HELP - Rover V8 Lucas 14CUX Hot Wire problems

[richard sails]

I am trying to diagnose an issue with poor idle and lacklustre acceleration on a 4.5BV Non-Cat engine.

So far I have checked the following;

While engine is idling, pulled each plug lead, a very minor change in rpm for all eight, more easily spotted on the re-connection, I think the stepper motor was controlling the idle rpm and masking any power loss.

Checked the header temps with an infra-red thermometer, all similar.

Checked the plug colours, look like running rich overall BUT the spark area is grey/white suggesting more lean running recently.

I checked the base idle and found it quite high at around 900rmp, tweaked it down to 700 rpm which has made no difference

Stepper motor, removed and cleaned

Throttle Pot; voltages checked and in range, smooth voltage changes through range, closed 0.33V, fully open 4.68V

Water and Fuel temperature sensors, in range and approximately correct at ambient and at various higher temps

AFM, Air flow signal voltage spike settles to 0.5V value very quickly (I would guess less than 250mS) before starting engine, 1.8V at tick-over, HOWEVER CO idle trim value which I believe should be between 1 and 1.5V, is measuring absolutely zero volts.

MOT two days ago showed HC at 112 ppm (max is 1200 ppm). previous MOT showed HC at 542 ppm

Rovergauge confirms no faults seen as such, road speed input confirmed, rpm data good, throttle position good, ECU confirmed going into stationary mode soon after vehicle stops. Engine temps and fuel temps believable, BUT engine appears to be running hotter than I would expect, sitting idling the engine coolant temperature climbed to 100 degrees C but dropped back to 91 on a short drive. Stepper motor output appear consistent with observed idle revs.

To me everything points to a weak air/fuel mixture, possible an inlet manifold leak or something to do with the CO idle trim value from the AFM. I cant find any leaks but I know that they can be hard to find.

Can anyone enlighten me on what the ECU actually does with this voltage, looking on line it appears that if a later AFM is fitted that does not have this trim output, a variable resistor is added into the loom to produce simulated value. Could this lack of voltage be the cause of running lean?

HELP, any suggestions, advice or information greatly accepted.

Cheers

Richard

Edited by richard sails on Friday 25th September 00:19

[666 SVT]

Hi Richard, I'm in Billinge near Wigan and have a spare air meter you can borrow to rule it out if you are not too far from me?

Regards, Col

[Belle427]

Have you verified the ignition timing?

[richard sails]

666 SVT said:

Hi Richard, I'm in Billinge near Wigan and have a spare air meter you can borrow to rule it out if you are not too far from me?

Regards, Col

Thanks Col, I am in Preston so not far.

I will continue investigating and might take up your kind offer if it looks like the AFM.

[blitzracing]

If you unplug the AFM and measure the trim voltage across the loom plug it should have near 5 volts on it. This voltage drops as you plug the AFM in because the trim setting is just a 5 k variable resistor that pulls the 5 volts towards ground as you lower the resistance so the voltage drops. A wrong trim voltage will cause the mixture to be wrong up to about 2400 rpm, but above that it wont have any effect on the mixture, so wont affect full power (or more to the point lack of it). The CO trim is the manual way of controlling the long term trim value on non cat cars, just like adjusting the idle mixture on a carb. Certainly your HC will get much higher or lower depending on the trim voltage. Unfortunately as you don't lambda sensors you cant check the full load mixture, but Id agree a timing check is worth while to check you have full advance first off. If you repeat the plug leads off test, then simply unplug the stepper so it wont control the idle.

Edited by blitzracing on Friday 25th September 11:05