Airbox thermostat failure...

Warren561 · Post by **Warren561** » 19 Oct 2009, 20:47

Finally got around to trying this mod on my 2000 S70 NA.

Surprisingly my air box intake is very different than any of the pics posted in this thread.

Instead of an air box thermostat that pushes the flap around, there's a vacuum diaphragm. I realized this only when I saw the vacuum hose attached to the "cold" (aka fresh air) side of the intake.

When vacuum is applied, the flapper closes of the "cold" side and only permits "hot" air (from the top of the exhaust manifold) to enter the air box. When the vacuum is removed (aka default), the flapper closes the "hot" side and only lets "cold" air into the air box.

The other end of the vacuum line attaches to the top (post filter) side of the air box (near the fuse box). Apparently, this is where the "thermostat" is. It regulates vacuum from the intake manifold to the flapper diaphragm. It appears that my "thermostat" is working properly, as I observed (when cold), the flapper closes the "cold" side. Then, after the air box is warmed up, the flapper closes the "hot" side.

The problem, is that the flapper's default position (no vacuum) does not fully (100%) seal off the "hot" side. It leaves it cracked open a tad.

I guess I'm going to need to figure out how to plug the vacuum line, and then somehow (glue?) the flapper so that it completely seals off the "hot" side.

Post by **tjts1** » 20 Oct 2009, 00:51

That sounds like a plan. The 98+ S70/V70 all have the system you describe. Its actually kind of an intelligent setup. There is some evidence to suggest that at low load, HOT intake air can actually improve fuel economy. Hot air is less dense so the throttle can be opened further reducing the vacuum in the manifold. At high load you want as much oxygen going as possible so cold air is the way to go. You would also need a good ECU that can take advantage of the constant switching between hot and cold air. But seeing how it no longer seals all the way I would just fix it to only breath cold air. Keep an eye on your fuel economy. I suspect you won't gain as much as 850 owners. There might even be a negative effect. On the plus side, with cold air you should be able to use 87 octane all the time.
cheers
Justin

beewerks · Post by **beewerks** » 20 Oct 2009, 05:57

Does that mean that I can run my '94 850 n/a that has had this mod done on 87 as well? I haven't heard of being able to use the cheaper fuel before. Please advise.

I plan on building a cold air intake with a KN air filter pretty soon if that makes any difference.

Post by **RobTheModd** » 27 Nov 2009, 13:48

Hey guys, not to start up an old thread again, but I experenced a little... Smoke from the *silver* pipe, Hot air pipe coming from the motor.
This is what I did,
I put the wooden dowles in to adjust the flap only to the coldside. *to allow the cold air in and not the warm air*
upon starting my car I noticed alot of smokish stuff coming out from the area where the metal pipe connects to the butterfly. (airbox)
When I removed the wooden dowles and put it all back together the smoke *while still there* was MUCH less. Soooo... what I would like to know, *as it was not there yesterday when doing my cam seal replacement* is what do you do with this silver pipe. either to reduce the smoke or just in general.

Since wouldnt the smoke just build up?

Any replys would be great as I would love to get this airbox mod done and over with as soon as possible.

Btw, it was a near clear smoke. like barely white to where its hard to spot when everything is hooked up like it is from the assm line.

Post by **Ozark Lee** » 27 Nov 2009, 14:00

My guess is that you had some oil that built up on the hot air intake pipe as a result of the seal leak. The heat will build up to a higher temperature without the airflow through the hot air pipe. If that is the case it is one of those things that will go away with time. Push comes to shove you can just take the hot air pipe off altogether - once the airbox modification is made it no longer serves any useful function.

...Lee

Post by **RobTheModd** » 27 Nov 2009, 14:29

I took the pipe off all together and let it run for a bit. Decided to take it for a ride. got in the car and it smelt like exhuast. Is this normal with this mod? I havent found anything about it. Or is this just because It was sitting for awhile and the air coming out of the engine had time to build up and go into the cab?

Also, lee, I know you have your pipe still on correct? Would that be best bet to just leave it connected so that at least it is diverting the warm air further away from the cab?
And where does the hot air go when you cap it off from the engine?

Post by **Ozark Lee** » 27 Nov 2009, 16:55

Rob,

I think you fundamentally misunderstand what is going on with the hot air intake. Not to feel bad about it since it is not obvious until you remove the heat shield. There is no exhaust that flows through the hot air intake pipe (unless you have an exhaust leak) The corrugated silver pipe simply plugs into a hole on the heat shield. As designed, the air box thermostat closes the cold air intake off and draws its air supply from the silver pipe that is drawing its air over the exhaust manifold. The exhaust manifold presumably heats up quicker than any other part of the engine.

As the car came up to temperature the wax thermostat capsule in the air box was supposed to melt and allow the door to swing to the cold air side so that only the cold air was supplying the intake air. It was a great plan but the wax capsule thermostats only lasted a couple of years and the whole system then forever got stuck on the hot air side.

As far as where the hot air goes, it is just like the rest of the heat off of the engine and the exhaust, it is there but it is cooled through convection. I clearly cools better while the vehicle is in motion but there is no harm to having it idle either.

...Lee

Post by **RobTheModd** » 27 Nov 2009, 17:39

I kind of figured it was something like that. Sorry father was over when I was attempting this mod and was riding me about EVERY detail of it. even got into an argument over it haha.

Anyway car seems to ride a bit better after this mod, MPG is still a wait and see process.

I did how ever misunderstand the hot air. I was too myself quite curious about where it went, but i figured hey if it worked ok for everyone else. why should mine be different.

Post by **jmmxc** » 02 Jan 2010, 12:22

I am very interested in this topic. I just mod my air box by leaving it in cold are in position, pushing the spring mechanism with a piece of wood). I have been driving around for couple days for a total of 100 miles... The car apparently runs fine; but I am getting suddenly the trouble code "P0422 Main Catalyst Efficiency Below Threshold (Bank 1).

To all out there, what does that exactly mean? It seem to be related to either that cat or the forward Ox sensor but I am not sure.

Is this related to the fact I just switched the air box to full cold in, this after so many years of the t-stat not working.

Thanks,
jmmxc
Volvo 850GLT, 1996
130K miles +

meteg · Post by **meteg** » 02 Jan 2010, 16:25

hello,

what you have experienced is only a coinsidence, the airbox thermostat system or the position of the valve(in hot or cold) has no effect on the catalyst.there is no relation between them..

Airbox thermostat failure...

Re: Airbox thermostat failure...