Looking for some help with my engine.

[radarcomp]

Hi guys,

I am having a problem with my engine and can't figure out what the problem is. After driving for a short period the short term fuel trim goes to +25% and then a short while later -25% before settling down to around zero plus and minus around 1.6%. The Long Term fuel trim permanently sits at -0.8% and my idle is normally steady at 640rpm.

When I come to a stop and the fuel trim goes through the roof the idle bounces up and down and sits at around 900 rpm. Anybody have any ideas about what could be causing this?

 

[davyboy]

Fuel trim (?)

 

[radarcomp]

Fuel trim, read by diagnostic scanner. The fuel trim determines how much fuel is delivered when driving and at idle. As I understand it the oxygen sensors fluctuate between rich and lean and this determines the fuel trim. I'm struggling to understand why I have this problem so if anybody has any ideas about what could be causing it I'd be grateful to hear them.

I serviced the car when it started happening and it's no better. If I blip the throttle a few times when it happens and take the revs up to about 2000rpm the car settles down to a normal idle and the short term fuel trim sits at around 0% give or take 2%.

 

[fuzzy-hair-man]

Just a though, I'm just learning some of this stuff...
O2 sensor needs to be hot to create a signal, perhaps it's reading lean when cold so adds the +25% then as it warms up and gets a proper signal it takes it away, similar when you rev it to 2000rpm?

Is the O2 sensor fluctuating on idle / when it happens? perhaps check the power to the O2 sensor? is it a factory manifold with the sensor up near the engine or is it further down?

 

[frank]

the fueltrim is the corrections that the ecu is having to make, should be zero% ideally

 

[davyboy]

Franks your man fuzzy.
Thank you Frank.

 

[radarcomp]

Thanks for the input guys. Fuzzy it happens after I've driven for a while so the car is warm. The car is absolutely fine when driving. The short term fuel trim is as would be expected constantly adding fuel and holding it back. So a lean and then rich combo.

Only when I come to a complete stop the idle goes high, the needle bounces all over the place and the short term fuel trim increases to +25% and then decreases after a short while to -25% before returning to around zero. One thing I have noted is that when this occurs the voltage of the 2nd lambda stays completely still and doesn't fluctuate the voltage until the fuel trim has returned to normal or if I race the engine a few times above 2000rpm.

I'm suspecting that there is extra air in the exhaust around the 2nd lambda but can't be sure. Do you think I'm right frank or am I barking up the wrong tree?