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

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Christoph Bollig - 22 Jul 2005 09:31 GMT

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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Tony - 22 Jul 2005 18:21 GMT

> Hi everyone,
>
[quoted text clipped - 7 lines]
> another car. The problem is still there, so I concluded that it's not
> the ignition.

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.

> 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.

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.

> 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
[quoted text clipped - 3 lines]
>
> Christoph

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dave - 26 Jul 2005 12:48 GMT

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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Christoph Bollig - 26 Jul 2005 17:41 GMT

Hi Tony and Dave,

Thanks for your advice. I will reply to both of you in one message:

On Fri, 22 Jul 2005 12:21:14 -0500, Tony <tonyjnospam@nospamvisi.com>
wrote:

> Christoph Bollig wrote:
> > Hi everyone,
[quoted text clipped - 12 lines]
> It could be that the balance of the I5 engine masks the effect of pulling plug
> wire on #2.

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.

> > The problem was there some time ago, then went away suddenly while
> > accelerating and came back a few weeks later. Unfortunately, it
[quoted text clipped - 5 lines]
> Test while idling:
> Pull oil dip stick up. Engine should begin running very rough.

No effect.

> Replace dip stick.
>
> Remove oil fill cap from valve cover. Engine should stall (quit)

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

> 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.

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.

> Compression test engine.

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?

> Exchange that #2 plug with another plug and see if the problem is still with
> #2

As mentioned, all plugs were changed.

> Exchange injector with another.

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 :-(

> Maybe a vacuum leak from a hose or at the injector causing problem.

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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dave AKA vwdoc1 - 29 Jul 2005 14:35 GMT

> Hi Tony and Dave,

>> Exchange that #2 plug with another plug and see if the problem is still
>> with
>> #2
>
> As mentioned, all plugs were changed.

So the spark plug that was oily could mean a problem with the engine.

>> Exchange injector with another.
>
[quoted text clipped - 3 lines]
> take off the cylinder head if that happens. Not the type of thing I
> want to learn at the moment :-(

Still might have to test injectors. :-(

>> Maybe a vacuum leak from a hose or at the injector causing problem.
>
> Do you mean a leak where the injector goes into the cylinder? Any way
> I can test that?

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

> 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 :-(

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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