Problem running 1997 A4 TDi from cold

Hi All,

I'm new to this forum so apologies if this is old ground, but I have just
bought a '97 A4 TDi Avant and found that I've inherited some problems that I
didn't notice on inspection. (once a sucker...)

When starting from cold the engine fires up okay, but as soon as you drive
off under any sort of power or even rev it, it will run roughly - on what
sound like three cylinders - for moment then die. On restarting it will
continue to run roughly, producing a considerable amount of white smoke that
appears to be unburnt/partially burnt diesel - especially if you attempt to
rev it a bit harder. After a while it will return to smooth idling. Once the
engine has been idling for a few minutes every thing's fine - no smoke,
performance is fine, if possibly a little uneven whilst not up to
temperature. There don't appear to be any symptoms of a cracked head or
gasket and once it's up to temp blows very little smoke at all.

It's been in an out of a local garage a couple of times in the last week -
first time, an injector was replaced due to a DTC on its needle lift sensor
(I never saw the fault, but I was shown the report from the Diagnostic
tool.) Next, a valve associated with the fuel pump that regulates the
pressure to the injectors was found to be faulty, and the injectors were
being supplied with too much fuel pressure (does this actually matter? Isn't
there a cam driven pump for each injector in VAG diesels to supply pressure
to the injector valve?)

On each occasion, the mechanic says he has no problems starting it from cold
but it fails for me the next morning (maybe it's a bit colder in my driveway
than his workshop?)
One thing I find odd is that despite the temperature in the morning over the
last being about 1°C or so (in sunny Scotland) the glow plug lamp never
stays on for longer than the second or so the power up test runs for. The
manual says the lamp and the glow plugs should come on when the engine temp
is below 8°C. So I'm wondering could all this simply be a product of a dodgy
temperature sensor? The running temp of the engine is indicated on the dash
as 90°C, but is the sensor that gives that reading the same as the one that
tells the engine management ECU to turn on the glow plags and do whatever
else it should do when the engine's cold?

Finally, there appears to be a leak from an exhaust coupling between the
manifold/turbo and the rest of the exhaust system - could there be an issue
with exhaust back pressure or something like that?

I don't profess to know much about diesels, but the big flaw in most
theories'is the fact that the engine starts fine, and only has problems if
you exert it without letting it warm up a bit first.

-Dan

dan wrote:
> Hi All,

>
> When starting from cold the engine fires up okay, but as soon as you drive
> off under any sort of power or even rev it, it will run roughly - on what
> sound like three cylinders - for moment then die. On restarting it will
> continue to run roughly, producing a considerable amount of white smoke that
> appears to be unburnt/partially burnt diesel - especially if you attempt to
> rev it a bit harder. After a while it will return to smooth idling. Once the
> engine has been idling for a few minutes every thing's fine - no smoke,
> performance is fine, if possibly a little uneven whilst not up to
> temperature. There don't appear to be any symptoms of a cracked head or
> gasket and once it's up to temp blows very little smoke at all.
>
Could be glow plugs- they are meant to stay on for a while after
starting.Post the query on the forums at http://www.tdiclub.com, there's
a lot of knowledge there.

> It's been in an out of a local garage a couple of times in the last week -
> first time, an injector was replaced due to a DTC on its needle lift sensor
> (I never saw the fault, but I was shown the report from the Diagnostic
> tool.) Next, a valve associated with the fuel pump that regulates the
> pressure to the injectors was found to be faulty, and the injectors were
> being supplied with too much fuel pressure (does this actually matter? Isn't
> there a cam driven pump for each injector in VAG diesels to supply pressure
> to the injector valve?)
Not on a 1997 TDi. It has a conventional rotary injection pump. Only the
later 'PD' engines have individual pump-injectors.
>
> On each occasion, the mechanic says he has no problems starting it from cold
> but it fails for me the next morning (maybe it's a bit colder in my driveway
> than his workshop?)
> One thing I find odd is that despite the temperature in the morning over the
> last being about 1°C or so (in sunny Scotland) the glow plug lamp never
> stays on for longer than the second or so the power up test runs for. The
> manual says the lamp and the glow plugs should come on when the engine temp
> is below 8°C. So I'm wondering could all this simply be a product of a dodgy
> temperature sensor? The running temp of the engine is indicated on the dash
> as 90°C, but is the sensor that gives that reading the same as the one that
> tells the engine management ECU to turn on the glow plags and do whatever
> else it should do when the engine's cold?
>
That's a very good point. I think the sensor is seperate. (ie there are 2).

> Finally, there appears to be a leak from an exhaust coupling between the
> manifold/turbo and the rest of the exhaust system - could there be an issue
> with exhaust back pressure or something like that?

It's not going to help :-)
>
> I don't profess to know much about diesels, but the big flaw in most
> theories'is the fact that the engine starts fine, and only has problems if
> you exert it without letting it warm up a bit first.
>
> -Dan
>
>
>