Forum » General Tuning Discussion » Rich Idle. Is it Dangerous?

Rich Idle. Is it Dangerous?

General Tuning Discussion

Discuss all things tuning in this section. News, products, problems and results. 

Page 1

Lets talk about rich idles.

The only real concern Ive heard about rich idles other than fowling plugs and excessive carbon build up is the possibility of washing the rings clean of oil. Is this even possible at idle with the small amounts of fuel entering the engine?

I have tuned my car to be extra lean 1.05-1.10 lambda to be extra safe, but ive noticed that my car likes to idle alot more at 0.80-0.90. Sometimes will even jump to 0.77ish before the ECU's short term fuel correction catches it.

How dangerous/likely am i to wash my rings down running so rich at idle?

What engine is it? The risk is higher with direct injection and a small bore.

It’s a BMW M52, 2.8l, low compression, 84mm square bore/ stroke, Stock internals, 740cc injectors, 91 octane. Etc

Can idle speed also change this affect at all?

I assume a higher idle speed will be less likely to wash the rings away...

Why don't you run it at lambda 1.0 and then tune injection timing and spark timing for best idle quality?

Remind me: An M52 is electronic throttle or no? What ECU is this, stock? The difficult thing is that you don't even have an emission analyzer to see how well the fuel is mixing. On port injection you can look at HC, CO, and O2 concentration to help understand how it is mixing.

With all due respect, Raymond, I can't think of many petrol/gasoline engines that like to idle at stoich. I'd say shoot for .92(ish) lambda depending on fuel and you may chase your tail less.

The newer the car is, the more important for it to run at stoich for emissions, unless you include unusual ones that run secondary air injection like an rx-7 or a 70s Porsche even with a warm engine.

Of course they also do oscillate a bit for emissions reasons (catalyst efficiency) but it's usually possible to run there if you have stock heads and cams. Now it's easier and less work to run rich of a stoich. OP has a special concern about oil dilution and presumably carbon buildup which is probably unwarranted, and I agree that running at .92 lambda isn't going to hurt anything.

All that being said, typically the trick to idling at stoich is less spark and more air, while also adjusting end of injection timing if that's what the ECU controls. If the car has electronic throttle it's a lot easier because you aren't turning screws on the throttle body or jacking up the idle air valve duty cycle. If you have VVT, make sure you are at 0.

So in that case you might warm idle at .92 lambda at 800rpm no accessory load. Adjust spark to MBT as best you can, with idle air getting you to 800. Now retard the spark in 2 degree steps, up to say 10 degrees. Watch idle drop. Then add air back in through the electronic throttle, or the throttle body air bypass screw with a fixed idle valve duty.

Now idle speed should be 800 again due to more air. Adjust injection timing for leanest AFR (if running open loop) or lowest pulsewidth (if closed loop). Now lean it out in steps to as close to lambda 1 as it will go. Combustion will be more stable with retarded spark and more air than with MBT spark. This is called idle torque reserve.

OR just do whatever the stock ECU did, if you can find out the idle timing (might be oscillating), idle air duty or throttle position, and injection timing. Sometimes the timing is in the service manual or on a sticker under the hood.

Cold start is more complicated.