Idle Fuel Trim Pissing me off...

[eclipsegsx1736]

I've had this really annoying idle fuel trim problem for about a month now. I know that since I have a turboback with no cat, and an intake, that the car may run a little leaner, but this problem I have doesn't make sense. I reset the ECU and SAFC to 0 (stock injectors, stock turbo). Pretty quickly the idle trim starts to go lean, so I richen the SAFC 1% at a time. This continues until I have my SAFC at idle at +10% or so, which is ridiculous, and the LTFT is still around 5 to 10 (lean). All my other trims are fine, they're at 0 or -2 etc, and my SAFC is only at +2% or +3% at those RPMs. Here's what I discovered: when the car is cold and warming up (closed loop) the STFT at idle is very rich (around -15 or richer). Once the car warms up, I'm back to my lean problems (trim is around 5 to 10 with my SAFC richened +10%). The only other abnormal thing is that at idle (once the car is suffeciently warmed up), the blinky A/F gauge doesn't cycle normally, instead it goes back and forth so fast that it just blurs all the LEDs...it never used to do that. If I BARELY give it gas to raise the idle to 800-850 rpm, the blinky gauge is completely normal, it cycles smoothy back and forth like it should. Is it possible that a slightly loose downpipe to O2 housing connection could cause vibration enough to make the O2 sensor act like this? When I let the idle sit at 750 without using gas, the downpipe to O2 area seems to make a vibrating/fluttering sound, maybe the connection is loose. It goes away if I raise the idle to 800-850 by giving it some gas. I can't tighten the bolt on the downpipe because there isn't enough room (the downpipe is so wide I have to use an open end wrench), I don't have the right tools to reach there and actually do anything... What gives?

 

[spyderturbo007]

Pretty quickly the idle trim starts to go lean, so I richen the SAFC 1% at a time. This continues until I have my SAFC at idle at +10% or so, which is ridiculous, and the LTFT is still around 5 to 10 (lean).

All you want to do is get the FT's around 0. You want to add together the STFT and the LTFT and get them close to 0. Also, every car is different, and some of them are really weird. For example, I'm running 650cc injectors and the normal calculation "says" that my lo throttle setting should be around -30%, but I'm right around -10%. WTF

What I'm saying is that you just want to get the lo throttle setting wherever they need to be to set your FT's.

Here's what I discovered: when the car is cold and warming up (closed loop) the STFT at idle is very rich (around -15 or richer). Once the car warms up, I'm back to my lean problems (trim is around 5 to 10 with my SAFC richened +10%).

Just remember that your car won't enter closed loop until the O2 sensor warms up to operating temperature. Until that point, you will be running rich while the ECU waits for the O2 sensor to warm up.

The only other abnormal thing is that at idle (once the car is suffeciently warmed up), the blinky A/F gauge doesn't cycle normally, instead it goes back and forth so fast that it just blurs all the LEDs...it never used to do that.

I am going to guess that your idle Ne Point is set at 1k ? If so, you need to rev the car to 1k and set the FT from that point.

Is it possible that a slightly loose downpipe to O2 housing connection could cause vibration enough to make the O2 sensor act like this?

I don't know, but I can't see how that would cause a problem.

 

[FORMONTOYA]

Is it possible that a slightly loose downpipe to O2 housing connection could cause vibration enough to make the O2 sensor act like this?

i had a leak between the turbo and o2 hsg. setup said it was a non gasket connection, however i had a slight leak that would suck in air before the o2 sensor making it think that it was running lean and then dumping in a ton of fuel to compensate, but never quite getting in line. since you could have a possible leak (o2 to downpipe) after the sensor it may not be affecting it, but it is close to the sensor and you might want to go ahead and tighten the connection.

jim

 

[spyderturbo007]

i had a leak between the turbo and o2 hsg. setup said it was a non gasket connection, however i had a slight leak that would suck in air before the o2 sensor making it think that it was running lean and then dumping in a ton of fuel to compensate, but never quite getting in line. since you could have a possible leak (o2 to downpipe) after the sensor it may not be affecting it, but it is close to the sensor and you might want to go ahead and tighten the connection.

jim

Good thought, I can see how a leak before the O2 sensor would do that. All that rushing exhaust gas would create an area of low pressure outside the exhaust. That would pull air in from outside and spike the O2 sensor with oxygen, thereby causing a drop in voltage.

Good thinking, coupled with some real world experience.

You get a rep point!