Lil' Wheezer: Fun with a Siezed Volvo Airbox Damper

Emissions controls on cars are great things, they keep cars from emitting vast quantities of pollutants that make breathing a miserable experience. They are also quite awful, since they add complexity to a vehicle, decreasing reliability, and, in some scenarios cause emissions to be worse than no controls at all.

Take, for example, the air damper in B230 and Volvo. The damper moves back and forth to control air coming from outside the car or from over the exhaust manifold, thus keeping the air entering the engine above ~55 degrees F but below whatever the exhaust temperature is. This is for several reasons. First, the computer apparently lacks an in air temperature correction, meaning that temperature extremes lead to inaccuracy in airflow estimations, which in turn, result in lean or rich running conditions (hurting economy and also making for worse emissions if not controlled). Second, without warmed air, ice can form in the intake under cold, humid conditions, causing intermittent failures. The downside to using a simple wax motor thermostat is that there is no indication if it fails, and, if it fails in the hotair only mode, it ends up making emissions much worse and will eventually kill the mass flow controller with the extremely hot air from over the exhaust manifold.

Like a lot of other people, I'd been wondering about some stuff my volvo 740 was doing. The fuel economy has been off since day one, and I noticed that the car got really really lazy when it was hot, and it tended to start to stumble a bit in traffic jams on warm days. Also, I was annoyed that it just barely passed emissions, with hydrocarbons being just a few ppm below failing.... So I decided to take apart the air box and test the thermostat.... It certainly didn't seem like it was working.

Getting the air box out was a bit of a chore. It's held into to the engine bay with a bunch of big silicon rubber grommets. They don't like to let go. Then the damper assembly is jammed into the side of the airbox, and requires some careful prying with screwdrivers to get some figures to unlock, then the friction between the conical assembly and the airbox housing must be overcome, all without snapping any 25 year old plastic. Fun times.


Freed of the housing at last, the thermostat was frozen at about 20% cold, 80% hot.

I dug through the parts drawer and found an equivalent shaped bolt and installed it in the airbox, and enjoyed a summer of powerful and smooth motoring.

The thermostats are pretty cheap. As I write this, there's one inbound from IPD volvo.

I'm considering putting a cheap mechanical thermometer in the airbox so I can check it more easily