My old '88 Escort has the old heat riser system on it where has the duct from the exhaust manifold to the breather to help with warm up time, so a few weeks ago I decided to set it up where the trap door would stay open all the time, allowing heat from the exhaust to enter the breather constantly. The only problem with doing this is that on warm days in town the temperature of the engine rises a lot faster than it did before. Since doing that I've had fill-ups at 46.748 mpg, 41.976 mpg and 44.199 mpg. On the tank that got 46.748 mpg we also had really nice temperatures several days in the 70 and lows in the upper 30's and 40's. The next tank the temperatures were pretty cold, we also had some snow that hurt FE and on the last tank most of the morning temperatures were in the lower to upper 20's with high's in the 40's and 50's. I've been very happy with the results that it has yielded. The two higher mpg tanks are at or near what I usually get during the summer. I'm hoping to be able to run this way during the summer without any overheating problems. If I can there's a good chance I can break the 50 mpg barrier.
What I would do is bypass the thermostat for the flapper, and connect it straight to engine manifold vacuum, so if the engine is running, the trap door is always closed. Now I just gotta get my lazy butt to take the air cleaner assembly apart to do it.
hmm thats odd, when my chevettes stuck open all the time i got crappy milleage, like went from mid 30's to low 20's... i should probably get a new one but thier $12 last time i checked. I gotta remember to get one off a junkyard car next time....I just disconnected it for now and back up in the mid 30's