Nissan Almera 2000 QG18 Engine Rich at idle

[leonard]

Hi all
this Nissan runs rich at idle 775rpm. intermittently, some times. If I do a wot,it revs ok, but the engine shudders when it get to idle.
then when I look at the ltft its a 20% and stft is +-3% (it was ltft 8% stft around -+3%)
The o2 sensor goes up to 800mV and stays there until I rev it up to 2000rpm, then the o2 starts switching normally,but at idle it is reading rich, taking hose off to let more air in brings it back to normal

fuel pressure 50psi

Its being 20 years since I did any fault finding on cars, the last 20 I have spent in the electronic repair business, so I am a bit rusty
Thanks for any help

Topic

[Gumpy Guss]

Air filter clogged? Drippy and not squirty injector?

[leonard]

Air filter is clean, but the injector is a possibility, but as soon as I set up to test, its working
Thanks for your reply

[college man]

Is the engine at operating temp?

[Kevin Criswell]

leonard, are you sure it is a positive 20%? That means the computer is widening the pulse width of the injectors by 20% to compensate for a lean condition. The LT FT should be -20% if the O2 is showing 800 mv (rich).

Leaking injectors and air blockage usually show up as negative percent fuel trims and intake leaks usually show up as positive percent fuel trims. One thing to think about is the ECM uses the MAF when mapping out the fuel stragedy, you might want to try cleaning the MAF sensor (they make a special cleaner available at almost all parts stores) and seeing if it is dirty, throwing off the fuel mapping.

[leonard]

I also did a fuel pressure test,about 50 psi, and when I switched off the engine, the pressure stayed at 50psi, it did not drop, If an ejector was dripping, would the pressure drop?
but as my fault is intermittent, I would have to do this test with the fault present

[leonard]

you are correct it should have read -20%
my mistake

[leonard]

yes it at normal temp, after running for about 20 minutes
thanks for your reply

[Kevin Criswell]

[quote=”leonard” post=76045]you are correct it should have read -20%
my mistake[/quote]

Good deal, you had me lost as last year’s easter egg 😆

Okay is there any feeling of a misfire when it is doing this?

One thing you could try, is unplugging each injector one at a time when it is showing rich and see which one makes the biggest change to your fuel trims. You might have an injector that is not closing all the way at lower pulse widths but working fine at higher ones. Also dont forget to check your MAF sensor a dirty one can drastically mess with the fuel strategy at idle, same for a MAP sensor if your Nissan has one.

[leonard]

HERE IS TWO CIRCUIT DIAGRAMS THAT MIGHT BE USEFUL
these two cicruit are not for electronic throttle control types

Attachments:

• QGENGINECONTROL.pdf
• YDENGINECONTROL.pdf

[Author]

Replies