Gridlock Mikey

OK then, we know the theory behind them, and we know of the supposed consequences of running too rich or lean.

Now i've been in a few peoples cars with AFR meters in them and it seems that so long as the lights are in the orange/red for the majority of the time, all is well

At stand still the things look like a disco light and tell no-one nothing.

My question is this, how the hell are you meant to use this gauge properly and apart from the thing staying green all the way through the rev range, are there any other combinations of lights to look out for?
i.e someone mentioned that thier car was a little lean just as the turbo kicks in and then runs rich up to the red line. Good or not? who knows?

I'd like to know because when I get asked, i'd like to know the answer. You all know my view on gauges etc but AFR meters are a total mystery to me and to many of my friends it seems.


Mikey

NENO

LOL at Mikey I think that's my post your refering to.

The lights represent a value of oxygen in the exhaust. A turbo car needs plenty of fuel ie a rich mixture to help guard against det. and the AFR tells you just how rich it is.

On Wide Open Throttle it should be rich (blue light with the Dawes AFR) all the way to the red line meaning there is ample fuel, you could argue this is excessive fueling but at least you can be sure it's not too little (green would be OK but red is danger).

AFR's are only designed to work on WOT.

I can't remember the values for the Dawes AFR but I'm sure someone will let us know.

nom

Well... AFRs aren't only for use on WOT - I use mine considerably on cruise to make sure it's running at 15.4:1.
So, they're only any use when running open loop. It just so happens that the standard ECU runs closed loop below some boost or other (120kpa absolute or something?) so you can get 'disco lights' - if you have such an AFR meter that shows things that way - when it's running closed loop as it's searching for 14.7:1.

There. Bet you're confused now

Gridlock Mikey

That'll be that stoichiometric (sp?) point then, i.e the perfect air to fuel ratio?

So say a gauge is reading lean, what can a person do about it? Apart from worry about it? Another example of a gauge that unless you FULLY understand what it's telling you, you will spend most of the time worrying instead of enjoying the car. I would have said that the same is true of an engine that is running too rich (How can an engine have too much fuel?)

I suppose it's a trade off is it? running too lean is worse than too rich (I presume) so AFR meters tell you more about underfuelling (Green lights) where as there doesn't seem to be anything to say you are running too rich (It's ok to be in the red so if it's red it's ok)
Is that about the long and the short of it?

Mikey

NENO

Well I hadn't heard of anyone using an AFR for that purpose nom, but I'm sure you are right.

Mikey yes lean is more dangerous than rich, because if it's running lean on full boost the temp in the combustion chamber will rise and can get to the point where the piston melts. The excess fuel actually helps to keep it cool, rather like water injection.

Running too rich is bad for the engine long term as the extra fuel breaks down the film of oil causing premature wear of the bore.

As for the Dawes lights, the following is off their website, http://www.dawesdevices.com/airfuelmeter.html

First light: Red= .78 to .90v

Second light: Yellow= .90 to .94v

Third light: Green= .94 to .98v

Fourth light: Blue= .98v and up

Perhaps someone else can take this further and point out anything I've misunderstood or forgotten.

I read a post by John Banks that I can't now find that explained what voltage became dangerous.

Rich Curtis

Interesting thread on a confusing subject (for me anyway) -

I am considering myself whether to get a dawes AFR just in case I have a dodgy batch of fuel / air leak etc. that may lead to a weak mixture and detting.

But I though I read a while back though that the sensor (is it the lambda sensor on the exhaust?) only reads correct when cold, hence would be of limited use.

I have also seen poeple quoting a wideband permanent lambda - is this one that also works when hot?

Sorry if I've made things even more confusing by talking rubbish

Cheers

Rich

Dizzy

imho if your worried about problems with your car get a knocklink. if the knocklink reports problems then the afr will give you a possible reason. You could be running lean quite safely! (although I'd never recomend it).

I got the AFR first because I needed it to set up my Dawes to make sure I wasn't going to lean out when I upped the boost. Again to reiterate... the AFR will tell you a potential cause the knocklink will tell you if you have a problem.

[edited coz I've no idea what an ADR is ]

[Edited by Dizzy - 7/23/2003 1:33:50 PM]

Leslie

If you decide to get some more horses by fitting a Dawes device to raise the boost, it is a good idea to make sure that the mixture is not too weak at the higher airflow through the engine. Normally the ECU will cope with that for a reasonable boost increase. The AFR indicator will show how the mixture is doing and is checked at WOT which is the most critical occasion. The Knock Link will show if you have reached detonation with the increased boost. This will show whether you have overdone the boost increase and you can do something to save your engine. The Dawes AFR indicator is fine and you can also get others such as the Oz one which is a DIY construction with 10 leds and costs only about £12 delivered. When the ECU is in "closed loop" operation on a light throttle opening the lights run up and down (disco effect). This is the ECU trying to run a mixture close to Stoichometric as was mentioned. On opening the throttle the AFR will show one light according to the voltage output from the Lambda sensor. This will indicate the mixture strength the engine is running at. The higher the voltage the richer the mixture.

Les

LeoneTurbo

At WOT, is the engine ECU still using the readings from the oxygen sensor or is not (ie. completely open loop)?

If it is not, the ECU cannot compensate for sensor or injector wear at WOT?

nom

After a certain 'point' (don't know whether it's MAP or MAF based on the JECS), the ECU switches from closed loop (reading the lambda & adjusting fuel mixture to suit) to open loop (using look-up tables). The look-up tables are largely rpm/MAF based. The higher the rpm & flow of air, the more fuel, basically.
There is good reason for this - as Lambda sensors get hot, they read lean. This is obviosuly not a good combination...
We actually end up with something which is quite convenient, if there's an AFR meter to look at. If there's something wrong, the AFR meter begins to read lean at WOT, so we know to check it. If there was something wrong & it used the lambda, it would just adapt & we'd never know. This does, of course, happen the other way round with the lambda sensor, in that when they being to 'go off' (which in this case is read rich), the ECU compensates & fuel economy plummets. Even more than usual .

Gridlock Mikey

EH!!!!

Floyd

iTrader: (9)

The other problem with the AFR, Mikey, is that when the AFR gets hot it reads lean (this isn't the same as what NOM just said!) so it looks lean but there isn't a problem.

Work that one out!

F
PS get an AFR and a Knocklink, yes really!