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        General >> OBD Advice >> Long term fuel trim
        
(Message started by: nigelt on Jun 17^th, 2005, 8:43am)

Title: Long term fuel trim
Post by nigelt on Jun 17^th, 2005, 8:43am

Can anyone tell me if LTFT should be going up and down with the revs or should it be constant? Mine is going from 12% at idle to 0% at 3000 rpm... ???

Thanks

Title: Re: Long term fuel trim
Post by scorpio_man on Jun 17^th, 2005, 10:53am

hi there

we need a bit more info. are you checking the Long Term Fuel Trim and not the Short Term Fuel Trim? that would move about a bit. also are you checking it right away? just after start up? fueling is open loop for about 180 seconds after start up. that's another point, is the car going closed loop after this?

are you using VE software from alex p? far easier to post a scan so we can look at it that way.http://www.fordscorpio.co.uk/obdfaq.htm

http://www.fordscorpio.co.uk/fuelmonitor.htm

Title: Re: Long term fuel trim
Post by nigelt on Jun 17^th, 2005, 3:17pm

Hi Scorpioman,

It is while driving along in closed loop and yes it is the LTFT and not the STFT. At Idle the LTFT is at about 11% while STFT is at about 4%. Then when driving along the LTFT will drop to 0% at about 3000RPM while STFT wil go +- say 5%.

Now I have to admit that its not a scorpio but a Mondeo mkII which has the similar ECU V But this site is the only place I can seem to get knowledgable answers from.. :) Hope this won't exclude me... I am using an elmscan unit and not the VE software mores the pitty...

Nige

Title: Re: Long term fuel trim
Post by scorpio_man on Jun 17^th, 2005, 3:41pm

hi there

you're ok, i scanned many a mondeo! ;D

i can't say i've looked at the ltft on the move before, but the figures quoted seem ok. is there something wrong with the running of the car?

give us all the info you can. if we can't answer here, i'll post it to our mailing list. this might take a wee while, maybe mid next week for a responce.

hth

Title: Re: Long term fuel trim
Post by nigelt on Jun 17^th, 2005, 4:03pm

Thanks for that scorpio_man,

Yes there is a problem with running. I have a misfire during light throttle, or what seems to be a misfire. But I also have and issue when first moving off in the morning, the car seems to stumble and intermittantly lack power and jerking. It also jerks during deceleration. However, It seems worse on colder mornings than on the warmer ones. It all started after having replaced the timming belt. So what have I checked/replaced.

Timming checked with timming light and the locking bar to prove not done it wrong.

Replaced
Plugs
Leads
Coil
Maf
Fuel pressure regulator
Fuel Filter
O2 Sensor
Crank sensor
Engine Temp sensor
DPFE Sensor (but now blocked up EGR valve to rule out)

Checked for
air leaks
PCV

Hmmm.. Car starts ok, idles ok, has power when you need it drives at speed ok but at 40mph and below does not cruise well worst at 30mph. Infact its at its worse when haven been driving for about 10mins at 70-80mph and then have to drop down to 30 for village.

I now think ecu but don't want to go there untill I have proved its nothing stupid... If this was a naturally asperated car then I would say fuel/air or timing.

Hope this is enough info to start with..

Thanks

Nige ;)

Title: Re: Long term fuel trim
Post by scorpio_man on Jun 17^th, 2005, 5:32pm

hi there

just a quick one for now. what about the Throttle Positioning Sensor? you can check this with a multi meter. set it at 5 volts. it should be a smooth curve. can't remember if it's up or down, though.

btw, was there any error codes? even if they don't seem related?

Title: Re: Long term fuel trim
Post by nigelt on Jun 17^th, 2005, 7:25pm

Oh yes I checked that as well and that's ok.

No error codes either...

What I did notice tonight on the way home, as the air temperature is a great deal warmer. The car was a great deal better. It could be the IAT but it always seems to be giving a correct reading to me...

Nige

Title: Re: Long term fuel trim
Post by nigelt on Jun 20^th, 2005, 12:27pm

No I have to admit the air temp may have some affect but still drives like pants... still feels like its being choked..

Title: Re: Long term fuel trim
Post by scorpio_man on Jun 20^th, 2005, 1:34pm

hi there

i'm going to post it to the mailing list. i'll post back if there any responce.

Title: Re: Long term fuel trim
Post by scorpio_man on Jun 20^th, 2005, 3:44pm

hi there

from our list..............

The interesting thing is that the guy says it's bad when first moving
off in the morning, especially when cold. It's in OL at this time, which
would make me think it's not a sensor issue. I'd be tempted to revisit
the possibility of air leaks, inc throttle body joints.

When it goes CL the trims will try to compensate for the leak, but may
not eliminate the symptoms he describes.

The fuel trim readings are just the value at the snapshot time the
samples are taken. The STFTs are changing many times a second, much
faster than the scanner sample rate, so you never see their real time
action when scanning, IYSWIM. Not sure what the adjustment cycle is for
the LTFTs, but it will need to move reasonably quickly if the STFTs are
not getting the O2 sensors to switch. I'm not surprised that he sees
these moving while driving.

What does he mean by "blocked up egr valve"? He should have just
disconnected the vacuum control line from the top of the valve, and
plugged this. If done incorrectly there may be a source of leak here.

As I say, air leak would be my favourite on this one (particularly as
the idle LTFT is positive).

Title: Re: Long term fuel trim
Post by nigelt on Jun 21^st, 2005, 12:42pm

Hi Scorpio_man,

Thanks for that quick respone... I initially thought it might be an air leak from information gathered on other web sites. But I have to admit I thought I would be able to hear such a leak, but will look again. One thing I did notice the other day was a hairline crack in the exhaust manifold and wonder could this effect the O2 sensor. Although air should be going out not in, but between exhuast strokes? any ideas?

I can put a Osciliscope on the O2 sensor as I own one..

Also I Blocked up the EGR valve by removing the gasket and replacing it with one without a hole in it. This was in case it wasn't closing properly. I also plugged the vacume pipe going to it as well.

Title: Re: Long term fuel trim
Post by martin_rowe on Jun 21^st, 2005, 3:05pm

any crack up stream of the o2 sensor will caust problems, or down stream for that matter if its close to the sensor, if its only a slight leak the sensor will compromise up to a point, check the voltage on the black wire, it should oscilate when at running temp. (0.1 - 0.8v)

Title: Re: Long term fuel trim
Post by nigelt on Jun 29^th, 2005, 3:30pm

Hi Guys,

Could this stuttering/power loss be something to do with my timing belt not being tight enough between the two cams or between crank and cams? As this all came about with the changing of my timing belt.

Nige

Title: Re: Long term fuel trim
Post by nigelt on Aug 5^th, 2005, 12:03pm

Just to let you know... I found out why my LTFT was sitting at 21% at idle..... The PCV... there seems to be a valve missing which is supposed to only alow air in one direction and it should be metered. There seems to be no such valve ...

Nige