DIY: Externally adustable air box flap for P-80 NA w/pics

Post by **dosbricks** » 05 Mar 2016, 13:33

Many folks have discovered that almost all the thermostats on an NA airbox intake eventually fail in the heated air position choking off the the larger fresh air port. Last fall I decided to make a way to adjust the flap from summer to winter mode incrementally as the season changed--and with an external adjuster. I just did the same for the S70. This is a quick and easy DIY.

Photo #1 shows the end result.

Photo #2 shows the dowels I previously used to switch the flap from winter to summer (or vise versa), alongside the device required to make the new adjuster.

Photo #3 shows the parts:
3/16"x 2" screw (brass plated is better), stamped-steel 3/16" wing nut, 3/16" self locking nut, and lastly, the type anchor used to screw into wood to hold a regular threaded screw.

Tighten and epoxy the head of the screw into the wing nut. Drill a 21/64" hole for the furniture anchor and epoxy it into the airbox securely. Insert the screw and add the self locking nut for more bearing surface on the flap and to prevent the adjustment screw from accidentally being backed all the way out.

Photo #4 shows the flap set for summer operation with full open fresh air and the heated air nearly closed off.
Photo#5 is the flap adjusted for winter operation with hot air open plus enough cold air to allow the engine to fully breath.
Photo #6 is looking down from above the air box. It's tight but there is just enough room to get my hand down there to turn the screw.

Post by **matthew1** » 08 Mar 2016, 12:11

Entered into the Repair Database. Great little DIY, Dos!

Post by **oragex** » 08 Mar 2016, 15:22

Smart idea. My intake is slightly different

This mod is important. Stuck flap - when the thermostat fails - causes a significant decrease in mpg in the area of 10-15% because then the intake air lacks the pressure that builds up with speed.

Have you found if a thermostat replacement part? The only ones I found are for earlier Volvo models.

Also, how do you adjust the flap? Usually I have it fully open in the summer (100% fresh air), and only 30% open (30% fresh air) in the winter.

Post by **dosbricks** » 08 Mar 2016, 19:21

Thanks Matt.

Oragex, I did not try to find a replacement thermostat. I would rather control it myself and know what it's doing. The flap is adjusted by turning the wingnut. The spring keeps the flap pressed against the end of the wingnut/screw.

Post by **erikv11** » 08 Mar 2016, 21:10

oragex wrote:... This mod is important. Stuck flap - when the thermostat fails - causes a significant decrease in mpg in the area of 10-15% because then the intake air lacks the pressure that builds up with speed ...

Since the airbox "mod" hit MVS about 10 years ago (e.g. https://www.matthewsvolvosite.com/forums ... php?t=8830), the group-think has been that the flap is important because if the thermostat fails to force it to the hot side, then the engine gets only very hot air off the exhaust, hence lower mpg (cooler air is denser so more O2 per unit volume). And I would expect the MAF doesn't car what the pressure is, it just measures how much air is flowing. Anyway, first time I have heard the air pressure theory, interesting idea. What about all the cold air intake enthusiasts (e.g. K & N intakes), would that sacrifice mpg?

Volvo 9142616 is the airbox stat for all pre-98 P80 cars, including OP's.

This is such a clean fix. I have been pulling the corrugated hose every spring and replacing it in the fall because I like to have the preheat function in the winter, we actually had the throttle plate freeze a couple times when the flap was wedged over to cold. But in a warmer climate I would go with this method.

Post by **precopster** » 09 Mar 2016, 04:54

I just rammed a self tapping woodworking screw straight through the box and through the flap.

Engine hasn't received warm air from exhaust manifold in 4 years however Melbourne Winters ARE very mild.

My methods are sometimes crude, rudimentary and without finesse however I fix my cars the same way

Post by **oragex** » 09 Mar 2016, 21:11

I think that 'pressurized' intake air is related to the intake air volume. I have once removed the intake air duct that's behind the front grill, and drove on the highway for a few hours. I noticed the mpg went down by a lot during that trip. In winter time, with no warm air the engine is more peppy, the opposite goes if I let only hot air into the intake.

For about two years I had the engine pinging in the summer, and the top of the engine was getting really hot to the point of smelling plastic, just like you mention. Perhaps the reason was the stuck flap. This summer I'll let the flap get only fresh air, I'm curios is the pinging will cure.

Post by **dosbricks** » 10 Mar 2016, 14:55

There is no doubt some merit to your "ram air" theory. But the engine is also more peppy anytime it's not running just off the hot air side because of the smaller diameter of the hot air tube/inlet and the reduced density of hot air. So there are multiple factors, which taken together, really hamper performance if the engine is on a reduced diet of oxygen.

When I first read about this issue and dealt with it using the dowels, my wagon suddenly found about 20 more hp. So thanks MVS!