So, I moved from Corpus Christi, TX (no emissions testing) to Houston, TX (they test emissions here) and what do you know, everything passes except the scam-of-your-life emissions test. NO2 is too high, they said, and offered to run a diagnostic test for an additional $50 (scam, scam, scam). So I ask if the diagnostics test will pinpoint my problem. "Naw", he says, it'll just show the "area of concern" with a '91.

My '91 does not have an O2 sensor that I can find. I've looked on the CAT conv. I did see something that looked like one right next to the oil filter but I'm not sure if it's an O2 or temp sensor. Also, it does not have an EGR valve.

Questions:
1. What causes the NO to rise?
2. What can I do about it?
3. Is there anything I can buy chemically, to temporarily improve the results?

One other thing, I've read in a search that I can retard the timing but I'll be darned if I can find a way to do it. Any advice here?

 

NOx is usually an indication of a high temperature in the conbustion chamber. It can be caused by advanced timing, carbon build-up, old rusty water in cooling system, a bad air themostat, etc. The first thing to do is check the tune-up components, plugs, rotor, cap, timing. A fuel system treatment (and some WOT driving) might help if you have carbon build-up.

In the old days, they would install a "NOx kit" that was simply a plug over the vaccuum advance to lower the timing.

 

every US market volvo since the late 70s at least has had an O2 sensor. it should be on the top front end of the cat on the left side. Volvo calls it a Lambda Sond in their drawings.

I'm curious why you think emissions testing is a scam? Willfully pumping poisonous fumes into the air is just plain wrong and evil. Multiplied by the millions of cars in a large metropolitan area, it is the recipe for smog.

 

Pierce, thanks for the info. To answer your question, first let me say I'm with you regarding the "poisonous fumes". It's the "willful" part I'm disagreeing with. Make no mistake, we are not willfully pumping anything into the air. We are merely utilizing what the Good Lord (or mother earth as some folks say) has given us. It's not my fault that we end up spewing all this stuff into the air but what REALLY ticks me off is how we here in Houston are forced to deal with this crap. It is nothing more than a money making scam. For example, I failed the inspection after it "had" to be put on a dyno because this is the "only way" to check a pre-1996 Volvo. Then this assenine tech says to me "it failed NOx but we can run a diagnostic on it for $50. IMHO, when they fail it, they should be REQUIRED to tell you what's wrong with it. I know other states you can get the emissions checked as often as you want, but here in Houston, you get one more chance to get it right or you get to pay for the inspection a second time. AKA SCAM in my book.

Another example; When you just can't get your car to pass emissions, ultimately you'll "trade" it in for a "newer, more reliable, more up-to-date auto". Do you know what happens to your old car? The mexicans bid on them and they get them REAL cheap because nobody wants a car that won't pass emissions. What we see down here are vehicle "trains". We call them tijuana trains. They are nothing more than one car pulling two more cars, and they are headed to Mexico. So what did we accomplish by refusing to allow your old car on our highways? They got sent to Mexico. Here's the ironic part...guess which way they wind is blowing all that smog...right into the good old U.S.A. SCAM, SCAM, SCAM.

 

Pierce, BTW I found my O2 sensor...I think. As I was talking with the Volvo Houston parts guy he was explaining that according to my VIN, mine should be located on the manifold (which I searched for all over). Anyway, as he was speaking to me, the sun came out and shone directly thru the two heat guards covering the manifold and I could see a wire laying in the bottom. I think that's it. I cannot find one on or near the CAT. I'll remove the guard Friday and let you know.

I'm curious though, I've been hearing alot about "heat" being the culprit. Do you think switching to a colder spark plug might help? Geez, come to think of it, I don't even know if the correct plugs are in this baby.

 

around here in smog-test happy California, if I have a car that I suspect is going to have a problem passing smog, I bring it to one of several auto shops I trust, they have their OWN smog test equiepment, and will pre-test it without hooking up to the state system, and will repair any problems.

the volvo 240 we've owned since new in 1987, with about 400K miles, had trouble passing smog 3 years ago and had to be driven hard til it was really really hot then tested immediately... so last year (we get tested biannually now), my mechanic who had recommended a new cat, yeah, 25 years and 400K miles, sure, stick a new cat and O2 sensor in there, its due..... yup, passed with flying colors. CO and NoX numbers were near zero.

pre-OBD-II cars here, they do a thorough visual inspection of all the smog systems, they check the gas tank seal and carbon vapor filter works by putting a special gas cap on thats connected to a vacuum pump and pressure tester, then they stick a wand up the tailpipe and run the engine for several minutes at idle, and several more minutes at 2500 rpm(I think?)... enhanced smog areas like the LA Basin, and much of the SF Bay Area, they use the dynometer treadmill testers on everybody, and 'drive' the car at 25 and 45mph for the full test with a wand up its butt measuring exhaust gasses. If your car has OBD-II (1996+), you have to be sure to drive at least 50 miles after the last time you did an OBD-II code clear for the 'monitor' registers to show valid data before you bring it in.

if they didn't do this testing, too many fools would be hacking off their converters, bypassing EGR, and basically doing any hack they could to get their old smoker running, and LA would look like it did in the early 1970s when the average visibility was around 1/4 mile of toxic yellow smog nearly daily.


anyways, if your 1991 740 is the turbo, your exhaust down pipe is #12 in this diagram, and the O2 sensor is screwed into the hole #23 on the cat (#22)


if you have a 1991 740 non-turbo, your downpipe looks like #1 in that diagram, but the O2 sensor should still in the same #23 on the cat per a green-book diagram I don't have online.