What does it mean to run lean?

could someone explain what it means to run lean, and how would it show up on an emissions test printout? What values would be high or low?

Reply to
Tube Audio
Loading thread data ...

I'm not sure that running a bit too lean *would* show up in a clean, direct way, on the typical not especially strenuous emissions tests. Running lean means that not enough gasoline is being put into the mixture, in proportion to the air. It isn't good for your valves, and on modern cars with closed-loop engine controls it means that something has gone wonky with a sensor or two.

Running lean enough would show up as excess HC or (unburned) hydrocarbons at the tailpipe. Yes, fuel going unburned because of too much air -- an engine is meant to work within a certain range, at that temperature and pressure. You might see it as high NOx (nitrogen oxides) because of higher combustion temperatures as well. (Running

*very* lean, possibly enough to cause driveability problems, can decrease NOx, as I (mis)understand it, but sends HC through the roof because of misfires.) Having the last smog slip around as a baseline for comparison is helpful.

O2 and/or mass-airflow sensors are among the things to watch for in your onboard diagnostics readout, if your car has them (what make, model, year, and engine are we talking about?) and I guess so could timing that is off beyond the computer's ability to twiddle it.

Reply to
Ad absurdum per aspera

Are you talking about a tailpipe test or a code?

Reply to
Steve Austin

yes, tailpipe

Reply to
Tube Audio

"Running lean" means that the air-fuel ratio is skewed so there is too much air per unit of fuel being run through the engine. It shows up on a tailpipe test in several ways, primarily as too much oxygen in the exhaust stream, and secondarily as too many oxides of nitrogen. The NOx comes about because the combustion process gets much hotter under lean conditions, and that causes some of the nitrogen in the intake air to oxidize.

Tube Audio wrote:

Reply to
Steve

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.