I have been doing a bit of trawling and I found this over on Audiworld. It is a good test procedure to see what is wrong.
Originally posted by UrS4boy
Welcome to the "joys" of UrS-car ownership. This is a likely a minor problem but it will take a bit of time, effort and money to solve.
Sounds to me like the problem could be either a Power Output Stage (POS) problem or a coil problem. Coils usually are fine at idle and then have a problem under boost, whereas the POSs will have problems at idle AND under boost. However, the only way to tell is to test each component:
Generally, missing is either a fuel or an ignition problem. Fuel problems can come from dirty/malfunctioning injectors, clogged fuel filter or bad fuel pump.
Fuel filters are cheap (from VM Autohaus in Burnaby, at least). Unless you know the fuel filter to be good, I would just replace it now. Order a filter and the two copper crush washers. Start the cold car and then pull Fuse No. 17 in the drivers side end of the dash. This stops the fuel pump and the engine will run until the pressure at the injectors is too low. Then turn off the ignitiion and slowly break the fuel filter connections (I think one of the wrenches is a 14 mm) slowly, with a rag to catch the remaining fuel drips. R&R the fuel filter see if the problem goes away.
Injectors can be checked by removing them and testing the spray pattern, looking for one that is just dribbling instead of producing an atmomizing spray. This can be done in the engine bay, provided some precautions are taken.
I don't think its the fuel pump so leave that alone until the ignition system is confirmed good.
The ignitions system is comprised of the ECU (computer) that controls the delivery/timing of the spark, the Power Output Stages (POSs) which are transistorized switches controlled by the ECU, the coils, the coil to spark plug boots and the spark plugs.
On an unknown (or a new to you) car, it is probably smart to install new plugs and maybe spark plug boots. The ONLY plugs that work without problems are the Bosch F5DPOR. If necessary, follow the noobie procedure I wrote up on Audiworld:
http://forums.audiworld.com/s4s6/msgs/114097.phtml
If you haven't done the plugs and boots, then do them and then retest the car. If you still have a problem, then it is either the one of the two POSs or a coil (or coils).
The POSs are both three channel. One does cylinders 1,2 and 3; the other cylinders 4 and 5.
The first POS check is at idle. Remove the injector connectors one at a time, noting when the idle roughness gets worse. If it does that eliminates that cylinder as the problem, reconnect the injector. When you disconnect an injector and the roughness is the same, note that cylinder number and reconnect the injector. Its either going to be a POS unit or the injector (hopefully there is only one cylinder with a problem).
The second stage of this test is to swap the connector on both the POS units and redo the injector disconnect test. If the missing cylinder moves, then it is a POS. If the missing cylinder is the same, then it is likely an injector (assuming you have installed new plugs and boots)or coil.
If the miss does move, you have one free position on one of the POSs that you can use. For example, if the miss was on cylinder 4 at the start and then it moves to 1,2 or 3 after the POS connector swap, you can swap the connectors back and move the cylinder 4 input and output pins to the empty position 6. You can also do this if the miss was on 1, 2 or 3 and then it moves to 4 or 5. You just leave the connectors in place and swap the pins on the new problem cylinder. If after you swap the connectors and the miss on 1, 2 or 3 goes away, its because the missing cylinder is now connected to the spare channel 6.
If none of this works, it may be more than one POS channel. In which case, you need at least one new POS. The correct POS is a 4A0 905 351A, however, the cheaper, 4A0 905 351 POSs from a 92-97 V6 wasted spark (fires two cylinders at once) ignition system will work on our cars, as a substitute for our 4A0 905 351A.
To check the coils, the simplest way is to swap in a known to be good coil pack (two POS connectors and two (one?) ground connection). If problems go away, then you know its one (or more) of your coils. Otherwise, to test your coils,
you need to remove the coil pack cover as in the spark plug replacement instructions. Inspect all the coil wiring for signs of cracking (a known failure mode). If the wires are cracked, replace the problem wire (same or thicker gauge) by soldering and heat shrink tubing (remember to get the tubing over the wire BEFORE you solder - (don't ask)). If the wires appear fine, then with a friend, and an old fashioned coil spark tester connected to one boot at a time, check to see which of the coils has the weakest spark (the one that only jump the smallest gap). This will be your culprit. Replacement coils need to be spliced in with soldering and heat shrinking.
Sounds to me like the problem could be either a Power Output Stage (POS) problem or a coil problem. Coils usually are fine at idle and then have a problem under boost, whereas the POSs will have problems at idle AND under boost. However, the only way to tell is to test each component:
Generally, missing is either a fuel or an ignition problem. Fuel problems can come from dirty/malfunctioning injectors, clogged fuel filter or bad fuel pump.
Fuel filters are cheap (from VM Autohaus in Burnaby, at least). Unless you know the fuel filter to be good, I would just replace it now. Order a filter and the two copper crush washers. Start the cold car and then pull Fuse No. 17 in the drivers side end of the dash. This stops the fuel pump and the engine will run until the pressure at the injectors is too low. Then turn off the ignitiion and slowly break the fuel filter connections (I think one of the wrenches is a 14 mm) slowly, with a rag to catch the remaining fuel drips. R&R the fuel filter see if the problem goes away.
Injectors can be checked by removing them and testing the spray pattern, looking for one that is just dribbling instead of producing an atmomizing spray. This can be done in the engine bay, provided some precautions are taken.
I don't think its the fuel pump so leave that alone until the ignition system is confirmed good.
The ignitions system is comprised of the ECU (computer) that controls the delivery/timing of the spark, the Power Output Stages (POSs) which are transistorized switches controlled by the ECU, the coils, the coil to spark plug boots and the spark plugs.
On an unknown (or a new to you) car, it is probably smart to install new plugs and maybe spark plug boots. The ONLY plugs that work without problems are the Bosch F5DPOR. If necessary, follow the noobie procedure I wrote up on Audiworld:
http://forums.audiworld.com/s4s6/msgs/114097.phtml
If you haven't done the plugs and boots, then do them and then retest the car. If you still have a problem, then it is either the one of the two POSs or a coil (or coils).
The POSs are both three channel. One does cylinders 1,2 and 3; the other cylinders 4 and 5.
The first POS check is at idle. Remove the injector connectors one at a time, noting when the idle roughness gets worse. If it does that eliminates that cylinder as the problem, reconnect the injector. When you disconnect an injector and the roughness is the same, note that cylinder number and reconnect the injector. Its either going to be a POS unit or the injector (hopefully there is only one cylinder with a problem).
The second stage of this test is to swap the connector on both the POS units and redo the injector disconnect test. If the missing cylinder moves, then it is a POS. If the missing cylinder is the same, then it is likely an injector (assuming you have installed new plugs and boots)or coil.
If the miss does move, you have one free position on one of the POSs that you can use. For example, if the miss was on cylinder 4 at the start and then it moves to 1,2 or 3 after the POS connector swap, you can swap the connectors back and move the cylinder 4 input and output pins to the empty position 6. You can also do this if the miss was on 1, 2 or 3 and then it moves to 4 or 5. You just leave the connectors in place and swap the pins on the new problem cylinder. If after you swap the connectors and the miss on 1, 2 or 3 goes away, its because the missing cylinder is now connected to the spare channel 6.
If none of this works, it may be more than one POS channel. In which case, you need at least one new POS. The correct POS is a 4A0 905 351A, however, the cheaper, 4A0 905 351 POSs from a 92-97 V6 wasted spark (fires two cylinders at once) ignition system will work on our cars, as a substitute for our 4A0 905 351A.
To check the coils, the simplest way is to swap in a known to be good coil pack (two POS connectors and two (one?) ground connection). If problems go away, then you know its one (or more) of your coils. Otherwise, to test your coils,
you need to remove the coil pack cover as in the spark plug replacement instructions. Inspect all the coil wiring for signs of cracking (a known failure mode). If the wires are cracked, replace the problem wire (same or thicker gauge) by soldering and heat shrink tubing (remember to get the tubing over the wire BEFORE you solder - (don't ask)). If the wires appear fine, then with a friend, and an old fashioned coil spark tester connected to one boot at a time, check to see which of the coils has the weakest spark (the one that only jump the smallest gap). This will be your culprit. Replacement coils need to be spliced in with soldering and heat shrinking.
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