Long Term Fuel Trim at 9%

[Gti-Jay]

Doing some regular maintenance on my mothers 10yr old, 50k Chrysler minivan (i'm in the US) It hardly gets used. Just for when she visits me from Blighty.... So anyway, did all the usual gubbins. Oil, brakes, coolant, power steering flush, plugs and leads and finally a transmission flush.. However i caused a misfire by accidently bending a spark plug when thumbing around on getting one of the rears in (V6 3.8L) as they're kinda hard to get too ! So instead of messing around with all of them again, especially as one had to be re-tapped after fusing itself to the cylinder head, i picked up an $50 ODB2 reader. Found the cylinder that was misfiring and pulled the plug that was causing the issue, re-gapped, and all seemed good.. But then i took a look at my fuel trims and i noticed the LTFT was at 9%. BTW I'm no mechanic, just a YouTube have a go hero ! Maybe it was like that before i messed around with plugs and wires?? I can't see any obvious vacuum leaks. Seems to run ok, Those crappy minivans are always a little rough, in comparison to say a modern German engine, that i'm more accustomed too.... Is 9% long term fuel trim anything that i should try and get to the bottom of ? or just let it be.. I'm hesitant to mess around with those plugs again, searching to see if there's another gapping issue, cos well one plug hole i had to tap to get the new plug in, and i could be SOL, and the three rears are hard to get to, I'd never be able to chase or helicoil the threads on one of those if things went south. Could a smaller gap, cause a long term fuel trim to be off? Or does it have to be a small vacuum leak somewhere? Or is 9% a perfectly fine number to live with? Seems its within spec. There's no DTC showing up. I'd think 9% is high enough to raise an eyebrow over.. But again, i'm no mechanic.

Edited by Gti-Jay on Thursday 31st January 21:18

[GreenV8S]

Could be an exhaust leak - or maybe it's just an aftereffect of the misfire which will clear up when the long term adaptive has had a chance to re-adapt. If the problem has been cured, you should see the short term adaptives working against the long term ones.

[gamefreaks]

It’s a V6, so you should have separate LTFT readings for both banks.

Are both banks around the same trim or is just one of the banks lean?

Does the lean condition reduce at fast idle? Say 3k revs?

Does the car have a misfire?

9% is a bit high and worth trying to resolve, but it’s within the range that the ECU can compensate for. At around 20%, the car will start throwing engine lights and limp-home modes.

Edit: Just read it’s got a misfire. Fix this first. The O2 sensors will see the unburnt Oxygen and the ECU will treat this as a lean condition.

Edited by gamefreaks on Friday 1st February 08:36

[edd344]

A good article I found on this with a few starting points for you

https://www.autoserviceprofessional.com/article/94...

[Gti-Jay]

gamefreaks said:

It’s a V6, so you should have separate LTFT readings for both banks.

Are both banks around the same trim or is just one of the banks lean?

Does the lean condition reduce at fast idle? Say 3k revs?

Does the car have a misfire?

9% is a bit high and worth trying to resolve, but it’s within the range that the ECU can compensate for. At around 20%, the car will start throwing engine lights and limp-home modes.

Edit: Just read it’s got a misfire. Fix this first. The O2 sensors will see the unburnt Oxygen and the ECU will treat this as a lean condition.

Edited by gamefreaks on Friday 1st February 08:36

No i found the misfire and fixed it. It was due to a plug incorrectly gapped. Its not throwing any codes since i cleared it and fixed the gap. I've done perhaps a 100miles since. The strange thing is my ODB2 reader, which is an inexpesive Foxwell NT301 only shows bank1 and not bank 2 ?? unless maybe i need to highlight bank 1 and press enter to go to bank 2... I'll check that out when i get home.

[Gti-Jay]

edd344 said:

A good article I found on this with a few starting points for you

https://www.autoserviceprofessional.com/article/94...

Thanks Edd, this helps a lot.

[Gti-Jay]

so plugged in this el cheapo odb2 reader in again, now its been a couple of hundred miles to see if its re-adapted.. Still at 9% and went for a drive this time while keeping an eye on the stats, it sort out jumped around from as low as 7% to 12% so i'm guessing its probably fine...

Short term was around -3% to + 3%

I couldn't' for the life of me find how to read bank 2, so stuck with looking at bank1... Could be that i only have a $50 reader ? who knows.