Volkswagen: 99 VW Passat misfiring, vw passat, miss fire

Question
John,

Recently my 99 Passat (1.8T AEB engine) has started misfiring.  It slowly began doing it during this past winter when the engine was cold, and now it is a constant problem.  Original engine trouble codes showed misfires on cylinders 2 and 4.  I have replaced all the fuel injectors, checked the coils, replaced the spark plugs, and confirmed compression on cylinder 2.  After I replaced the fuel injectors, the misfire code for cylinder 4 disappeared but cylinder 2 is still misfiring.  Since this car is no longer under warranty (approx 130K miles) I do not wish to take it to the dealership ($$$$)  but unfortunately my usual mechanic can't figure out this problem.  The car still runs but it shudders at idle and feels like it has about 20 hp.  As for repair history, the only event that may have something to do with this problem occurred around Christmas.  The timing belt snapped about 40K miles early and I had the engine rebuilt with all new valves and one new head.  It ran great for about a month or so before the misfiring first began when the engine was cold.  Only in the past month has it started misfiring constantly.  Any advice you could offer is greatly appreciated.  Thank you!

Eric

Answer
Firstly, with the engine idling, unplug each injector in turn, this will enable you to isolate which cylinder(s) are missing. the trouble codes help but you can double check using the above method. when you unplug an injector, there should be a noticeable change in engine noise and increase in "shuddering". If it has four individual coils, trying swapping the coil from the miss-firing cylinder to a working cylinder and unplug the injectors one by one. If the miss-fire moves with the coil, replace the coil. Should the miss-fire remain the same, then the ignition rectifier pack may be breaking down, this is a unit on top of the airbox, with two multi plugs on it (one 4 pin and one 5 pin). These also break down, but are fairly expensive so i hope this is not the fault. Another problem may be the "air mass" meter, if it is reading incorrect the ecu will think the engine is running rich and adjust the fueling appropriately ( can cause same fault if there is an air leak in the intake system, so check that before the air mass meter).

If your technician has a code reader, ask if it can check the "measured value blocks" or live reading's. if so, group 003 field 3/4 will tell you the air mass reading in mg/r or mg/s, this should be no lower than 2.5 and no higher than 4.5 on idle. Also display group 006, field 4 shows altitude correction, this should be around -5% to 5%.

Hope this helps.
(ps, coils can check fine with ohm meter, but be faulty under running conditions)