safulop

When I bought my S4 in 2010 it nearly failed smog because of hydrocarbons. I replaced the O2 sensor which had gone soft, and was running a bit rich.

A recent smog check almost failed again, this time both the hydrocarbons and NOx are too high. Is this my Cat? It is expensive and I want to be sure before I spend the big bucks on a rebuilt Cat.

-Sean

dr bob

Sean--

High HC is caused by misfire (raw gas passes through without burning). High NOx is caused by lean mixture. The two symptoms together point to dirty injectors. You can replace them (sure-fire solution), try to clean them in place yourself, or find a local place tat can do a BG or similar force-fed cleaning in place.

I think Roger has the 19# injectors in sets, with the correct bottom o-rings for the S4. His prices are competitive with the open market for those parts, and you are sure to get the right stuff from our 928 vendors.

There are services like Witchunter that will clean your injectors for you. There are threads on that service if you search here a bit. I threatened to build my own cleaning fixture but haven't gotten around to it yet. My old injectors are on the bench, staring at me, daring me to find a solution other than tossing them in the original-parts bin still dirty. If I do that, they will be inop by th etime they might be needed again in another 80k or so.

Ed Scherer

E.g.,

• WitchHunter Performance
• Fuel injector cleaning service in Pacific Northwest

safulop

Bob are you sure? This car just had a full inspection at my local German car shop, and they didn't detect any misfiring or hot Cat from unburned fuel. It idles smoothly, runs strongly, there is no detectable symptom in the running order. E.g. it does not bog, hesitate, or flood. Mechanics I've talked with here both at the Porsche shop and at smog shops are saying these are both bad CAT symptoms, especially the HC, but NOx is also cleaned by the Cat. I'm not against cleaning the injectors, but the engine is running so well it seems strange.

-Sean

safulop

Also, looking over the PO's records, it looks like he replaced the injectors in 10/2006, the car has gone less than 25K miles since then.

-Sean

safulop

And I also see from the records that the car was almost failing smog with high HC back in 2007, so not much has changed except the NOx is increasing. Car has had intake refresh a few years ago as well, there is no indication of lean running. Certainly the mileage is poor enough, I am running about 12.5 mpg in all-city driving here in Fresno. Highway mileage is 20-21 mpg.

Lizard928

MAF?

outbackgeorgia

Wow,
That's really bad mileage for an S4 in my opinion. I just measured 18 city, over 24 highway. I do have a 5 speed, and my commute of 42 miles daily is pretty slow, but I suspect there is something wrong with yours. Seat of the pants "running strongly" isn't an indicator of a properly tuned engine.
Dave

Speedtoys

On 2.54 gears, solid smog numbers, I'm 18average, 16 up and down 3500 of mountain roads 2x a day, and 22hwy.

dr bob

Sean--

I'm not "sure" of anything, just playing your symptoms. Might be as easy as needing plugs/caps/rotors/wires. Could also be an air lek. Or a weak MAF.

safulop

I've had it analyzed more than once since 2010 at a local shop with long 928 expertise. And it has been tuned up and worked on more than once. I'm not just going with my gut here. Opinions vary about S4 mileage, Dr. Bob advised that 12.5 mpg is to be expected in heavy city driving with all short trips. 21 mpg was achieved at 80 mph on the interstate, but at a higher elevation so less fuel is used. It seems within normal parameters. 18 mpg city sounds impossible for an auto S4, the EPA rating is 15 and that's very optimistic.

safulop

MAF has been rebuilt in the recent past as well. Maybe a tune-up would be a good place to start. You mention an "air leak", where would that be? Intake?

Speedtoys

Build the MAF replacement air leak tool, I did. Gave it to bill ball to help others with.

safulop

Qu'est-ce que c'est?

safulop

And what's the story with "high-flow" Cat? If I'm going to replace the Cat one of these days, might as well make improvements.

-Sean