I have been running the 730 ecm for the past few months and have been experiencing the problem of leaning out. What happens is if I stay at any constant throttle position the mixture will some how lean out. It will happen in cruise on the highway and in town. If I am just taking off from a stop light and reach a cruising speed you can slowly watch the 02 sensor readings go down to .1 volts.

I have totally disabled lean cruise highway modes. No fuel is supposed to be subtracted or spark added. Everything will read normal. I do have long tube headers with a non-heated 02 sensor. I do not think that is the problem because it will dip down to .1-.2, then I can give it a small amount of throttle (5%) and see the readings go any where from .4-.8. I have put in my other 730 ecm's to see the same problem. I also had this engine running beautifully on MegaSquirt as well in closed loop.

It also will lean out after PE has been activated. I will give it a large amount of throttle and then let off. The readings will almost immediatly go down to .1-.2. Unless I give it more gas. I am totally stumped on this. I just decided to run it in open loop on the rich side. I am still getting 23 mpg mixed driving though.

 

Try less timing.
Running too much timing, lets the heat build, thus showing an indicated leanness. Note the word indicated. The better the vaporization, or atomization, the leaner the indicated mixture will be. An O2 sensor is just measuring one element of the combustion process. It can be misleading to draw too many conclusions from partical data.

 

I will try running less timing, The most it sees is 37*-40.* The O2 sensor doing that makes it very hard to run CL.

 

36-37 is max at WOT. At cruise, should be closer to 28-30. May vary by application, just rule of thumb there.

 

I will lower it lowre to those numbers suggested. I am just used to hearing how I should be using quite a bit of advance for cruise. Even the stock setting has about 37*.

 

No, I guess that's right. Sorry, my bad. Cruise #s around 40 are fine...

 

I am going to try it anyway. I left the WOT alone, I will see how much mileage is affected. Less spark does take less fuel, but to little spark will lead to an increase in throttle position while driving.