Why is my vehicle making a whooshing noise after about 15 minutes of driving and then not accelerating?

Question

I've had a recent issue with my 1990 B2600 RWD manual transmission Mazda pickup truck. I will start it cold and drive at highway speeds for about 10 - 15 minutes when all of a sudden a whooshing sound suddenly comes from under the cab and I cannot accelerate when I push on the gas pedal. I can somewhat maintain the speed I am at and am able to go another 10 minutes or so to my destination, but I cannot get above 60 - 70 mph. I can stop and accelerate normally from stop lights. Also, when I arrive at my destination I notice a burnt smell. I pop the hood and see smoke coming from the outside of the air intake manifold cover. The engine does not get hot (the temperature gage is where it normally runs). No warning lights ever come on. I have noticed a bad vibration that emanates from inside the muffler (from putting my hand on it when the engine is first cranked), but nothing else out of the ordinary.

Coolant system seems to be okay, and spark plugs and wires were replaced about 10,000 miles ago.

Any ideas what could be causing this?

Answer 1

You may have air under turbo pressure escaping from an air duct due to a loose or broken or missing clamp. The 10-15 minute delay could be the time it takes for the rubber duct to warm up and get soft and flexible enough to expand away from the point of attachment. (Happened to me once, but only made the whooshing noise when climbing hills.)

Go to the turbo and find where an air duct is attached to the turbo output. Is there a clamp? Is it fully tightened? Is the rubber duct fully seated on the turbo? Now follow the duct all the way to the intercooler, checking carefully along the way for clamped joints. Is it firmly attached to the intercooler? All clamps tight?

Now go to the other side of the intercooler. You know the drill by now. Continue following air ducts all the way back to the engine.

I think you'll find a barely attached air duct somewhere. Re-seat it, tighten the clamp and you're good to go.