VOLKSWAGEN Golf AHW Engine - Golf Idle Problem
I have got a Golf 1999 AHW engine 1.4 16v with an idling problem. The car drives perfectly apart from a slight hesitation at low throttle and uneven tickover. You can hear it popping out the rear exhaust. I've cleaned the throttle housing/re-calibrated/installed basic settings and there are no fault codes. The ECT is reading the same as the temperature gauge, map sensor reads 4v with the engine off, 3.8V throttle wide open and 1.5V at idle.
If I unplug the front oxygen sensor, the car runs perfect and all the faults are gone. I renewed the oxygen sensor but the fault is still the same.
The only clue I have is that the injector duration at idle is approx 5ms, so I think maybe there is an air leak, although I can't find one! I've also blanked off the EGR valve and the servo hose, but still no good. Where the servo hose comes off of the throttle housing, there are two small vacuum take-offs. One is for the EGR valve but the other is just open. I tried blanking it but no good. I've checked the compressions. They are all approx 14 bar. Checked the manifold vacuum at idle. The only way I can get it to idle perfectly is to unplug the front oxygen sensor sensor. Could you help?
The slightly low compression will cause a reduction in engine vacuum, but 17in is not excessively low, though 18-19in is better. If you have access to a signal generator, then substitute 1.0V for the map sensor output to see if this cures the idling fault (or make your own potentiometer to give a 1.0V tap). If you can suck 19in by mouth or have a vacuum generator (you can always use another car's vacuum for this) see what the idling is like and what the map sensor reads then.
The map sensor may be reading a little high, but Autodata values are not always spot on. The injector duration of 5ms is very high, 3.5ms is normal here. If the emissions are okay, then the error must be in your measuring equipment. Injection duration is best measured with a scope; some meters read the duration incorrectly.
You may never get it to idle exactly to any set speed, since inevitably the idling mixture is constantly being altered as dictated by the O2 sensor output making the engine speed to change slightly as a result. If there is a fault, then it looks like being the ECU programming with respect to its response to the O2 signal input.