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[Andy A]

My long term fuel trim stays rock solid at 7.03% and doesn't change even with revs. Is this normal?

I thought it adjusted to match what the STFT was doing.

Thanks

[TheBoy]

STFT should react quickly. LTFT reacts much, much slower.

However, if LTFT is that far out, something is badly wrong.  LTFT for an Omega on petrol should never deviate from 0%

[Andy A]

STFT should react quickly. LTFT reacts much, much slower.

However, if LTFT is that far out, something is badly wrong.  LTFT for an Omega on petrol should never deviate from 0%

That's what I thought. I have run it for the last few days with the scan tool connected on short journeys but wormed up to temp and the LTFT never changes from 7.03. Any suggestions what graph to look at next to diagnose the problem?

Thanks

[omega2018]

here is a good general article on fuel trims http://www.autoserviceprofessional.com/article/94982/Fuel-trim-How-it-works-and-how-to-make-it-work-for-you?

[Andy A]

here is a good general article on fuel trims http://www.autoserviceprofessional.com/article/94982/Fuel-trim-How-it-works-and-how-to-make-it-work-for-you?

Thanks for the link. 

[omega2018]

you're welcome, make sure you read page 2

[Andy A]

Will do. 

[TheBoy]

STFT should react quickly. LTFT reacts much, much slower.

However, if LTFT is that far out, something is badly wrong.  LTFT for an Omega on petrol should never deviate from 0%

That's what I thought. I have run it for the last few days with the scan tool connected on short journeys but wormed up to temp and the LTFT never changes from 7.03. Any suggestions what graph to look at next to diagnose the problem?

Thanks

Is this for the 4 pot in your profile, or a V6. V6 makes it easier to diagnose due to 2 sets of data.

That link was misleading on page 2. On an Omega not on LPG, LTFT should *never* drift from 0% (or 127/128 on non EOBD compliant cars).

The usual reasons you have a LTFT > 0% (ie, running lean, the most common fault) is an airleak in intake systems or exhaust (particularly if pre cat), a suspect MAF, or knackered lamda.

With a V6, due to 2 banks, you can rule out MAF if its only happening on 1 bank.


Get the MAF reading. At idle, a 4 pot should be approx. 12kg/hr, a small V6 about 13kg/hr, and a proper v6 around 14kg/hr.  Additionally, careful check for any unmetered air being able to get into the intake or top end of exhaust.

[Andy A]

STFT should react quickly. LTFT reacts much, much slower.

However, if LTFT is that far out, something is badly wrong.  LTFT for an Omega on petrol should never deviate from 0%

That's what I thought. I have run it for the last few days with the scan tool connected on short journeys but wormed up to temp and the LTFT never changes from 7.03. Any suggestions what graph to look at next to diagnose the problem?

Thanks

Is this for the 4 pot in your profile, or a V6. V6 makes it easier to diagnose due to 2 sets of data.

That link was misleading on page 2. On an Omega not on LPG, LTFT should *never* drift from 0% (or 127/128 on non EOBD compliant cars).

The usual reasons you have a LTFT > 0% (ie, running lean, the most common fault) is an airleak in intake systems or exhaust (particularly if pre cat), a suspect MAF, or knackered lamda.

With a V6, due to 2 banks, you can rule out MAF if its only happening on 1 bank.


Get the MAF reading. At idle, a 4 pot should be approx. 12kg/hr, a small V6 about 13kg/hr, and a proper v6 around 14kg/hr.  Additionally, careful check for any unmetered air being able to get into the intake or top end of exhaust.

It is a 4 pot. I will check the MAF reading later and have a good look around the engine. I'll spray some propane gas around the intake joints to see if it gets a response. I'll let you know later what I find.

[Andy A]

TB, I've just checked the MAF reading and its giving me on average 3.3g/s at idle. Just around the normal 12kg/hr according to the online converter that I used.

Sprayed some propane gas around all the intake joints and the engine rpm stayed the same. So seems OK.

Looked at the data for both pre and post cat lamda sensors and they are both fluctuating between 0.20V and 0.87V.

Thanks

[TheBoy]

The post one shouldn't be

[Andy A]

The post one shouldn't be

TB, what should the voltage range be for on the post lamda sensor to be working correctly?

Thanks

[TheBoy]

The post one shouldn't be

TB, what should the voltage range be for on the post lamda sensor to be working correctly?

Thanks

So it stays Lean when the system is closed loop.

[Andy A]

The post one shouldn't be

TB, what should the voltage range be for on the post lamda sensor to be working correctly?

Thanks

So it stays Lean when the system is closed loop.

Did a bit of research and so far as I can tell the Lamda sensor post cat should always be a steady voltage usually between 500 to 700 mV. Is this correct for the Omega 4 pot?

Thanks

[Kevin Wood]

Did a bit of research and so far as I can tell the Lamda sensor post cat should always be a steady voltage usually between 500 to 700 mV. Is this correct for the Omega 4 pot?

Thanks

IME there are times when it is and times when it isn't. Clearly, until the cat is nice and hot, it will just mirror the front lambda sensor. After that, there will be times when the ECU is checking the cat efficiency where the rear sensors will cycle too, but with a delay from the front sensors. Other times, they will be pegged at lean. The only thing that can really tell if the signals are correct is the ECU, and if it isn't happy it'll raise a trouble code.