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Losing power when braking

Sometimes when I brake to come to a stop, my car loses power: the whole thing shudders, RPMs take a dive. It doesn’t start immediately on braking. It starts when I am almost at a stop, but not at a full stop. If it’s relevant, I generally brake quite gradually, starting several car lengths away. So there are several seconds between onset of braking and onset of power loss. It starts while the brake is depressed. It does not stop when I let up on the brake. It does stop when I depress the accelerator. If I am idling at a light, it might happen. If I am just going slow without braking, it doesn’t happen. Braking is fine in all circumstances except coming to a full stop.

'93 Mustang, 4-cyl, 2.3L, manual, fuel-injected

It’s been doing this for years, actually, and yes, I’ve been just dealing with it instead of getting it checked out. Usually a little rev of the engine gets her through it. Last night, it actually stalled, several times in fact. It restarted, but it did take cranking it for a while for it to restart.

Now that I think about it, my battery might be up for replacement. It’s probably around 7 years old, and the last 6 have been very light driving, maybe 3k per year. But like I said, this is an ongoing problem. What should I be thinking about besides the battery, and what is a mechanic likely to tell me?

*Losing power. Fingers faster than brain.

You may have a vacuum leak from the engine to your master brake cylinder.
Look at the rubber hose from the master cylinder and follow where it goes. Inspect it for cracks and make sure it is well connected.

Hey RemcoW, thanks for that. I’ll look into it. If I have a vacuum leak, wouldn’t I have this problem every time I brake and at onset of braking?

Along with the vacuum RemcoW suggested-

If your battery is weak, the lower rpm’s of idling could cause a drop in voltage and cause a rough idle. Do you have a dash volts gauge?

But- this could also be a fuel delivery issue (filter, pump, injectors,) or an air delivery issue (filter, air flow sensor?,) or even a spark delivery issue (coil, plugs, wires.)

How is the maintenance history? When was a tune up done last?


Hi eddo, and thanks. I maintain some things (plugs, filters) based on mileage and oil based on age. So, some things are old but don’t have many miles on them (eg, the battery). When I take it in for something major, I always ask them to look for any other problems. It was just in some a/c work, and they didn’t report anything, but maybe they just didn’t look. I do have a volts gauge, right next to the tachometer, but I am drawing a blank on what it does. I’m usually focused on how the car feels and trying to rev it up to keep it from dying. Gauges become less important during those times :).

Vacuum leaks could definitely behave intermittently. If it is cracked, you may not always lose the same amount of vacuum.
You could try this: get the car to idle in park, open the hood and carefully spritz a little WD40 or starter fluid around suspect vacuum lines. If you notice that the idle briefly changes when you spray, you’re in the general vicinity of a problem area.

Eddo’s battery idea is also something you’ll want to investigate.

In addition to the suggestions mentioned, try disconnecting the vacuum line to the brake booster (that frying pan shaped can just behind the master cylinder), clamping it, and seeing if the problem disappears. Your brakes will get hard and need more force, so don’t perform your diagnostic test drive past any playgrounds. The test will either confirm or eliminate a vacuum leak in the booster.

Hey Remco,

I get that vacuum leaks can be intermittent. This isn’t so much intermittent as under certain specific circumstances.

And yeah, I want to test the battery. It might be low mileage, but damn, it’s old.

Vacuum leaks can also be susceptable to specific conditions. A vacuum leak in the booster valve or line can be affected by the amount of vacuum being pulled by the engine as well as the pressure, rate, and displacement of the booster diaphragm. A vacuum leak in an engine line will be susceptable to the engine vacuum as well as even heat, since these lines are elastomeric and subject to heat expansion. Even the temperature of the gas going through the line could make a difference, as that affects the temperature of the elastomer.

From your description it sounds like the Idle Air Control valve is dirty/defective.

The IAC valve allows the engine to idle anytime the accelerator is released. One way to determine if it is the IAC valve is, if you slightly step on the accelerator when stopping and it prevents the engine from stalling the IAC valve is the problem.

You can try removing the IAC valve and clean it along with the idle air bleed port on the throttle body to see if that fixes the stalling problem. If not, then the IAC valve needs replacing.


When the engine is idling, tap on the IAC with a screwdriver handle. If the engine stalls, the part is defective.
You should clean the throttle body. A dirty throttle body can wreak havoc with your idle.
If the battery really is 7 years old, replace it immediately. You’re on borrowed time.
Do you hear any hissing at idle with the hood open? If you do, you’ve got vacuum leaks.

Well, I am leaning toward the Vacuum leak.