Emissions, Cat and Lamda

[Davi]

Trying to work out why my Dad's car is failing the emissions test. Engine runs beautifully, idles fine and is responsive, not excessive fuel useage but it's getting very high results.

Putting a temp. gauge on the cat gives readings at the front 20 deg hotter than at the rear, which if I'm reading right indicates a rich mixture. Sticking a digital multimeter on the oxygen sensor give a reading of 0.46v constant at idle, 0.82v constant at 2000rpm.

Now - can a (very) good quality digital meter pick up the change in voltage quickly enough from the oxygen sensor to display the fluctuations from high to low that we should be seeing, or are those results the mean output of the sensor?

Basically just trying to avoid replacing the cat then finding it was the oxygen sensor, or visa versa! Haven't gone as far as drilling holes in the cat for a pressure check yet as I he doesn't like the idea of shoving a self tapper in the resultant holes if there is nothing wrong with it!

So - thoughts, suggestions, anything helpful?

[GreenV8S]

I wouldn't bother with an exhaust backpressure test based on those symptoms. If the cat was blocked sufficiently to cause running problems at idle, it'd be completely obvious in driving.

[Davi]

that was his feeling as well - it's bloody annoying it's failing the test as it really does run beautifully!

[Avocet]

When you say "high" readings, could you put some numbers on that? I might be trying to teach granny to suck eggs here, but remember that a high lambda means a WEAK mixture - and vice versa!

Also, as far as I'm aware, most lambda sensors go rich-lean-rich-lean continuously (as you say) in just under a second per cycle - so you ought to be able to see it on the cheapest of analogue multimeters.

[That Daddy]

What car is it? and what are the readings of the failed emissions test?

[stevieturbo]

Avocet said:

When you say "high" readings, could you put some numbers on that? I might be trying to teach granny to suck eggs here, but remember that a high lambda means a WEAK mixture - and vice versa!

Also, as far as I'm aware, most lambda sensors go rich-lean-rich-lean continuously (as you say) in just under a second per cycle - so you ought to be able to see it on the cheapest of analogue multimeters.

Do not attach an analogue multimeter to the lambda sensor, it will damage the sensor.

Use only a digital instrument, preferably a scope, to see how fast the sensor is switching.

But as the others have said, what were the fail readings ?

[Davi]

I'll give my Dad a ring later and grab the readings off of him - can't remember off the top of my head, just that it failed!

Car is a 2002 Laguna. Multimeter is a v expensive Fluke so would hope that could pick up the fluctuations but not certain, good point on the 'scope instead of a meter. I've got one that was given to me ages ago, never had it out the box yet and don't think it's ever been used!

Edited by Davi on Monday 19th May 07:50

[Davi]

OK so we appear to have misplaced the readings - which is helpful

What are the thoughts on the cat temperature variations front to back - I'm beginning to think just bin the cat and replace it, but it's easy to say that when it's not my money paying for it

[rev-erend]

Why not test the temps on the exhaust headers - to check in one is hotter / cooler than the rest..

You might just have a sticking injector and some of that super injector cleaner might do the trick..

[Davi]

rev-erend said:

Why not test the temps on the exhaust headers - to check in one is hotter / cooler than the rest..

You might just have a sticking injector and some of that super injector cleaner might do the trick..

worth a shot!