Rich9928p

Most shops have a CO meter and can tell you if you're running too rich. You connect the meter to the tubes beside the air filter housing that enable "sniffing" before the cat on older S4s. The newer 928s eliminated the tubes and you need to pull the little bolts just in front of the cat to sniff the exhaust.

One of the LH failure modes is to running a rich A/F mixture. The symptoms can slowly sneek up until the car is grossly running rich and ruins the cat. The attached photo shows John Eifert creating a Nurburgring decal out of the soot from Johnny B's loaner S4 at Euro 2003. The car was running so rich the fuel economy was terrible. It later died and wouldn't run any longer in this over rich condition. My buddy Achim Mueller rebuilt that LH and the S4 is no longer polluting the air of Sweden.

heinrich

Nicole is correct. Cold cat == ZERO cleaning. And btw, try some very high-octane fuel. Like fresh air. All my 928's do as you describe.

marton

mine is an 81 so no cat but the exhaust stinks, everybody complains. I have my parking place in an underground garage with about 40 other cars - the ventilation is not so good so I am not so popular....
Hard to describe the smell, not really rotten eggs....

i have to have an exhaust gas test every year so I guess mixture issues would show up? They usually have to slightly adjust the mix and slow running every time. It always starts at the first attempt.


marton

toofast928

Try changing fuel brands first. The fuel your using may have an abundance of sulfur.
928's also run on the fat side of fuel richness. Does the exhaust burn your eyes?
As far as the O2, it isn't operational until the exhaust temp is 400f. So replacing it won't help a cold start smell.

perrys4

Thanks for all the info and input. I will be trying some of these things.

9xauto

Every old Porsche stinks up to the 90's, because older models were not designed to use detergent-gasoline with ethanol. Modern catalytic converter could help a little, but not that much. In reality the exhaust must be recirculated more like in Prius and other "Green" models.

jeff spahn

Nice job bringing an 11 year old thread back from the dead.

dr bob

Jeff, we are talking about your car here. Does it still do this?


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K has made the same comment about my car, mostly because she doesn't stand behind here own car (4Runner) when she starts it cold. Regardless of make, catalyst cars need a certain bit of running to get the cats up to temperature before they start really polishing the fumes. More modern cats and engine management systems seem to do a slightly better job of this.

This also guides the manufacturers who tell us to immediately drive off (read: get it out of the garage...) after the engine is started rather than letting the engine come up to temperature more slowly. They want to get the cats to 'light off' sooner, important in a drive-cycle emissions test for sure.

And for those who make a decision to run without cats at all, your cars --always-- stink.

davek9

Before you do anything, what is the AFR?
If you do not know than your pissing into the wind.

FredR

Interesting to see this subject appear. I have always opined that my 928 produces more "fumes" when starting than any other car I have owned. I tune the idle for 14:1 AFR as I understand that gives the best idle and corresponds to the specified CO emission of 0.5% [?]. I have no CATS fitted and run open loop.

Whether or not this is some 928 characteristic or something more specific to my motor I do not know. My 6 litre 12 cylinder Jaguar motor was much less noticeable as are our two 6 cylinder 4x4's. I tend to think it is some wear issue causing more oil to pass through at idle but have never found any indicators to suggest a problem with seemingly excellent compression and leak down values.

It will be interesting to see if there is some kind of consensus here.

Rgds

Fred

jeff spahn

It doesn't seem to be too stinky. AFR at idle once warmed up is stoch. When first started its around 8ish but settles down in 10 seconds or so.

WallyP

No idea whether it is correct or not, but I have always associated it as (more compression)=(more stink). And, yes, different fuels smell different.

dr bob

Incorrect ignition advance is often evidenced by a different tailpipe stink too, probably for the same reasons. A little headgasket or other coolant leak offers a very sweet-smelling tailpipe cloud on initial start-up. Oil puddled around worn valve guides has a particular start-up fragrance too. For carburetor cars, getting the choke adjusted right helped a lot with start-up tailpipe smells. Many US cars were fitted with little sheet-metal "choke stoves" to pass exhaust-heated air to the intake when cold, alleviating cold driveability issues and misfire stink.

The Good News is that electronic fuel injection and ignition systems include cold engine compensation, Regardless of that, the first minute or so of cold start 928 exhaust in the garage will make your eyes water.

Magnus Rostadmo

My -87 S4 varies between instant start, and i mean instantly first piston over top - or mostly some hesitance as in 5-8 turns before I fires. Cold and warm. Have this small misses every 5-8 secs at idle. New plugs, ign-leads, rotors & caps, coils and checked all ground points. The hesitance at full boost went away after the new parts but I still think it doesn`t got the 320 hp it should have - but i haven`t any other car to compare with. And - my exhaust smells like hell even though its running lean - go figure.... Get a new LH-brain perhaps ?

9xauto

I have been fixing and tuning classic Porsches since 1997 and I noticed the smell have gotten worse. It is not even catalytic converters, it is the additives including ethanol in the gas. Plus earlier fuel injection systems where too simple and non reactive to the gasoline change. Exhaust does however has to do with timing and other factors. Modern engines have knock sensors and excellent reactive ECU's to adjust and manage engine's performance unlike the cars from the 80's and early 90s. I can tell you one thing however, as the early Boxsters and 996's rolled out so is the exhaust issue disappeared. If you open any Porsche user manual from the 80s and 90s you will find paragraphs where Porsche warns against the use of ethanol and detergents in their cars. However we not alone. Every vintage makes including Ferraris have similar issues.