5 cyl engine running on 4 cylinders (Audi 100, '89)

Hi everyone,
My 1989 Audi 100 is not running very well and during idle id doesn't sound too smooth. Especially at lower revs, the power is done and it's
not smooth either. It feels as if it is running only on four cylinders (5 cyl engine) and sometimes it gets even worse as if only three are left working. Some time ago, I pulled out HV wires during idle and it is definitely cyl no 2 which is not working. However, the spark is there and I exchanged the distributor cap and all HV wires with another car. The problem is still there, so I concluded that it's not the ignition.
The problem was there some time ago, then went away suddenly while accelerating and came back a few weeks later. Unfortunately, it decided to stay with me now.
I am a bit stuck now. I assume it has to do with the fuel injection, but I don't know where to start. Is there any typical problem I can check?
Thanks a lot for any advice,
Christoph
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Christoph Bollig wrote:

How does the plug look? Similar to the others? It could be that the balance of the I5 engine masks the effect of pulling plug wire on #2.

Before throwing money at ignition parts you may want to consider other causes. I would be suspicious of engine vacuum.
Test while idling: Pull oil dip stick up. Engine should begin running very rough.
Replace dip stick.
Remove oil fill cap from valve cover. Engine should stall (quit)
If these do not perform this way you need to isolate and fix the vacuum leak before other things. The crank case breather tube is a chronic problem on the I5s.

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Compression test engine. Exchange that #2 plug with another plug and see if the problem is still with #2 Exchange injector with another.
Maybe a vacuum leak from a hose or at the injector causing problem.
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later,
dave
(One out of many daves)
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Hi Tony and Dave,
Thanks for your advice. I will reply to both of you in one message:
wrote:

The plug looked different to all other ones, more oily. We concluded that this was because it was not working.
When I pulled the wire on any of the other plugs, there was a clear difference. Only on #2 there was no effect.

No effect.

When I open it, it feels like a vacuum is pulling the cap down. However, when I remove it, there is no effect.

The pipes around the engine were all replaced relatively recently.
I did the same test on another Audi with the same engine (but a bit older, 1988 model). It was exactly the same behaviour (feels as if vacuum under cap, but no effect when I take it off). That other Audi is running fine with less than 10L fuel consumption per 100km.
Taking all this into account, I still believe it is a problem specific to that cylinder. I forgot to mention that all spark plugs were replaced, still no change. I might also be worth noting, that when the problem went away, it went very suddenly. It was a clear transition at which the car suddenly performed well again. Unfortunately, the problem came back and I cannot reproduce the "get better" part again.

Do you mean a compression test? I did one a few months ago and all cylinders were normal, except that #3 was down slightly. Can compression come and go like this?

As mentioned, all plugs were changed.

I am a bit reluctant to pull out the injectors, after I read somewhere on SJM Auto-Technik ( http://www.sjmautotechnik.com/ ) that some part of the injector could fall into the cylinder and one would have to take off the cylinder head if that happens. Not the type of thing I want to learn at the moment :-(

Do you mean a leak where the injector goes into the cylinder? Any way I can test that?
I was wondering whether there could be a problem with the injection pipes or somewhere in the system. Maybe some dirt clocking it up?
I was hoping there might be some simple test I could try on the injection system. But if the diagnosis is difficult, I will just have to face it and bring it to the experts :-(
Thanks anyway for your help,
Christoph
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So the spark plug that was oily could mean a problem with the engine.

Still might have to test injectors. :-(

Spraying carb cleaner at fuel injector may cause engine speed to change possible a vacuum leak.

That can sometimes be less expensive (less parts to purchase but more labor) and usually faster results.
good luck, dave (One out of many daves)
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