Vacuum powered airbox thermostat modification

jchrisloyd · Post by **jchrisloyd** » 20 Nov 2007, 11:24

Variation on the Airbox thermostat modification.

I have a 96 850 GLT, 132,000 miles and the when looking at doing the airbox thermostat, my thermostat is vacuum powered. My mileage has been around 20mpg regardless of city or highway.

When looking at the flap that closes off the hot air, the flap did not close completely, therefore hot air was always being introduced.

Since the thermostat area is different, there was no way to force the flap open with dowels, etc. I screwed a long screw through the themostat housing to force the hot air flap closed. I have seen little written about this type of airbox thermostat.

This made a large difference in acceleration, power and hopefully my gas mileage.

Post by **hausmeister** » 20 Nov 2007, 11:56

As I also tried this ( https://www.matthewsvolvosite.com/chat/p ... 9522#49522 )but I did not notice any (positive) differnce.

What I found out about the vacuum powered valve is that it is either fully open or fully closed. I believe the older version is different and also operates "in between".

Anyway, it did nothing for me, except me wanting to put it back in as soon as it got cold.

Also you do not have to modify the airbox, just simply dismantle the duct, remove the flap and block the hot air side if you want to do this. That way it can be reversed any time.

jchrisloyd · Post by **jchrisloyd** » 20 Nov 2007, 13:24

Thanks hausmeister,

I noticed you have an S70, while mine is a 96 GLT. I was looking on FCP, and noticed an Intake Air Temperature Sensor, and saw a few references on the net that this sensor determines when to put vacuum to the vacuum type airbox. I do not know if this is correct, and if so then my 850 does not have this sensor.

I'm starting to wonder if the prior owner might have put a newer style airbox in my car. They had it connected to the vacuum tree, and therefore it was always in the "hot" position.

any thoughts anyone?

Post by **MadeInJapan** » 20 Nov 2007, 14:32

Have you seen this? Click and scroll down (of course, everyone got off topic, but it is closer to what you're asking): https://www.matthewsvolvosite.com/chat/p ... hp?t=13141

Post by **tjts1** » 20 Nov 2007, 18:16

jchrisloyd wrote: I'm starting to wonder if the prior owner might have put a newer style airbox in my car. They had it connected to the vacuum tree, and therefore it was always in the "hot" position.

any thoughts anyone?

I'm really surprised you have a vacuum operated airbox on a 96 GLT. Our base 96 850 5 speed has the regular thermostat as pointed out in the above link. I thought those showed up on S70/V70s.

Post by **hausmeister** » 22 Nov 2007, 05:13

jchrisloyd wrote: I'm starting to wonder if the prior owner might have put a newer style airbox in my car. They had it connected to the vacuum tree, and therefore it was always in the "hot" position.

any thoughts anyone?

On mine there is a valve/switch/Thermostat in the top airbox cover. from there are hoses to the vlave below and to the vacuum tree. Maybe its not connected correctly? You can test the calve by sucking on one of the hoses and see what happens. If i remember correcty, it should open to hot air if you apply vacuum.