ROG100

My car failed the emission test to day!!!!

The inspector said I needed new cats $$$$$$$

Everything apart from the NOx(ppm) was fine and well within standard.

High Speed (25mph) standard was 751, reading = 1532
Low speed (15mph) standard was 829, reading = 1694

Anyone who knows about emissions agree with the diagnosis?
Any other suggestions would be well received.

Car was driven for 4 miles before the test - car was at normal temperature.
12 months ago readings for NOx were High = 264 Low = 459

Randy V

Change the oil, fill the tank with super premium and get that sucker red-hot before you bring it in for a re-test. Let the car idle while you're waiting to enter the test station.

Also, you may need to replace your O2 sensor.

Others here will have more suggestions for you.

Realist D.

Randy's suggestion is spot-on. Driving the car for only 4 miles before an emissions test was a big mistake. You need to take the car on the highway and get it as hot as you can before the test. Drive it in a lower gear on the highway for 15 minutes or so, then leave it idling while you wait for the test. Also, run the A/C while you're warming it up, but don't run the A/C during the test.

SharkSkin

NOx indicates high comustion temp. Could be too lean, could be malfunctioning EGR. Maybe you could sneak some water injection in there if the above checks out.

DG84S

My '84 failed last year for high NOx. I too was told it I needed a new catalytic converter, but I didn't accept that advice, since the year before it had passed with low readings on all measurements. Like yours, my other readings were within acceptable range. A cold engine would have resulted in high HC readings from a too rich mixture. High NOx is the result of elevated combustion chamber temperatures. I replaced the oxygen sensor and retarded my iginition timing (I had advanced it earlier in the year and it was out of spec), and it passed with NOx on the low end of the acceptable range. I know you can't adjust the timing on a '93, but you can replace the O2 sensor and investigate the sensors and circuits that manage ignition timing to make sure they are working per spec. Installing a new catalytic converter won't solve the problem that is causing the high NOx.

Tony

My 87 and i think most 928s will pass smog without CATS! Wont pass a visual inspection obviously but it will pass the sniffer, mine did. I highly doubt its the cats on a 93 car!

Go take it for a good thrashing, get things nice and warm. Make sure your airpump belt is on?

HTH

Bryan

A lean mixture can cause low HC and CO but high NOx. Check your intake tract for air leaks. Broken/abraded/split vacuum lines, injectors sealed tightly into manifolds, and generally make sure the system isn't letting unmetered air sneak in.

Bryan

ROG100

Thanks for all the suggestions.
Changed the oil, kicked in the A/C and drove for over an hour. Great excuse for driving.
Took it back for the free retest. Car was cooking hot - straight onto the dyno - killed the A/C. Car was tested immediately.
Failed again - 15mph = 1874 25mph = 1439
The first was worse than last time, the second slightly better, however both way out.
I will probably take it to Zimms to have it checked for the air pump, O2 and MAF.
Thanks for your advice - it was worth a try.
Roger

borland

A good catalytic converter will exhibit a crackling sound for several minutes after shutting the engine down. More so with only a short warmup. Lots of short distance driving can kill a cat sooner than later.

If you don't notice any activity after parking the car, then your cat is probably dead and needs replacement.

Randy V

Next step is replace the O2 sensor - easy and relatively inexpensive.

I still doubt it's due to a dead cat.

Steve J.

Roger - Here's the trick to passing Texas Emissions HC reading: Make sure for the 15mph run it is in a lower gear. I barely squeaked by last year 15mph, 1304rpm, standard-148, my car-146. This year it barely failed so they did a second run with the 15mph reading in a lower gear: 15mph, 1761rpm, standard-148, my car-76 ppm HC! New air filter, plugs, oil change and hot engine. Maybe run a tank of gas with a bottle of Techron through it, too.

John.

Try the 02. My twin turbo will pass with a small cat and the 02 plugged back in. It has no smog pump.

ROG100

Steve J,
The HC are fine the NOx is the problem.
John,
New O2 on Monday and try again.

User 4221

Get a bottle of RXP at a Kwik Kar. This will greatly help the NOx

dr bob

Don't Forget:

HC is the result of misfire, and is only a function of mixture if it's so lean that you get a misfire. HC is the --unburned-- hydrocarbons that pass through. If you want to see how the mixture is, look at the CO reading. This is why the oxygen sensdor is used to adjust mixture on the fly; otherwise they'd have a HC sensor for that.

NOx is due to high combustion temps, period. Misture too lean (what was that CO reading ahead of he cat again??) or timing too far advanced will do it, as will poor fuel. If the CO and HC readings --ahead of the cat-- are normal, then NOx should fall in fine unless the timing or fuel quality is wrong.

Most 928's could probably pass with no cats at all if the ignition was in perfect shape and the injectors were clean.


My '89 was on the border of passing NOx. The ride operator agreed to do the 25mph test in second gear, rather than allowing thge auto trans to go to fourth on its own. I argued that the dyno wasn't putting enough load on the car to get it to hold he correct gear, and after a little discussion he did it in second and we passed just fine. Worth a try!