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LTFT Bank 2 High

2004 Tahoe 5.3Z Flex 235K miles

Overall vehicle runs good but does have a few misfires at idle in bank 2. The only issue is at idle the LTFT bank 2 is around 11.7. Bank 1 is 4.8.
When going 65 @ 2000 RPM both banks are often exactly the same around 11. STFT works perfect around 0, jumping ± 4. In addition the O2 sensor voltages seem normal jumping around from .2 to .8 volts on both banks…

I have heard that older engines may have a higher LTFT but I am curious is there is anything else I can try to get LTFT down closer to 0?

From my understanding O2 sensors are detecting a slightly lean condition causing the fuel trims to be a little high. My intake gaskets are new and I have performed a smoke test and it shows no leaks. I have a new MAP, CPS, Injectors, Fuel Pump, FPR, Plugs, Cables. All relearns performed, fuel pressure dead on spec.

Ideas I have to optimize my fuel trims are:

Clean or Replace MAF
Replace O2 Sensors (235K miles)
Determine if I have a small exhaust leak on bank 2.

Does anyone have any feedback or input?

Re your ideas: all are good.
I clean the MAF and throttle body every 100k anyway, so worth doing.
At 235K miles the O2 sensors have earned their retirement.
However, I’d maybe check how rapidly the front sensor is switching, to get an idea of how tired it is.
Besides affecting fuel trims an exhaust leak can affect your long term and short term life plans.

So is the CEL being lit? I’d guess not since you didn’t post that nor codes. If you are reading misfires from the scan program and bank 2 shows more than bank 1… with 235K miles that would not be strange.

My 04 Avalance with 140K shows similar.

Cleaning the MAF likely won’t help because it would affect both banks.

O2 sensors might be getting lazy… watch the data on a plot rather than just the number and see if any of the upstream sensors look “lazy” I.e. don’t switch very quickly.

An exhaust leak won’t likely cause a lean condition but an injector or 2 that is clogged or getting lazy might indeed force the LTFT to go positive.

I’d consider running a cylinder balance test - unhook each injector with a vacuum gauge attached and note the RPM drop and vacuum reading - to determine which injector might be in need of repalcement. I’d also run a compression test to determine how healthy each cylinder is, especially if one cylinder is weaker than another on the balance test.

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It means the engine computer is having to inject 11.7% more fuel to meet the demands of the oxygen sensor than it otherwise would, just based on the amount of airflow going into the engine. Since the problem is only on one bank, I doubt this is the MAF. It has to be something wrong with just that bank. Clogged fuel injector, vacuum or intake manifold air leak on that side, valve clearance problem, faulty ignition system on that side, something like that. Are there any misfire codes? If I had this problem and a general visual inspection of the engine compartment and a complete vacuum system test didn’t turn up anything I’d probably try a fuel injector cleaning treatment first. You may have to do a compression test at some point.

CircuitSmith, Mustangman, George…

Thanks for feedback. Oh and Merry Christmas!!

  1. I purchased new O2 sensors for upstream and I am going to replace today. Thus I will skip testing O2 response times. Is it true downstream sensors will not effect fuel trims? I am leaving them as is for now.

  2. I forgot to mention that I did perform compression and leak down testing earlier in year and all cylinders were good. Around 190-200 psi with about a 5-8% leak. Cylinder 1 was lowest around 170 psi with leakage around 8%.

  3. In addition I have used ACDelco intake foam cleaner to clean valves. Also sea-foam intake and fuel additives. Also my injectors are new (1 year) but we’re not OEM.

  4. I also have moved the coil packs around between bank 1&2 and it did not change anything. Never codes out to a specific cylinder.

  5. I agree with MAF likely not issue as it’s common to both banks but I will clean it anyway. May need it.

  6. CEL is off. Codes are normally clear but occasionally it throws a P0300 random misfire code that clears itself. It never shows a 301-308 ever.

  7. I have never performed the cylinder balance test. I did the coil or injector unplug to see if I could hear no change but never recorded vacuum or rpm changes. What is expected ? If all are similar are they likely all good?

  8. I will try some propane or soapy water again around bank 2 intake. If a leak is occurring up top or on hose wouldn’t it effect both banks? Thus if a small leak exists it has to be where it only hits bank 2 right?

  9. After O2 sensor swaps I am going to try an exhaust leak test procedure off YouTube for the fun. Basically blow your shopvac into tail pipe and spray soapy water at head back. If I find anything I will seal it… when sealing it reverse the shop vac to vacuum and seal. Would a small exhaust leak pre O2 sensor not cause a false lean condition resulting in more fuel from injectors? If so could that cause a few misfires?

  10. Overall my gut tells me there is likely a very small vacuum leak that at idle contributes a higher % of the overall air flow (eg 20%), but then as throttle opens up that % drops (eg 3%). This would help explain why at higher speeds both banks even out?

Yes, downstreams are there to check the cats

You note the change. If it is consistent, all is well. The reason to do fuel injectors is to find a bad or clogged injector. You verified the coils are just fine with a coil swap.

Vacuum leaks are pretty much universal bank 1 to 2 since all cylinders see the same vacuum. Stranger things have happened so check it out with a little propane.

If the leak only occurs when hot, you won’t find it. I’d suggest the potato trick. Plug the exhaust with a potato of the right size once the engine is warm. It won’t be a perfect seal but it will build back pressure like crazy. Listen for the hiss, you should be able to feel it with your hand or a paper towel. Not sure if this year has cast iron exhaust manifolds or tubular… the cast iron ones tend to crack.

Good luck and Merry Christmas!

I think it’s iron. After putting the potato in pipe do I let it idle or turn engine off?

You let it idle with the potato in the pipe.
Or, what I do is have a helper wear a heavy glove and hold their hand over the pipe.
Save our spuds. :yum:

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Ok everyone need more advice…

I tried to swap the o2 sensors today and gheeez why does this stuff never go well? Ok I could not get either to budge. I used liquid wrench several times. I used the o2 sensor tool socket with the slit. I even connected a 24 inch breaker bar and only managed to cause the socket to strip over once. So I stopped. I watched some vids and tomorrow will go by a strong /long 22m box wrench. Per vid I heat it up with propane blue flame and using a box wrench work it slowly…


I am also considering just taking my CAT off to make it easier to replace sensors. However it is 1 big piece for both banks. I am considering cutting it dead center of vehicle where it crosses over from bank 1 CAT over to pre muffler. This will be past the bank 1 downstream o2 sensor. I am thinking this will make it easier to remove both sides and put back. Once finished I will wrap it with exhaust repair tape or a pipe clamp… thoughts?

When I needed to budge O2 sensors on my 12-years old (at the moment) Pathfinder, I removed covers/shields first, made sure I can get to sensors with a socket/bar, then started the car and let it warm up, after that it was much-much easier to remove sensors. In cold state they were rock-solid in place, after warming up I removed them within minutes.

Thanks. I have seen many on YouTube use propene torch to heat up. I will try that tomorrow after I buy a box wrench…

I pull stubborn sensors by cutting off the cable and using a 6pt deep well socket. You’re replacing it anyway. The split socket is only for installation of the new one.


Ok sorta a good day. I bought a set of metric xl wrenches at Harbor Freight. 11 piece going up to 26mm. ($20 with coupon). The 22mm 15 inch worked great. Replaced the upstream sensors no problem although bank 1 was tight on and off. I used below sensors as replacements.

After changing I test drove and initially both banks looked good. After driving about 30 minutes I started seeing bank 2 go high again. I parked and at idle (550 rpm) it looked like attached pic… Bank 1 great, but Bank 2 high LTFT. I saw LTFT at idle go as high as 17.

I tried to attach a vid but not allowed. So at idle (parked) I ran idle up to 3000 rpm and below is what happened…

Bank 1 STFT (.9) and LTFT (3.4) increased to around 10.5 and then settled back down to where it started as idle returned to 550 rpm. LTFT followed STFT.

Bank 2 acted backwards. STFT (.5) increased to 14, while LTFT (12.5) decreased to -7.5. Both returned to 0.5 and 12.5 as idle returned to 550 rpm. So as STFT went up the LTFT went down and visa versus?


$26 is way too cheap for one good upstream sensor… you bought two for that price…

at least the problem is persistent and sensors are unlikely to be at fault, but I do not know how long these two will last, given their price

you have non-OEM injectors, which would be suspect, plus check your injector o-rings, as air can leak around these if you reused o-rings when swapping injectors (they are not reusable BTW, replaced on every disassembly)

at this point, I would swap injectors between banks 1 and 2 and clean them and fuel rails up while doing that, replacing o-rings in the same time

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Keep the original oem sensors, you may need to re-install them. The unusual bank 2 fuel trims as a function of rpm could be caused by misfires. to the extent you can swap the ignition system & fuel system components between bank 1 and bank 2, that might be worth a try. I think that occasional p0300 is an important clue.

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I have them but they are old.

Today I am testing for exhaust and intake leak. I swapped intake gaskets 1 year ago and it went perfect with a good smoke test but now my confidence has waned. I am really hoping I find a leak at bank 2 exhaust manifold as that would explain a lot. If neither are leaking I am going back to cylinders. The cylinder with most misfires is #2 so I will swamp coil, wire, plug, etc.

Another suggestion I was given is possible clogged bank 2 cat. It’s original cat and while I see the possibility (gas backing up on 02 sensor) I don’t think this is likely…

Exhaust leak may be a good explanation to the symptoms you experience.

I has similar issues with trims skewed on one bank when I had a leak between the cast iron manifold and the first cat, then yet another leak between the first cat and the pipe to the secondary one, where the flange rusted out and created huge leak.

Sensors were smelling oxygen downstream and engine was dumping so much fuel, that air/fuel ratio sensor was all in black soot.

Update: 12/27

Today I did exhaust leak test and intake leak test.
Exhaust was pressurized with shopvac and I sprayed entire system on both banks. Bank 2 was 100% with no issues. Bank 1 had a leak at exhaust manifold leaving head on rear at cylinder 7. Also a small leak at upstream O2 sensor bung. I am going to seal these with gasket seal / repair material.

I then checked for intake leak. I setup scanner to read fuel trims graphs. First I sprayed brake cleaner around intake gasket, then head gasket, then injectors, then throttle body, then all hoses on both ends. There was no increase in idle bore any fuel trim changes. I then repeated this test with propene bottle and again found nothing, no fuel trim changes.

At my wits end I test drove again for 30 minutes going down interstate. Set cruise on 75 @ 2000rpm. Trims looks good on both banks. Both banks LTFT would rise up to 9-10 but when added to STFT remained under 10 most of the time… on occasion and only for a couple seconds the S+L would reach 14 but quickly returned under 10.

Not sure if it’s important but I noticed both LT trims would drop exactly the same time. For example Bank1=9.4, and Bank2=10.2 and both together would drop to say 4 or 5, then return up again.

The main takeaway is I could not repeat the test from yesterday. At idle bank 1 trims were less than 2, while bank 2 were around 6.

I still have misfires at idle. P0300 codes, but I know what it’s not… It’s not…

Air - no leaks and MAF/MAP/IAT working.
Fuel Pump - brand new with spec pressures
Fuel Regulator - 1 year old and tested
Fuel Filter - new
Plugs and Cables - 1 year old, set at .0400 spec.
Crankshaft Sensor - just replaces and relearned
CAM Sensor - replaced 1 year ago
MAP - replaced 1 year ago .
TPS - new throttle body 1 year ago.
Pressure - all cylinders passed compression and leak down. C1 worse at 170 psi.

Next Steps:

1- injectors are new but cheap from amazon. May install the original injectors back in?

  1. Coils - have moved around and tested but maybe all are weak?

  2. Cable Harnesses

  3. ECU

My guess is one of the injectors on the bank 2 side has a slight issue which you are noticing at idle speeds. You might try swapping out the injectors on that side using the old ones and see if that changes things.

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I would bet on injectors, pretty much as @Cougar suggested.
Clean your old ones before you put them there.
You can find next video handy:

I did pretty much the same before, but instead of using pressurized can propellant, I filled the hose attached to injectors with carb cleaner, then used air compressor to push it through while supplying electric pulses.
For me, the spray pattern changed from “patchy”, to “kinda ok” to “very good spray” in only few cycles.
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