waveform, that engine does seem that it should not be running at all?

I recorded the camshaft and crankshaft waveforms together also without the WPS installed. Looking in those waveforms, the flywheel missing teeth I suspect are at 180 degrees apart, and in conjunction with the cam sensor signal, the actual data the waveforms provide to me seem useless? This was why I put the WPS in to provide some clarity as to what is actually occurring in the cylinder and when!

I then used an overlay on the scope waveforms because I could not convince myself that I was getting the angles of degrees rotation correctly interpreted, hence the overlay has built in TDC markers and additional markers at 30 degrees each, therefore once I'd established where TDC was using the WPS, I could then using the 30 degree markers and the rulers from the overlay try to determine how many degrees the timing was out!

Now another confusion I gave myself was always referring to the gear teeth as 6 degrees apart each? This rule I was using to determine the above timing shift, but nothing made sense, and this is why...

When I dismantled the engine today, the first job I completed was to remove the crank sensor. The star bolt is in at an angle facing upwards towards the inlet manifold. I found it impossible to access this bolt. I could see it but could not put a tool on it, therefore my only option was to remove the driveshaft so that I could remove the sensor.

For anyone who ever needs to change a crank sensor on these engines, the driveshaft can be removed completely without dismantling the suspension assembly.

After removal of the crank sensor I used the timing tool to locate TDC. I then removed the blanking plugs in the rear of the camshaft housings. Looking at the camshaft tools designs, it would appear the camshaft slots must be horizontal to locate and secure them.

Before the timing chain was removed, the timing tools were offered up to the camshafts, the misalignment of each camshaft by visual representation of the timing tools appeared to be only a few degrees out in each camshaft.

Now after removing the timing chain cover and inspecting the chain, visually I didn't find any chain slack! All the components appeared in tack and in good order. After removal of the chain I didn't notice any wear in the links, and truthfully, I could be wrong, but felt it rather impossible for that design of chain to stretch the distance shown by the cam tools being inserted as above.

I now looked at the camshaft gears and found nothing obviously wrong with them, and then remembered to count the teeth. The camshaft sprockets each had 36 teeth, which equates to 10 degrees each tooth, and the crankshaft sprocket having 18 teeth equated to 20 degrees each tooth. Now if one didn't know that, and being honest I didn't, then the interpretation of the scope trace assuming each tooth was 6 degrees, then the results would be very innacurate!

Now this next part I don't know is correct or not, but the overlay software records 54.8 degrees after TDC for the camshaft sensor to trigger. Now is this timing inaccuracy recording both the crank and cams? The camshafts didn't look that far out of time based on the tools being fitted, hence I came up with this sinario...

Timing... 56.0 - 1.2 = 54.8 degrees
54.8 - 40 = 14.8 degrees
14.8 / 2 = 7.4 degrees per camshaft

Very much open to further discussions...

Added more information.

Referring back to the text above I can see I got some understanding wrong in regards to the camshaft sensor operation. There are two methods of operation, one is positively triggered and one is negatively triggered. It seems I got them the wrong way round in operation!

The laptops I'm using won't allow me to overlay previous and post repair scope traces on the same pc. Sometimes even the ruler legends won't display the data correctly, which must be down to my laptop?

Anyway moving on... I originally started off by just wanting to see the difference in valve timing correlation between a chain either stretched or slipped, and a good properly timed engine, but then progressed into seeing other areas of thinking that initially caused more confusion that anything else!

Valve timing understanding using a scope for me is somewhat difficult and not that straight forwards. Here is why...

The scope waveforms of the crank and camshaft of this particular engine setup show as any other the crank and cam. The crankshaft waveform displays the number of teeth per 360 degrees of rotation, but that is the crankshaft teeth and not the actually gear teeth of the timing chain on the crank, which is completely different in the number of degrees of rotation. Also as explained above the camshaft gears teeth are completely different in their degrees, hence when examining the scope waveforms, what is seen and being thought about is not actually the same as the valve timing of the engine?

If one now refers back to the waveform above with one cylinder WPS operation shown, the number of degrees of crankshaft rotation to the point that the camshaft sensor signals the PCM to switch off the trigger signal, the ruler legend records 76.46 degrees ATDC.

If we now look at the scope waveform that records 720 degrees of cylinder operation with the use of the WPS, the data records the crankshaft rotation to the point of the camshaft sensor signal instructing the PCMto switch off the trigger has been recorded at 70.91 degrees ATDC.

If we do the math here, 76.46 - 70.91 = 5.55 degrees

If this approximates to 6 degrees of the inlet camshaft timing out of phase, then the above estimate I did reproduced here...

56.0 - 1.2 = 54.8
54.8 / 40 = 14.8
14.8 / 2 = 7.4 degrees

Now if we truncate the figures, there is 1 degree difference!

My own opinion here is that the timing chain did not "stretch" to cause this correlation problem. I've seen this before going back to the early 1990s and I would lean towards the hydraulic chain tensioner "spring rate" having weakened inoperation, along with a delay in oil pressure build up to maintain the chain tension.

For clarity, I'm not saying the engine oil pressure rise time is slow, I'm saying the tensioner spring is weaker and more reliant on engine oil pressure to maintain the correct chain tension, which has not occurred and allowed the slack in the chain and jumped teeth on the gear wheels.

Also worthy of note is that the exhaust camshaft timing can't be seen on the scope, and at cranking or engine idle no real evidence is available to see if a problem is present, however, if the engine speed to raised (but this is with caution) and not without risk because the valve timing could jump out of time further and cause engine damage, when the piston is near TDC on the exhaust stroke, the pressure significantly rises if the exhaust valves are open when they should be closed. I noticed this on the scope at higher engine speed.

I almost forgot to mention that looking at the waveform before repair, the cylinder 1 compression was recorded at 10.50 bar (152 psi) and the repaired engine waveform recorded 14.57 bar (211 psi)