P0171 and P0174 lean codes, can’t seem to fix them.

[Avyn Nix]

You can pull the vacuum hose off EGR valve with engine running, and feel the end of the hose with your finger, should have NO VACUUM at all
Then put your own vacuum hose on the EGR valve and suck on it, no there will be no exhaust, or there better not be, lol
As you "pull a vacuum" by sucking on the hose, the idle should start to deteriorate, stumbling, release vacuum and idle should return to normal, if it hesitates to return to normal then EGR valve could be sticking a bit

Purge valve leak should show up on IAC valve test, so not an issue if idle dropped

Would be a long shot but exhaust manifold leaks on BOTH banks could cause False Lean, these suck in fresh air, which O2s see as "air in exhaust" so lean burns
Check a few spark plug tips to see if they are darker brown this would indicate False Lean so engine is actually running richer than it should
But this can also be a computer issue

LTFT(long term fuel trims) can take some time to come down after repairing an issue
What are the STFT(short term fuel trims) on both banks, say when cruising along at 40MPH steadily?

The way Fuel Trims work is this
Computer calculates Open Time for injectors, and that is STFT 0%
It then ADDS LTFT to that, in this case STFT 0 is really STFT +25%
So if you don't see STFT as -10% or so then problem is not fixed

LTFT is there so the system can "age gracefully", lol, and computer does not need to RELEARN the system at each startup
There will always be small air leaks, MAFs get dirty and fuel pressure will vary over time
So LTFT of +10% is not a concern

This is why computer adds LTFT to its calculation before using it, it is adjusting the air/fuel mix for Long Term use in this specific vehicle

So if there was a problem like a vacuum leak which caused STFT to go high it would also cause LTFT to go high over time
When air leak is repaired then when the high LTFT is added to computers calculation the STFT would then go into -10% STFT because its now too "rich" with addition of LTFT and then this -10% will pull down the LTFT over time

Ron,

Thanks for the info on how the fuel trims work. We were not seeing the -#'s like you stated..

I'll go get the data at 40mph after i get back from the parts house.

I put a syringe on the egr vacuum port and was able to pull both a vacuum and hold pressure, I'll do the running check later..
I also checked the EGR vacuum solenoid, it appears to be stuck open, can freely pass air through vacuum ports and the breather port on the top, this would easily be a vacuum leak. Soap test did not find it due to it under a plastic cap and covered by a small foam filter.

Everywhere that was not power washed has a thick layer or fine silt on it, the area where injectors go into the head, the EGR vacuum solenoid, the valley cover on the engine was over an inch thick of silt.

Thanks for the speedy replies, off to get a part ....

 

[Avyn Nix]

40mph

60mph

Before tank fill up and injector cleaner

 

[Avyn Nix]

Update!

Replaced the EGR vacuum solenoid...

Drove truck and got a P171 P420, then reset the scanner and got a P174 code. in addition to the 171 & 420.

One thing to note, I filled up and added a bottle of LiqiMoly Jectron injector cleaner, dont know if this plays any role.

So, is the CATS that are bad?

Thanks

 

[RonD]

Looks like the repairs are pulling down the LTFT

After warm up
Check O2 sensor B1S2 that's the downstream O2
It should be steady and 0.75 to 0.85volts
That tells you if Cats are OK

O2 sensors have a numbering ID
All upstream O2s are Sensor 1's(S1)
All downstream O2s are sensor 2's(S2)

Bank 1 is B1 passenger side on a V6 or V8 and the only side on a 4cyl
Bank 2 is B2 drivers side on a V6 or V8 engine

So a V6 with 3 O2 sensors will have
B1S1, upstream
B2S1, upstream
B1S2, downstream
Dual exhaust and some later models have B2S2, a 4th O2 downstream sensor


O2 sensors only detect oxygen in the exhaust
O2s generate their own voltage AFTER they are heated up above 600degF
0.1v = high oxygen
0.9v = low oxygen

Upstream O2s will jump around a lot as computer fine turns air/fuel mix, its trying to get an average of 0.4v

Cats use unburned fuel and remaining oxygen in exhaust to get hotter than the exhaust "air" to burn up the toxic gases/emissions
So the O2(s) after the Cats should have very low oxygen levels and be steadier than upstream
0.8v average

Downstream O2s can/do effect LTFT
If downstream O2 is showing lower voltage, under 0.75 then computer will add extra fuel to try and get it Hotter so it can burn up more oxygen(and emissions)
This means LTFT has to be higher as this represents current state of this vehicles system, failing Cat(s)

But this will set a code or codes, usually

P0420 Catalyst System Efficiency Below Threshold (Bank 1)
Is that kind of code, lol

If you haven't changed the Downstream O2 do that first
If you have then yes it looks like a Cat is starting to fail

You are in Calif so you need CARB(California Air Resources Board) approved Cats, they are of course more expensive, bummer

 

[Avyn Nix]

Here’s some videos of the voltages for the O2 sensors.

https://youtube.com/shorts/oXS5LBqAmrk?feature=share

https://youtube.com/shorts/a0S2cfl5AaU?feature=share

 

[RonD]

Yes B1S2 is running too low at 0.6v
Computer is adding more fuel to try and bring voltage up

FYI, at idle computer has to run higher STFT to prevent engine from overheating
Its why newer vehicles have to have Auto stop/start, there is just no way to get low enough emissions on a gasoline engine at idle, besides shutting it off

Another FYI, B2S2 is at 99 because there is no B2S2 on your vehicle, thats the "default"/no sensor found display
Why they don't use N/A is beyond my understanding, but its like that on most OBD2 readers

 

[Avyn Nix]

Gotcha...

I have not replaced the D/S O2s yet

Will be picking up some new ones tomorrow

As for the Cats, luckily I bought this rig from Nevada, so I will be putting on some 49-state cats, plan to ship some to a friend that lives in NV, that's the plan at least

Thank you very much for all of your help!!

 

[RonD]

It may just be the downstream O2 is too old, so not a Cat issue, O2s get lower and lower voltage after 12 years old or 100k miles

But computer will still send the extra fuel regardless, all based on downstream O2 voltage

 

[Avyn Nix]

Update, put in the new downstream O2 sensors and the fuel trims got worse, and the O2 sensor voltage seems to be down even though all of them are new.

And the lean code appears to be gone but the P0420 for the CATS is still popping up which tells me it's the CATS for sure, however, I don't know which CAT it is.

Is it the upstream CATS or the downstream CAT?

Thoughts?

 

[RonD]

Downstream O2 sensor(B1S2) is not plugged in, it shows 99%

Check it
If it seems plugged in put in the old one and see what it shows
New parts are a gamble these days

 

[Avyn Nix]

Downstream O2 sensor(B1S2) is not plugged in, it shows 99%

Check it
If it seems plugged in put in the old one and see what it shows
New parts are a gamble these days

Replaced the bad O2 sensor. Still getting P0420 for bank 1 cat inefficiency. Pretty sure it’s the cats now but I’m wondering if there’s a way to trick the downstream O2 sensors into thinking the cats are alright.

 

[RonD]

Not really
Some use stand offs to try and reduce the Oxygen the downstream O2 sees
Google: downstream O2 without cat converters

Cat converters burn up most of the Oxygen as they burn up emissions, so the downstream O2s sees very little Oxygen so has a higher steady voltage, 0.75 to 0.85v
But computer sends extra fuel thru into exhaust to keep the Cats HOT, if downstream O2 doesn't "respond" to that it may set the code again
Computer has to see voltage changing at the O2 so can't just feed computer 0.8v and be done with it, lol

 

[Avyn Nix]

Not really
Some use stand offs to try and reduce the Oxygen the downstream O2 sees
Google: downstream O2 without cat converters

Cat converters burn up most of the Oxygen as they burn up emissions, so the downstream O2s sees very little Oxygen so has a higher steady voltage, 0.75 to 0.85v
But computer sends extra fuel thru into exhaust to keep the Cats HOT, if downstream O2 does "respond" to that it may set the code again
Computer has to see voltage changing at the O2 so can't just feed computer 0.8v and be done with it, lol

Ok that was the original plan was to feed the 0.8v but now I’m thinking about getting those O2 sensor spacers, have you used them or heard of them?

 

[RonD]

No, just replaced the Cats

 

[Avyn Nix]

Ok, will do.

Thanks for all the help, much appreciated.