cb951

My '86 951 just failed inspection for the first time ever for high NO. It had been sitting for about a month (driven only sporadically for the last year). It was up to operating temp, but just barely. Also a very humid, rainy, cool day.

Car is completely stock with the exception of a 5-angle head rebuild after the head warped just enough to let coolant into the oil. (Rod bearings changed then also.) Stock chips and original cat. Vacuum lines also inspected/replaced. All of this was done about a year/1000 miles ago.

Here are my readings (rolling dyno at about 15mph):
NO-- std:1323 reading: 2085
HC-- std:173 reading: 133
CO-- std: 0.98 reading: 0.52
CO2-- 14.5
O2-- 0.5
rpm-- 1388


From reading various RL posts, I suspect the O2 sensor, a vacuum leak, or possible cat. The car runs very well, though. Very smooth. No apparent loss of power at all. I do have a Fabspeed 3" catback, cat bypass, and high-flow cat ready to install, but I wanted to pass inspection before it goes on (with the bypass).

Any ideas?

Techno Duck

Someone might want to correct me if i am wrong, but i am pretty sure high nitrogen oxide levels are because you have no cat on the car. Put the cat back on and it should pass without issue.

You could richen your AFR a little as you have some room to play with on the hydro carbon levels..but i think its easier to just put the cat on!

JerryW

Had the same issue on my S2 -- High NOx. Ended up having to put a new cat on it.

reno808

chris why do you just pay extra to you pass.

cb951

The original cat is still on the car (never removed).

marky522

High NOx is caused when combustion temps get to high, those numbers seem REAL high. It could be 02 sensor running the car lean, Cat not taking care of the NOx (are these 3 way cats???) Or like you said a Vac leak causeing a lean condition. I would start with the easiest, check for vac leaks, Smoke machine works best for me. How old is the 02 sensor?? Remeber according to Porsche the 02 sensors on these cars are a 30k service item...

Mark

cb951

I replaced the original O2 sensor around '95, about 30k miles ago. Might be time for another about now anyway.

nickg

if you addlike 5 gallons of e85 to the tank it will fix the nox problem...

Funn944

high internal cylinder temp !!

nickg

which is why if you run some e85, it will pass. the alcohol will burn cooler.

cb951

After doing a little more research online, it seems possible that my recent (1k miles ago) head gasket failure (coolant in oil) might have allowed minute amounts of coolant into the #4 combustion chamber, which may have led to O2 sensor and/or cat failure.

Does that seem possible?

kdjones2000

A master mechanic told me that the only way to fix my NOX problem when I failed emissions was to replace the catalytic converter. Did that, and it has passed everytime since.

They can't sell you a used cat, unfortunately (At least in AZ). Get one off of here, and be happy you saved $1k.

Good luck,

Keith

marky522

Your NOx problem was caused by your cat, but that isnt the only thing that causes NOx to be high.

Mark

tk's944

I have a cat for sale, if you are interested send a pm.

Mike951

I happened to see your post this morning. The whole emission testing thing in NJ sux when you start running into problems like we are having. I decided to do the following:

1. Tear apart intake system and inspect for failures leading to vacuum leaks. One car I had ended up with a hole burned in the J-boot

2. Change O2 sensor......still trying to find out if an O2 from an '86 Turbo will fit and work in my 1988 Turbo S

3. Replace the cat...PITA

Keep in touch...my car has 68k original miles!!!