99 Mazda Protege with rough idle

The idle speed is higher than normal, about 800-900 RPM. Whenever I stop for the traffic light, the engine will behave as if someone is pressing the gas pedal intermittently.

Anyone has some clue? It has new spark plug wires and coil.

It might be a vacuum leak. That could make the idle rise and fall.

You might also clean the Idle Air Control (IAC) valve. It controls idle speed.

Thanks for the reply. Sounds like a very serious problem.

The car has 140k miles. It has been a very reliable car except for this.

Its likely to be a very minor problem. Worst case would be a leaking intake manifold gasket. On a little Protege even this isn’t really a big deal.

Give folks a clue about how handy you are and its easy to provide some simple things to do.

If it’s a vacuum leak it’s probably a bad hose. Not a major problem.

The IAC is not a huge deal, either.


I am reasonable handy. I can do things such as replacing starter, water pump. tec.

Which hose? The air hose between air filter and the throttle body looks good.

How do I check if the IAC is good or not?


First, I’d just say that I’d get new fuel & air filters on there is they aren’t already. I’d also take a quick read of fuel pressure if you have a gauge. Actually, if you have a vacuum gauge you can find out pretty easily if you have a vacuum leak.

That said, what you describe sounds like a classic small vacuum leak.

So first, the next time you’re stopped and it is surging, if its safe, put the car neutral or park and remove your foot from the brake. If the surging stops every time you come off the brakes and starts whenever you hit the brakes then you probably need a new brake booster.

If that checks out turn off the car and pop the hood. Start at your air filter housing. Just pull the air filter (replace if needed), and pull the whole intake tract - should just be a big black rubber tube - removal likely with worm-type clamps requiring just a screwdriver or small socket. Carefully inspect that big black intake tube for splits or cracks. Many of these will not be obvious at all - they can be razor-thin splits that hide in corners & creases and that you’d only see by pulling, prodding & bending. (While you’re at this you have access to your MAF sensor - sticks into that tube or air filter box in some kind of housing - has two really small little wires it. Blast those with some MAF or electronics cleaner - don’t touch the wires though).

Now start looking all over the top of the engine - most vacuum tubing is just very small black tube. Find anyplace that a piece of this, large or small, connects to the engine. Inspect all of those for breaks, disconnects, whatever. The ends are especially prone to cracking/splitting and you need to get at both ends. Once again the problems can be hard to see. One thing you can do is pull one and plug up its hole on the engine completely with your finger or something. If this cures the surging then the leak is associated with that bit of vacuum stuff.

There should be a schematic diagram of the vacuum system on the underside of the hood. It is schematic rather than a literal map of the engine compartment, but you can use it to help you find all of the vacuum connections.

If none of that gets you anywhere, run the engine and take either an unlit propane/butane torch or can of carb cleaner or something similar and start hitting all of the seams at the throttle body and intake manifold (basically all of that stuff on the opposite end of the intake tract). If you hit a gasket leak there the idle will respond.

The IAC valve sits right on the throttle body somewhere. They’re normally easy to remove and then you can clean yours - use some throttle body cleaner. While you’re there you may as well clean the throttle body and plate too.

None of this is complicated. Just plan to spend a few hours hunting.

Hi Cigroller,

Thank you so much for the nice suggestions. I will check that out when I find time in the weekend.

I really appreciate your kind help very much!