Thread: Running Rough

A fuller explanation from an English site.

This is a fairly crude affair, utilising a air bypass screw (called the base idle setting) and a simple stepper motor controlled air valve to keep the idle steady as the engine loads vary. The stepper motor has 180 different step positions, and each time the ignition is turned off, the stepper motor pulls the air valve wide open, by being pulsed 200 times. As only 180 steps are available, it will always reach a "home position", and from this point the ECU keeps track of its position by counting pulses from the home position. One weakness of this system, is if the stepper motor sticks, the ECU looses its correct position, as there is no feedback to say where it is which leads to an unstable idle. Another weakness is how it crudely controls the tick over. If the engine is running above the required RPM at tick over, a burst of pulses is sent to the stepper motor to reduce the air supply. The ECU then waits a few seconds for the mechanics of the engine to respond. This time "constant" depends on the engine dropping its RPM in a fairly controlled manner. Further tweeks then take place if the RPM is still outside tolerance. A problem occurs if the engine RPM drops faster than predicted due to fueling errors, or ignition problems, so the engine RPM drops too far. After the wait time the ECU detects the RPM is now too low, and winds the stepper motor back again and waits again, at which point the RPM goes too high. The process then repeats itself, so the idle remains very unstable.
Road Speed sensor.

I repeat the stepper has no effect on mixture .
Regards Philip A

PhillipA thanks for clarifying the IAC issue. I had checked "the air bypass stepper", (I just didn't recognise the abbreviation IAC) in the course of my checks of the other sensors. Interestingly the screws on the plate were very loose so I removed it and cleaned it, proved it was working and replaced it. I was getting a faultering tickover but it was classic over rich mixture, I just wanted to eliminate everything else first. It had only just been serviced with new spark plugs a yet they were as black and sooty as a chimney after only 300 Kms driving it from Sydney to Canberra. I think on the Hume Highway the ECU and MAF would be just following the Mapping programmed in so the mixture probably wasn't too bad but once the revs drop below 2500 coming into the city. The mixture settings on the MAF have more influence and mine was set very rich hence the sooty look. I will do some more plug checks after different driving conditions to see if it looks right.

Cheers

Gromit

"You just gotta be lucky sometimes!"

Well Phillip my stepper motor IAC or Air bypass valve was jammed wide open on my first 93 disco and it was blowing black fuel rich smoke which made your eyes water. After a few enquiries on the Forum here (a few years back now) I was directed to the IAC by more than a few forumites. I found it not only jammed open but loose as well, so I installed a new stepper motor and cleaned the seat in the plenum and bolted it all up tight. Guess what, ran beautifully, no black smoke, no sickening smell of fuel, and fuel consumption was less. Maybe you have a special setup on yours, but mine and plenty of others have found that attending to problems with the IAC that idle improved and richness went away, maybe you can explain how this is so, Regards Frank.

If the IAC valve body was stuffed to the point of not providing manifold vacuum to the fuel pressure regulator, then indeed the higher fuel pressure resulting would cause extremely rich idling. Not a fault of the IAC valve specifically but the vacuum hose connection sucking air.

Agree That it was the looseness letting unmetered air in, not the IAC valve.

I thought about this later. Just like a vacuum hose being disconnected. In addition maybe someone richened the MAF to compensate for the leak which would make it idle OK but then be rich everywhere else.
Regards Philip A

BTW Tank, I had nearly your situation last week where my stepper was playing up and it still is to some extent. I think I had/have a poor contact in the plug and sometimes it seems to step back but not forward and open up. The only effect is that the idle went up to 1200RPM but otherwiae it ran perfectly. I think in your case someone did somewthing subsequent to the leak, or maybe it was simply no vacuum for the Fuel Pressure Regulator.