Surgiblade's 88 S4 failed the CA smog test last week after passing 2 years ago, with the failure being very high NO (1300 PPM, limit 772). HC and CO were good, in fact very good, far better than mine.

So, to me, that indicated a lean condition, such as an air leak, perhaps bad O2 sensor. Today he came over for a look-see. We checked out all the vacuum and breather lines - all good. Looked at the plugs to make sure they were the right heat range and gapped OK - all good. The car ran strangely, like a miss but not real definite. We decided to install the new O2 sensor Surgiblade brought with him, since it had never been changed.

After that, the car ran with the same sorta miss. So, we pulled spark plug cables one at a time. When we got to cylinder 1 - no change. A plug not firing should give high HC, so it occurred to me to check the injector to cylinder 1. Sure enough, the wire clip on the plug was missing and the plug was loose. The injector was not firing. Borrowed a wire clip from a Ford injector plug. The Porsche clips are held on by a dab of melted plastic - that often fractures and the clip slips off. The Ford's wire clip is bent on the tips so it cannot slide off like the Porsche ones do. See drawing at the end of the post.

Anyway, until I found it, an unclipped injector hadn't occurred to me as a possible cause of high NO. But it makes perfect sense as that cylinder was pumping air but no fuel. We'll see what the re-test shows tomorrow.

The miss sounded odd to me. Is this because it was not the more typical plug miss where unburnt fuel may create a small "pop" in the exhaust? Anyway, I think we found the problem. Surgiblade says the car has more power now too.

Porsche clip.......Ford clip

 

Nice catch Bill - locked that one away in my brain.

 

Not so sure that would cause high Nox Bill, he has been pumping "cool" air thru that cylinder and out the exhaust. The O2 sensor would actually see this as a lean condition if it even picked it up. This would result in the richening of the mixture if anything. Nox is usually a result of hight combustion chamber temps & I don't believe a non-functioning injector would cause this.

 

Bill
It does seem odd that a non firing injector would cause high Nox.....since Nox is only made from high temps during combustion....which typically is an over-lean condition....or really hot plugs....maybe the O2 sensor + the injector together might do it......but with excellent HC......that means all the fuel is burnt...which also makes sense on the NOx....I dunno?

 

Make sure that the air pump is working, and that the air is getting to the cat.

 

Isn't the EGR for nox?

Disconnected injector, no telling how it confused things. Does the engine computer have a o2 limit it can correct before going into some limp home mode?

 

High NO is usually high combustion temps or lean running as noted, also excessive timing advance. If he's running junk fuel the EZK may not be able to retard the spark enough to compensate.

 

No EGR on S4. Pre-'80 only, IIRC.

 

OK, so the injector is not a likely explanation. The air pump is working and hooked up.

This quote got me thinking extra air could do this...
"The catalyst needs a reducing atmosphere to strip the oxygen from the
nitrogen (NOx to N2) and use it to combine with CO (CO to CO2). If there is
excess oxygen in the catalyst, the NOx to N2 conversion does not take place."

 

Bill,
That may be it, for with one injector not firing, there is an extra 12.5% cool air passing through to the cat .... the first half of which is the reduction zone. As 20% of that dead cylinders vol is O2, that combined with the temperature drop may effectively quench the NOx conversion.
The tail end oxidation zone of the cat ( a redox device) is unaffected.

 

Thank you, Garth.

 

If the o2 sensor is working properly, the ECU will compensate for this. As I said above, this will cause a 'lean' condition and the ecu will richen the mixture to compensate. That added fuel will heat up the cat, believe me.

 

Yes, I mostly agree - and believe you .... the false lean will be compensated for by the O2 sensor response - to the extent of its compensation range: also agree that this extra enrichment will be oxidized by a good working cat, which will further heat it; however, this scenario will cause the back half of the cat to heat in deference to the inlet ... that being the oxidation zone.
My speculation was focused of the cool charge from the "air only" cylinder thermally quenching the NOx conversion in the front half of the cat - where reduction occurs: this reaction is very temp sensitive IIRC.

 

Well, had we not changed the O2 sensor too, we'd get a more definitive answer to this. The car was running about as well as it could on 7 cylinders, so I think the O2 sensor was OK. Let's see if the NO comes down on re-test.

 

Bill:

I am surprised that you didn't notice one sparkplug out of character with the rest when you gaped the plugs? I am guessing that this issue was intermittent.

Good luck on your retest.

Ken