KV6 engine racing

[KWIL]

The brass throttle spindle has a short length where there is a flat on both sides. this allows the various pieces which are on the shaft to be fixed and should not rotate about the shaft itself.


The pieces can slide on but do not rotate about the shaft.

[SD1too]

Thanks Ken, I understand what you mean now.

Simon

[Synchromesh]

Sometimes thick grease is useful for locating air leaks. When carburettors were the norm they used to apply it to the outside of butterfly spindle bearings and flange joints as it sometimes provided a brief fix that located the leak.

[SD1too]

I woke this morning with an idea that if the traction control butterfly sensor was incorrectly telling the ECM it was reducing airflow, then the idle speed stepper motor might try to compensate.
Sadly not; I could not find any fault with the circuit.

I have read and observed that when the ignition is switched off, the stepper motor moves to a position suitable for the next starting of the engine. When I then start the engine, it idles perfectly as expected.

So with the engine stopped I unplugged the stepper motor so that it could not move from this proven position. I turned the key and the idle was good so I opened the throttle slightly, the revs rose and I closed the throttle again. As the stepper motor must have remained in its idling position I expected the revs to drop back to their previous level.

But to my amazement they didn't. I plugged the idle air intake and the engine promptly stalled. How could this be?

The engine speed stays at whatever level I set by moving the throttle linkage; 1,600 rpm, 2,600 rpm, even 3,000 rpm. If I stop the engine and restart it runs with a near perfect idle!

The only sensor I cannot check is the dual purpose intake air temperature/manifold absolute pressure (IAT/MAP). But even if that's faulty, how is the engine getting huge amounts of extra air? A worn throttle spindle would not provide enough area.

Simon

[Bob Walker]

Please, just make sure its not your car mats! I had a racing away KV6 and the night before it started happening I hoovered the car, the mats are really heavy and the drivers was catching the accelerator, drove to work the next morning and I had to switch off the engine at every roundabout and there are eight on the way to work (no, I don't live in Milton Keynes!!). Took the car to my friendly local garage who couldn't come up with a solution, drove home and it dawned on me what I had done. Went back to the garage to tell them I had found out what the issue was and all five mechanics extracted the Micheal out of me!! Never lived it down since!

[Synchromesh]

When you unplugged the stepper motor you assumed as there was no power to it it would not move. I am not familiar with the R75 stepper motor but it may provide positional feedback to the ECU. If so it would be held in the demanded position when connected and if air passing the valve tried to move it the ECU would hold it steady. With it disconnected the valve may be free to move by the air passing through it. How many pins are there in the stepper motor electrical connector.

[SD1too]

Originally Posted by Bob Walker

Please, just make sure its not your car mats!

Thanks Bob, that cheered me up last night after a long day of unsuccessful fault finding. I'm pleased to say that my driver's mat is firmly latched onto the hooks which Rover thoughtfully provided.

Simon

[SD1too]

Originally Posted by Synchromesh

When you unplugged the stepper motor you assumed as there was no power to it it would not move.

Thank you Steve for your continued interest and suggestions. This is what RAVE says in answer to your first point:

"The bipolar stepper motor ... will hold its position and resist external forces to move when stationary."

The shaft can be moved in and out manually but it takes a really firm push. The frustrating thing is that it's impossible to tell what the motor is doing (without diagnostic live data) because as soon as the engine is stopped, the motor returns to its "referenced" starting position which provides a tiny air bleed through the valve.

The motor has two windings each of which can be energised in a positive or negative direction, so four terminals in the connecting plug. There is no feedback to the ECM from the stepper motor. The ECM obtains engine speed from the crankshaft position sensor.

Simon

[Synchromesh]

I bought my V6 new in 2000. My service record shows that I cleaned the idle control valve in 2004 at 39000 miles because the idle was erratic. I had to clean it again in 2006 at 52000 miles. Ever since 2006 the idle has been perfect and it is now at about 122000 miles. It is a long time since I did this but I vaguely remember the plunger was difficult to move before I cleaned it but easier after I cleaned it. I think the thread below is similar to your problem. Unfortunately as is often the case the solution has not been posted. Note how he pinches the rubber tube.


https://www.mg-rover.org/threads/kv6...revving.95119/

[SD1too]

Originally Posted by Synchromesh

I think the thread below is similar to your problem ... Note how he pinches the rubber tube.

Yes, Greg's findings are very similar to mine. Like his, my engine is definitely racing on air supplied via the idle control valve. For currently unknown reasons, my ECM is telling the stepper motor to provide huge amounts of air, far in excess of what's needed for a normal idle.

Today cb750Chris arrived armed with his T4 and TOAF. We spent five and a half hours going through everything together in great detail. Unfortunately TOAF wouldn't talk to my ECM but T4 provided valuable live data. It was a great frustration to us both that it gave no clue as to why the engine speed remains high following depression of the accelerator pedal.

I'm making arrangements to temporarily replace the manifold pressure sensor since this tells the ECM the volume of air consumed by the engine which of course is directly related to engine speed. I'm not keen on diagnosis by substitution as regular readers of my posts will know, but I have little choice with this problem.

Simon