High Positive Fuel Trim -sometimes-
- Thread starter Jonas
- Start date
My LTFT was at about +10 tonight. I did the thing where you rev the engine for a few seconds and it went down to +7. Seems like a small vacuum leak to me, but I'm still a bit confused about this subject. Would a rebuilt engine and a new cat have anything to do with it? That's the only recent change, but it's a pretty big one.
gathermewool
Site Donor 2023
5.5 isn’t bad. Every car is different.
I always like to see +/- 4%
fuel trim is basically a comparison between what your MAF reading is for intake g/s to what’s in your exhaust, as measured by the front O2 sensor. The ECU/ECM, in your case, is adding, on average, 10% more fuel than your MAF is reading. This relationship assumes both your MAF and O2 sensor are operating properly.
With a positive trim, you’ve got a leak somewhere letting in unmetered air —> MAF + leaked air in = what your O2 sensor sees and corrects for.
Its more important that you find your fuel trims with the engine loaded, ie, while driving.
Last edited:
It definitely gets worse the longer I drive. It starts out perfectly smooth, can't even feel it running for about 30 minutes to an hour. By the end of my shift tonight, it was vibrating at idle in drive and in park. LTFT when I left was +5.5, when I got home it was +9.3. It doesn't seem like it's going to die on me, but the vibration is annoying especially since it starts out so smooth. I'm not entirely sure the high fuel trims are the problem, but the car just seems to get fatigued after a while.
I'd at least wait for a couple tanks of gas before concern.
Variations in ethanol content can change it a few points.
Variations in ethanol content can change it a few points.
It's entirely possible that your MAF is getting lazy when hot, but it's pretty easy to check. If you check your MAF g/s reading, it should be 1 g/s, per 1k RPM, per cylinder. So a 4 cylinder engine at 1k RPM should report AROUND 4 g/s unloaded. 8 g/s at 2k, 12 g/s at 3k, etc. The fact that your trims get higher at the engine warms is a bit strange, usually a warm engine is more efficient and therefore should mask most issues that cause high fuel trims.
High fuel trims indicate that the PCM is having to add fuel from it's base table to maintain a stochiometric air/fuel ratio as measured by the primary oxygen sensor. This is usually caused by a vacuum leak, however it can also be caused by a sticking EGR valve or improper valve timing.
I see P0172 (Bank 1 rich) codes all the time on the Toyota 1ZZ engines. It's almost impossible to catch them in the act, the root cause is shrunken intake gaskets that only leak when bone cold. After 2-3 minutes of running, the cylinder head warms enough to seal against the intake manifold and the fuel trims look perfectly normal.
End of the day, if you aren't throwing a P0172 code, my first stop would be replacing the B1S1 air fuel ratio (oxygen) sensor. This is the most likely spot for a "lazy" sensor to give erroneous readings and therefore skew trims and effect driveability.
High fuel trims indicate that the PCM is having to add fuel from it's base table to maintain a stochiometric air/fuel ratio as measured by the primary oxygen sensor. This is usually caused by a vacuum leak, however it can also be caused by a sticking EGR valve or improper valve timing.
I see P0172 (Bank 1 rich) codes all the time on the Toyota 1ZZ engines. It's almost impossible to catch them in the act, the root cause is shrunken intake gaskets that only leak when bone cold. After 2-3 minutes of running, the cylinder head warms enough to seal against the intake manifold and the fuel trims look perfectly normal.
End of the day, if you aren't throwing a P0172 code, my first stop would be replacing the B1S1 air fuel ratio (oxygen) sensor. This is the most likely spot for a "lazy" sensor to give erroneous readings and therefore skew trims and effect driveability.
Mine is the 2.4 L engine with the intake manifold on the back facing the firewall. The bank 1 oxygen sensor is brand new it was replaced with the new catalytic converter. This problem showed up after the engine was rebuilt. The engine vibrates between 800 and 1200 RPM but is smooth above and below that. I don't know what's wrong but it's very annoying because I sit and idle a lot.
I meant to say sensor 1 not Bank One. There's only one Bank haha
Right, I was just using the 1.8 as a case study. I personally haven't seen any issues with the 2.4 and intake gaskets. The vibration between 800-1200 could be almost anything, but you say the engine was rebuilt? I assume that means they completely removed it?
I only ask because it's common to not get the engine quite square when it's reinstalled, and that can send weird vibrations through the chassis. This was much less of an issue when most cars had the powertrain mounted to a subframe, however when the motor mount bolts directly to the frame fail it can cause very strange harmonics.
If you're positive that the problem started after the engine work, an easy test is to loosen the mount to frame bolts 5-6 turns, start the engine, rev it a couple times, then tighten the bolts while the engine is still running. This can help center the powertrain on it's mounts, and can help a lot with vibrations.
The MAF is under-reporting according to your rubric. It was showing less than 3 gps at over 1k rpm. It also fluctuates randomly. It reports anywhere from 1.8 to 2.3 at idle, but that could be variation in idle speed. Idle is pretty consistent, but it jumps to over 1k when I put it in park, then settles to about 750rpm.
The fuel sock is a fine mesh bag inside the fuel tank that is attached to the fuel pick up tube.
Keep in mind, this is a very "rule of thumb" guideline. There's no harm in pulling the MAF and using an approved cleaner on the elements to see what happens. Make sure and disconnect your battery before cleaning, and reconnect it after. This will force the PCM to relearn the fuel trim tables (usually, some PCM's store trim tables in non-volatile memory and require a scan tool to reset).
I've tried that, but long term fuel trim started at +5.5 and gradually started going up again. The fuel trims do go down when accelerating under load, sometimes as low as +2.3. I'd like to pinpoint the problem before I throw anymore money at it. I'm willing to buy a new MAF only if I know it will fix it.
You said you had a rebuild on the engine and a catalyst replacement?
I'd check and make sure that there aren't any exhaust leaks between your head to exhaust manifold, manifold to collector/catalyst inlet and that all gaskets and/or bevel seals and bolts are installed and snug to spec.
Anything out of place can cause the surrounding air to pulse in through the slightest gap between exhaust cycles. This will enrich oxygen readings artificially and cause a monitor to show fuel enrichment as you OBD computer *ENRICHENS* the fuel load as it seeks stochiometric balance. (Good fuel:air ratio for maximum efficiency)
A loose or missing manifold bolt.
Missing crush gaskets, or the funny ring between a collector outlet to catalyst inlet. Hell, check to make sure Lambda F/A or plain O2 sensors are snug and not cross-threaded.
If there is raw air ingress, your computer is going to match it with what it thinks is the proper load of fuel in the moment and will artificially cause your LTFT and STFT to spike.
It may also be slight help to have an injector function test done after you're absolutely, dead on b*lls certain that filtrationon on your fuel system, pressure regulator and fuel pump are in spec. I am not experienced with your make/model, but I am certain, much more brilliant minds can assist more.
I'd check and make sure that there aren't any exhaust leaks between your head to exhaust manifold, manifold to collector/catalyst inlet and that all gaskets and/or bevel seals and bolts are installed and snug to spec.
Anything out of place can cause the surrounding air to pulse in through the slightest gap between exhaust cycles. This will enrich oxygen readings artificially and cause a monitor to show fuel enrichment as you OBD computer *ENRICHENS* the fuel load as it seeks stochiometric balance. (Good fuel:air ratio for maximum efficiency)
A loose or missing manifold bolt.
Missing crush gaskets, or the funny ring between a collector outlet to catalyst inlet. Hell, check to make sure Lambda F/A or plain O2 sensors are snug and not cross-threaded.
If there is raw air ingress, your computer is going to match it with what it thinks is the proper load of fuel in the moment and will artificially cause your LTFT and STFT to spike.
It may also be slight help to have an injector function test done after you're absolutely, dead on b*lls certain that filtrationon on your fuel system, pressure regulator and fuel pump are in spec. I am not experienced with your make/model, but I am certain, much more brilliant minds can assist more.
Why was it rebuilt? Was it related to oil consumption? Stripped headbolts?
Oil consumption. New pistons put in and new cat. Old cat was shot from oil/misfires.
Similar threads
