Richard S

I need to lower my NOx as I failed the Ca smog test today. First of all, here are the results:

Speed......RPM.......%CO2......%O2......HCppm.....CO%......NOppm
15mph.....1259.......12.4.........3.3.........33..........0.02........1188 (FAIL)
25mph.....2058.......12.4.........3.2.........27..........0.03.......... 495 (Pass)

Background: 87 5spd, Devek L2 headers, fuel pressure at 55lbs, known good LH, MAF about 4yrs old, 2 high-flow cats (tested on Dennis Kao's car and passed CA smog with good numbers), copper plugs (wr6).

So my only failing number is the 15mph NO. The max allowed is 767, vs my reading of 1188. I also question the high %O2 at 3.3.

From reading some past threads and other google results, high combustion temps cause high NOx. So, what do I look for as the culprit?

Running lean? If so, why?...i.e. air leaks? (but wouldn't that cause a high idle?)
Weak MAF?
Carbon deposits causing hot spots?...(if so, whats the best way to clean?)

Open to suggestions.

Thanks, Rich

GlenL

You're right in that high cylinder temps and lean mixtures create NOx.

My _guess_ is that your headers scavenge too well. There is more fresh air and less exhaust in the cylinder leading to a leaner and hotter burn. EGR was used waaay back to improve emissions as it produced a cooler burn and less NOx.

Are the O2 sensors good?

Jim M.

NOx high, possible cause or recommended action: Replace converter and O2 sensor; verify tune-up and correct any faults, check lean AFR, leaking exhaust valves, EGR, lean AFR, leaks, run-in converter.

dr bob

I wouldn't be suspicious of headers as a cause at 15MPH just-off-idle rolling test. I convinced my smog-checker to do the test with teh trans held in 2nd. That was good enough to pass when numbers were really close.

Richard S

I forgot to list new O2 sensor.

Rich

Hilton

I'd suspect aging MAF - they go lean as they get old and outside calibration.

If the cats tested ok on Dennis' car, and you have a new O2 sensor, then its likely the MAF.

The 15mph test is lowest rpms, and I think its the upper and lower ends where the MAF is most wrong (the voltage range gets reduced), as the O2 sensor compensates to an extent in the middle.

Its getting richer up the range - could try re-taking the test in a lower gear to keep rpms higher? Or test with a different known-good MAF.

edit: just saw Dr Bob's post above about lower gear too.. simple cheap fix But with your car running around lean, you should get to the bottom of it regardless, as it'll be easier on the head gasket and other internals.

Louie928

You might try plugging the exhaust a bit with something just for the smog test. The increased back pressure will let more residual exhaust stay in the cylinder and lower combustion temp. I heard of another owner who had high NOx and got by by lowering the engine temp. He did the test with fans blowing over the engine or something similar.

123quattro

I was just going to suggest what Louie did. Install some back pressure. If you wanted to be really clever you could install some 1/8" plate between the collectors and exhaust with something like a 1.5" hole in it. That, or try and richen it up a little, but not too much as that will drive up CO. Or, if you have adjustable cam pulleys retard your cam timing 5 degrees. That will give you more EGR at low speed, which will drop in-cylinder temps. Honestly I think you are kind of chasing your tail because of the cats though. If you slapped on a set of non-high flow versions I'm sure you would clean up emissions across the board.

John Speake

Under the conditions for that test you will be working on the O2 loop, so a moderately aged MAF will be compensated for.

So look at the other possibilities that have been suggested.

61tony

2ND Glenn L

ptuomov

Without any practical experience on the topic, I think nox would be cured by running the engine in the Paris Hilton mode.

Is there any way to make the car run richer in the closed loop mode?

Is it possible that some of the cylinders are running lean because of a dirty injector? I think that there is cleaners that you can but in your gas tank. I don't know if they work.

Can you retard spark timing with an acceptable cost/effort? Retarded spark lowers the combustion temperature and nox emissions.

Maybe you could find low-octane gas that has no ethanol. This might make the knock sensors automatically retard the timing and lower the combustion temperatures. Just speculating.

Is it easy to run water injection manually with these cars for a little while to get the glowing carbon deposits out of the engine?

RyanPerrella

try plugs too

copper is reported to be better then platinum

douvillier

High O2 is a lean mixture indicator, your low CO2 indicates a rather low efficient engine and your high NOX indicates your engine is running high combustion temps. Two of the major reasons for lean mixtures are intake leaks and clogged fuel injectors. The reason the engine computer cycles between rich/lean when running is because the cat needs lean mixtures to deal with HC and CO and a rich mixture to deal with NOX. If you are running lean the cat can't lower your NOX. I would first get your fuel injectors flushed and de-carbonize your intake/combustion chambers.

douvillier

Of course due the easy/cheap fixes first. Check for intake leaks first.

Richard S

Thanks for the great suggestions, keep them coming!

Fuel injectors are also new, sorry for forgetting that important fact.

The lack of backpressure has me thinking....2 years ago I smogged with a stock cat modified to fit the headers and the NOx levels were nice and low. The stock cats are much larger and I assume more restictive. With the current hi-flow cats I assume there is less restriction and back pressure. As Louie and others have suggested, maybe a bit more of a restriction would help keep some residual exhaust gas in there. John's idea of fashioning a sheet-metal gasket to go between the header and cat flange, with a smaller diameter hole in it would be easy to make.

A couple other questions:

Does race gas lower temps inside the chamber? i.e. 100 octane unleaded?

Does bumping up the fuel pressure have a noticeable effect? i.e. push it up to 65lbs, will it run richer to make a difference for NOx without putting it over the limit for being too rich?

While the goal is to pass the smog test, I also want to know what is causing all this and hopefully find a long term solution.

Rich