I just wanted to take the time to briefly talk about a long term timing/ fueling issue that I have recently fixed.
Hoping that this can help others when going through the frustration of diagnosing running issues.
Obviously with an older car you start suspecting everything being a potential issue and causing running issues.
So generally when cold it seemed to be at about 80%, then slowly it would start to have pinging issues and randomly at different points in the rev range.
No real issue when starting, it isn't a new car so I wouldn't say it was a hard starter.
It felt under powered and would have regular periods of pinging unless I was super conservative on the throttle to reduce or avoid the pinging.
The attempted cure
In the mix of it all I had a faulty Coil ICM that was causing issues then died, it was replaced and the car ran again.
As I had a few spare parts I played around with different throttle bodies with different TP sensors.
Replaced plugs, leads, rotor arm, rotor cap, Fuel pressure regulator, fuel injectors, temp sensors, lifters, cleaned lots of parts including the distributor and so the list was getting vast.
I was at the stage of replacing the TPS and the MAF and rushing a conversion to coil pack and new style MAF.
The tune was also considered as a possible source of the problem as the car runs a Mk4 head gasket and a VWMS inlet mani.
The actual solution
Rather than continue to read endless posts on VR6 symptoms and repairs I researched what each component did what on the system. Through my research I believed that the Engine Speed sensor/ Crank Position sensor would start to cause starting issues and cutting out when coasting to a stop. Now I have experienced the cutting out a few times but that could have been due to the weak idle when the revs were dropping when coasting to a stop.
Recently if I depressed the throttle to floor and turned the ignition to on then took my foot of the pedal and started the car it seemed to run better, seemed to have some pull back.
This is what happens to you when you start trying silly things to cure a problem you really have no idea about.
Everything seems to come back to TPS or MAF, nothing made a lot of sense. Since VW Classic Parts had the Genuine Crank position sensor and it wasn't super expensive I decided to place an order for it with a few other items. You know this thing isn't hard to fit so it wouldn't damage my pride too much if it didn't make a difference.
To support my previous belief the car started up the same it had done in recent memory.
I test drove the car to temp and gave it a bit of a thrashing, no pinging.
The nice VR6 sound had returned a little as well, it sounded a lot cleaner and no pinging.
The information I had read focused a lot on the early MAF (plat. wire) and or the throttle sensor being an issue, it is very easy to convince yourself of the issue especially when you're making the cure fit the symptoms. The engine spend sensor was affecting timing really badly at times and obviously fueling, with all the informing I covered I think the most important point to keep in mind is that the Crank position sensor senses what event the engine is at to accurately time and fuel the engine. It is heavily replied upon from what I understand.
During this issue, it covered quite a period of time as it isn't my only car, the problem was intermittent and sometimes I would have good power.
I think the Corrado is a bit of Champion for being able to carry on when it was obviously having trouble.
Remember it still ran and didn't have a starting issue so it didn't present itself as a bad crank sensor.
It's great to have the engine running cleanly and not going through those pinging cycles, a lot of the problems I have read might just benefit from changing the Crank position sensor???