Technical Idle speed too slow/stalling from cold

[capers626]

Hi all,

I have a 1993 Fiat Uno Mk2 999cc 45ie with single point injection (Bosch unit) and am experiencing problems with idle speed, which is too slow when engine is cold and so causes stalling at junctions and roundabouts .

It is easy to restart car but the fault continues until temperature gauge (inside car) begins to register as slightly warm.

Have changed stepper motor on injection unit (genuine Bosch used replacement from a scrapped car with same size engine and similar age) and this seemed to cure the issue as revs were a lot higher via the automatic choke. However, back to square one a few weeks later and the same experience after disconnecting battery terminal for an ECU reset.

Also fitted a new coolant sensor (the blue chamfered one) but didn't appear to make any difference.

As far as I know there isn't any manual adjustment for the idle speed or fuel mixture in the case of Uno's with the electronically controlled single point injection system, and any such adjustments should be automatically made by the ECU.

Accelerator cable seems to be correctly adjusted with the right amount of tension.

Am not sure if there is a problem with the ECU, Lambda sensor, thermostat, ignition coil or something else?

Any advice or help appreciated

Cheers,

Andy

 

[jackwhoo]

Clean the passages that lead to and from idle stepper motor.
It could well be the second hand stepper motor has failed since you fitted it.
Especially as symptoms the same.

You are correct there is no manual idle speed control.

Make sure there is a little slack in accelerator cable so throttle switch is in idle position.

 

[capers626]

Hi, thanks for reply, will try and clean the pipes to and from the stepper motor/injection unit to see if that cures the problem. Are you suggesting there might be a slight fuel blockage which shows up when the car is cold but disappears when engine temperature rises ?

It may be the (replacement) stepper motor has failed but although the symptoms are the same as before, I am not sure it is the culprit. Reason being if the ECU reset is carried out, the idle speed is a lot higher on start up, and only reverts to the 'barely ticking over' setting after a number of days - the motor is not constantly stuck in one position all the time.

A faulty sensor (or other component) seems to be fooling the ECU into transmitting the signal for a slow idle speed, if I have understood the correct operating sequence for the fuel injection system. The exact way in which the various components interact (and in what order) is a bit confusing to me. This is also assuming the ECU itself doesn't have some kind of internal malfunction.

Haynes manual extracts:

"The air to fuel mixture is regulated by values obtained from the ignition coil (engine speed), engine coolant temperature sensor, throttle position switch and the Lambda sensor. No adjustments to the fuel mixture are possible".

"The ECU...its function being to control the fuel system under all operating conditions, including starting from cold".

Any further thoughts or insights into this ongoing issue?

Cheers,
Andy

 

[jackwhoo]

No not suggesting that.

The idle stepper motor just controls the size of an air passage that bypasses the throttle butterfly plate that is closed at idle.

Larger opening more air going to engine , ECU detects more air and adds correct about of fuel, faster idle.

Smaller opening less air less fuel , slower idle speed.

If you remove the idle stepper do not force the end in or out you may break it.

Check the throttle position switch is at idle when throttle pedal released and make sure there is slack in the cable with throttle pedal released.

 

[capers626]

Thanks for further advice, there is slack in accelerator cable and tension seems fine, as mentioned in my first post. Will check throttle position switch later today and report back.

From memory, a diagnostic check can be performed via plug in to ECU. If this showed an idle speed fault would you be suggesting a different solution?

Cheers,
Andy

 

[jackwhoo]

No

 

[jackwhoo]

I have previously seen the very same problem you have on single point injection system. The idle speed control stepper motor could close but not open.

 

[stami]

At the past had same problem car died cold or idling to low and car electrician found that the pins from Bosch ECU that control stepper motor was faulty.
When i replace the ecu problem solved.
I think you should check it unless your stepper motor is faulty.

 

[Louie Bee]

999cc ah...

What's the vacuum advance diapram like?

Loads of posts on here about Blowing (not sucking) to check if it's shot.

In the pic in this post remove pipe 7 from 6 and blow down the exposed bit of metal on 6...

https://www.fiatforum.com/uno/29820-distributor-replacement-fire-1-1ie-engine.html?p=372408

blow about as hard as a wistle, not as hard as blowing up a baloon. you should not be able to bloe for more than about a second before hitting firm resistance.

part 6 can be replaced without replacing the whole distrubutor.

 

[peterdt0260]

What is the. temperature for coolant sensor to activate

 

[peterdt0260]

What is the. temperature for coolant sensor to activate

for a uno. 1.400 1993

 

[MathiasDKling]

What is the. temperature for coolant sensor to activate

Should be a variable resistor, lowering its resistance as temperature rises. The ECU does a calculation of the required amount of fuel, from this value, and a few others.