As some of you may recall, I have recently replaced the head on my 2.6 Trooper with an AMC head. Its all back together, and runs beautifully. But there seem to be a couple problems.

It failed the California smog test today. The NOX was a bit high-- not too much, but enough to fail. Everything else tested within tolerance. The tech suggested that I retard the timing from 12?btdc to 9?btdc, and that that would bring the NOX down enough to pass. Anybody have any experience with this?

What causes high NOX? Might there be some other area which is the problem, and the high NOX is only the symptom?

The other problem is the idle speed. The idle speed wanders. I set the idle speed after I did the timing, and it was perfect. But after driving, when it comes back to idle, it might drop to 1200 RPM and stay there, or it might drop to 400 RPM, and stay there. Simply put, the idle speed wanders when it returns from a higher speed. That is to say, it returns to a different idle speed. If I let it sit at idle, it remains constant, though not necessarily at the speed I want. It does not return to the same idle speed two times in a row. Any ideas?

All help will be appreciated.

Ken

I ran "smog test NOX" by the wikipedia, yes it's snowing here and I'm bored stiff. Anyway there is a wealth of info there.
http://en.wikipedia.org/wiki/Automobile_emissions_control
Was that the emissions tech that told you to retard the timing? I may be speaking knowing nothing (no doubt true) but more info should help this. Does the engine have fuel injection? Doesn't sound like it with the idle speed floating all over the place. If it does have fuel injection then this is where I'd begin, no doubt the two symptoms are related.
Brian Rodgers

Oxides of Nitrogen are formed when Nitrogen gets extremely hot and bonds with oxygen. When running correctly the engine uses the EGR system to put some inert gas back into the engine to keep combustion temps down and prevent nox emissions. You might want to try the timing trick or try checking out the egr system to see if everything is intact. The engine could be running a little lean. Where did it fail? At idle, cruise or somewhere else, did they tell you?

I'd start with the EGR system too. The CA emissions rigs used a different computer, and an extra thermal switch on the EGR valve. If you have the FED version (check the ECM label) you might have to "fiddle" to get it past.

As for the idle, it's possible that the throttle body may be worn/dirty. When you had the motor apart, did you clean out the air bleed ports ? Did you check that the manual throttle stop was properly adjusted ? (that's the screw on the front of the throttle body, that _usually_ has a dab of red paint on it) You probably already know that screw isn't used for setting the idle speed, and the one on the rear side is. I imagine the TPS could play a part too...

YOU SHOULD NOT HAVE TO MESS WITH THE TIMING IF EVERYTHING ELSE IS WORKING CORRECT, RETARDING THE TIMING MAY MAKE IT PASS AND IT ALLSO CAN MAKE IT FAIL, RETARDING THE TIMING WILL CAUSE THE COMBUSTION TEMP TO GO UP ,THAT IS NOT WHAT YOU WANT TO CONTROL NOX,NOX IS PRIMARILY CAUSED BY COMBUSTION HEAT , FIRST GET THE IDOL STABLIZED , CHECK THE AIR PIPE GOING ACROSS THE VLV CVR FOR A HOLE BURNED IN THE BOTTOM SIDE ,IT WILL CAUSE THE IDOL TO BE ALL OVER THE PLACE , THEN EGR MUST BE WORKING CORRECT AND THE TPS ADJUSTMENT, IF ALL THIS IS WORKING , YOU PROB, HAVE A DEAD OR MARGINAL CAT, YOU CAN CHECK IT BUT FIRST MAKE SURE THE AIR INJ SYSTEM IS WORKING OK ALLSO, AFTER YOU GET IT SETTLED DOWN, GET YOURSELF AN INFARED THERMOMETER AND HOLD THE ENG SPEED AT 1500 RPM FOR ABOUT 2 MIN AS STEADY AS YOU CAN, KEEP IT AT THIS SPEED AND CHECK THE INLET TEMP OF THE CAT AND THE OUTLET TEMP ,THE OUTLET SHOULD BE AT LEAST 350-400 DEGREES HIGHER AT THIS POINT THAN THE INLET, IF IT IS VERY CLOSE INLET AND OUTLET REPLACE THE CONVERTOR , IT HAS THE OLD GM PANCAKE CONV, AND THEY ARE NOT KNOWN FOR LONG LIFE AFTER 125000 OR SO. , I RPLACE THEM ALL THE TIME WITH THE NEW HONEYCOMB THREE WAY CAT AND THEY NEVER FAIL TO PASS, WITH READING AS LOW AS ANY 2000 MODEL VEH. JERRY

Brian, good information. It always helps to understand how the system is supposed to work. It helps in the diagnostic process.

Chris, I agree, I think, retarding the timing might work, but its a risk. I'm pretty sure the EGR is working well, though I'll take a harder look there too.

Here are the numbers from the test. They test at two speeds, 15 and 25 mph, both at 1975-2000 rpm. The HC measures 15 and 10 (max is 121 and 101, with average 35 and 24). CO measures 0.02 at both speeds (Max is 0.70 and 0.90, with average 0.11 and 0.09). The NO measures 1046 and 935 (max is 1006 and 866, with average 332 and 274). So, its doing great on the HC and the CO, and just a bit over on the NO. But thats all it takes to fail.

The engine might be running a little lean? I thought that was controlled by the ECU. How can I adjust that? The ECU is deffinitely a CA model, btw. When I had the throttle body off I was after another objective. I didn't adjust anything or specifically clean anything. Yes, I'm adjusting the idle with the rear-facing screw.

Last night the idle seemed to be better, more consistent. Perhaps the dirt or whatever, cleaned itself out with driving. Is that possible?

Jerry, I'm always happy to see your replies. The timing was right on, 12?btdc. You're right, I shouldn't have to fiddle with it to make the Trooper pass smog, but I'm willing to do it if I know it will pass. I can always reset it afterwards. The large air duct, going from the air filter to the I-TEC unit, does have a hole in the bottom. I've plugged it up with some gorilla snot a long time ago, but I'll check that. The TPS adjustment? I'm used to doing that on my truck (F-350 diesel), and was able to do it on the 2.8 Trooper (sort of), but I have no experience on the 2.6 Trooper.

So, lets start with the basics. Where is the TPS on this engine? What is the adjustment procedure? My manual (Haynes 1641) makes no mention of the TPS that I've been able to find.

Would the Cat possibly cause the NO to be high? I replaced it once, 5-6 years ago, along with the rest of the exhaust system. Could it be that time again?

In looking back over my records, this same thing happened in 2003. It failed with high NO numbers, and passed 11 days later with average numbers. The only record I have of maintenance I have for those 11 days is replacing the O2 sensor. But that was only about 7000 miles ago. Could it need replacing again? Might I have damaged it or its wire lead during the head replacement? That O2 sensor lasted 3+ years, 20,000 miles.

Gerry, good point about the Cat. I wasn't aware that you could determine its effectiveness with a point and shoot IR unit. I see in the latest ad that PepBoys has an Actron unit at $50. That should be good enough for my purposes.

I've rambled long enough, asked more questions than I should in a single post. Still hoping for some input, though.

Ken

Jerry, I used an IR thermometer just a moment ago. I had driven the Trooper several miles, then had my wife hold the RPMs between 1500 and 2000. I took 7 readings, just to be sure. The average of the 7 is 403? at the inlet to the Cat, and 327? at the outlet. Those are measured within 2 inches of the cat.

Just to be sure, to verify, the outlet should be 350? higher than the inlet? That makes sense if the cat is burning unused gases.

Am I looking at a new cat? This one LOOKS to be in good shape. It's not even rusted. I think it's a Catco brand. These things are too expensive to be replacing every couple years. Do I need a new one again?

Thanks Jerry,

Ken

HI KEN ,THAT CERTAINLY POINTS TO A BAD CAT, THE PROBLEM WITH THE AFTERMARKET CATS , THEY DONT GENERALLY HAVE THE SAME AMOUNT OF SUBSTRATE ASSY IN THE CAN AS THE ORIGINAL, AND TWO YEARS IS ABOUT ALL I HAVE SEEN THEM LAST , UNLESS THE LAW ON AFTERMARKET CONV HAS CHANGED RECENTLY , THEY WERE 24 MO 24000 MI, AND THEY WILL GET VERY WEAK OR JUST BE DEAD , DONT BE TOO SHOCKED AFTER YOU PRICE ANOTHER ONE , THE PRICES ON CONVERTORS HAS SKY ROCKED IN RECENT MO,S , HAVE YOU CHECKED THE AIR INJ SYSTEM TO MAKE CERTAIN IT IS RUNNING AIR INTO THE EXHAUSE SYSTEM THRU THE PORTS IN THE HEAD .. JERRY