High NO at 15mph - everything else is normal on Acura

Maintenance/Repairs
21

I am sorry, mine is 1999 - it doesn’t seem to have an EGR?

22

Okay.

From the horses mouth.

Tester

23

With no egr system there’s only a few common possibilities for the cause of the high nox. overly lean, engine running too hot, timing too far advanced.

Here’s some info how engine timing can affect nox emissions.

That your nox emissions are highest at the lower speed (15 mph), that would be consistent with a vacuum leak of some kind causing the mixture to be overly lean. The adverse effect (leaning out the mixture) of a vacuum leak is more at lower rpms than higher rpms. At higher rpms there is a lot of air flow into the engine anyway, so a little more is barely noticed. Have you done all the common sense vacuum leak tests? Any lean codes in the computer memory?

24

There had been no check engine light for a while - the last was on Auto tranny.

Yeah, this is the last part of the investigation holding me from re-test. Will speak with a mechanic today. Parts store was saying it is the dirty Air filter - but looking at the records, after it was changed in 2013 NOx didn’t come down (also added NOx at 25mph)

2005 @15mph, NOx 126 @40k miles (25mph 34)
– no records
2011 @15mph, NOx 246 @77k miles (25mph 72)
– 2011 timing belt was done @80k miles- m/c said he tuned it then
2013 @15mph, NOx 442 @91k miles (25mph 78)
– Dec 2013 Air filter changed
2015 @15mph, NOx 445 @107k (25mph 44)
– May 2017 O2 sensor was replaced with NGK
2017 @15mph, NOx 462 @122k (25mph 34)
2019 @15mph, NOx 693 @134k (25mph 8)

25

All those other years numbers are useless. All that matters is you can’t pass now. If you don’t know what to do have a shop fix what needs to be fixed before you run out of retesting time.

1 Like
26

It’s quite a mystery at this point. If the problem was at 25 mph I’d guess it was the cat. But since it passes easily at 25 mph, hard to make an argument the cat would be the culprit. Keep us informed there OP, best of luck.

27

It is indeed a mystery - this is also what the m/c told me.

The plastic radiator was changed a twice but the temp sensor was not. Would this be a good investment at 135k and 19yrs of age?

28

It depends on what that sensor does. I presume you mean the sensor the computer uses to determine coolant temperature as part of its air/fuel mixture calculations That part rarely fails, and can easily be tested in place. The test may require the use of a Honda/Acura scan tool though.

In addition, ff that sensor was faulty or inaccurate, it would affect the HC emissions at least as much as the NOX. And your car’s HC emission levels are nearly perfect.

Basic tune-up stuff makes sense prior to any emissions testing. Spark plugs, engine air filter, checking valve clearances and ignition timing, fuel trims.

29

@George_San_Jose1: Thanks for your expert advice.

  • what do you mean by fuel trims?
  • when they do valve clearance adjustments, do they do ignition timing - because valves were adjusted by an expert shop last yr? Otherwise, timing was done with timing belt in 2011.
30

That’s an easy way to check (using a shop scan tool) if there’s any issues going on with the air/fuel ratio. such as would be caused by a vacuum leak, inaccurate maf sensor, etc . The measured “fuel trim” for a given operating condition is the % reduction or increase in fuel the computer determines is needed based on the pre-cat O2 sensor, compared to how much fuel it thinks it would need based only on the maf and coolant temp sensors.

not normally.

As a diy’er I have always checked ignition timing as part of changing the timing belt, but I doubt that is part of the shop procedure. Valve timing (set by the timing belt position on the sprockets) is a different engine operating parameter than ignition timing and they two are independent for the most part.

Have you done some google research to see if there’s any known failure modes for a cat that would cause it to not work at 15 mph (and presumably lower rpm) but work fine at 25 mph (and presumably higher rpm)? maybe there’s a weird cat failure mode you are battling.

31

deleted :tada:

32

There is no known failure - one’s I saw had EGR
Also the engine rpm was about the same at about 1800rpm irrespective of 15/25mph.

33

He used an additive whichreduced NO at 15mph CRC 05063

Another people use often - especially there is a code through OBD:

34

If the CAT were the culprit, it would have been at 25mph too!

35

The word I am getting is lazy O2 sensor, car is not warmed up enough.

Given the historical readings, the CAT seems to be getting dirty too - { Recommended ratio of one 450ml bottle of Cataclean® Liquid Science to approximately 15 litres of fuel}

36

Fuel additive worth a try I guess. I noticed the PPF poster, besides the fuel treatment, was extra diligent on the second testing to thoroughly warm up the engine and cat before testing started. To increase your odds, probably want to do that also. there’s no way to know at this point if the reason for the poster’s dramatic drop in nox at 15 mph was due to the fuel treatment or the well heated engine and cat. But at least you got something to go on now.

37

It passed:
2005 @15mph, NOx 126 @40k miles (25mph 34)
2011 @15mph, NOx 246 @77k miles (25mph 72)
2013 @15mph, NOx 442 @91k miles (25mph 78)
2015 @15mph, NOx 445 @107k (25mph 44)
2017 @15mph, NOx 462 @122k (25mph 34)
2019 @15mph, NOx 693 @134k (25mph 8)
2019 @15mph, NOx 146 @134k (25mph 646)

NOx came way down to 2005 level but went up at 25mph

HC was 10 & 9
rpm was at 1800 at 15 & 25mph

I warmed it up by:

  • driving 9 miles regular
  • driving 3 miles on D2
  • Mechanic had it warmed up for 1 min at 2700 rpm before the test.

I never paid attention to warming up prior to this and took it for granted.

38

Good for you! Congrats on a diagnosis job well done.

39

Thank you @George_San_Jose1 and others for your support.

These are the things I did:

Any idea why NOx went up at 25mph?

40

I don’t see any 2019 before/after data for 25 mph posted above.