Negative Fuel Trims at Idle

[El Andrew]

Hey guys. Not a huge problem, but this one has me scratching my head a bit. I've got negative fuel trims at idle in the 5-10% range (moves around a bit, but that generally where the LTFT settles).

 

Car is a FG XR6T which until the weekend was tuned by a workshop under the previous owner, standard injectors, X-force cat and dump, K&N air filter and no other drivetrain mods.

 

Car had the above fuel trims and occasionally goes a bit rough at idle when putting it into gear.

 

Over the weekend I put in reconditioned F6 injectors (no known issues with existing injectors, but was concerned they were a bit maxxed out in the tune and wanted a bit more headroom) and given the above issue, thought a new fuel pressure regulator wouldn't hurt either.

 

Issue is still there and maybe slightly worse. I really thought a faulty FPR or a leaking injector may have been the cause, but it seems not.

 

Other info/observations:

O2 sensor is new

I also out new plugs in on the weekend, but the old ones looked fine and identical to each other

No misfires, fault codes, etc

On my BA (which had the same wideband in it) it would quite rapidly oscillate between extremes of lambda 0.98 and 1.02 at idle. On this car, the extremes are more like 0.96 and 1.04 and seems to take longer to bring them back into line.

On mild acceleration, the trims trend back towards zero (obviously heavy acceleration is irrelevant as the car goes into open loop) but it has to pull fuel again once off the throttle

Disconnected the pipe to the airbox on the off chance the filter was clogged and restricting airflow, but didn't change the trims

Engine temp data looks all good, so don't think it is a temp sensor or thermostat issue triggering a cold strategy.

Fuel economy is good.

When I bought the car in July and then again before the warranty expired in September it had a full health check including compression

No fluid contamination or loss

Car seems to go very rich on heavy throttle, like more than what is being commanded in the tune

Plenty of power, throttle is responsive - if it wasn't for me having the WB in the car and also looking at fuel trims is never suspect there was an issue (other than the occasional rough idle).

 

So overall it seems to have some fuel control issue but also I seem to have eliminated the main culprits?

 

My thoughts on next checks are:

Maybe there's an aftermarket pump in it that is too much for the reg? Unlikely given stock injectors, but you never know. I've got a near-new Walbro 255 I kept from the BA to out in, so might as well do that and eliminate something else.

Clogged return line resulting in excess fuel pressure? If I do the pump I can point the return line at a container and see if it is returning plenty of fuel at idle.

Pressure test the fuel system

Pull each injector plug and check the impact on trims? If one leans the trims more than the other maybe that one isn't firing reliably? Given the issue exists on two sets of injectors it might be a wiring issue rather than injector in that case.

 

Any suggestions on other stuff to check?

 

Sorry for brain dump!

Edited 19/03/19 03:57 AM by k31th
OP requested to change title

[Puffwagon]

Lol it's probably the wrong forum to ask that question!!!

 

Without looking at the tune or logging data, a fix for what you're saying would be to minus 2 from the high slope and plus 3 on the low slope. There is a bunch of stuff that needs checking and confirming before more specific advice can be given.

 

You've got pcmtec yeah? Flash it back to stock, swap the injector data over for your new injectors and start from scratch. If you need help with it, pm me your email and we can cover all of it. I'll be happy to answer questions for you, no matter how basic they may seem.

[El Andrew]

Thanks mate. Yeah I guess I just assumed I was chasing a hardware problem but maybe not. That's a good idea to put in a stock tune with F6 injector parameters and see how it idles from there.

I'll also do a comparison with the stock strategy to see if I can see anything obviously funky that's been changed. I'll get in touch after that!

[Puffwagon]

You gotta keep an open mind when chasing issues and unless you're feeling deadly, you'll almost always want to return to stock when fixing someone else's tune.

 

It could be hardware related and it might be fixable with tuning. You'll need to check stuff and go from there. 

[4321]

If it has a different fuel pump it might have to much fuel pressure (there are 2 restrictions in the return setup) get the fuel pressure checked before you try to fix a hardware problem with a software fix

[El Andrew]

Thanks mate. I've set up a tune file that is stock except for the injector data which I'll chuck in before I go to work. Will treat it nice cos of the dump and cat.

I guess if the fuel trims are spot on then it was a tune issue, and if there's no change I have a hardware issue.

[Puffwagon]

The fuel trims will be off because of the dump and cat. Not 10% off but it won't hold stoich like a stock system will.

[El Andrew]

Cool thanks, I'll keep that in mind too. I'll let you know what it does.

[El Andrew]

Flashed in stock tune in the school car park this morning and then drove to work. No change to trims so I think the tune not the cause at this point.

(Puff, if you are happy for me to I might flick you the tune and see what you think. There's a couple of things in there where I think I'd prefer to do it differently but would like a second opinion).

[Puffwagon]

No worries, just pm me your email (I'll reply straight away) and I'll have a look. Send a log too so I can see what the afr is doing when you put it into gear.

 

There are two ways I'd approach it, if I wanted to fix it; Check the fuel pressure and the rest of the fuel system etc and make sure that's all fine, or just change the slopes a very small amount. If it isn't using excess fuel in everyday driving, there really isn't a problem. Sure you can change the tune to get the trims closer to 0 but it really doesn't matter.

 

Trims will trim and providing it's not getting under say 11.5:1 at WOT I would just leave it.

 

Lol it probably wouldn't hurt to give everything a once over anyway, if someone has been messing with it before you got it.