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Will a bad MAF sensor cause high NOx?
not usually, lean mix and/or high combustion temps are a great cause, clogged EGR is a very safe bet for a first look. Remember all the pipes need to be clean, not just the valve. Check the diaphragm and ALL vac lines as well. A small air leak can really upset things, grab some pliers and pinch each hose close to the manifold and see if the idling changes. If you pinch the one for the idle control, it will speed up!!
If MAF was playing up you would have issues at various rev ranges, lack of power or similar symptoms.
 

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Discussion Starter #4
i disconnected the MAF while the engine was running and it did not stall, really the idle got a little better. so i ordered a new MAF, it will be her on the 12th. any thoughts?
 

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i disconnected the MAF while the engine was running and it did not stall, really the idle got a little better. so i ordered a new MAF, it will be her on the 12th. any thoughts?
I see your parts cannon has fired for no discernible reason. It won't stall with it disconnected. It will have "funny" power delivery, low end is down on power, high end is down and mid range will be ok, which is what its supposed to do.
Idle can be good, bad or unchanged, depends on age of the engine.

Clean the EGR system.....you have 6 days to do that before the new MAF arrives.

idle with maf disconnected loads default settings, which will get you home, and idle is usually affected when disconnected. Doesn't prove, or dis prove the MAF at this stage.

If the MAF is faulty, the NOx will usually be out across the rev range, EGR will knock it out at idle, usually 800-1000 ppm. Can also put it out at higher revs but not usually as high readings. EGR diaphragm leaks will make a big difference to readings too.
 

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Discussion Starter #6
i am sorry because i have not given all the information in this post


I have a 2005 chevy tracker. It came from the factory with a 2.0 suzuki engine with 92,000 miles automatic transmission. It has failed emission 3 times for NOx.

6 months ago, had the top of motor rebuilt. Coolant is good, fan is running, car does not over heat.

1 time fail, changed the Cat Converter.

2 time fail, changed the EGR Valve and cleaned the housing

(when I picked the car up the check engine light was on for the 1st time, then the mechanic cleared the code that caused the light. He said it was because of the change of the EGR valve. but i saw that it said it was for the MAF sensor before he cleared it)

3 time fail.


With New CAT Converter
Idle Engine under load
HC ppm 7 7
CO% 0.02 0.01
CO2% 15.2 15.1
CO + CO2% 15.22 15.11
O2% 0.0 0.0
NOx ppm 422 1021
Lambda 1.00 1.00

With New CAT and EGR
Idle Engine under load
HC ppm 3 1
CO% 0.03 0.01
CO2% 15.1 15.1
CO + CO2% 15.13 15.11
O2% 0.0 0.0
NOx ppm 829 365
Lambda 1.00 1.00
 

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First let me touch on the check engine codes.

IF a faulty MAF sensor causes a code to be set, that code will come back after it is cleared - for all you know the code may have been caused by the switching the ignition on with the MAF sensor disconnected, rather than the MAF sensor itself being defective.

Second - EGR - were the EGR passages cleaned - unless I'm mistaken that engine has an EGR passage through the cylinder head, so that there are passages from the exhaust manifold, through the head, through the intake manifold, to the EGR valve, and also from the valve back to the intake - if these are clogged, you would still be looking at high NOx test readings.
 

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First let me touch on the check engine codes.

IF a faulty MAF sensor causes a code to be set, that code will come back after it is cleared - for all you know the code may have been caused by the switching the ignition on with the MAF sensor disconnected, rather than the MAF sensor itself being defective.

Second - EGR - were the EGR passages cleaned - unless I'm mistaken that engine has an EGR passage through the cylinder head, so that there are passages from the exhaust manifold, through the head, through the intake manifold, to the EGR valve, and also from the valve back to the intake - if these are clogged, you would still be looking at high NOx test readings.
I agree, its those passages people don't clean that cause the issue. Theres one tiny one in the intake mainifold everybody misses that causes a ton of problems. The head ones usually pretty obvious that its gunked up and just scraping the crud out where the valve is doesn't cut it, you have to do the ENTIRE passage. Hot carbon is not your friend when it comes to NOx readings.

Looking at the figures, it still looks like the EGR system is the cause of the problems as when the valve was replaced the readings go up at idle, drop under load which to me signifies it wasn't working first test when you replaced the CAT, but it was at the second test, and that the passages are clogged.
If it was "just" the EGR valve, the readings would have dropped at idle and high. O2 and CO2 are good, Lamda is 1.0 that signifies the MAF is functioning correctly in my book.if Lamda was off, then I'd be looking at MAF.
I bet the mechanic only cleaned the housing, not the passages from the exhaust manifold, right thru to the inlet manifold.

I think your mechanic set the MAF code because he had it unplugged to get to the EGR valve to replace it. If you don't have a MAF code since you reset it, then your MAF is not faulty. By unplugging it to check, you should have tripped the MIL and have a code.
 
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