Cold Start and Rough Idle Issues Golf MK3 GL 2.0

[timiteo]

Hey Guys,

When I start my car in the morning (or when the engine is cold) it starts but the idle is very rough (low, fluctuating revs even to the point of stalling the car).

When I rev it around 2500 for a bit it gets better but only after the engine has warmed up does the problem go away (I can pull over and turn the car off then on again and it sits there and idles at 900 fine).

Does anyone have any any ideas as to what the problem may be. I have cleaned the MAF, and the throttle body to no avail. Also, there is the smell of fuel, more so than is normal, when I start the car cold and it has the problem described.

Look forward to some help, Tim.

[h100vw]

Coolant temp sensor and lambda probe.

Scanning with VCDS would be a good move first.

Gavin

[timiteo]

Excuse me for being a noob, but how would one do these checks, and also I don't have a VCDS to scan.

[h100vw]

Can you use a multimeter?
You can see the output of the probe with the meter set to Volts, When the car starts from cold it will rise steadily up towards a volt. Taking a couple of minutes. Then as the water temp rises, it should then be swinging from about half a volt to a volt around once a second. This is the ECU controlling the mixture in closed loop mode.
You won't see this really well with a digital meter because of the sampling rate being too low. You can see the trend though with the voltage rising and falling.
If you don't see this happening then the ECU doesn't know the car has warmed up and it will allow a rich mixture condition to develop, which could be your bad idle.
Not sure why restarting the car would clear it though?
Alternatively, you could just replace the temp sensor because they are cheap and see if it fixes the problem.
Might be worth looking for someone with VCDS local to you, that really is the best way to go. You can see if both the temp sensor and probe are working without lifting the bonnet.

Gavin

[timiteo]

Oh, restarting doesn't clear it I was just trying to emphasize that the problem only occurs when the engine is cold.

I would have no idea where someone with a VCDS would be in my locality. You say replacing the temp sensor and checking the lambda probe; perhaps you could enlighten me to their location on the vehicle if possible, so I can take a look.

Cheers, Tim

[h100vw]

Lambda probe in the exhaust, look down the back of the engine from the drivers side. You can see the plug on the engine mount IIRC, just by the brake servo.

Temp sensor is in the top hose outlet of the cylinder head. With 4 wires coming out of it.

Gavin

[timiteo]

Just bought OBDII Cable to go from car to computer, I took your advice on not having to lift the bonnet. Should expect it in a few days.

So just load up VCDS-Lite (Free I assume?) and do a scan?