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metromizer 06-18-2008 05:59 PM

Fooling the ECU to burn lean...
Is there a simple way to fool my ECU into reducing the amount of fuel it tells the injectors to pass? Sort of a flip of the switch 'lean burn' mode?

Here's how I would use this lean burn mode, the goal is use less fuel:

I'm in top gear, the road is flat, the engine is humming along at 2800rpm in closed loop mode and under a constant load while traveling at 65mph. I'd flip a switch, that puts the engine in this 'lean burn mode', maybe an LED on the dash lights up (to remind me to switch it off while climbing a grade).

I would likely need to experiment to find what my engine likes, how much fuel I could take away under 'lean burn' trim. Maybe I would read voltage from one or both existing o2 sensors, maybe I would add a stand alone cylinder head temp probe under #2 spark plug and gage on the dash (to keep from melting the thing down).

My '96 (3) Cylinder TB injected engine is basically stock with (2) 4-wire O2 sensors (maybe the ECU averages between the two?). The car retains it's stock cat, it needs to pass California emmisions every two years. I am no electronics guy, I only know a little about the way EFI works, and have only used O2 sensors to keep an eye on carb jetting on a race car... a single wire narrow band that gave only marginally useful feedback (just another tuning tool like reading spark plugs). I can wire, solder, etc, but switching over to a Megasqirt system is out of the question.

Just brain storming here... please chime in with ideas and concerns, I'm thick skinned (hopefully not thick headed):cool:

diesel_john 06-18-2008 06:18 PM

metromizer, there is this thread. Also i have not searched the site for keyword, leanburn yet.

I have in the past made a little 2 channel box that used two 3v batteries and a simple variable voltage divider(megaohms, batteries last longer) to add 0.1 or 0.2 volts to the signal from the o2 sensors to the ECU. i rigged it up so it was switchable to be in series with the 02 signal or out (02 direct to the ECU). then i monitored the actual voltage from the 02 to see what really was happening. The best improvement i ever got was about 10%. (2 mpg) The ECU likes to see the 02 voltage cross over the .4v to .5v range every so often or it will turn on check engine light, so there is a limit as to how far you can go. before a check light. Some engines get a little better mileage around 16.5 to 1.(ref.johnnygrey)

another test i did is ref. in the above thread. using a zero to 10 ohm rheostat to drop the voltage to the injectors until the ECU can no longer lengthen pulse at which time the engine leans out. with this method i run open loop and unplug the o2 sensor and discount the engine check light. i have continuous read out on the o2 to avoid going below .25volts. obviously this brute force manual mode, your mileage may vary. (poor man's rheostat Craft shops sell heavy Ni-Chrome wire)

AXMonster 06-18-2008 06:24 PM

You'd probably need to adjust the O2 sensor's output so that the ECU thinks its running stoich when its actually running lean.

Here's some interesting info:

There is a circuit that can be utilised to adjust the O2 sensor output, but I can't find the link.

Edit - Found it http://www.alternative-energy-resour...tallation.html

And available from the designer here

SVOboy 06-18-2008 06:55 PM

Just because you can get it to burn lean doesn't mean you'll save any gas though, and that's the trick. Cars like the VX, HX, and Insight used vtec-e and a host of other things to make lean burn efficient...normal cars might not like it, :p

AXMonster 06-18-2008 07:18 PM

How well does/did it work John?

diesel_john 06-18-2008 07:22 PM

Voltage divider worked good on a 5.0 HO mustang.(batteries lasted for weeks) 10% improvement
The rheostat is still on my 5.0 F250 don't have any improvement data because my pulse width mileage computer gets very confused when the pulse width goes to 80% and the injector still wouldn't open. But i have range enough range to stall the engine so i am sure lean burn is in between there somewhere. It is difficult to hurt these old cast iron engines at low throttle settings. never had any problems with detonation.:D

AXMonster 06-18-2008 07:26 PM

That answers my question ;)

cfg83 06-18-2008 09:12 PM

metromizer -

Welcome to EM! Here's mine (I flip a switch just like you said) :

My Eagle Research EFIE

I don't use it much except for testing. I don't want to make it part of my overall system until I can verify emissions compliance.


diesel_john 06-18-2008 11:00 PM

Because all part throttle, never had any problems with detonation.:D

MechEngVT 06-19-2008 08:50 AM


Originally Posted by metromizer (Post 36048)
My '96 (3) Cylinder TB injected engine is basically stock with (2) 4-wire O2 sensors (maybe the ECU averages between the two?).

The dual-O2 sensors is part of the OBD-II scheme to diagnose the proper functioning of the catalytic converter. One of the O2 sensors is upstream to detect pre-cat exhaust oxygen content and should switch back/forth past stoichiometric to let the cat function properly. The downstream O2 sensor is to detect that the cat is functioning properly and should not switch, but read constant...cant' remember which voltage. I have heard that some folks "fool" this sensor by putting it in an empty coke bottle with just air inside it to stop a MIL lamp with a bad cat.

I don't know much about Metros, but if you don't have a knock sensor that will automatically pull back timing if your engine pings you might get some gains by doing what you propose. If any timing gets pulled out to fight ping you'll probably not see much benefit. You also don't want to run lean-burn for too long because you may cause damage to your catalytic converter. The cats require the switching between lean/rich to properly function as both an oxidation and reduction catalyst. The cat needs a lean mixture to oxidize HC and CO, but then needs a rich mixture to reduce NOx. Without running rich, your cat can't reduce NOx so not only do you emit more NOx, but the cat heats up and can become brittle and crumble from only oxidizing HC/CO which produces heat.

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