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After having trouble codes P0171 & P0174, I tested my o2 sensors. The two after the catalytic converters are down (~ 0.1mV, no fluctuation). What are the odds that this can happen at the same time?
I tested them with a voltmeter when engine was hot. The 2 on the front were fluctuating between 0.1 and 0.8 . Confirmed with my odb scanner tool.
Also I forgot to mention that I had 2 other fault codes only once, but I don't remember. P015* or P013*.
What are the odds that this can happen at the same time?
Very low that both would be bad.
Originally Posted by Haaghkhen
I tested them with a voltmeter when engine was hot. The 2 on the front were fluctuating between 0.1 and 0.8 . Confirmed with my odb scanner tool.
Also I forgot to mention that I had 2 other fault codes only once, but I don't remember. P015* or P013*.
You can't test O2 sensors like this. They switch much faster and make a high frequency wave form. You need a special scanner to test them. If you watch the following videos (I know they are long.), you will become an expert on this testing and on your problem. (The later video walks through the diagnosis of a car with the exact same codes you have.)
HOWEVER; on our cars, these codes are really common when we get a dirty MAF sensor. Are you using an oiled air filter, like a K&N? (They are really just a tool of the devil...)
From: Poway, Ca. where GOD and the sun always shines
I own a 2001 Z28 (Sara) with 50K original miles, I had my repair shop change both my front and rear O2 sensors just due to time (19 years). I noticed (in months to come) that the amount of carbon in my tail pipes was reduced significantly, and I picked up a couple of miles per gallon (don't understand how). I purchased AC/Delco OEM units, and the price was reasonable, and the labor charge was less than 1 hour.
Sooooooo, after some scanner tool check, my "ltft" is up to 25% on both sides. If I increase rpm, it lowers to about 15%.
From what I understood, I have to check for vacuum leaks. Anything else I have to look for?
I asked not because I have a crystal ball, but we've seen this a lot. A whole lot....
You have to do a few things.
1) Throw that air filter in the garbage. It's going to continue to cause problems. This problem will come back again unless you move to a stock filter. K&N is promising you 5HP by using their filter, but... it is fouling your MAF slowly and you are loosing 15HP because of that! Then the next month, you loose 16HP and then the next month its 17HP...
2) You need to clean the MAF again.
What did you use to clean the MAF the first time?
A single application of MAF cleaner is often not enough. To tell really where you are, you need to get some magnification and look at the MAF elements. If you see any blackness on the twisted wires, then you still have a problem that is causing degradation.
When I did my last (and final) cleaning, I had to take the MAF apart and (VERY carefully) swab it with a Q-Tip and use the cleaner to get all the K&N gunk off. (<- BTW, this is really delicate and can destroy the MAF if one isn't careful.)
From: Poway, Ca. where GOD and the sun always shines
"Throw that air filter in the garbage. It's going to continue to cause problems. This problem will come back again unless you move to a stock filter."
I totally disagree, I have been using a "Holley" Power Shot filter that I oil before installation, I clean it from the top down, and oil the pleats (very sparingly) from the bottom up. Using this process, I have had no problems whatsoever.
I do agree with your statement about the 5hp increase.
"Throw that air filter in the garbage. It's going to continue to cause problems. This problem will come back again unless you move to a stock filter."
I totally disagree, I have been using a "Holley" Power Shot filter that I oil before installation, I clean it from the top down, and oil the pleats (very sparingly) from the bottom up. Using this process, I have had no problems whatsoever.
I do agree with your statement about the 5hp increase.
So, why do you do this, then? What benefits is this device and all the work it takes to maintain it giving you?
(For a while after I tossed my K&N, I put a K&N sticker on my car. I thought it ironic that people would think I had the part on the car and there was no way in the world anyone would ever know!)
@wssix99 I used a MAF cleaner, and it's totally clean.
How do you know? Did you look at it under magnification? Did you do a proper test of it? Here's a method. You need to plot your lbs/second from the MAF by RPM and make sure you have a straight line: (Example 3) https://support.alldata.com/sites/ma...ing_110918.pdf
Originally Posted by Haaghkhen
But I will see to replace the K&N.
At a minimum; while you are diagnosing this problem. You'll want to go back to known quantities and take out all things that could cause the problem.
Originally Posted by Haaghkhen
If someone is interested, here are some pictures of my scan numbers.
The MAF number looks low, but a proper curve will confirm.
It was a vacuum leak. I don't know what's this hose, but it's cracked at the bottom and the inside is in a bad shape. Any thoughts?
Here are some pictures of the culprit and the white one I put for test:
That's the PCV hose. To the left of your white patch is the PCV valve, wrapped in the larger black hose. There's no vacuum there, but the Venturi effect could pull some air in at that point, behind the MAF but it should only be a small amount of air. The size of the MAF orafice is MUCH larger than that hole.
I'm pretty sure you still have a dirty MAF. I expect that MAF is still most of your problem. If your codes go away, maybe this leak is what is putting you over the top?
I've wanted to get a baseline curve of my MAF for a while. Here it is:
^ At idle, you should be much closer to 1 lb/min. (A dirty MAF will read less air going in vs. actual.) The blue lines are what I measured and extrapolate based on my car's measurements. The red line is a trend line adjusted to intercept at 0. (A stock sensor should be reading at least what the red line depicts.)
That is much better! At least; that could mean that 35% of your combustion air was going through that hole. (Unless you did another cleaning before these numbers - then it could have been less, which is what I would have expected.)
^ If your numbers are lower, that may be OK. The plot of your numbers is pretty straight, which would indicate a good sensor. If you are testing in a warmer environment than I am in, the air will be less dense and the measurements will be lower due to the ambient conditions. FYI - My IAT reading was 80 degrees, but the air in my garage was around 50 degrees.
Originally Posted by Haaghkhen
I also have to figure out why there is a hole in the hose and why it looks kinda burnt or dry inside.
These hoses wear out a lot. They can chafe and the removal/reinstall of the PCV valve can stress them out. That passage also gets a lot of oil in it.
Have you changed your PCV valve ever? If not, it could gum up and allow air to flow backwards and maybe oxidize some of that oil.
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o2 sensors down at the same time? 
