Hello everybody, I have a big problem, although I changed the spark plugs and coils , when in full gas the car is sluggish between 3000 and 4000 rpm and the acceleration is not smooth. The check engine light is on because I have a milltek full exhaust and no cat. On vag-com the only problem is the lambda sensor located after the cat. It doesn't say anything else, maybe you can help.

PS Please excuse my english, it's not that great in case you didn't notice

Thank you

If you have a full miltek that should have a sports cat which means you still have the cat, unless you fitted a decat pipe from the turbo.

Are you sure you have a decat(Sorry if this is common sense but checking)

Some Sports cats(cell100) will make cell light come on. Then also if a O2 sensor goes down the air/fuel will try and recover but depending on failure may not be able to thus restrict power to save engine, and if not already done so, throw cell light as the car is in safe mode.

I have a decat pipe from the turbo (THS) , I will do another vag-com test today...

The CEL has been on since I have fitted the exhaust and it was no problem, until two weeks ago.

Also not trying to be funny, but do you have 2 lambda sensors? If so, are they possibly returning the same value?

same as above if you still have 2 lamda sensors you need to blank the first one off so the later has a higher reading or the light will stay on...

also why would you go for a decat pipe???

Edited 7 July, 200916 yr by westallc

I'm not sure where Constanta is off-hand, but I know it's not in the UK!

Constanta is a city in Romania

Today I hope that my friend with vag com will help me, how can I blank the first lambda sensor?

The easiest thing to do would be to just unplug its cable and fasten it away from the exhaust. Slightly harder would be to do that, and find a short bolt with the right thread to replace the sensor.

The first sensor is the one before cat, or the one after?

First time when I had a remap I didn't have the lambda sensor after the cat because the were flames on the exhaust when racing. I changed the map with one from "upsolute" and I instaled the sensor. I had no problem until 2 weeks ago :(

Before the cat; the one nearer the turbo.

All the flaming proves is that your old map was over-fuelling on the over-run. It is/was quite common on turbo-charged competition cars that used fuel injection on the over-run as a crude anti-lag system.

I thought that the first sensor is the one who is takin' care of the oxygen and fuel mix. If I blank it, don't you think I'll have problems with overfuelling or maybe I'll have to much oxygen in the mix?

As Westallc said above, you need to get the second sensor to return a higher voltage to the ECU than the first, so the ECU thinks the catalytic convertor is present and active. With both connected and a decat, they'll return the same voltage.

I thought that the first sensor is the one who is takin' care of the oxygen and fuel mix. If I blank it, don't you think I'll have problems with overfuelling or maybe I'll have to much oxygen in the mix?

No the second sensor(post cat) is taking the actual reading of the fuel air mix. This is what will tell the ECU to adjust fueling.

No the second sensor(post cat) is taking the actual reading of the fuel air mix. This is what will tell the ECU to adjust fueling.

sorry, this is plainly incorrect. The front (wideband) sensor is controlling the fuelling, the rear post-cat sensor is simply looking for any excess oxygen coming out of the cat, indicating the cat isn't working correctly. The rear sensor has no influence on the fuelling, it is purely there for OBD2 emmisions monitoring.

Dont mess with the front sensor, (blanking it off or removing it), as this will upset the fuelling and make the engine run badly. The system uses the wideband sensor to control the fuelling in closed loop all the time, richening the AFR at high load/boost - with Vagcom etc you can monitor the "target" and "actual" lambda values to see this working.

sorry, this is plainly incorrect. The front (wideband) sensor is controlling the fuelling, the rear post-cat sensor is simply looking for any excess oxygen coming out of the cat, indicating the cat isn't working correctly. The rear sensor has no influence on the fuelling, it is purely there for OBD2 emmisions monitoring.

Dont mess with the front sensor, (blanking it off or removing it), as this will upset the fuelling and make the engine run badly. The system uses the wideband sensor to control the fuelling in closed loop all the time, richening the AFR at high load/boost - with Vagcom etc you can monitor the "target" and "actual" lambda values to see this working.

Point taken, and thanks for explaining it.

Something else then has caused the difference of running that needs checking before messing with the sensors.

oh yea, sorry I wasn't helping with the original post... oops..

I think I'd be trying it with the MAF sensor (airflow sensor) disconnected and see if it's better - MAF sounds like a likely candidate to me.

Thank you guys

The problem is the alternator, I'm not sure if this is the corect term, but it is charghing max 12.8 V. That's why I thought that the sparks or the coils are the problem. Next week I'll have a new alternator and I hope that everything will be alright

Thank you

Is the drivebelt tension OK? I'm asking because a loose drivebelt can give you low charge with an otherwise perfectly good alternator.

also worth checking the connection between alternator body and engine and the earth strap from engine to bodyshell and earth strap from battery negative terminal to bodyshell.

Maybe try physically connecting a seperate cable from the alternator body directly to the chassis.

Next week I'll check everything you advised

Thank you again