ECU Fault Code 48?

[Mr Plow]

Hi All

My MIL light randomly keeps coming on. I can go for a few hundred miles after resetting it without it coming on or only 20 miles. I have finally got round to getting a fault code reader. The only code that the car is displaying is code 48. This indicates "Stepper Motor fully open above 500 rpm or fully closed above 750 rpm". Comments are:

"Sticking stepper valve, incorrect base idle speed adjustment, air leak on non cat cars, incorrect stepper motor adjustment, incorrect throttle butterfly adjustment, rough running because of fuel or ignition or mechanical faults. A faulty road speed input can also cause this problem"

To me the above means that it could be pretty much anything and I'm looking for needle in a hay stack as they say!

Does anyone have any advice of where to start? I'd rather not ignore it and either take the bulb out or stick some black tape over the light when it comes on.

Thanks in advance.

Liam

[977]

Giving it a good clean is almost free and very easy... ? If it sticking due to carbon deposits you may be getting occasional grief..

[Mr Plow]

Thanks for your reply.

When I found out what the code meant I took out the stepper and gave it a quick clean. It looked pretty clean already though.

Will give it a better clean when I have some carb cleaner. Do they need replacing very often? Although I surpose if its an intermitent fault its more likely to be sticking than the part failing?

Liam

[977]

Not heard much about them breakingt. Try putting some PTFE on the thread when I fit it to ensure it doesn't leak any? It could be something else like the base idle (i.e where the car naturally idles before the stepper motor adds more air) as described. If the base idle is set too high then the stepper motor will be totally closed and still not able to bring the idle down to where it is supposed to be.

[gavgavgav]

Mr Plow said:

Thanks for your reply.

When I found out what the code meant I took out the stepper and gave it a quick clean. It looked pretty clean already though.

Will give it a better clean when I have some carb cleaner. Do they need replacing very often? Although I surpose if its an intermitent fault its more likely to be sticking than the part failing?

Liam

Did you also clean the aperture that the 'cone' goes into? Removing to clean is a pain & I found a bunch of cotton buds soaked in cleaning fluid removed a lot.

[blitzracing]

That error code simply means the tha idle is outside its range whatever the stepper motor does to try and bring it back into range it cant. So the question is what is the current idle and is it stable? It should be around 800 rpm, although this value is not that fixed (give or take 100 rpm,) its part of the ECU programming so its chip dependent. I suspect the error value of 750 rpm peak is for the Range Rover fuel map, not the TVR one, so take it with a pinch of salt.

Air leaks or a high base idle with make it idle above 800 rpm. The base idle setting can be easily checked by blocking the air feed to the stepper (or even clamping the feed pipe with some big pliers) and seeing what the tickover drops to- its should be around 600 rpm with no additional air feed from the stepper.

Edited by blitzracing on Wednesday 29th February 15:21

[Mr Plow]

blitzracing said:

That error code simply means the tha idle is outside its range whatever the stepper motor does to try and bring it back into range it cant. So the question is what is the current idle and is it stable? It should be around 800 rpm, although this value is not that fixed (give or take 100 rpm,) its part of the ECU programming so its chip dependent. I suspect the error value of 750 rpm peak is for the Range Rover fuel map, not the TVR one, so take it with a pinch of salt.

Air leaks or a high base idle with make it idle above 800 rpm. The base idle setting can be easily checked by blocking the air feed to the stepper (or even clamping the feed pipe with some big pliers) and seeing what the tickover drops to- its should be around 600 rpm with no additional air feed from the stepper.

Edited by blitzracing on Wednesday 29th February 15:21

Thanks for the info. I will take a look next time I'm with the car (hopefully tomorrow) and report back.

The idle is a little eratic so makes sense.

Liam

[Mr Plow]

Bit of an update on the below.

The idle isn't stable, I have checked the base idle and that is correct.

When you first start the car it idles at approx. 1200rpm. I presume this is about right until the car has warmed up?

It seems to be once the car warms up I start to get issues. If I reset the fault code reader it will come back with code 48 once the car has warmed up. Once warm the car idles at 1500 - 1800 rpm when I stop at junctions etc returning to 900rpm after approx. 10 seconds. This also means that shunting as I drive along is terrible.

I'm thinking that the stepper motor is ok but it is receiving incorrect signals from the ECU, i.e. ECU getting info from somewhere else that something is wrong and asking the stepper motor to compensate.

As I only seem to have the problem once the car is warm should I start looking at temp sensors, I'm thinking the ECU may be being told that the car is still cold so add more fuel etc. Although the dial on the dash doesn't indicate anything out of the ordinary.

Any help would be appreciated.

Cheers

Liam

[shpub]

To be honest this is exactly where an ECUmate would come in as it will tell you if the ECU is moving the stepper motor, whether the ECU is actually going into idle control mode (which I don't think is happening) and why.

If I was a betting man I think you have a problem with the road speed sensor to the ECU...

[blitzracing]

As the man says- the TVR "speedo calibration unit" is a box of electronics under the dash that generates an all or nothing pulse as the car moves. It should be 0 volts below 3mph, and then produces a fixed pulse train above 3mph to show the car is moving to the ECU. This should then alter the way the stepper controls the idle. I think the speed pulse causes the stepper to lift the idle a bit, so there is less engine braking between gears, but Ive not been able to replicate this acurately. It should be easy, just let the car roll along with the clutch depressed and no throttle and see where the idle sits. It should then drop as soon as the car stops rolling (not 10 seconds later). The electronics itself in the calibration unit is pretty flakey, it can easily produce a car moving signal when the car is stopped due to a poor circuit design and earthing issues, so the idle control becomes very eratic. As Mr Heath says his ECU mate ( a throughly recommended tool!) will display this signal, otherwise you can use an osciloscope, but thats the hard way. The Speedo calibration unit is under the dash, and there is a modification you can do to make it less likely to produce an incorrect signal, or it may just need the joints resoldering on the internal PCB.