Would having a minor exhaust leak leak cause a decrease in gas mileage? When I bought my truck the exhaust manifold gaskets to the heads were shot and it was leaking alot of air through there. I bought new gaskets and installed them but the drivers side still has a leak. Probably because it was ran so long like that before I got it and the manifolds were all pitted up. Now the noise caused by this is one thing, and doesnt really bother me a whole lot, but if this is decreasing my gas mileage then I would like to do something about it and fix it.
Next will having a vacuum leak cause a decrease in gas mileage? I'm pretty sure it does but just want to make sure. I think I might have a vacuum leak because when I am driving speed limit on the highway you can hear a little bit of a whine sound coming from the engine. when you let the gas off it doesnt do it, and when you give it gas it either doesnt do it or you cant hear it over the exhaust? But definatly a little something there when just cruising. Is this a vacuum leak?
That doesn't sound like a vacuum leak to me - vacuum is lower as you get on the gas. Manifold vacuum is supposed to be highest at idle. If you have a good enough vacuum leak, you will have problems with really rough idle, poor cold-running, bogging on acceleration, and even dieseling, to name a few. If you have a whining noise with higher RPMs I'd be looking at your belts or alternator bushings. As for that exhaust leak, I'd doubt it affects your mileage too much if at all. The gas is already burnt by the time the exhaust can find the leak.