Erratic tick-over

[LenChim]

4.5 chim. I have an erratic tick-over. When the engine is warm It slows down to almost nothing then revs up again, ut other times it runs at 1500 rpm. I sometimes smell petrol. How do I check that the stepper motor is working? Can I put a voltage onto one of the pins and see it move? The car sometimes seem to shunt if I am moving in slow traffic. Oh by the way the car seems to go very well apart from that.

[bobfather]

Have you cleaned the stepper motor? That's the first step

[Steve_D]

LenChim said:

....... How do I check that the stepper motor is working?...........

You need RoverGauge. It is free software but you can buy it along with the connection cable and full instruction for (i think) £35 in the classifieds.

With RG you can see what the stepper is doing and command it to open or shut so you can check it is functioning. You can also use the RG command function to help power the stepper piston out of the body so you can clean it completely.

Steve

[LenChim]

As you can see I have cleaned the stepper motor, although it was not ever dirty to start with. Now I will have to see if I can get a RG software and cable.

[LenChim]

[Steve_D]

Cleaning the end is OK but that only addresses the seal of the plunger against the housing (you did clean the housing?).
With RG you can power the stepper to eject the plunger completely for a proper clean.

Steve

[Classic Chim]

As these damn things are such a pain in the arse can someone explain the electro mechanics of the thing.
So electrical pulses push and pull it, does the shaft turn (spin) inline with the pulses or just move in a straight in out with no circulating. Mine would spring off if held in my hand and manually moved with RG, ECUMATE actually as it was years ago now. Was that normal?

How does the shaft move via a small electrical charge.
Sorry for the school boy questions but I’ve learnt all I know on here

[Steve_D]

The plunger has a male screw thread. Inside the body is a stepper motor with a female screw thread inside which the plunger fits into.
A stepper motor rotates a number of degrees when it receives an electrical pulse so an ECU or controller can send a set number of pulses and know how far this will move the plunger.

Steve

[Classic Chim]

I googled stepper motor cross section drawings.

Yep I get it now.
So if your plunger is not gummed up and clean is it the motor that starts to fail when sent the pulses, something similar to a sticky relay. Worn contacts internally. Heat effected most of all!
So the pulse comes to it but the motor fails to move the shaft and out of sync it then becomes until re set again via engine ign off.

I don’t even posses one anymore

Thanks Steve

[blitzracing]

Its not just gum- there is a metal ball bearing at the rear that supports the shaft, and this can corrode with nasty gasses that get blown back into the plenum. Mind you from the symptoms, I doubt its the actual stepper causing the issue.

[ChimpOnGas]

Classic Chim said:

I googled stepper motor cross section drawings.

Yep I get it now.
So if your plunger is not gummed up and clean is it the motor that starts to fail when sent the pulses, something similar to a sticky relay. Worn contacts internally. Heat effected most of all!
So the pulse comes to it but the motor fails to move the shaft and out of sync it then becomes until re set again via engine ign off.

I don’t even posses one anymore

Thanks Steve

I'm of the mindset that says....

"If you can get away without running an idle air control valve (of any type), you absolutely should"

If you think about it an idle valve of any type is really just an ECU managed vacuum leak, and it was always drummed to me by the old heads that taught me carb tuning that...... "you absolutely must eliminate all vacuum leaks before you can even consider starting the process of tuning any fuel system".... These highly experienced mechanics were really quite instant over this rule so it stayed burnt into my brain to this day.

I believe Powers Performance understand this fundamental rule better than most, by all accounts it would seem they've proved they can nail the tricky cold and hot starts and catch stalls controlling idle speeds just using dynamic ignition timing management (scatter spark), apparently this frees them to run a system that has no requirement for an idle air control valve of any sort which I can't help feeling is only a good thing

To me the Powers Performance MBE system is superior to others in this respect, personally I've ended up running scatter spark on my Canems system which does the idle management thing brilliantly all on it's own (when configured correctly), but in my case the idle valve still sits waiting in the wings to catch a stall and help with cold starts especially.

My experience with idle air control valves is less is more, the less involvement they have and the less you rely on them the better! This takes me right back to what those highly experienced mechanics drummed into me all those years ago....Ie "When tuning an engine for nice drivability and a super stable idle, all vacuum leaks are bad".

It seems David Hampshire who designed the Canems system also understood an idle air control valve may introduce certain undesirables all of its own which is why he added the Idle Valve Fuel Correction feature to the software...



The thinking behind the above feature is simple, ie "If you're going to add air, you better add fuel too.... or you'll be sure to run lean which will clearly have a serious negative impact on idle quality and drivability!

My guess is the 14CUX does this fuel adding trick too, it must do as the narrow band lambda system isn't nearly as responsive as the wide band systems most aftermarket ECUs sport (mine included). However, perhaps when it retracts the stepper motor to let air in the 14CUX is then equally slow to add the required extra fuel too...... which would clearly contribute to the shunting problems our cars are famous for

The old boys I worked with back in the early 1990's learnt their craft on a diet of carburetors and distributors during the 60' & 70's, TBH they knew nothing of fuel injection and ECUs but they sure as hell knew how to make an engine run nicely. The fundamentals of engine tuning remain unchanged no matter if your working with a ancient SU carb or the most sophisticated injection system going.... because the basic principles of an internal combustion engine remain completely unchanged too...... so all the same rules must apply.

If you add air you better add fuel too, and fundamentally all vacuum leaks are bad anyway, so with idle air control valves...... less is definitely more and if you can eliminate the thing completely as Powers Performance have managed to.. then that can only be a good thing!

[Steve_D]

This stepper is not particularly powerful so if the thread on the plunger and other bits are gummed up then the stepper either does not move or misses some of the pulses. As you say the ECU does not know this has happened so does not know where the stepper is. At ignition off the ECU sends way more pulses than needed to close it to make sure it has a datum for the next engine start however if it is gummed even this does not help.

Steve

[ChimpOnGas]

Steve_D, do you know what the 14CUX does with the fueling when it allows air into the engine via the stepper motor?

It would be interesting to understand what the Lucas engine management engineers did in this respect, if anything?

Perhaps Blitz has the answer?

[macdeb]

I had a helluva game with the stepper, wouldn't go back to rest at same point etc. Did away with it completely and had the start up map changed. OP, have you checked plugs? I changed down a grade and it smoothed out the idle perfect at 650 rpm.

[blitzracing]

ChimpOnGas said:

Steve_D, do you know what the 14CUX does with the fueling when it allows air into the engine via the stepper motor?

It would be interesting to understand what the Lucas engine management engineers did in this respect, if anything?

Perhaps Blitz has the answer?

The stepper air feed is metered by the AFM, just as if you open the throttle plate, so the load mapping based on air feed takes care of the correct fueling whatever the stepper position is.

[N7GTX]

blitzracing said:

ChimpOnGas said:

Steve_D, do you know what the 14CUX does with the fueling when it allows air into the engine via the stepper motor?

It would be interesting to understand what the Lucas engine management engineers did in this respect, if anything?

Perhaps Blitz has the answer?

The stepper air feed is metered by the AFM, just as if you open the throttle plate, so the load mapping based on air feed takes care of the correct fueling whatever the stepper position is.

Are you sure? The additional air going into the engine will result in a lean mixture so the oxygen sensors will detect this and correct it. Or the ECU will be programmed to inject a touch more fuel to ensure the correct mixture to match the air. This happens on modern systems when you switch the aircon on or use the electric power steering. Or is the 14CUX too old for this modern stuff?

[Steve_D]

N7GTX said:

blitzracing said:

ChimpOnGas said:

Steve_D, do you know what the 14CUX does with the fueling when it allows air into the engine via the stepper motor?

It would be interesting to understand what the Lucas engine management engineers did in this respect, if anything?

Perhaps Blitz has the answer?

The stepper air feed is metered by the AFM, just as if you open the throttle plate, so the load mapping based on air feed takes care of the correct fueling whatever the stepper position is.

Are you sure? The additional air going into the engine will result in a lean mixture so the oxygen sensors will detect this and correct it. Or the ECU will be programmed to inject a touch more fuel to ensure the correct mixture to match the air. This happens on modern systems when you switch the aircon on or use the electric power steering. Or is the 14CUX too old for this modern stuff?

As Blitz said the extra air passing through the stepper has to also pass through the MAF so the ECU knows the4e is more air passing so increases the fuelling.

Steve

[N7GTX]

Thanks Steve, the system is more ancient than I thought

[davep]

IMO in its time and given the technology available 14CUX was pretty clever where idle control is concerned.

When calculating the Target Idle value for stepper motor control and the amount of adjustment required the 14CUX code takes into account, amongst other things:

- AC On or Off
- Neutral or Drive if Auto
- HRW On or Off
- variations in idle RPM due to changes in coolant temperature and resultant fuel calculations.

Target Idle is recalculated every time the ECU completes its checks of all other sensor values and after every 80th pass of the main control loop.

Idle Mode, where the stepper and base idle values determine induction air flow and the resultant AFM value, is only entered when road speed is less 3 MPH, throttle is closed and engine speed is greater than 50 RPM.

The amount of code in 14CUX dedicated to the Stepper Motor control function is extensive, and complicated.

[Classic Chim]

macdeb said:

I had a helluva game with the stepper, wouldn't go back to rest at same point etc. Did away with it completely and had the start up map changed. OP, have you checked plugs? I changed down a grade and it smoothed out the idle perfect at 650 rpm.

That’s exactly how mine works. And the grade down too which does help keep things smoother.

GOG Dave.
My scatter spark is switched off. It’s just fuelling and timing that maintains idle. Cold start map etc.

I agree the 14CUX is very sophisticated and when working is all but as good as any.
If reliable steppers etc can be found there’s a good case for keeping it. Tvr engine bay heat is probably what starts the faults not the system. If we could control the heat better they’d still be fairly reliable.
Things like AFM do last well considering it’s proximity to the exhaust if you think about it.