K5 C28 STVA Fixed - Keep it Simple!

Over the last couple of weeks I have been getting the dreaded C28 fault shortly after start up. Sometimes just stopping and restarting cleared it but in the last couple of days it became more persistent. Anyway I scoured the forums, Haynes manual etc and have now fixed it but wanted to share my experience with other members as the fix was far easier than the rabbit hole I was about to go down. It's a bit embarrassing (especially as an engineer but in my defence I am new to bikes) but the moral of the story is keep it simple and don't forget the basics.


When the FI fault displayed I made a jumper cable as described in other threads and found the dreaded C28 code showing. I duly did internet research and consulted the Haynes manual which told me to check the resistance and continuity of the STVA motor. I had infinity to ground and 16 Ohms across the motor which is outside of the 7-14 (approx) expected range but only a little bit. These readings were found to be constant as I worked through the wiring to the ECM. Continuity from the STVA connector to the ECM was also correct.


The fault diagnosis and internet advice was then to investigate maybe using another ECM or replacing the throttle body. I thought I'd also maybe try to drive the motor by putting a false signal to the motor and starting looking at that. Whilst I was looking into the what I would need to do if I changed the throttle body I came across the bit about STV synchronisation in literally the paragraph before the one I started at a couple of days ago. Here I found about moving the valves via the nut on the end of the gearbox, I knew not to try working the actual butterflies by hand.


That was the eureka moment where I realised I had jumped in too deep with multimeters etc without a simple check of will the valve turn by hand.


Anyway, I tried turning the nut using just my fingers but found it a bit stiff so using an 8mm socket I slowly worked it open and shut between the stops. I then powered up the ignition and the STVA moved (only a little bit and quite slowly) and I knew I was on the right track. With the use of WD 40, hot soapy water, a nylon brush and some moly grease I was able to free up the whole mechanism and now it works perfectly.


So my advice is, don't assume the worst and check the basics as you may actually be lucky like myself and not need to replace expensive components.


The arrows on this picture show where I cleaned and greased. The nut on the end of the gearbox and the arrow bottom left seemed to be the most crusted and respond the most to cleaning.


k5 stva.jpg


Also I found the correct pins in the ECM coupling to be the other end but I may have just mis-interpreted it. You are looking for the black wires with red tracer.


2v26wp5.jpg


Hope this may help some people


Gaz

[Craig380]

Really useful post, thanks for that. It is a complex mechanism so any stiff / dry / corroded points could cause the STV position to not quite match what the ECU wants to see, and throw the code.

[Fen Tiger]

Yes, interesting post. Am thinking of how to prevent a future problem by keeping the mechanism lubricated with a spray of WD40, GT85 or ACF50. Good to know what faults arise with age.

[Craig380]

Originally Posted by Fen Tiger

Yes, interesting post. Am thinking of how to prevent a future problem by keeping the mechanism lubricated with a spray of WD40, GT85 or ACF50. Good to know what faults arise with age.

You can get at all the joints with a long straw on the spray can, and a bit of squinting without lifting the tank, I've just done it ...