Hi, I've got a problem where when I come off the gas pedal and dip the clutch, the idle speed drops sharply, the engine almost dies before the rpms bounce back (to anything between 1200 and 1600 rpm) before settling back to 900 at idle. The revs occasionally will oscilate a couple of times before settling down. What I've done: I've reset the throttle stop as per Haynes (when I bought the car this had been adjusted such that the stop was not in contact with the lever, ie the throttle plate was too tightly closed. This is now correct as per the Haynes guidance. I've reset the idle and CO following the FAQ, took a while but I got there, cheers John. Checked the ISV operation, per the Haynes. Cleaned the ISV, soaked in petrol overnight (this made no difference to the problem) So where I am at the mo: the idle adjustment screw is fully wound in, idle is at about 900, or just over (which seems a bit too high to me but the adjuster screw is already fully wound in, and it does this funny rpm dip thing when I come off the throttle and dip the clutch. If I come off the throttle, coast for a bit on overrun and then dip the clutch the idle falls back to 900 just fine. One thing I did notice is that the secondary throttle plate has a stop screw also, and the lever does not rest on this stop screw (it is a small screw, looks not possible to adjust it, on the driver's side of the throttle body lever mechanism). Don't know if it should be resting on the stop?? I'm not sure this is relevant in any case. I think if I can work out why my idle speed adjustment screw needs to be fully screwed in to give a reasonable idle speed I think I'd be well on the way. Sounds perhaps like a vac leak but I've gone all round the intake tract several times and eliminated all possible leaks (replaced the hoses and hose plugs, plugged the lines out to the airbox and brake servo. One other thing, if I disconnect the ISV the engine dies straight away, so the engine is obviously getting a significant amount of the air that it needs at idle through the ISV. Is that right?
Adjust the base throttle stop just to the point where the plate is not jammed and lock off the 8mm AF nut. Check the condition of ISV bypass screw and o ring. Replace if worn. Disconnect the ISV and start engine. You may need to keep the throttle open to keep the engine running. Adjust the idle bypass screw to achieve 950rpm. Reconnect the ISV. Restart the engine. Blimp throttle. Engine revs should drop to 1100rpm into a dashpot before returning to target 900rpm. If the bypass screw is turned fully clockwise, the ECU will have to control the ISV to its maximum to maintain target idle. The PID in these old things are not that good and will struggle causing you to enter into a idle roll. By adding enough base bypass air, the controller will have less work in controlling the ISV and you will maintain a good idle +/- 100ish rpm.
thanks Toyo, what do you mean by dashpot? I'm thinking of a dashpot as being like a mechanical damper, stopping cupboard doors from slamming etc. There's no mechanical damper on my throttle body. Do you mean the ISV works electrically like a dashpot by "catching" the idle rpms at 1100 rpm and then gently decreasing them further? If so that makes perfect sense and I'll have a go tonight. From experience of my tinkering to date, following this test method is going to mean sky high idle once I reconnect the ISV, which I think tells me the ISV is my problem, jammed wide open or such like... Maybe that the PO noted a problem high idle and, rather than replacing the ISV, he just screwed the adjuster screw to compensate?? is there any way to unjam an ISV, or do I need to get my chequebook out? Used or new? ta Drew
Ja, The dashpot is ECU calibrated to operate the ISV. You cannot adjust this but can set the base conditions to ensure this is achieved. @950rpm base idle the engine should not rev to 3000rpm or more with the ICV connected. If that is the case you may have a problem with the ISV.
golf, vento, passat, corrado and seat models 2.0 8v 2E engine ISV and AFM both fit the MK2 digi, though with the afm make sure it has a CO adjuster screw as some dont
this problem was 100% user error, the engine is much more sensitve to the idle speed screw than I had expected. So I followed the above instructions and all is well, I did tweak it a small amount again yesterday as the revs were high at idle (just over 1000) so I set the idle screw to about 900 rpms with the ISV disconnected, which gave me 950ish reconnected. then I reset the CO much happier thanks
