Hi all,

This is my first post on the forum, I just picked up a 92 6-speed and am glad to be back! This is my 4th Vette, but the first in about 6 years.

The car does have high miles (128K), but runs like new, no smoke at all, no oil usage, and what appears to be a bone stock setup aside from a Borla stainless system. Without doing anything, I got the following numbers:

HC reading 60, limit 117 = pass
CO .80, .65 = fail
NO 929, 821 = fail

I was in line for about 50 minutes at idle, so I thought maybe the O2 sensors and cats had cooled down too much. I did an oil change, added fuel injector cleaner, and tried the old denatured alcohol trick. This time I tested early in the morning before the heat and humidity cranked up and got the following:

HC 97 of 117
CO .87 of .65
NO 877 of 821

The converters were tested using a temp gun and the outlet temps were 100 degrees hotter than the inlet temps, indicating that they are working. EGR is working, no codes, nothing disconnected, etc.

The only driveability issue I've noticed is that if I'm running in too low a gear (say 4th on the highway) and let off the gas, I'll get a pop in the exhaust. Sounds like fuel igniting inside the exhaust. I only did this to help blow out the motor on the way to the test, I never get a pop driving normally.

Any more ideas on getting my new pride and joy a passing grade? I'm borderline failing, and new converters will probably get me over the hump, but the cheapest GM replacements I can find are $450 a side! Summit has aftermarket direct replacements for $170 a piece. I'd just hate to spend the money and still have a failed test. If somebody could point me towards more diagnostics it would be greatly appreciated!

 

Kind of a strange combination, high CO usually indicates it is rich, high Nox can be caused by running too lean or too hot. Usually if CO is high it will passs the Nox portion. Since the Nox seems farther off I'd make sure your EGR is working first. Make sure the air injection is working if it has it, good luck and let us know how you do.

 

Yeah, I thought it was a strange combo too. Especially since the HC was originally half the limit! I don't know much about cats other than I'm not a fan, but is it possible they're doing their job on HC, but not CO and NOx? I don't know if they are all or nothing, or if parts of them can work while others are weak? Could I still have the temp increase between the inlet and outlet but have non-working cats?

Both cats have AC part #'s on them w/ some tack welded plates. I don't know how to decode these numbers, but from the looks of them alone, I'd say they're likely the originals.

Driver's side tag:
GYU31
170253
P/N2512900

Passenger's side tag:
GYU31
174253
P/N2512901

The P/N lines are obvious, but I didn't know if there was a date code hidden in one of the other lines somewhere.

 

Scan to see if there are any codes, active or historic that mgiht point in the right direction. I am thinking 02 sensors are possible.

 

It's rich - scan it and see if the BLM's are dead on or favoring Lean. If they're dead on, the CAT's shot (and there are 3 parts doing different things so one or the other maybe worse than the other). If it's lean, start thinking of all the places air might be getting into the engine - other than through the throttle body - and you can usually come up with a list of things to check or repost. Assuming the CAT's good the EGR would be the prime culprit for the high NOX. Therefore, you're going to need to make sure it's working and with no trouble code or Check Engine Light, the ECM thinks that it is. Maybe someone can give you the trouble tree chart so that you can see how it all works, but I wouldn't even bother until you know the status of the CAT and fuel delivery will give you good idea about that.

 

I had trouble getting my 96 to pass SMOG here in Cali...the only parameter that kept failing was the NOx. Ran a bottle of SeaFoam through it, and it passed on its first try with flying colors. Reduced my numbers by almost half (mid-400s to mid-200s)...

 

Thanks for the input guys. I had already scanned for codes, but came up empty. The EGR was cleaned and is working properly. I will do a run and record the BLM readings to see where they stand. I'll post my findings. Thanks!

 

OK, got it warmed up and into closed loop mode, then captured the data. I had a real good run, but then my batteries died in the AutoXRay . Anyway, another run still got me some results. I'm not sure how to interpret the new info I have, so any help will be greatly appreciated. I hit the trigger as I was coming down a hill to a stop, then you can see the Vette pick up speed and spew out all sorts of data. The right bank seems to be below 128 most of the time, but stays relatively consistent. The left bank seems to jump around more. If there's another type of driving I can record to help shed more light, just let me know!

http://www.redneckperformance.com/Scan20060716Vette.csv

 

Averaging out my numbers for a 35 to 0 to 35 run I came to these averages:

Left O2 sensor = .51V
Right O2 sensor = .51V

Left BLM = 123
Right BLM = 123

Left Short-Term Integrator = 130
Right Short-Term Integrator = 131

I'm guessing these numbers overall are fairly close to normal and not what you'd expect to see from a failed emissions standpoint? Would it be better to do the same type of data collection at idle or at a steady cruising speed? Does this point to dead cats as the reason for my inflated CO and NOx numbers?

 

Looks like dirty or maybe even one or two weak injectors to me. The PCM picks up the rich condition, but you integrator swings too lean. I'd ohm them out hot and cold, maybe consider a few bucks on a good cleaning if they all peg around 16 to 17 ohms. Rich of course is hard on the CAT so even if you have to replace it, you might want it deadon first.

 

SunCr, you are the MAN!

I will check the injectors hot tonight when I get home (nice hour-long drive from work), then again in the morning after it cools off. Do you think the data you see from my run is bad enough to cause the failing numbers?

The reason I ask is because when I was changing the oil, I peeked inside the valve cover and noticed that I had roller rockers. That makes me wonder if I've got a different cam as well as a re-programmed computer. Maybe a higher-ratio rocker alone could cause problems? Getting something used, it's hard to say what all I've really got in there. I'd like to tear it apart over the winter and possibly do the hot LT4 cam kit, but for right now I just need to get it registered. I'm very interested in doing my own custom programming, is there a good site that describes the process? I'm finding bits and pieces, but no good comprehensive info.

 

Obviously I don't know what's been done to your car and I could be wrong about your injectors. A broken valve spring or other valve train problems can also set up a Rich/Lean condition. Dirty and/or weak injectors will cause it to fail though - and the California Air Resources Board was willing to take some action over it when it started showing up out here - ostensibly due to using MTBE as a fuel additive. GM then extended an injector warranty on everything from about '96 to 2000 for 200,000 miles (though most of the problems were seen on the 6's using poppets in lieu of a standard, solenoid activated injector). In fact, if you live out here and do have an injector problem on an otherwise stock motor, might not hurt to raise a little stink with CARB or filing a complaint with NHTSA or just railing at a factory rep. In addition, if you bought this car here, it's the seller's responsibility to get it smogged and that can't be waived by contract. You can also check the smog history at http://159.145.15.175/vehtests/pubtstqry.aspx

 

That's great info! I'm in Ohio and we don't have any type of on-line registry like that I'm aware of. I bought the car in Georgia and where the previous owner lived, emissions testing wasn't done. He thought that maybe the cats were gutted, but the temperature difference proved that there was at least SOMETHING in there. He was as helpful as he could be, but he only had the car a little over a year and wasn't really sure what all had been done to it previously. It really does start to pull hard above 4K, I don't know if that's a characteristic of the stock cam or not.

I'm going to test the injectors, put some new cats on it, and try again. Unless I find an obvious problem w/ the injectors, I'm hoping that fresh cats alone will get me a passing grade and then I can take my time dialing it in from there. I'm under the gun to spend money attempting to make repairs to keep trying before my 30 days is up! I'll keep everyone posted on my results.

 

On a hot motor, I got about 13.4 ohms on each injector. #3 was a little bit higher at 13.8. I'll re-check in the morning when it's cold, but is this within the range I should be expecting so far?

 

It's within specs which is greater than 10 ohms, but 16 to 17 ohms is what I would consider normal. Lose the resistance and it takes all oomph out of it and it ends up lean. Post what you get cold.

 

After cooling off overnight, I got readings of 12.6-12.7 ohms on all the injectors, but #3 was once again a little higher than the others at 12.9.

 

I'd say they're shot - anyone else?

 

Well, I took the day off and got the new cats installed. While I was in there, I put in new O2 sensors and a new PCV valve just for good measure. Can't notice a bit of change in driving, but check out these new numbers:

HC = 10
CO = 0.00!!!
NOx = 396

Apparently the original cats were shot. I looked like when I saw that I'd be spending a grand for a pair of GM cats, and that was through some of the popular discount dealers on-line! I had some gift cards from Summit Racing about to expire and went w/ the Catco units for $335 out the door.

But even though I passed E-Check with flying colors, I would still like to get this thing tuned as perfectly as I can, especially if maybe a problem upstream caused the original cats to die in the first place. Plus it's good to know as I dig into the wealth of knowledge out there for the cam change I hope to do over the winter.

Thanks for all the suggestions. Any more thoughts on the condition of those injectors?

 

Great news! But I'll tell ya' one thing - those NOx numbers are only ~30 points below failing in CA, so something is still "off"...

 

Wow, our NOx limit in Ohio is 821! Although I guess I should be happy that I've got one of the few vehicles here in Bubbaville that, even if close, would pass CA emissions!!!

With the HC so low and CO non-existent, what else could cause high NOx? Cam? Weak injectors? Re-programmed chip? Those are the 3 areas I'm going to focus on next. Over half the battle now is trying to figure out what exactly I have. I thought about taking it to the dyno as the easiest/quickest way to see how close I am to a stock setup. This winter I can tear it down when we've got a foot of snow outside, but right now is prime driving time, plus it runs so good!!!