1996 850 smog failure high NOx

tomstir · Post by **tomstir** » 07 Jun 2009, 16:49

Hey gang....this is my first post....I have been reading info here for the last few years and it has helped out a lot...but I have now run into a problem that I cannot figure out....smog failure!!!!

In February I gave the car a tune up and in May went in for a smog test. The car failed with high no of 1793 at 15mph and 2246 at 25mph.....4 times the gross pulluter limit. I took the readings to the mechanics at my work (trucking co. with smog tech) and the two that I talked to said that the catalytic converter in those cars have three chambers and one of them is for the no...and that section has failed. I replaced the cat and took the car back and it still failed..the numbers are a bit lower...1539 and 2612...still way too high. The other numbers are well below the limit.

I searched for cracked/broken vacuum lines and even sprayed carb cleaner all over the engine looking for vacuum leaks. The car runs great and there aren't any cel's...the coolant temp is right in the middle where it has always been....I am running out of ideas/suggestions and am hoping someone can shed some light on my problem.

thanks
tomstir

Post by **misha** » 07 Jun 2009, 17:15

When you said tune-up...did you meant on performance tune-up?If you did,i think that's the problem.

Post by **vegasjetskier** » 07 Jun 2009, 17:23

Getting any fault codes?

tomstir · Post by **tomstir** » 08 Jun 2009, 04:10

Thanks for the replies. No codes.....or at least no cel. I don't know what a "performace" tune up is. Could you explain what that is and how it could effect smog........I changed the cap,rotor,plugs/wires, cleaned out the flame trap, flushed the cooling system and changed the thermostat.

One odd thing that did happen a few months ago was the car didn't start on the first crank like it usually does....kinda like you turned the key and as soon as it fired you turned it off.....and a cel came on...I drove the car home and the nest day disconnected the battery and the light never came back on. Do you think that could be something?

I have read that there are some vacuum lines that are very difficult to see that may be cracked...does anyone have a description of where these are?

Tha car seems to run fine and the fuel mileage is on par with what I have seen in the three years I have owned it

Any more ideas?

thanks

Post by **vegasjetskier** » 08 Jun 2009, 09:07

There are several things that can cause high nox emissions. The general cause is that the combustion temperature is too high.

http://www.discountconverter.com/cataly ... r-&News=98

If your car has it, you might want to check that the hot air diverter valve (not sure if this is the right name, the 850 guys would know) is not stuck in the hot position. EDIT - It's called the airbox thermostat - here's a thread on it: https://www.matthewsvolvosite.com/forums ... f=1&t=8830 These fail all the time and most people just glue the door in the cold position.

You could throw new parts at it, but you would probably be better off taking it to a smog shop that can diagnose it properly even if you have to pay them a few bucks. They can narrow it down to what's really wrong so you don't pay for a lot of parts you don't need.

Post by **Brucebo** » 08 Jun 2009, 10:42

You may also want to check your Oxygen sensor. You may have out of wack fuel trims.

Hi NOx is causes by high combustion chamber temperatures, and/or knocking. One way to cool things off, at least for a smog test, is to use 100+ octane unleaded "racing gas", which burns more slowly, mixed with 20% by volume denatured ethanol (from Home Depot) (which raises octane even more, and is "oxygenated" so it has less energy per volume and also reduces HC). I tried this on my '89 Jeep Cherokee, with no other modifications, and it reduced my NOx by 75%.

-B

Post by **misha** » 08 Jun 2009, 17:27

tomstir wrote: I don't know what a "performace" tune up is. Could you explain what that is and how it could effect smog........I changed the cap,rotor,plugs/wires, cleaned out the flame trap, flushed the cooling system and changed the thermostat.

That shouldn't be a problem.
All of that you replaced...cap,rotor,plugs,wires....is called tune up.
Performance tune up would be...performance ecu,higher capacity injectors,k&n air filter,performance fuel pump...and all of this would affect smog test because the engine with performance tune up runs richer than normal.

tomstir · Post by **tomstir** » 11 Jun 2009, 04:05

Vegasjetskier---I have riveted the airbox thermostat closed...fresh air only.

Brucebo---lowering the value by 75% might just get me to pass but I would like to fix the problem for good.....if I did what you suggested are there any negative side effects? I would hate to pass smog and continue to drive "as is" and make whatever problem I have worse.

misha---thanks for the definition.

Anyone have any ideas on vacuum leaks...maybe I missed something in my "visual inspection". Would you guys think that using propane to try to find a vacuum leak would be better than carb cleaner? I have a tank for the bbq and could just lug that out to the car and try to blow on some vacuum lines. Are there any "common" lines that leak?

How about testing the o2 sensor? Anyone ever done that? If so how was it performed?

thanks guys.....keep the suggestions coming
tomstir

drsugg · Post by **drsugg** » 11 Jun 2009, 08:18

It would help to know the rest of the readings. High no is a result of high combustion temperatures. One possibility is a head that has been milled to much. Excessive carbon deposits are another. The other readings would really help a lot, but since you did not post them, I will assume the were a pass. If it were in my shop, I would be verifying cam/crank time as it is something you can see and would effect readings. Fuel pressure is another item to get scratched from the list too. The mass is a possibility, checking readings with it unplugged may eliminate it. Look forward to more info from you.

Post by **misha** » 11 Jun 2009, 16:52

tomstir wrote:....Anyone have any ideas on vacuum leaks...maybe I missed something in my "visual inspection". Would you guys think that using propane to try to find a vacuum leak would be better than carb cleaner?Are there any "common" lines that leak?

Check the one at the intake manifold(passenger side).It's about between power steering and intake manifold,and check the vacuum tree...they are "common".
The one at intake manifold could be tricky...could look ok,but not be ok,i replaced mine about a month ago.
You can try both...propane or carb cleaner...both will point you to your leak(if you have it).