My eco-conscious image is taking a real beating since my 2006 HCH-II has recently developed a very foul exhaust odor on cold start. Under all other conditions it is sweet and pristine.


Seems like a cold-start enrichment process or component gone amok, dumping too much gas into the engine. Anyone have ideas about where to start? No error codes are showing and the car starts, runs and drives just like always, just a very big stink in the mornings.


BTW, it has 151K miles, got new plugs at 95K and has a recent good air filter. I probably should replace the O2 sensors at this mileage as normal maintenance since a bad signal from one of these could certainly cause the ECU to open the injectors too much.

 

What's the odor?

 

Unburned gasoline ... like standing behind a 1965 Corvette ...

 

Heh. Fair enough. So a running rich smell vs. raw fuel? The source is definitely the exhaust pipe and not a source of raw fuel like the evap cannister?

Did you replace the plugs with the expensive NGK/DENSO OEM style plugs ($14 EA at most parts stores)?
Did you replace all 8 plugs?
Has anything changed about the routine? How or where it's parked, etc.?

The upper O2 sensor is about $200 locally, and they generally code when they go bad. The downstream sensor is closer to $55 or so. I recommend that you ONLY use the DENSO OEM style sensors.

 

Yes, over-rich smell like back in the days of carburators. It's very recent, and only service in past couple of years has been oil changes and trans fluid. I do have a main bearing seal leak, but that wouldn't cause a smell until the engine was hot and even then not a gasoline odor.

As mentioned, it's only on cold starts and the car has no other symptoms or warning lights appearing. Driving patterns and usage are unchanged.

The Denso 'upstream' sensor at $90 seems like a reasonable first step (https://www.rockauto.com/en/catalog/...pe+spout,10653) since it's prob overdue anyway after 12 years and this many miles.


A sensor seems most likely, but I was thinking of cold start enrichment process as well. Some cars have a separate enrichment 'nozzle' to add fuel at start ... I don't know if Honda uses that method or just has the ECU vary the injector pulse width to open the injectors longer.

 

Nice. Those prices have come down substantially. Might as well replace the downstream too at $35 (DENSO 2344350).

Mine cost me $250 combined. I didn't have time to wait (mine had failed, coded and voltage output was all over the map) for Rockauto, which was about $170 at the time vs. $200 local.

Good first step.

Also probably worth checking the MAF sensor and cleaning it if needed.