TMWTP

Tried to search, no luck. Just bought a new-to-me '91 from another member, and it's having issues passing CO smog. Car runs and drives great, everything seems to work. Hydrocarbons are barely over the limit, and NOx is pretty high. Today we tried Seafoam and BG44k, with no luck. Referee said it was due to an aftermarket exhaust with an ineffective cat. After speaking with the previous owner, and reading through the records (going back to '92), there is no evidence that the cat is aftermarket. Has anyone else dealt with this, and what was the ultimate fix? The car will go in to 3Zero3 Motorsports next week for porfessional help if it doesn't get figured out by then.

matt33

Pretty easy to tell if the cat on there is stock or if its a sports cat (search for some photos/parts, it will be obvious when you see how different they typically look). If you are running a stock cat, its possible it is worn out, they break down over time.

Getting the car very hot right before the test can also help. I have passed smog in CA with a 200 cell sports cat (barely) and and also stock cats. Have never passed with a 100 cell cat. Records wont necessarily show a cat or secondary muffler change as its so easy to do most ppl do it themselves.

There are other things that can impact the sniff test - air/fuel mix/etc. Once in that territory, unless you know what you are doing, you really want a shop with 930/964 experience.

Matty

urquattro20Vt

TMWTP

Thanks for the offer. I may take you up on that. Let me know when the next meet up is.

ralt

I would guess its not the cat. I dont have a cat and passed emissions here in memphis several years back before they ended testing. Get a good mechanic to go over motor. have valves been done recently?

matt33

I don't believe there is any way you could (legitimately) pass in CA without a cat. (possible that the standards are/were very different between CA and Memphis?)

100 cell, 200 cell and stock cat on the same car yields very different results (personal experience back-to-back).

That isnt to say the cat is the problem.. but if your cat is gone, you probably have no chance (assuming CO test standards are similar to CA... which they may not be).

If the oil is reasonably fresh, O2 sensor ok, AFR confirmed and the engine HOT when you do the test.. I would definitely take Brandon up on the offer to borrow a known good cat. If nothing else you will eliminate that as the cause.

Matty

ralt

I guess the really good news is that I dont live in California!!!

matt33

And the really bad news is that I do!!!

It's getting out of control here and effectively forcing a lot of cars off the road because they become un-smoggable. Some of the smog stations even enforce the visual inspection meaning you fail with aftermarket components even if the emissions are fine (cannot understand the logic of this!). Drives a premium for <1975 911's to build track cars as they are exempt.

Metal Guru

A lean mixture will cause high nox. Check your mixture; you want to be at stoic (afr of 14.7 to 1) at idle and steady-state cruise.

jeff91C2T

Is your o2 sensor plugged in? If it is, is it actually running closed loop?

Regarding the need for a cat, I've been running without one for 20 years and it gets by with outstanding results (well below the requirements). I've had an issue once and it was because the lambda control loop was not working. Getting this straightened out is a much longer post...


​​​​​​

urquattro20Vt

TMWTP

Thanks to urquattro20vt, I got the necessary equipment to pass. Thanks for all the input.

matt33

Congrats on getting through it...

What was the issue and solution?

TMWTP

Cat