safulop

I bought my S4 in 2010. It has always had low city mileage (from 12-12.5 mpg) here in Fresno. It also barely passes smog due to high HC. This seems to be a fueling issue but the highway mileage is at least 20 mpg, which is spec. Folks here weighed in and we figured it was maybe due to the Autothority chips. Never really figured it out.

A few weeks back I had some work done on the HVAC system, trying to trace vacuum leaks. The only one found was a weak vacuum in the main line to the heater valve. This was fixed.

At the same time I tried to have stock chips installed to replace the Autothority, but the ones I got didn't work for some reason. Ended up having to put the same Autothority chips back in.

Since that day, the mileage has improved. We are now seeing 13-13.5 mpg in all-city conditions. I am a bit stumped. I can't wait to see the next smog check (never thought I'd say that) because I am hopeful the HC numbers will not be so high.

Can anyone tell me what happened?

-Sean

dr bob

Sure. A slight lean misfire at idle/low-load due to the air leak. The misfire gives you the high HC (it's unburned fuel passing through to the exhaust). It also gives you the poor mileage. The leak is small, and manifold pressure goes up as load increases (freeway driving) so the leak passes less air, as well it's a much smalller proportion of total air so less chance of misfire.

Funny how a small thing manifests itself in bigger ways.

safulop

Now that I think of it, it did used to have a slight stumble in the idle before. Always thought that was due to dirty injectors or something. Everything has to be perfect, or nothing is.

-Sean