Airbox thermostat failure...

If you own a N/A 850 or S70 (and maybe others), you may wish to check your airbox thermostat. What I discovered three weeks ago shocked me about my own 850.

For the 1.5 years i've owned my 850, i've been trying to figure out why i've been getting only 15-16mpg city and highway combined (mostly city stop and go). After a while i've gave up and just thought thought this is just normal. I've done all the stage 0 maintanance items over at volvospeed and somehow i can't even drag another 1MPG out of the car. Auto-Rx and everything, just can't get more for some reason.

So about three weeks ago i've opened up my airbox to check the filter. Now something was bothering me here, after 12000 miles, the air filter is still surprising clean. It was unusal, because in the environment that I live in, the filter would've been naturally much dirtier by now and i know the filter isn't the dust and crap get through. So after reading some TSBs on airbox thermostat replacement on AllDataDIY.com, I thought:

"If the car's thermostat failed, that would mean that the airbox has been sucking in hot air from above the exhaust manifold all this time and maybe thats why the filter is not dirty "

So I proceeded to take out the airbox thermostat. It was pretty easy, just open it up and at the bottom at where the air enters the box, there is a flat black plastic tab... pull it up and pull out the thermostat. Now you can't just leave it like that, because when the thermostat is cold it sucks in hot air... so I took a small fuel clamp and shoved it between the spring (that the thermostat would push against) and the where the plastic tab was, acting as a spacer, so that it would push the valve open to the cold air side. I did this so that I can run the car with cold air while I figure out this thermostat.

Well the first thing I did was take the thermostat home and inspect it. I cleaned off the dirt and crap on it and pulled out the center pin. I instantly saw why it failed. All the dirt and dust from the outside air accumulated on this thermostat, got past that seal where the pin goes in and contaminated the wax inside. I also noted that the pin was stuck when I applied hot water to the thermostat to try to simulated it.

Well three weeks later (and 300 miles later)... (today) OMG!! This car in the past three weeks have just amazed me in performance and fuel economy. The car really pulls now on the freeway and just so much more peppy. Today I filled up my tank and calculated everything, my combined MPG climbed 3MPG! I swear it could be more MPG if I didn't keep having my foot on the pedal (having too much fun with the newfound power).

I opened up the airbox today and the first things i've noticed is the airbox itself isn't as hot as it 'normally' was after driving it for a while and the air filter itself was suddenly DIRTY! For some crazy reason, in only 300 miles, my air filter literally doubled it's dirtiness!

I can't believe that i'm happy that my air filter is finally dirty! I highly recommend that every N/A 850/S70 check their airbox thermostat, because for me it really made a difference, a very noticable one!

Is there any harm in running my car in the winter in Iowa without the hot air circulation to the airbox???


It looks like I have the same thing going on. Admittedly, it's only 30 degrees outside, but after a long drive, I pulled the hot air hose to the exhaust off of my airbox and gave it some throttle.. All of the suction to the intake was coming from the exhaust side and 0% from the cold air inlet.

Not only does this feed hot air into your intake, but the size of the hole is physically smaller.

I have everything apart in my house now and I've got the thermostat piece in the fridge. How in the world do you test it? I ran it under hot water (110 degrees F) and the center pin pushed out only about 1mm

judging by the way this valve works, it'd have to be pushing a lot further than that to flip the valve over to 'the cold side'

My thermostat isn't filthy dirty or anything.. It just seems like it's not working (but I don't know how to test it..)

Well, I have to thank you for the tip. I ended up using two pieces of wooden dowel to hold the flapper over to the cold side.

I'm not going to dragrace anyone, but it noticibly affects driveability! The car went from being the second slowest in my fleet to being the second fastest.

And since people put cold air intake systems on cars all the time, I guess I'll just not worry about the hot air.

The car also seems to idle a bit smoother better now as well. Hopefully I'll experience the increased fuel economy too.

Thanks!



yes, that's snow on the ground

Where exactly is that piece you're holding in your hand? I've looked at my S70 and don't seem to find it. Maybe mine doesn't have it? However, if it does, I'd like to check mine to and make that modification. It doesn't get that cold here in E. Tenn and I'm sure that if I can keep hot air out of my intake it would do my gas mileage and umph a world of good.

Remove the air cleaner assembly.

Remove the air intake body from the air cleaner housing by depressing the two tabs. That bit is inside.

Or check out
http://www.matthewsvolvosite.com/chat/phpBB2/viewtopic.php?t=8779&highlight=airbox

Bill.

Okay, gotcha...thanks. I'll take a look in the next day or two...raining here and dark. Hopefully tomorrow will be sunny out and I can get a good look.

just take the top of the airbox off and reach into the place where the air comes in. The piece I'm holding is the fitting that both the cold air and hot air hook up to. It removes from the airbox in my case.

To remove the airbox, I just unhooked any plumbing to it, and undid two vacuum lines on some sort of valve that's screwed to it. I also undid the wiring connector that plugs into the valve. Then I was careful, but just pulled up and popped out two lower plastic connection points.. then the airbox slid in, toward the engine to release another plastic-in-rubber mount on the fender side.

It was pretty straightforward. Once I got the piece I'm holding off of the airbox, I was able to take it apart and see how everything worked.

If you're wanting the quick fix, just work the flap internally with your hand and hold it so that the cold air side is open. Then wedge the dowels or whatever you want into the spring assembly to keep it set at cold air only.

Strange I've never really noticed this...I've had the airbox out of the car a number of times.

These might only reside in NA cars.. I'm not sure.. My Volvo 240, 960, and 850 all have them. The silver colored ductwork from the exhaust manifold to the airbox is the giveaway I guess.

On several of my 240s, the metal tubing was gone, so I was just sucking in air from the engine bay, rather than off of the exhaust.

I don't have that silver pipe...and do have turbo on both cars...will take a look anyway. Thanks!

MIJ, I just had the air intake box and the air intake tube (no body) out this past week when doing the PCV system and replacing the vacuum lines. I have absolutely now recall of an air intake body or valves. I did depress the 2 tabes and pulled the airintake tube out of the air intake box. It is a staight shot. If there is an air intake valve I "shur as heck did miss it.
so now got me curiosity roused. will look too.
Must say though with the K&N air filter I need to keep braking to keep the car going slowly. So air the car is getting there.

Thanks, dasapir. I'm thinking that our Turbo'ed cars don't have the warm air switch with a thermostat in the bottom of our air boxes.

Well, I just went out to my car and nope, there is no warm air valve switch. Wheter this is just on turbos or on non-turbo S70 I cannot say. What I can say is that the switch is not there on my 98 S70 turbo. And thank you.

I notice they don't say "just run the car without the thermostat"

However, they also say that 100% hot air all the time kills MAF sensors... I'll let you know if I have trouble with 'cold starts' It was 29F tonight and my car was at operating temperature within 2 miles of my house.


I've survived one winter with a non-working glowplug in my TDI bug haha!


From: http://www.volvoclub.org.uk/faq/EngineF ... ction.html

Air Box Thermostat Change. Note: Non-turbo cars have a thermostat inside the air box that shifts air intake from heated air to cold air when the engine is warm. This is done to improve combustion at cold starts. Early turbos also had this thermostat, ending around 1988; later turbos do not. [Query:] Can I change the air box thermostat in 7xx/9xx cars with an integral lower air box/thermostat combination? [Response: Steve Seekins] You can change just the thermostat element on any of the cars that are equipped with it. You only need to change the flapper if the hinges are worn or broken, or if the gaskets are destroyed. See below for instructions on thermostat removal on 90+ cars with integral air boxes. [Editor] You can buy a replacement thermostat from an aftermarket supplier for about $10.
[Editor] Spring is an appropriate time to test the function of your airbox thermostat and change it if needed. The thermostat can seize, allowing hot air to enter the air mass meter and dramatically reducing its lifetime. Note that many turbos do not have these thermostats since the turbo itself adds heat to the intake air. [Tech Tip from Walt Posluszny] I stuck a wireless household thermometer in the airbox of our 95 960 (152,820 miles) today. The outside temp was 70F and the temp in the airbox steadily rose while we were driving up a fairly constant 4 percent grade for 4.5 miles. Halfway up the road the thermometer registered 160F before it literally melted down. I pulled the air mixing box, and jammed the air door to open/cool air, replaced the still-operable remote thermometer, and took it up the same grade. The temperature declined to 73.5F in the airbox even though I was climbing the same 4% grade. Lesson: check your airbox thermostat and replace it if needed.

To test the thermostat:

Remove the air intake pipe from the grill hole to the air box extension.
Start your engine. As the engine warms up, the flapper inside the hole should move to the fully closed position in moderate to warm weather. If it does not, then you need to change the thermostat. See the FAQ section for test temperatures and flapper positions.
To change the thermostat in a 90+ car with the assembly inside the airbox :

Remove the air box from the car.
Spray PBlaster around the air pipe joint where the extension tube enters the airbox. This joint will be very tight.
Gently depress the tabs and twist to loosen and remove the extension tube assembly holding the thermostat mechanism. Be careful that you don't crack the tube or the box while working on it.
Once this is free, you will be able to see and access the brass thermostat.
Depress the spring using either plier jaws or a two-pringed body clip remover tool. Push this down sufficiently to expose the thermostat piston rod.
Pop the thermostat out and insert the new one in its place. Make sure the rod goes in the hole at the top of the spring.
Make sure the flapper assembly operates correctly.
Clean everything up, lube the assembly with non-silicone lube, and re-assemble.
The only risks are being a little too brutal and cracking the plastic, or breaking an air box clip on removal. Be careful.

I remember buying metal blanking plates for small block chevy intake manifolds from Summit racing. It blocks off the EGR function from the car.. I guess I never heard of any issues when using those. We don't have emissions testing in Iowa. Plus, I imagine the EGR valve is already letting in some heated air on my car. The turbos apparently make enough hot air to not need either device! Lucky you