A4 rough running

737800

I have a 98 automatic A4 1800cc petrol non turbo engine code ADR. it failed the nct on emissions so i had the cat and lambda sensor replaced today (second hand), he tested the emissions and they would be a pass for the nct test.
Problem is though its very rough running at around 2000 rpm now and accelerator really needs to be pressed to get it over 2000 rpm sounds almost like its choking (not enough air or too much fuel or something) i have checked the air filter it looks clean and the plugs do not look fouled. It had this problem before but nearly as bad..... any ideas??

Kind regards.

Buford T Justice

You should be using a new lambda sensor in the first case imo

737800

What does that sensor do?

El Guapo!

Definately should be using new sensor anyway.
Check for air leaks too. They're well known for perished vacuum hoses and/or clogged hoses

Buford T Justice

737800 wrote: »

What does that sensor do?

It analyzes the exhaust gases and adjusts the fuel mix in the engine accordingly to ensure the car is running properly. A Faulty sensor will cause you problems like this

737800

On the test it gives "lambda: 1.02" is this an indication that its working or not working? can these sensors be running poorly or is it a case that they work one day and then stop working?
Because it is a 98 i do not want to put too much money into it so i do not want to buy one on the off chance it might work!

shamwari

737800 wrote: »

On the test it gives "lambda: 1.02" is this an indication that its working or not working? can these sensors be running poorly or is it a case that they work one day and then stop working?
Because it is a 98 i do not want to put too much money into it so i do not want to buy one on the off chance it might work!

The Lambda in the emissions test relates to the gas mixture read during the test. It is not an indication of how well or other wise the Lambda sensor itself is working. That lambda reading you have there seems to be within the prescribed limits.

From reading your opening post, it seems that your car may have developed a fault condition since you had those parts replaced. You should bring it back as something is clearly wrong. Also agree that a 2nd hand Lambda sensor is not a smart move, but if your mechanic has a suitable scantool, it should be possible to determine if it is the cause of your current problem or not.

737800

The fault did seem to be there before the parts were replaced albeit on a much lesser scale, the engine hesitating at around 2000 rpm but no problems once you got past that rpm.
However today since the parts were replaced at times it really struggled to get over 2000 rpm and at one stage was unable to do so and eventually stalled but started immediately after.

shamwari

You need to have it looked at properly because there are several things which could cause that type of a fault. If you have any kind of a misfire then this could well wreck that cat again, so I wouldn't drive it any further than you need to. Fitting second hand parts adds even more unknowns into the equation.

LookBehindYou

Did you check the MAF ?

737800

MAF?? what's that in layman's!?

Buford T Justice

Mass Airflow Meter / Mass Airflow Sensor.

tossy

You replaced a sensor with an unknown unproven second hand sensor and problems started,so before you go diagnosing other problems either revert to original sensor and see if problems go away or have the car scanned with a diagnostic tool and see what codes come up.