Hi,
had to go for the smog test yesterday, thanks to the guy measuring twice we finally got lambda value at upper limit at high rpm over 2000 and passed.

But would like to understand better what this is all about and where to look.
It is a 850-94 NA engine with 270k kms on, no major repairs (no A2/A6 DTC), air filter renewed at 240k kms, MAF cleaned once two years ago.Is cleaning causing lean or rich mix? and high lambda?

Lambda value is about oxygen level at O2sensor, one before cat in this 850:
- is my original sensor getting old and/or dirty (is dirty sensor causing high lambda?) or
- is my exhaust leaking before O2sensor or
- is my air intake leaking after MAF or is MAF getting old/dirty,or
- is my cat getting worn out
-does ECU reset by power off have any effect

all the measurements at over 2000 rpm (high limit value)
CO=0%(0.3),HC=11ppm(100), lambda=1.03(1.03),CO2=14%(-), O2=1.2%(5.0)

Br Tapsa

I always have people check the vacuum lines/elbows for leaks, along with the vacuum tree and intake manifold gasket. Any air leaking into the manifold after the MAF will cause the engine to run rich and mess things up.

The CAT is still good. A leaky exhaust will not cause it. The O2 sensor could be dirty.
Don't disconnect the battery to delete messages, do it the proper way via the diagnostic boxes in the engine compartment. The only code that will not erase is the B-2 1-2-2 for the cruise control.

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My name is Klaus and I am a Volvoholic.

Hi,
found no clear leak indications, but have to wait for warmer evenings to go thorough/replace all lines propperly.
Did tap the HO2S sensor (black/grey) with voltmeter and got 0.45V at ignition on, increasing voltage ( to 0.7V)when enrgine running and getting warmer and then it started show values between 0.1 to 0.8V i.e. working about as it should. I will take a look at the sensor when it gets warmer to see how dirty the sensor is.
br Tapsa

Hi,
It's good I can exclude cat and exhaust and I share the opinion about vacuum leak, to be checked. A minor leak wouldn't do much over 2000 rpms? would it.

But then the logic? A leak after MAF is adding air and causing leaner mix and higher lambda (>1). Is there a control feed back loop making the engine to run rich instead? based on HO2S signal for example.

Anyway the mpg is down last year, based on this or the change in use of the car since the new V70D5AWD is used for longer distances mainly.

br Tapsa

The logic is not totally perverted. The ECU has the amount of air from the MAF plus readings from the front O2 sensor. The additional air from a leak will show up as a "lean" condition on the O2 sensor, so the ECU lengthens the injection time to increase the fuel. As you apply throttle, the vacuum level in the manifold decreases, and the air leak is not as bad, giving a very rich environment.
A friend of mine has a NA engine which had a very bad leak. First, the O2 sensor became very dirty and failed. The sensor was replaced, turning off the CEL, but the engine was running so rich at lower rpms (1500-2000) that the plugs fouled and caused a no start condition. All is now good with a new elbow.

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My name is Klaus and I am a Volvoholic.