It Begins: The S3 1.8T into my '85 Mk2 GTI

[GOLFBALLS]

Wow. The plot thickens as the tuning gets more mystifying!

The tuners skyped up with the qpeng guys who were very helpful and showed him how to best use the system. After tweaking around with the settings and richening the mixture, etc it has not changed the leaning out issue, which in turn has led them to go back to looking at the fuel system!

Apparently, if you gradually apply the revs it doesn't lean out, but under load and with 100% throttle on the dyno it leans out on boost???!!!

I thought 'ok, last thing left to do is delete the digifant II fuel accumulator/reservoir still sitting there'. So off it came, and guess what? Absolutely no difference! AAAAAARRRGGGGGGGGGHHHHHHHHH!!!!!!!!!!!!

The ONLY thing left that I can possibly imagine that is restricting fuel flow now could be the fuel filter (which is a digi II RV spec one). I'm going to make a dash back to work tonight to pop in an S3 spec one and see what happens.

So far I have tried;
- new fuel tank and brand new VDO VR6 fuel pump
- new VDO fuel pressure regulator
- new fuel filter (mk2 GTI RV spec)
- ECU map adjustment

If anyone can think of any other possible causes, PLEASE chime in...

Cheers,
Wade

[Dscott]

well generally when things like this go wrong, we have to go back to basic's...

Spark plugs?

[Jarred]

how is the timing when it comes on boost?

Could it be something simple like a vac leak somewhere?

I spose you've already checked all that, but i figure worth mentioning.

HTH

[h100vw]

Wade, what diameter are the S3 fuel pipes, compared to the mk2s? It's not a volume rather than pressure thing is it? Hunt down the fuel flow figures and see if it is moving enough juice, cc's a minute, to the engine bay. Doesn't really explain why a slowly increasing throttle works OK, unless there's something amiss with the way the FPR is working.

Gavin

[GOLFBALLS]

Hmmm. The vac lines i already replaced and also the plugs. The mk2 fuel hard lines are the same diameter and all the other things i've replaced are genuine OEM brands from work and AMK specific.

The timing is fine, and if I unplug the boost controller (n75) and run it at low (wastegate) pressure it's perfect - just 90bhp short! Seems to be only an issue when on full boost at max pressure...

[MattyT]

Sounds like a pretty maddening situation to be in! I dont know enough about engines to assist sadly. So long as its getting enough dinojuice it should be okay??? Personally I would try running a full S3 fuel system from tank to donk (if you arent already?).

[GOLFBALLS]

Sounds like a pretty maddening situation to be in! I dont know enough about engines to assist sadly. So long as its getting enough dinojuice it should be okay??? Personally I would try running a full S3 fuel system from tank to donk (if you arent already?).

Well it pretty much is now...

I just spent an hour or so removing the whole fuel reservoir/external pump assembly from under the car and have now even fitted the S3 spec filter just to be super sure. So its basically the in-tank VR6 pump feeding directly to the filter, and straight up to the fuel rail, to the stock S3 injectors and brand new S3 spec FPR, then back around. Simple as, and still the same.

If I plug in the N75 and run it at full boost and bury the throttle, I can just hear it start to sputter as it spools up, at which point I immediately button off to avoid engine kaboomage.

With the N75 disconnected, its revving up beautifully and getting enough fuel, but feels gutless obviously due to the low boost pressure

So, that's it. Theres' nothing left it could be but the ECU....

[Dscott]

one point, which n75? n75J right from the s3? cos i've read alot about the splutting and surging with an unsuitable n75.

just trying to help

[GOLFBALLS]

one point, which n75? n75J right from the s3? cos i've read alot about the splutting and surging with an unsuitable n75.

just trying to help

Hmm, well it was the one fitted to the engine when I got it and is a genuine Audi part, but I'll get the part number off it and double check it in ETKA. Thanks!

[Valver.]

You've certainly got a curly one! Again, apologies if I've missed something earlier in the thread, but have you logged what's happening with the fuel pressure by tee-ing into the rail entry or somewhere around there? It can quite literally only be a fuel pressure and delivery issue now.

Have you flow tested and/or had the injectors ultrasonically cleaned? Are they up to spec (440ccm IIRC?) or is something amiss here?

Is the fuel rail blocked with dislodged filter material (rare, but have seen it happen!)?

Have you gone right back to basics with a noid light on the injectors to ensure accurate pulse? And is the wiring consistent to all four injectors?

What's happening with voltage to the fuel pump?

Have you back-probed the injector outputs to check for voltage/spikes?

Don't spend too much more on Bosch parts - I work for Bosch Automotive and can get them to you for a fraction of what you've have paid so far