Low idle speed

[chriskay]

To start at the beginning: My car, a turbo, very occasionally refused to idle from cold & needed a bit of throttle. I suspected the coolant temp. sensor, so obtained a second hand replacement. When that was fitted, it refused to idle at all from cold. It turned out to be the wrong sensor, from a n/a car. I obtained the correct sensor, again second hand, & had it fitted. The engine now wants to idle from cold at about 400 RPM & dies if you apply any load. The idle returns to a normal 900 RPM as soon as the engine begins to warm up. I can manage this situation by applying a little throttle & left foot braking; easy enough in an auto. My question is this: do I have the original sensor replaced, or have I caused a problem in the ECU or the EZ 210K, both of which are fed by this sensor? I have tried disconnecting the battery for an hour in order to clear the memory, but no change. I have no reason to suspect that the present sensor is faulty, as it came from a reliable source.
If I were still able to work on my car, I'd change it, but it's a bit awkward to get at & especially in this weather I'd want the garage to do it.
Cheers, Chris.

[Iroll480]

It's easy to check if the coldstartsensor is ok if you have a multi meter. Measure the resistance of the coldstartsensor at different engine temps. The higher the temp of the coolant the less the resistance:
At -10c it should be 9400ohm
0c - 5800ohm
20c - 2500ohm
80c - 330ohm

[chriskay]

Thanks for that. I've measured the resistance at 0 deg. & 50 deg. & they broadly agree with yours. The sensor is not suspect (it was originally which is why I changed it). The question is; have I damaged the ECU by fitting the wrong sensor?
BTW, your figures are correct for a sensor for a n/a engine which has 1 thermistor. The sensor for a turbo has 2 thermistors so if you measure across the pins, the figures are double. If you measure from 1 pin to earth (the body of the sensor), the figures are as you suggest.
As far as I can tell, the sensor for the n/a engine has a blue top, for the turbo, it's black. Wish I'd known that before I started playing around.

Cheers, Chris.

[ChrisRidett]

From a purely electronic pov, I'd be surprised if connecting the n/a sensor would cause a problem like popping the ECU or EK.
Both of these lines are Voltage diffential lines ( the voltage level changes as the resistance of the sensor changes )

Typically this would involve an internal resistor( normally taken to +12V ) in line with the sensor and the ecu takeoff connected after the internal resistor and before the sensor.

Only punching a load of amps or arcing this line should cause a problem.

This doesnt mean the ECU or EK may not have already been having a problem.


However, looking at the diagrams, I would be inclined to check the Idle speed regulating valve.

Has it been thirstier on the petrol then normal?

[Iroll480]

BTW, your figures are correct for a sensor for a n/a engine which has 1 thermistor.

Those figures I quoted are from the turbo manual, so they are correct. I noticed it too that they are exactly the same as from a n/a sensor.
Other suspects are the throttle body. Maybe the sensor is not adjusted correctly or it is contaminated with sludge. Take of the breatherhose and clean the
inside of the throttle body and the butterflyvalve with a solvent like carb.cleaner. Do the same with the idlecontrol valve.

[chriskay]

Thanks for your inputs, guys. I'd be surprised if it was the throttle body or the idle control valve, since it was idling perfectly at 900 RPM before I started messing around with the sensor. Also, as soon as the engine starts to warm up after only 1/4 mile, the idling is perfect again. It only shows the problem on first start up in the morning, so as long as there's even the slightest bit of warmth left in the system, it starts & idles OK. I think I may have the original sensor re-fitted & see what happens.

Cheers, Chris.

[felyx]

maybe a vacum leak issue ? or maybe try the co2 adjuster on the air flow meter , it is blanked off with some resin with a flat head screw underneath (MARK THE ORIGINAL SETTING though !! , cheers felyx