STVA C28

[Higherford Kid]

Hi again folks,
I am guessing that this topic has plenty of old posts so maybe you can point me to them or offer up a suggestion.
I have been getting the C28 code a few times on my 2007 DL650 but I can reset by switching off and back on so no stress. But here's the puzzle ( for me anyway!)
The bike starts, runs and pulls well and I can ride all day on country roads keeping to 50 or 60 without a problem but when I get onto the motorway and take her up to 70, I get the evil eye after about half a mile.
Is the SVTA not opening fully at higher throttle openings or is it something else?, any ideas please.
I will soon be putting her away for the winter but would like to ride a bit longer and I don't really like riding with fault lights showing !
Cheers

[greywolf]

That's almost always the result of a sticking STVA linkage, usually at the stepper motor shaft that goes through the throttle body to operate the secondary butterfly. Here's an STVA video showing its actuation. Some WD40 on the shaft at the motor and the shaft bearings in the throttle body will often free things up. A C28 may involve sticky linkage on the other side or a bad STPS though that is rarer.

[Higherford Kid]

Thanks greywolf, brilliant video.
Does the sensor for the actuator only sense the full open position of the throttle valves ie. is it just a " switch" actuated when the actuator has reached the end of it's travel or does it monitor the whole of the travel from shut to open with analogue feed back ?
I am hoping I am not going to do any damage riding for a couple more ride-outs with this fault since it will only be affecting emissions ?

[greywolf]

The STPS reports the exact step out of all the steps the secondary throttle may be at to the ECU, not just open or closed. A C28 will normally be noted as the motor stuttering as it tries to position the secondary throttle plate where the ECU "tells" it to. The motor tries to step to the requested position but the stickiness or obstruction prevents that, the ECU reads the position of the secondary throttle plate, "sees" it isn't there, and signals the stepper motor to try again, etc., resulting in the perceived stuttering. After a few seconds of trying without success, the ECU throws a C28 and signals and FI.

Again, there may be other reasons for the failure than a sticky shaft like a bad STPS or a physical obstruction in the secondary throttle linkage, but the ECU is not getting a good report of the secondary throttle position for some reason.

[Higherford Kid]

Many thanks for the great explanation. I can understand more and more how this bike works which makes repairs so much easier. Modern bikes seem to depend more on electronics than I have ever been used to. I have had plenty of bikes, stripped engines and transmission without problems but these days it's all a bag of tricks.
Thanks again.

[mr_diver]

Had the same issue a long time back.

Loads of wd40 and wiggleing the end of the motor shaft by hand.
Followed by riding a day to work and back and more wd40 with all panels and tank still in place for a couple of weeks.
2 years later it hadn't happened again.
I always gave a good squirt of wd40/acf50 on the throttle bodies whenever the tank was off or the bike got a scrub.