924Board.org

[cessnaflyr]

Hello Gents-

I have an 1987 924S - fun car- love it but failed the emissions test yesterday. Had two tests run both failed NOx. I'm off by about 50 points for passing. Failed only on the 15 mph test- 25 mph was ok.

Went through and added a fuel system cleaner and drove the hell out of it. Saw the 25mph NOx raise (I assume from burning the fuel cleaner) on the second test.

Looking for advice- since it seems that the the 924s doesn't have an EGR, would plugs, wires, cap and rotor along with an air filter achieve any difference? The car is running great, except when she's cold and I generally have to keep my foot on the gas until it's warm or the car dies out.

Any tips or suggestions would be most helpful. Thanks in advance for your help.

[gohim]

The Dyno Test is hard to pass, and the technician actually has some control over whether your car does or not.

The technician can actually change the emissions content by the RPM that he runns your engine at during the Dyno tests. For instance, if you engine is running lean at higher RPM, he can select a lower gear to run the car in during the Dyno run. This can hold the ignition advance down, and help lower Nox. If your HCs are high, most cars are more efficient at higher RPM, so the technician could run the engine at a higher RPM (higher gear) in the Dyno tests to make the engine more efficient, increase the ignition advance and lower HCs.

Excessive Nox comes from normally comes from mixture too lean, or too much timing advance.

Since timing is controlled by the brain box and flywheel sensors, you can't retard the ignition timing to lower Nox (unless you are running an aftermarket chip with an aggressive ignition map).

It is harder to make minor changes to affect the emissions output in a 944 than a 924, because of the flywheel sensors and the way the fuel mixture is controlled. But there are some things you can try.

What do your HCs look like? I am guessing that they are below the maximum limitation, and you could increase fuel and raise HCs to lower Nox.

Replacing the O2 sensor may help if the O2 sensor has failed and gone rich, but when they fail, (from my experience) they usually go lean, and the brain box richens the fuel mixture to max to try to compensate, making the mixture too rich.

Colder range spark plugs. Are you running standard heat range spark plugs, or hotter plugs for some reason? What plug gap are you running and how many electrodes? Maybe if you are running the two or four electrode plugs you could switch to colder heat range, conventional single electrode plugs for testing and experiment with the plug gap.

Widening the spark plug gap should retard the ignition timing slightly. I haven't tried this before, and I don't know how much timing retard you will get, or if it will help, but theoritically it could help.

Premium Gas is more resistant to ignition/pre-ignition (knocking). You might gain a little by running two or three tanks of a good grade of premium fuel to clean out lower grade fuel, and during your next emissions test.

The important thing here is to not get branded as a Gross Polluter, which could mean a Smog Check every year at a Test Only Station, instead of every two years. Sounds like you are real close to passing, and a minor tweek (or two or three) will get you there. Choose a Test Station that will run a "pre-test", and not go straight to the real test, so Sacramento will not know that your car has failed an emissions test. You may get charged for a pre-test, but that is better than Big Brother knowing what you are doing.

Good Luck

[cessnaflyr]

Here's the failed test:



I'm thinking the O2 sesnor is the culprit. Here was the past test in 11/04




Maybe vacuum? I'm a little stumped.

[sequential]

REPLACE THE OXY SENSOR !
_________________
928 gts prototype
baby blue engine block
steam in 1,2,3,4 sometimes
cold star issues while on stands
112 whp with new 4 valve head and MIS 2

[gohim]

Your HCs are already higher than I would like them to be, and it looks like the only thing that has gone up since the last test was the Nox.

That tells me that your exhaust temps are up. You need to reduce the combustion chamber temps.

Like I wrote before, retard ignition somehow (premium gas?). And maybe a new cat. A new cat would allow you richen the mixture (lowering the exhaust temp)without them coming out the tailpipe and cause you to fail for HCs.