hey everyone,

car is running great (740 gle), no problems, except on emissions, i fail miserably on NOx, but HC and CO are good.

this is what i've done:

replaced cat
replaced catback exhaust (mufflers, crossover pipe, etc.)
replaced manifold to downpipe gasket
checked timing @ 10°
replaced pickup sensor in distributer


i figure if there was a problem with the FPR, i'd be high on HC. and if the o2 sensor was bad, my HC would really be high. my only other guess is the AMM, but i wanted some input. i've done all the basic maintenance before the emissions test, plugs, cap/rotor, oil change, air filter. only other thing is i have a small exhaust leak that i can't pinpoint. i suspect it's a crack in the manifold, but am not that concerned as it is very small.

any input is greatly appreciated.

Pull the vaccum line off of the FPR. See if any fuel comes out. Mine went bad. It was causing the engine to run rich. The O2 sensor was trying to compensate. That ruined the cat. Whatever the problem is, it will need to be found. So two other parts parts went bad on me because of the FPR. When I got that fixed, she runs like a charm. My wife drives it to two jobs.

yeah, i already checked the fpr vacuum hose, thinking it might be bad, but no fuel came out.

I hear you about FPR and O2 sensor not being the fault but why not look at your O2 output anyway. There is actually an LH2.2 test point connector along the passenger fender that can be monitored with an LED or any meter. When it flashes (goes alternately from 12V to ground) in a steady manner you have in theory achieved Stoicometric perfection. Of course you can also just look for the regular O2 sensoe output swings towards .1V and .9V with a digital meter. When I had my 88 740 and my daughter had an 87 240 it was my habit after a tune up or any work to tweek the MAF adjustment screw to achieve this. There is some info in the FAQ on the subject.

If the fuel mixture as measured by the O2 sensor is in line then maybe the NOX will come in line too. Just my thoughts.

I find it hard to believe that a minor (or major) exhaust leak can affect emission measurements at the arse. Then again maybe air is sucked into the exhaust stream in between exhaust strokes thereby upsetting the whole O2 sensor and cat chemical process, but that is just me rambling on.
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David Hunter

yeah, i was thinking about checking to see how the o2 sensor is doing. i also did think that if the manifold is sucking in air, it's throwing off the o2 sensor, and having the ecu send more fuel. BUT, i get pretty damn good gas mileage ($30 for 300km's), so i doubt i'm running rich.

i'll check the o2 sensor output and maybe grab another sensor.

thanks

I know nothing about the chemistry of combustion but seem to remember that lean running can cause NOX problem. Checking output of O2 sensor may reveal that.
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David Hunter

You can find the exhaust leak by attaching an air source e.g. shop vac or leaf blower to your tail pipe and back pressuring your exhaust. Attach the air source loosely so you don't pressurize it too much. Then squirt soapy water where ever there might be a leak. Don't be surprised if your find many tiny leaks you didn't suspect.

I don't think these small leaks have much of an effect on NOx. Now, you could have a leak that is actually sucking air in which will drive your O2 sensor crazy.

Recently, I had a high NOx problem on a '93 Nissan Maxima. After spending several hundred bucks at the so called "professional" shops with no joy, I took it to a backstreet smog shop where a mechanic tuned it while on the dyno. It came down to a simple timing issue. He suggested that my timing belt may be getting ready to go.
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this is a signature

Just a heads up im assuming your in California trying to pass the Smog Test, since your profile doesnt say, the Smog Checker is required to check your Timeing during the test, did you watch what they were doing?

Hi, if you failed because of high oxide of nitrogen (nox) the cause would be any of the ff:

*overheated engine
*lean air fuel ratio
*vacuum leak
*overadvance ignition timing
*defective EGR

Good luck

MM

a buddy of mine had this problem same thing your describing, i removed his Manifold as i was tired of him complaing only to find 2! cracks around where the turbo mounted on the inside/backside ( nearest to engine ), replacing the manifold fixed all his issues and probly saved him from a fire if he ever developed a oil leak on that side.

Howdy,

only other thing is i have a small exhaust leak that i can't pinpoint. i suspect it's a crack in the manifold, but am not that concerned as it is very small.

A cracked manifold can elevate NOx. There was a post on the Brickboard two or three years ago outlining the same problem. Everything that could be replaced was replaced and the NOx readings were still high with CO and HC within range. A cracked manifold was the problem. The crack could not be seen until the manifold was removed.

I can remember all that but I cannot remember who posted it.
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Mr. Shannon DeWolfe -- I've taken to using mister because my name misleads folks on the WWW. I am a 53 year old fat man. ;-)

yeah, i remember reading that a leak in the exhaust prior to the tailpipe can elevate the NOx readings, but didn't think it would raise them THAT much.

i tried searching, but couldn't narrow down my search with this google search addon, so i was picking up all rwd threads.

i guess i'll try replacing the manifold first. anyone know where they sell them new/aftermarket?