car failed mot on emissions

[skud vrs]

Car went to vw today for an mot.

It failed on the lambda readings the limits are 0.97 - 1.03 and i got 1.04

Any ideas thankyou

[skud vrs]

This is the mot slip i had.

Thanks

[piv34uk]

Get them to measure the oil temperature as it says it was bypassed, this way you know the engine is hot...also if they can get them to measure the rpm or at worst watch them do it.Its such a small amount that they may not have prepared the car properly before testing it, that said it could always be down to a small leak in the exhaust.

HTH

Phil

[skud vrs]

They had the car in there for an hour trying the emissions.

So the engine should be hot

[piv34uk]

Trying to think hard....... they have tested your engine as a AGU should be AUQ, from memory no difference in values...like i said before check your exhaust for leaks

[hussarman]

OXYGEN (O2) SENSOR. Used on both carbureted and fuel injected engines since 1981, the oxygen (O2) sensor is the key sensor in the fuel mixture feedback control loop.

Mounted in the exhaust manifold, the O2 sensor monitors the amount of unburned oxygen in the exhaust. On many V6 and V8 engines, there are two such sensors (one for each bank of cylinders).

The O2 sensor generates a voltage signal that is proportional to the amount of unburned oxygen in the exhaust. When the fuel mixture is rich, most of the oxygen is consumed during combustion so there is little unburned oxygen in the exhaust. The difference in oxygen levels between the exhaust inside the manifold and the air outside creates an electrical potential across the sensor’s platinum and zirconium tip. This causes the sensor to generate a voltage signal. The sensor’s output is high (up to 0.9v) when the fuel mixture is rich (low oxygen), and low (down to 0.1v) when the mixture is lean (high oxygen).

The sensor’s output is monitored by the computer and is used to rebalance the fuel mixture for lowest emissions. When the sensor reads "lean" the PCM increases the on-time of the injectors to make the fuel mixture go rich. Conversely, when the sensor reads "rich" the PCM shortens the on-time of the injectors to make the fuel mixture go lean. This causes a rapid back-and-forth switching from rich to lean and back again as the engine is running. These even waves result in an "average" mixture that is almost perfectly balanced for clean combustion. The switching rate is slowest in older feedback carburetors, faster is throttle body injection systems and fastest in multiport sequential fuel injection.

If the O2 sensor’s output is monitored on an oscilloscope, it will produce a zigzagging line that dances back and forth from rich to lean. That’s what a technician wants to see when he checks the O2 - think of it as a kind of heart monitor for the engine’s air/fuel mixture.

O2 Sensor Strategies: Unheated one- or two-wire O2 sensors on 1976 through early 1990s applications should be replaced every 30,000 to 50,000 miles to assure reliable performance. Heated 3 and 4-wire O2 sensors on mid-1980s through mid-1990s applications should be changed every 60,000 miles. On OBD II equipped vehicles, the recommended replacement interval is 100,000 miles. The O2 sensor’s responsiveness and voltage output can diminish with age and exposure to certain contaminants in the exhaust such as lead, sulfur, silicone (coolant leaks) and phosphorus (oil burning). If the sensor becomes contaminated, it may not respond very quickly to changes in the air/fuel mixture causing a lag in the PCM’s ability to control the air/fuel mixture.

The sensor’s voltage output may decline giving a lower than normal reading. This may cause the PCM to react as if the fuel mixture were leaner than it really is resulting in an overly rich fuel mixture.

How common is this problem? An EPA study found that 85 percent of the vehicles that failed an I/M 240 emissions test needed a new O2 sensor.

[fordfan]

First of all it helps to know what the lambda reading on the emissions test means.

For optimal combustion there is an ideal air to fuel ratio (AFR), which is 14.7:1 (stoichometric value), when there is this ratio the lambda reading is designated as 1.00.

Anything under 1.00 and the engine is running rich, either too much fuel or too little air. Over 1.00 and the engine is running lean, either too much air or too little fuel.

With this engine check the vacuum pipes and breather hoses for any splits, also as already mentioned check the exhaust for any leaks, the reason being that air could be entering the exhaust between the engine and where the gas testing probe is inserted in the tailpipe.

[Land_Raver]

Mine was high like yours so I replaced the o2 sensor gave the injectors a clean with the stuff you put in the tank & changed the engine oil & filter. Mileage was 61K on a 1.8T engine. The car then passed. Hope this helps Nick

[Jon_VRS]

Well mine is due it's MOT next week, so will see what it comes up with. No errors last time it was on VAG-COM but that probably means nothing when it comes to the MOT test.

[skud vrs]

Car went back to VW on Monday,

They cleaned the lambda sensor and then cleaned all the earthing points.

Once retested the lambda reading was 1.00.

Thank you all for your help.