GM Truck Club Forum banner

1 - 20 of 93 Posts

·
Registered
Joined
·
134 Posts
Discussion Starter #1
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 9, while Bank 1 is 4.8.
When going 65 @ 2000 RPM both banks are often exactly the same around 11.7. 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?
 

·
Registered
Joined
·
242 Posts
If those O2s are original.... ya. Well past time to replace.
 

·
Registered
Joined
·
134 Posts
Discussion Starter #3
Thanks. Yea they are original and that’s what I am thinking. They may have good voltages and seem to work fine but I have heard a lot on YouTub and Forums day that they only last 100-150K miles. I am not sure if they are a hard fail item or can degrade over time?
 

·
Registered
Joined
·
242 Posts
Can degrade over time. Particle build up starts to prevent them from sending strong signals.
 

·
Registered
Joined
·
13,351 Posts
They can do both; but they (front ones) will get lazy after 100 thousand. I change them when I change plugs.
 

·
Registered
Joined
·
2,765 Posts
X2 on everything above about O2 sensors. 235K is good service. Time to retire those guys. I have seen them go bad as low as 55K miles due to contamination. Also the exhaust leaks as well.
 

·
Registered
Joined
·
134 Posts
Discussion Starter #7
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...

Question:

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?
 

·
Registered
Joined
·
2,765 Posts
They have been in there for 235K miles. Stuff that old usually requires heat and patience to get them off with a 6 point socket. Every time I run into a part that won't break free I heat it up and then work it slowly. Personally I wouldn't cut anything as it could be more trouble and another can of worms and expense. Good luck
 

·
Registered
Joined
·
242 Posts
Agree with @silverado002. Heat and rust penetrate are your friend. Hit it for a few days with penetrant, then heat it when you try to take it out. And be patient.
 

·
Registered
Joined
·
134 Posts
Discussion Starter #10
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?
 

Attachments

·
Registered
Joined
·
242 Posts
Sorry, Gotta ask, did you perform relearn after changing sensors?

If yes, next step for me would be clean the MAF. If you’re confident you have no intake or exhaust leaks and the MAF cleaning doesn’t make a difference, you probably have a clogged/fouled catalytic converter behind bank 2. Wouldn’t be outside the realm of possibility since those O2 sensors were original.
 

·
Registered
Joined
·
134 Posts
Discussion Starter #12
I did have a relearn after the CPS change and it went well.

I eliminated MAF because bank 1 looks great and MAF applies to both banks. I did clean it with MAF cleaner 1 year ago but will do again.

I was 100% confident it was not an intake leak until now because I changed gasket 1 year ago and smoke tested it. But it is pointing to either Bank 2 intake manifold leak, or exhaust manifold leak pre upstream o2 sensor.

Today I am going to test for both to see if I can find either. I am really hoping it’s an exhaust leak because it would explain everything.

When you say clogged cat do you mean literally clogged so exhaust gases back up and float around the upstream 02 sensor?
 

·
Registered
Joined
·
242 Posts
That’s correct, but that’s normally accompanied by more severe symptoms like poor performance bad smells or smoke. Your trims aren’t terrible but high trims usually mean bad air readings or unmeasured air entering somewhere.
 

·
Registered
Joined
·
242 Posts
Ya the more I think about it, I believe you’re on the right path with exhaust manifold leak. That would cause a false lean, and the sensor on bank 2 would compensate with more fuel. That would explain why the new o2 sensors didn’t fix it.
 

·
Registered
Joined
·
134 Posts
Discussion Starter #15
Oh I meant to ask. Keep me straight on this...

In my situation it appears a leak is giving o2 sensors a lean condition (not false but wrong) driving up fuel supply to cylinders to offset. Right?
So it’s reading lean but causing a rich condition (more fuel in cylinder) right?

Does this extra fuel cause misfires? Or just not good for emissions and fuel economy?
 

·
Registered
Joined
·
134 Posts
Discussion Starter #16
Rebel you know what just hit me. My wife has told me the Tahoe is not as quiet as it used to be. The cabin is noisy to her.. I sorta ignored her but now realize that is another symptom of an exhaust leak. 1 more coffee and I am on the exhaust testing..
 

·
Registered
Joined
·
242 Posts
There we go. Ya bad for fuel economy but exhaust leak shouldn’t cause a misfire. Intake leak is a different story.
 

·
Registered
Joined
·
134 Posts
Discussion Starter #18
Well it’s so strange how nothing follows as expected. I am a DIY novice but I do understand the logic and troubleshooting . I am an electronics tech by trade. So I finished the exhaust leak testing and it’s the opposite of what I wanted to see. Bank 2 has zero (0) leaks from heads to exhaust tip. Not heads, manifolds, flanges, cat, pipes, muffler, nothing.. no bubbles anywhere..

However on Bank 1 I have 2 leaks. One on exhaust manifold at back corner near Cylinder 7. The other is on the upstream 02 sensor where it’s screwed into bung. I tried to tighten the sensor but no change. That one was very difficult to remove and install so I think it came cross threaded. I think I can fix both with exhaust repair manifold and gasket seal on sensor..

So that leaves only 1 thing left and I am going to eat a late lunch and then test it. That’s a intake leak on bank 2 side.. more to come...
 

·
Registered
Joined
·
134 Posts
Discussion Starter #19
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 nor 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 3, 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 replaced 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?

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

3. Cable Harnesses

4. ECU
 

·
Registered
Joined
·
2,765 Posts
Caution, be very carefull that you don't get sealer on the O2 sensor above or below the threads as there is usually a very small air port in the body of the sensor.
 
1 - 20 of 93 Posts
Top