O2 sensor voltage

[jmhaze]

I have a unusual problem on my 2004 Audi A4 w/ 1.8L Turbo. I get a P1128 Fuel trim lean about every other day after I clear it. I checked my O2 sensor readings everything is normal at idle but while driving when I decellerate the O2 voltage drops to 0volts and the ECM enters open loop. I think I might have a defective downstream sensor. I wanted to check with someone to find out if I'm right or if maybe something else might be causing this. I've already tried changing the MAF but it didn't change anything.
Thanks, John

[Crasher]

Possibly a crankcase ventilation hose is split, most likely the one from the breather hose T piece under the inlet manifold that goes up to the inlet manifold itself (part number 058 103 221 E) , that is assuming your car has an AMB engine. The hose tends to open up under boost so often it is only possible to check it unless you perform a boost leak pressure test so removing it is the best way to check it.

[jmhaze]

I did some checking and found the connection between the Y Pipe and the hose from the "T" had cracked. I have a new hose on order hopefully it works out. Thanks for your help.

John

[Crasher]

Yes, that would do it, that pipe goes all soft and then splits. We find this more on cars run on very long oil change intervals, we think the oil becomes very aggressive to rubber components.

[jmhaze]

Well I changed the hose but the O2 sensor voltage still goes to zero when decellerating. Any other ideas what could be causing this?

Thanks,


John

[Crasher]

On deceleration it goes into overrun fuel cut.

[jmhaze]

So is this normal? Also will this cause my long term fuel trim to climb till I set DTC P1128?

John

[Crasher]

The fuel cut off will cause the sensor to flat line. What do you mean by setting that P code?

[jmhaze]

After a couple days of driving it sets a P1128 code Indicating Long term fuel trim too lean. You clear it but it will come back in about 2 days. O2 sensor voltage at idle is about .440 volts and fluctuates a little. MAF indicates .5 lbs /min. air temp is a little higher than ambient but I thought not much air flow through the intercooler will cause this. Right now Long term trim value is at 14% but when it climbs to about 22% it will set the code. Is there any values you need to know? I can get them for you.

Thanks
John

[Crasher]

Sounds like a crankcase ventilation hose air leak or failed AMM