What problem can be overcome by advancing the timing?

Background: Car is a 1988 UK Audi 100 with 2.0 5-cyl RT engine. It is
extremely difficult to start. If I can start it, then it will run ok for
about 10-15 mins then cut out. Difficult to restart if turned off.
I have swapped the warm-up valve, ISV & fuel distributor for known good
ones but no change. Cold start valve ok, idle switch ok, no vacuum leaks
found. Plenty of fuel getting to the fuel distributor. Hall sensor ok -
anyway I always get a spark.

Recently I played around with different ignition timing settings. 18°
BTDC is the standard figure but I'm using regular unleaded for which I
believe the timing should be 8° BTDC. However, the best setting I found
was about 30° BTDC! With this it starts on the 2nd crank instead of
about the 30th. Cam timing seems in order and all my TDC marks line up together.

I've spent ages on this so any explanations of what's happening are most welcome.
Thanks, Gary

 

Sounds like you have leaky injectors flooding the engine causing no
start until is clears. Try holding the pedal 1 /4 down when starting to
clear. Use Tecron to clean injectors. Might need to replace the
injectors - worst case.

Dying after about ten minutes might be caused by an after market remote
starter or security system being installed. Not sure of the details but
check here:
http://forums.audiworld.com/v8/

There has been a recent thread like this in the last month.

Good luck.

Tony
'91 100q 5spd


Gaz B wrote:

> What problem can be overcome by advancing the timing?
>
> Background: Car is a 1988 UK Audi 100 with 2.0 5-cyl RT engine. It is
> extremely difficult to start. If I can start it, then it will run ok for
> about 10-15 mins then cut out. Difficult to restart if turned off.
> I have swapped the warm-up valve, ISV & fuel distributor for known good
> ones but no change. Cold start valve ok, idle switch ok, no vacuum leaks
> found. Plenty of fuel getting to the fuel distributor. Hall sensor ok -
> anyway I always get a spark.
>
> Recently I played around with different ignition timing settings. 18°
> BTDC is the standard figure but I'm using regular unleaded for which I
> believe the timing should be 8° BTDC. However, the best setting I found
> was about 30° BTDC! With this it starts on the 2nd crank instead of
> about the 30th. Cam timing seems in order and all my TDC marks line up together.
>
> I've spent ages on this so any explanations of what's happening are most welcome.
> Thanks, Gary

 

TonyJ wrote:
>
> Sounds like you have leaky injectors flooding the engine causing no

Thanks for your reply Tony.

The injectors could well be past their best as the car has done 184000m.
I will check them next - I've been putting off doing so as they are so
difficult to get to!

However, I still don't understand why the advanced timing 'fixes' it. I
managed a 3 mile journey on this setting (I couldn't get out of the road
previously)! Surely I should have experienced serious pinking, but I did
not. Further testing is needed at this setting but I don't want to risk
engine damage.

Gary

 

My understanding is that the ICU / ECU controlls the spark within a
normal range and that if it gets out-of-range input it will use a
default value. But i am not positive on this.

I think that the timing is supposed to be set at 8 degrees BTC. In order
to check this you need to put a fuse in the top of the fuel pump relay
when you are timing it. This overrides the ECU advance / retard inputs.

Gaz B wrote:
> TonyJ wrote:
>
>>Sounds like you have leaky injectors flooding the engine causing no
>
>
> Thanks for your reply Tony.
>
> The injectors could well be past their best as the car has done 184000m.
> I will check them next - I've been putting off doing so as they are so
> difficult to get to!
Try a bottle of the Tecron injector cleaner.
>
> However, I still don't understand why the advanced timing 'fixes' it. I
> managed a 3 mile journey on this setting (I couldn't get out of the road
> previously)! Surely I should have experienced serious pinking, but I did
> not.
Probably the knock sensor heard pinging and then the ECU retarded the
timing so you really wern't driving with it as far advanced as you think.

Further testing is needed at this setting but I don't want to risk
> engine damage.
>
> Gary

 

> My understanding is that the ICU / ECU controlls the spark within a

Tony, thanks for response, but with this age/engine in the UK the
ignition is TCI-H with an ICU. Timing is adjusted by moving the distributor.
My understanding is that the timing is controlled by an ECU on later
models where fully electronic ignition is fitted. I think only these
models have the fuse holder in the relay, and knock sensors too.
I'm measuring the timing with a strobe pointing at the flywheel.

Thanks, Gary

 

I have done the same with the strobe, flywheel timing by turning the
distributor on my current '91 100q and previous '87 5kq. Although the
control is direct I am fairly sure that the ECU continues to override
the timing by using inputs from sources including the knock sensor once
the fuse is removed from the FP relay. The result, I would think, is
that the timing would be retarded to the 8 or so advanced point when
driving. the 18 advence might only be real when you are first starting
the engine.

Tony
'91 100q 5spd

Gaz B wrote:
>>My understanding is that the ICU / ECU controlls the spark within a
>
>
> Tony, thanks for response, but with this age/engine in the UK the
> ignition is TCI-H with an ICU. Timing is adjusted by moving the distributor.
> My understanding is that the timing is controlled by an ECU on later
> models where fully electronic ignition is fitted. I think only these
> models have the fuse holder in the relay, and knock sensors too.
> I'm measuring the timing with a strobe pointing at the flywheel.
>
> Thanks, Gary