DksMester

Hi

Just bought a 2000 S4, what a nice car! But I wondered why it pulls in waves. Meaning if I put the pedal max down i 6th gear (and others but easier to notice i 6th), it pulls hard for a moment then not so hard for a moment, then hard again and so on. Does anybody have an idea of why this could be?

Please put a comment

Thanks
Jacob

complacentsee

Need a vag-com to fully diagnos. Is it stock?

I would guess a slight boost leak, or a bad EGT sensor. But you need to get the codes scanned/and or do some logging to determine whats wrong.

Is there an associated flashing check engine light?

donromani

Sounds like a screwy MAF (mass airflow sensor) to me. That was one of the first things I had to replace after I bought mine.

DksMester

Thanks for the replies!

Yes it is stock. No there is no flashing check engine light.
A slight boost leak, meaning the turbo wheel is not tight?
Hoping it's just a sensor. Sounds posible with faulty EGT, meaning the fuel is regulated in waves. Have some touble to see how the MAF should course this, but I have heard MAF is a common fault with these engines.

Please come with more comments. It is all usefull

donromani

The Bosch MAF is a finicky SOB. If your MAF readings are faulty then your engine will do all sorts of stupid crap that will reduce the output. Mostly it causes insufficient boost to be requested from the turbos and a corresponding reduction in fuel supply.

Mine sent a continuous bad signal, sapping the engine of 50% of the power. My friend in his stock 2.8L A4 killed me accelerating away from a light while I was still waiting for the replacement MAF to show up. Horrible shame.

A boost leak is common on these engines, not from the turbos but from rubber hoses, which tend to crack. You probably will not be able to see cracks until you take the hoses off, turn them inside out or otherwise stretch the rubber a little. You can do a boost leak test to find out if you have a leaky hose. The commons hoses to fail are the TBB (throttle body boot) and the the f-hoses.

You might also want to check on the Diverter valves as well to see if they are worn out.

complacentsee

Well, under most conditions the MAF works in conjuction with both primary oxygen sensors to determine the fueling. This is called closed loop mode; during this the car continually checks the output of the system, the exhaust gasses, to fine tune the air to fuel ratio. In Wide Open Throttle (WOT) the MAF is used alone and the system operates in open loop mode, meaning there is no check of exhaust gasses to determine if the correct fueling is supplied with the exception the EGTs. The EGTs just increase the fuel enrichment to make sure your exhaust gas temperatures don't exceed some magical number. Something around 1000C.

donromani

Did not know there were two systems depending on TP. Sweet. That matches my experience though, the car drove fine under normal circumstances, but if I floored it would basically choke.

CTAudiDriver

I would guess a faulty N75 valve (wastegate bypass regulator valve) or boost leak. A boost leak is when there is a hole in the system between the turbo and the motor meaning air is escaping the system. This will cause the turbo to spool to full boost, lose some pressure through the leak and then compensate by spinning faster to get back to the full boost level. It is not good for the turbo to be doing this because you can over spin it. I would start by checking for a boost leak, search AWE pressure tester, go to home depot build one, and check with an air compressor at 10 psi, (you can do more but you should hear a leak at that pressure, also a faint gurgling sound is normal and make sure you cap off the pcv system like it says) if you hear a leak spray soapy water on the area where you hear the leak and it will bubble up. This would be a good place to start. Check the hose clamps on the diverter valves other hoses.

A fualty N75 valve would do similiar things, It wouldnt regulate the wastegates properly causing your car to surge.

Try testing for a boost leak first and let us know what you find

CTAudiDriver

The guys that posted below are correct too, it could be your MAF sensor, if your car has the stock one I would replace it with a hitachi one regardless, the bosch ones on the early models are known to fail

DksMester

Thank you for the answers, very useful!
But im thinking a boost leak, would that make it pull in waves? You should think with a leak, that it will loose pressure constant, therefore just not preforme perfect. If the engine compensate for the leak and adds more boost, then the cycle should be boost, loosing boost, then boost. And not continue up in revs.
I feel it preformes perfect (therefore no leaks), just I could get rid of those lacks.
If I understand *complacentsee* right, then it could be the MAF as EPT is only safety when throttle full open. And if it is the EPT that kicks in then its good as then it works as it should. Or not good as then the engine is above 1000C.

But must say im a noob when it comes to this engine, so im glad you can enlighten me.

-Jacob