DTC - Lean mixture

[Golich]

I had a look at my mates (98)S4 the other night after he was complaining of it feeling a little low on power.

I did the usual check TBB, simply felt around it and decided we take it for a run with the VAGCOM hooked up to measure the boost pressure under load rather than remove the TBB.

I hooked up VAGCOM and found two DTC - lean mixture! Before setting off, I cleared the codes (engine idling - Ref Joshie's post and AMD's advice) and we went for a spin. Boost pressure came up as approx. 0.7bar i.e. I think the ambient pressure was 0.98 something and VAGCOM was recording 1.69 something.

When we came back I re-checked for re-occuring DTC - Nothing.

First off - does anyone know what the stock boost pressure should be?

Secondly having noted Joshie's recent post on the faulty MAF and lean running. Are there any other potential items that would cause this?

Having read Joshie and Nige's posts I'm thinking we'll have to take his car out to check the MAF readings and keep an eye out for a return of the lean mixture code.

Thirdly, I have given some thought to my logic of measuring the boost pressure to check for boost leaks and I now think that measuring the boost pressure is not necessarily the correct way to have approached this.

Am I correct in thinking that the process of boost goes somewhat simply like this.

1. ECU notes throttle opened
2. For this example engine happens to be cruising - so little boost pressure, therefore closes waste gate (if open).
3.Boost pressure is allowed to build as turbos spool up with increasing engine RPM.
4.ECU is constantly monitoring the boost pressure ready to send signal to N75 which in turn will activate the waste gate upon the requested boost being achieved.
5. However, there is a slight boost leak so the waste gate stays closed. Turbos continually spin to provide boost.

My question is this - This term ‘over stressing’ or ‘over working’ the turbos had conjured up the idea in my mind that the turbos were spinning much faster than they need to to make up for the leak in the boost pressure. But since the turbos are driven by the flow of exhaust gases how would they spin any quicker as a result of a boost pressure leak?

I suppose if the boost pressure achieves a level that it offers sufficient resistance to the driving exhaust gases then equilibrium is achieved across both sides of the turbo with the exhaust gases in effect slipping over the driving vanes of the turbo (if you will). Therefore a leak in boost side of the system will offer reduced resistance therefore allowing increased turbo rpm until sufficient boost pressure is achieved thus the waste gate is opened, but then this allows the turbo to spin as freely as the flow of exhaust gases permit - I’ve just come full circle in my train of thought [img]images/graemlins/crazy.gif[/img]

Someone please explain!

Incidentally I gave him a spin in my AMD chipped S4 and he is now desperate to get his car done.

[Dippy]

A standard Euro S4 will reach 0.7 - 0.8 bar over atmospheric.

A faulty MAF is one possibility, but I think there are others sensors which could generate a lean DTC. An air leak should cause a rich DTC if anything as LESS air goes in than the ECU measures.

Your analysis is correct: Charge pressure is controlled by the wastegates. If there is a leak (or DVs stuck open), then more airflow is needed to maintain the demanded boost, and so the turbos spin faster than they would normally. The wastegates stay closed where they would otherwise open because they are controlled by the ECU measuring demanded boost at the throttle.

However true overstressing of the turbos only really happens at the limits - trying to maintain their peak boost (around 1.3 bar) at full revs. In the worst case you get compressor surge which is what you describe.

Air leaks for chipped S4s are bad news. Air leaks for standard S4s may or may not be bad news, depending on how long the engine is run with the leak.

Since you have VAG-COM, you really need to monitor things like air flow (see Nige's post) and torque vs revs.