Erratic Idle, jerky when driving when foot lifted.

[Raptor50]

Here goes,
I own a 91 Passat 2.0Ltr which has an idling problem.

When the throttle is blipped it often drops to 300 RPM approx and sometimes stalls. If the car is left to tick over it is very erratic after several minutes.
When driving and the accelerator is lifted, at about 30 - 40 MPH and 1500 RPM approx the car becomes very jerky but is fine if the throttle is depressed and cruising along at a decent speed (provided that the revs are above 1500 - 2000 RPM approx).

Sometimes the throttle gives the impression that it is stuck open as the car surges forwards as if there is a demand from the pedal.
All of the problems appear to be on the very first part of the accelarator eg 1/2" to 3/4" of accelarator pedal depressed.

The car has been in the garage twice now (diagnostics) for which they have replaced the throttle potentiometer, distributor cap, plugs and fuel filter. They did comment that the CO2 pot has no adjustment and that the coolant temperature sensors were not working correctly but stated that it was not worth replacing these until the problem was diagnosed.

I have cleaned all of the rubber hoses and breather pipes and cleaned the carb butterfly with carb cleaner along with cleaning the 'Idle Stabilisation Valve'.
In attempting to fault find I have used a donar car and swapped over the top half of the air filter assy which includes the Air Mass Volume Sensor and the CO2 sensor, then I tried the coil assembly followed by the Idle Stabilisation Valve but not all at the same time with still no luck.
Since cleaning the revs are much higher.

Please....please....help as I don't want to scrap her as she looks nice and is worth keeping but I don't know what to try next.

Anyone out there who can help with advice?

[boschmann]

I would need to know if it's a 16 valve engine & which fuel system is on it. Does it have an all aluminum mechanical fuel distributor with braided steel fuel lines going to each mechanical injector (CIS-E) or a black plastic and aluminum air flow meter attached to the air filter housing & electrically operated fuel injectors (AFC Digifant)?

[Raptor50]

Thanks very much Boschmann for answering my plea for help.
It is a 8V and has the Digifant system.
What I did forget to say was that yesterday when it was running lumpy I removed what I believe to be a coolant sensor (which is located at the front of the engine block near spark plug 3, but it has 4 wires coming out of it) after having read another one of your responses and the car idling speed picked up and ran much better but when taking the car out for a quick check did not seem to have any effect when removing my foot of the pedal at 30 -40mph.

Hope this helps,

Cheers in advance,

Brian (Raptor50 man).

[Raptor50]

Sorry Boschmann,
I also forgot to mention that when the ISV is diconnected the engine dies instantly which I am led to believe that it should still run if not a little lumpy as it should drop to 500 to 550 RPM. The donar car that I was using does still runs with it removed as told it would.
I have also measured the resistance of the four connections with the connector unplugged and the measurements that I am getting with the engine coolant slightly hot are across 2 of them 270 ohms approx and across the other 2 connections about 130 ohms.

Cheers.

Brian (Raptor50 amn).

[boschmann]

If that sensor is located in the radiator hose flange it probably is a coolant temp sensor, it is different than the NA models as ours are only two wire. The engine should run terrible with it unplugged. Yours probably has two wires for the gauge & two for the engine management. You can jumper between a pair & see if the temp gauge goes up to determine which is for the gauge. If you have a brown/white & violet/black pair they should be for the ECM, resistance should be 2000-3000 Ohms cold & 200-300 hot on the correlating sensor pins. Unplugging the ISV cold should kill the engine, unplugging at operating temp shouldn't.

[Raptor50]

Well......what can I say!
Rang the dealer and had a sensor put by this morning.........wife picked it up and it was the wrong one (being a 2 pin instead of a 4 pin). She went back and I spoke to the man on my mobile while she was there and he gave her a 4 pin one. Several hours later I get home and ask to see it and guess what.........still the wrong one.
I rush back to the garage and get the man out to look at the one fitted in my car and then he looks on his system and then he tells me that there are 4+ different ones on his screen. They don't have any in stock so I now have to wait until Thursday as they have to order one.
Knowing my luck it will still be the wrong one.

Watch this space and I will keep you informed Mr Boschmann.

PS:- I did take a gamble yesterday evening and paid another guy some money to connect his analyser up to the car and he informed me that there was an error code (I believe it was G62 but I was looking over his shoulder) which he tells me that the coolant sensor was open circuit.
So everything is pointing towards it .........if only I could lay my hands on one.

I assume that there is only the one coolant sensor.......if so whats the one that screws into the block for?

If you don't come back on that question I'll keep you informed of the outcome when the sensors arrives.

Thanks for the advice so far,

Cheers,

Brian (Raptor50 man).