Air Fuel Ratio Issue
 

2000 4x4 Excursion V10 2V

So I have tuned a couple vehicles and one of the things that is standard on all the pcms I have done is when you let off the gas pedal while going down hill the AF ratio leans out to 22:1 or so.

Basically the pcm cuts fuel while coasting with negative load.

I am seeing this ocasionally on my V10, basically after a hard pull and I let off go pedal the AF ratio will lean out to about 22:1.

Problem is if I'm just crusing down the hwy at 65 or 75 mph and let off the pedal the AF ratio stays at 14.7:1 or so.

This affects fuel mileage and I don't think it's right. What can be done to fix this?

I believe the stock V10 is programmed to maintain stoichiometric (14.7 / 1) air fuel ratio at all times when in closed loop. That is why the tuners can give a little more power at WOT by going down to around 12.5 / 1 and letting it run open loop.

A good question for 5 Star would be if it can go open loop on coast also.

I know all that but when coasting with negative load it should shut off fuel like most all efi vehicles.

AF ratio should lean out to 22:1, mine only does this occasionally.