Polo 9N AZQ motor does not start

[PeterPolo9N]

It's a 2002 Polo 9N with 1.2 12v AZQ motor
Before the troubles started the motor ran ok for years, be it with faults on the catalytic convertor when checking with VCDS. Strangely enough, these had disappeared after one of the prvious fault resets I made, as well as the yellow 'motor trouble' indicator on the dashboard. Only issue remaining was a fault on the inlet manifold pressure (but intermittently), but still the motor ran ok.
I had just started the motor and it ran like always well, but only for 1 or 2 seconds. Suddenly it cut with a kind of shock and an accompanying short noise as if something broke. Since then, nothing any more. Checked the fault codes, but nothing new (only the intermittent inlet manifold pressure). Turning the key does make the motor turn around, by moments it 'wants' to do something, but it does not start.
I changed the inlet manifold pressure sensor for a new one (as that was my only fault) and the ECU senses that ok (the error returns immediately if I put the old sensor back / it remains away when I put the new one). So no faults are showing anymore.
Fuel looks to arrive ok, and ignition seems to be ok as well (unmount each of the plugs, make a mass against the motor block and start > ignition sparks show at the the tip of the plug). To check the fuel injection is ok and fuel gets injected, I disconnected the injection rail. Fuel is pulverized in a nice triangular spray and coming out of the injectors. Even more surprising... With the injection rail placed about 1 cm out of it's mounting, so with all 3 injectors in front of their holes in the inlet manifold, the motor is able to start and turn more or less for a few seconds until I cut all with the key. Of-course it turns completely square, but it turns !!! My conclusion was that this would have to be an issue with the incoming air, because with the injectors put in front of the holes, the motor sucks into it's cylinders beside the fuel as well some air that it can grab through these holes. So I mounted the injection rail back into place, and unmounted the throttle body putting it at a distance so that some air can freely pass into the inlet manifold... Nothing happens. I cleaned the throttle body and put it back, still nothing. I (re-)aligned it with VCDS... nothing. I put a throttle body of another AZQ motor on it, aligned it with VCDS... still nothing. Faults appeared of-course after having turned square with the injectors loose, but once everything is mounted back into place and the faults reset, no fault re-appears.
Next I am thinking of unmounting the whole inlet manifold and EGR valve circuit, but I have no idea this will get me any closer. If anyone has a nice idea, that would of-course be hulpfull ?

[Crasher]

The valve timing has jumped on the drive chain, you need to use the proper locking tools to position the crank at zero and then access the position of the cams with a pair of locking plates.



Inlet cam lock, hole aligned with M6 thread in head ready for setting bolt



Exhaust cam lock. Most often it is the exhaust cam which jumps and will not align with the crank or inlet cam

[Rob69]

That would explain the shock and noise you described when it stopped.

[PeterPolo9N]

Ok, thank you. Will verify that but need to buy that alignment kit first. Apparently it can be found on eBay for around 20€ + shipment so not the end of the world, even if I use it only once. Is such valve timing jumping on the drive chain happening frequently, in general ? And on this AZQ motor ? It's 15 years old and hardly has done about 105.000 km...

[Crasher]

"frequently" is not the way to describe the problem, it is a HUGE issue with chain cam engines due to the insane long life oil changes, these are OK on plain bearings but chains do not like it and wear. I have seen them fail in 25K miles. Make sure you use a good kit such as Febi and then do 15K Km oil changes intervals at most.

[PeterPolo9N]

Your analysis was completely right ! Indeed, the timing chain had jumped

This is the position of both camshafts with the locking tool inserted on the crankshaft. As the movement of the one to the right (inlet) is less then 1 dent, it looks indeed the chain was worn out (if not that slight movement less then 1 dent can not be explained). The camshaft on the left (exhaust) looks to have moved 2 dents

Attachment 35006

The root cause of the troubles is probably indeed the oil. This little car was the 2nd car in the family, even 3rd one for the passed 2,5 years, so it made not that much km. But off-course, it's not because you do only 3000-4000km a year you should not change the oil at least after 2 years. Meanwhile the oil change was 4 years ago...

Thanks for the perfect advise !!

[Crasher]

Liqui Moly are launching an additive that is supposed to help replace SOME of the anti wear additives removed years ago to keep catalytic converters happy.