Dear Car Talk:
My husband and I are both nurses, so we are trained to diagnose things. My husband is not only a great nurse but a fine backyard mechanic as well. But we have a problem with our 2015 Ford Escape that has us stumped.
When I stop at a red light, my car shakes violently, and the RPM drops below a thousand. When I put it in neutral or park, the shaking stops.
We took it to our mechanic, who hooked up a computer that showed no codes, no idiot lights and no warning light on the dash. They ruled out a bad motor mount and brake rotors being warped, and they cleaned the idle air-control valve. They are stumped, too.
Please help with this problem. If anybody can, you can. — Marsha (for Marsha and Robert)
I’d give you my diagnosis, Marsha, but I need to see your proof of insurance coverage first. Sorry, little hospital joke there.
It could be any number of things, Marsha. But I suspect you have a vacuum leak. The engine naturally creates vacuum every time a piston descends and sucks fuel and air into a cylinder. That “free” vacuum power is then used by other systems in the car. For example, it’s used by the ventilation system to control the flaps inside your ducts, and by the exhaust gas recirculation system to open and close the EGR valve.
If you have a vacuum leak, that means unwanted air is leaking into the cylinders. Because the mixture of gasoline and air in the cylinders has to be just right, a vacuum leak throws off that delicate balance by letting in too much air, and that makes the engine run rough.
There are various ways to find a vacuum leak. If it’s big enough, you can sometimes hear it hissing. If not, we sometimes spray combustible fuel cleaner where we think air might be leaking in. If there’s a vacuum leak nearby, that fuel cleaner will get sucked into the cylinders, and we’ll hear the engine rev up. Or we pinch off individual vacuum hoses, one at a time, to see if closing one off makes the engine run smoothly.
There are other tricks, too. But if your mechanic is as good as you say he is, he’ll know how to search for a vacuum leak. I will give you one hint for your mechanic: One often-overlooked source of a vacuum leak is the power brake booster.
The power brake booster uses vacuum from the engine to multiply the force your foot puts on the brake pedal. It’s the “power” in power brakes. But if there’s a hole in the booster’s diaphragm, that’ll create a vacuum leak, but only when you’re stepping on the brake — like when you’re stopped at a light. If the leak is a small one, when you put the car in park or neutral, you reduce the load on the engine enough to stop the shaking.
So ask your mechanic to set out on an expedition for a vacuum leak, Marsha. Hopefully he’ll find it, and your Escape can escape the shakes.
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