This Golf was presented to me with a “misfire” as the customer called it. In fact it was more of a judder or mild kangarooing under hard acceleration, also the performance is poor. It won’t spin its wheels in first gear in the wet!
With no DTC’s present I scope the MAF & get a nice clean 4.6 volts peak. I have tried a genuine new MAF aswell. I checked exhaust back pressure with an adaptor, screwed into the lambda sensor flange. There is no significant pressure increase even when revved hard. I found a fault in the “Over run pressure release valve” (dump valve). The diaphragm was porous so it was replaced. This has improved the kangarooing, but the performance is still poor.
A mitty vac check on manifold boost pressure is showing 7-8 psi at full throttle. I am a little unsure about whether this is adequate or not. Elsa Win gives 1.500 to 1.650 bar (absolute). Which I think means that I need to take atmospheric pressure away from that figure to leave 0.500 to 0.650 bar, which I think means that my 7 to 8 psi is adequate. There is a steady 20 inHG at idle too.
I have road tested with the waste-gate control hose disconnected, so that it should overboost if anything & it is still poor. I have done a smoke test of the inlet manifold & intercooler & associated pipes without finding any leaks.
I cant see any PID for MAP (boost pressure) & I can’t see that it has a sensor for this purpose. I wonder how boost pressure can be electronically regulated with no MAP info for closed loop feedback?
The down pipe has been dropped & the turbo spindle feels nice & free with no excess free play. The one thing that is clear & that is that the waste-gate valve doesn’t sit squarely on its seat. It appears that the waste-gate pivot spindle is worn & there is some free play in it that is “cocking” it over slightly. I am not sure if this is possibly the cause of my problem.
EDIT... I just remembered that it seems to reach a rev limit at approx 5000 RPM. It doesn't sound like a normal limiter though. It sounds "strangled" like a blocked exhaust. I have checked fuel pressure which appears to be fine.