Idle Way Too High.

Just replaced a failed O2 sensor and now the bike runs without any errors. But it idles at about 3,000 rpm.

When first started from cold, it seems to idle at about 2,500, which is too high even for a cold start fast idle, but as it warms up, the idle rises to at least 3K and then as I let the clutch out (in gear), the revs go up another 3-400. I assume the latter is normal for a VFR and not a problem if it was idling at the correct 1150. But when it's already racing at over 3K, the last thing you need is it getting even higher as you let out the clutch.

Anyway, something is obviously up here, but there are no errors being reported. The TCP sensor (controlled by the throttle cables) seems to be shutting completely as when the engine is not running, I can hear it snapping shut. But for some reason the engine is idling at a stupidly high speed and the ECU is not reporting any problem. In fact in all other respects the bike seems to be running perfectly.

Anyone any suggestions?

Have you tried disconnecting the battery for 20 minutes and plugging it in again?

Did  you  do  any else to  the  bike ?  change air filter ?

Have you tried disconnecting the battery for 20 minutes and plugging it in again?

The battery was disconnected briefly (a couple of minutes at most) since I first ran the bike and found the high idle speed. I'll try it again for a longer period. Is this a recognised procedure? I've not seen it in the manual anywhere.

Did  you  do  any else to  the  bike ?  change air filter ?

The air filter has I'm told been changed, but even if fitted badly, I cannot see how it would affect idle speed, let alone as dramatically as this. My plan is to check all that today.

I should point out that this bike was 'drowned' in a ditch and although it now looks immaculate, there of course has been some 'drying out' work done on it - not by me. When I got it there were several fault codes stored, but the only one that was real was the O2 sensor. Now that's been replaced, it generates no fault codes. So this is a process of tracking down the last little problem. I realised there would be a few issues and so I'm not surprised. Just a bit puzzled at the moment.

This is what I'm thinking about the problem.

THE thing about which to be most concerned when designing a TBW/RBW (Throttle/Ride By Wire) system is the possibility of the throttle being held open more than the rider is asking/expecting. That's one of the reasons why there's a lot of duplication within the system, so there's a failsafe, a backup if something should malfunction. With this in mind, it seems to me that if the engine is revving higher than the ECU thinks it should, it would generate an error. So if the throttle is apparently closed, but the engine is still running at 3K rpm, the ECU would notice that and report at error. Ergo, the ECU must be thinking the rider is asking for that rpm.

If the throttle actuator motor is in some way faulty so it doesn't shut the butterflies completely, it must see that as being an error.

If the TPS (Throttle Position Sensor) is reading wrong, so it's reporting butterflies shut, when they're actually still slightly open, again the ECU MUST realise this is an error.

If thee's an air leak, so more fuel is added to compensate, once again the engine is running far faster than the ECU is expecting, so there must be an error.

If the ECU thinks the twistgrip is closed and hence the engine should be idling, it will try to control the engine speed at its programmed idle speed of 1150 and whatever the actual fault, it must report an error if engine speed remains at 3K rpm.

The only thing that makes sense is if the TCP (Throttle Control Position) sensor that is actuated by the throttle cables is reporting the throttle open more than the actual position of the twistgrip. In this case the ECU cannot know about the discrepancy, only the rider will notice that. All the ECU thinks is that the rider is holding the throttle slightly open. It CANNOT know that the twistgrip is actually closed.

This is supported by the fact that even the cold start idle is too fast (about 2,500) and it then rises slightly as the engine warms up. This is exactly what would happen if you held the throttle slightly open. Idle speed would be higher than the automatic cold start fast idle speed which would then be irrelevant and it would of course speed up a bit as the engine warmed to full operating temperature - all the while NOT throwing an error because as far as the ECU is concerned, the rider is asking for that.

However, there are TWO TCP sensors. If only one was faulty, the ECU would throw an error and it seems unlikely to me that both TCP sensors would misread in the same way - which is exactly why all vehicles that use TBW systems run duplicate sensors.

So the only fault that seems to fit the symptoms cannot be occurring. Which means some part of the above analysis is flawed. If anyone has any suggestions about this, they'd be much appreciated.

OK, here's another thought. I must be wrong about the ECU noticing the engine speed is too high. If the ECU actually takes no notice of engine speed and simply does what it thinks is right, then the engine speed could be sky high and the ECU would take no notice. So, e.g:-

THE TCP sensor says the throttle should be closed, so the ECU shuts the butterflies and adjusts fuelling to suit the other sensor inputs, like manifold pressure etc, but NOT actually checking the result (i.e. rpm). In this case, if there's an air leak, the ECU would simply respond by increasing fuelling to suit which would result in a higher engine speed than expected, but since the ECU is not checking this, it would report no error.

So it looks to me like the ECU is not as smart as I first thought and is NOT checking engine speed and hence is not aware that even though it's supposed to be idling, in fact it's racing at 3K rpm and no error will be generated.

This then would be the flaw in my earlier analysis. The ECU does NOT check the result of its 'commands'. ECU says 'IDLE', but the engine races and the ECU doesn't even notice.

Does anyone have any more in depth knowledge of how the ECU operates?

Is  there  anything  in  the service  manual  about re-calibrating the throttle  after  ECU  or  TPS  replacement ? 

Is  there  anything  in  the service  manual  about re-calibrating the throttle  after  ECU  or  TPS  replacement ? 

Basically, no.

In the Troubleshooting section, it mentions idle too high as a symptom with the following diagnostic procedure:-

1. Check idle speed
2. check throttle operation and freeplay
3. check the TBW system

None of which is much help because:-

1. The manual section that relates to this just says it's fixed and cannot be adjusted.
2. The cables are not too tight and I can hear the TCP sensor clicking as you close the twistgrip, so it IS shutting completely.
3. Yes, good idea. But all the checks are based on the DTC error (fault code) that has been displayed. But there are no errors.

Looks like I'm going to have to pull off the throttle bodes and just have a look around, but it would be better if I knew what to look for.

If it started happening after you replaced an 02 sensor then that is where your problem lies

If it started happening after you replaced an 02 sensor then that is where your problem lies

Sorry if I wasn't clear. When I received the bike it was idling too high and there was an O2 sensor error. I've replaced the sensor, cleared the DTC and ridden the bike long enough to see that NO codes are now being generated, but it is still idling too high. So the O2 sensor replacement and idle speed issues are not related.

I think it's time to take the throttle bodies off and have a good look.