Bill Ball

Hope Jeff's cats do the trick. I reported on a NO only failure with aftermarket cats that were great a year earlier. Some good used factory cats resolved that.

Charley B

I don't know that the MAF has ever been touched. It seems to me when we found the bad LH on Bill ***** analyzer the MAF checked out OK. Of course my memory is as faulty as my NOX numbers.

safulop

One thing that can spoil the action of a catalytic converter is an exhaust leak between it and the engine. The leak causes the exhaust pipe to suck in extra oxygen which decreases the ability of the cat to scrub NOx.

Charley B

I checked the elbos, only one had a very very slight odor of gas.

Charley B

I was very careful to ensure the cat was fully seated and the flange gaskets were in good shape. There are no extra sounds in that area.

safulop

Seems like you've been all over this; the combination high NOx + good HC is troublesome and does not obviously point to lean burning. But all this emissions stuff is funny and I wonder about their measurement equipment sometimes. I once had a 944 fail smog in the morning and pass in the afternoon, after I drove it 20 miles up the road to a smog check in the next town.

dr bob

Valid exhaust numbers for combustion analysis are made on exhaust gas samples from before the catalyst. After the cats/tailpipe numbers tell you the sum of actual exhaust components minus the stuff scrubbed out by the cats.

With that limitation in mind...

HC is a measure of fuel that passes through the engine unburned. Ignition faults are the most common cause. Unburned fuel passing through is undoubtedly the biggest killer of cats, often causing so much heat that the tar and insulation on the floor over the cats catches fire. Short of that, the heat will destroy the reactive components and often melts the ceramic support medium.

CO is a product of successful combustion, ad a good indicator of air/fuel ratios. The "oxygen sensor" responds to increases in CO with a higher output voltage. In closed-loop control, this directly guides the operation of the LH controller and how it adjusts injector duty cycle. Note that CO does not change if a cylinder misfires; HC goes up from the unburned fuel, while CO stays relatively constant based on A/F in the cylinders that are still firing.

NOx is formed when combustion temps exceed about 2200 degF at any place or time in a combustion chamber. One lean cylinder can easily cause high NOx with a lean condition, while CO continues to indicate combusted mixture. Since the total CO causes the LH to adjust, one or two lean cylinders might cause a high NOx condition even while LH adjusts only slightly richer to get the total average CO number back in range.

Got high NOx? Look for vacuum leaks, including air leaks into the oil vent plumbing between cam covers, oil filler neck and cap, plus the vacuum hoses. Consider partially-obstructed injectors. Also consider oil accumulation in the belly of the intake, since oil carryover dramatically reduces effective octane ratings and increases chamber temps as a result.


Several folks have reported excellent NOx numbers after a BG fuel and intake system cleaning done. This s a combination of a serious injection cleaner forced through the injectors, and a subsequent intake manifold cleaning by spraying a cleaning chem in downstream of the MAF and sometimes after the throttle. There are similar systems under the Snap-On banner for instance. The combination of clean injectors, no false air, and elimination oil vapor carryover into the intake will do wonders for the NOx numbers and let the car run better.

Charley B

A set of factory cats from Richard and problem solved. Thanks everyone.

968M030

Sorry to hijack this thread but I am wondering if used OEM cats would help in this situation. I bought this car back in July from a guy who said that I would not pass smog. I thought I could outsmart him/California. He failed twice, now I have failed four times, always high nox. I changed the cat twice (second muffler shop said first muffler shop installed incorrect cat), had MAF rebuilt, new caps and rotors, adjusted timing, new O2 sensor, new timing belt. Second test was 7/13/15 and was close. Last week passed at 15 mph then gross polluter at 25 mph! Any comments on these numbers? This is getting stranger instead of more clear.

Hilton

Things to consider:

What's your fuel pressure? Is it in spec?
Is the LH a rebuilt unit? (the part of the LH which fails also contorls the O2 sensor loop)
Are the injectors clean?

OEM cats are excellent and will also hide a lot of issues, but new aftermarket cars should work fine if the car is running correctly.

Imo000

You are running "pig rich". Figure out why engine is getting so much gaaazoline.

dr bob

Seldom disagree with IMO, but the numbers say exactly the opposite. Of course the numbers are after the cats so we have to consider whatever work they are doing as a wildcard. Please carefully re-read my post #22 above, and also Greg Brown's guidance. The ver low CO number indicates a lean combustion condition. The marginally high HC numbers suggest a lean misfire. The very high NOx numbers indicate a lean combustion condtion causing high combustion temperatures. The common component in all of these is a LEAN mixture.

Causes include air leaks, failing or failed oxy sensor, failing/tired MAF, partially plugged/dirty injectors, oil in the intake manifold, and go on towards LH failure. You've addressed the MAF and oxygen sensor already. Continue through the rest of the list and emissions salvation shall be yours.

For Cali folks playing along, if your car fails a test once, be darn sure that you work with the testing technician on subsequent tests to abort any test before it completes if he sees the car failing. It only takes a few "fails" to push a car onto the gross-polluters list, which dooms it to a life of STAR testing (at minimum) from that point on. Might cost you coffee or donuts to get the tech to work with you, but it's way worth it in the long run.

Many folks dread getting a smog test. Instead, look at it as an opportunity to have someone look at what comes out to help determine how well the engine is actually running. My car ran fine from when it came to me with 22k on it. But over the years the NOx numbers slowly crept up, to the point where the car was barely passing. I'd been running top-tier fuel, drove the car regularly, everything seemed fine. Tested for vacuum leaks, monitored fuel pressure, scratched my head. I did an in-car pressure injection and intake cleaning, which helped some. Finally, new injectors as part of an intake refresh solved the issue. The rest of the intake refresh revealed no split hoses, broken fittings, oil puddling, etc that could be used to explain the sudden improvement back to factory new numbers.


So.... With everything you've done so far, the next step should be a full out of car pro injector cleaning, or just get a set of new replacement pieces. and know they are good. Talk to Roger or Tom/Mark about options they have on new or on already-cleaned and rebuilt nozzles, with new o-rings and pintle caps already installed. If you car has more than 75k since they were new or last out for full cleaning, you are almost undoubtedly overdue.

Imo000

Bobby.....I'm referring to post #24, the Oct. 14 report.

Now as for the other one, the problem might also be an non functioning radiator fan and the combustion temperatures could actually be high because the coolant temperature is high too.

968M030

I passed, and it was something simple. A buddy of mine sold me an OEM cat. I was looking at it and noticed how the air inlet was near the back of the cat. I climbed under my car and the MagnaFlow had an air inlet that was capped off, and mine was routed in front of the cat. I have no idea why the 2 mechanics that I had check it out did not mention anything about this capped off inlet. I brought it back to the muffler shop and they rerouted the inlet and sealed the hole in front of my cap. The nox went from 1769 to 131. Huge difference and I passed easily in all categories.

I knew my car was running great. Bummer that I purchased 3 cats, smoke tests, exhaust leak tests, new caps and rotors, etc., to the tune of $1500, and that was doing a lot of the work myself. I guess I learned something though, and hopefully am now helping others. I actually did help another customer at the shop I use who was having the same problem. With the air inlet in front of the cat, the cat can not get hot enough, and the air an not help scub the nox. All of the other numbers barely moved but nox at 25mph came way down. Interestingly, nox at 15mph went up, from 219 to 373.

Hilton

Factory cats have air injection to the cats before the cats.. no problem passing emissions testing like that.

My guess is they disturbed the o2 sensor wiring while fiddling with your exhaust.