Troubleshooting Idle Problems in Mazda Engines

frustrated drivers

Is your engine idling so low it feels like it will stall at traffic lights? Solve this very frustrating problem so you don't have to be that person who is always revving the engine at stop signs. This letter came in and tells the story very well:

I am currently having some car trouble with my 1993 Mazda 626. The car is idling very rough, as if not enough gas is reaching the motor because it feels as though it wants to turn off. I constantly have to keep giving it gas. When I'm at a traffic light, the car feels like it is going turn off so I put it in neutral and from there proceed to "REV UP" the engine to prevent it from turning off.

The problem began about a month and a half ago when it started getting colder, I live in the East coast. In the mornings, when I initially turn on the car to warm it up, it doesn't feel as bad. Once the car is warmed up though it begins to idle and feel very rough. Since it has been giving me problems I have given the vehicle a tune up; new wires, plugs, PCV valve, rotor, distributor cap, fuel filter and oil change. I did not however give it a new air filter.

Another example of the problem that I've noticed is when I initially turn on the vehicle the idle is rough, but manageable. However, I do noticed when I turn off the car and turn it back on for, let's say, a trip to the supermarket, I've noticed the idle is worse and it takes me a good 5 minutes of revving up the engine before I can take it out of park without it shutting off on me.

Furthermore, I've noticed that when I have my lights are on and my car is idling ruff and about to shut off the lights get very dim.

I've gone to see a mechanic and they cannot determine the problem unless they take the car apart which they said will cost me. Can you tell me if it's time to get a new car?

  • 1993 Mazda 626
  • 2.5 liter 4 cylinder
  • Automatic transmission
  • 112,000 Miles
  • Fuel Injection
  • ABS brakes
  • P/S, A/C, Cruise Control
  • Rack and Pinion Steering

Thank you, in advance, for your help.

Mazda Man in NJ


The problem described has an answer. Try this. First, check for stored Diagnostic Trouble Codes. A code will narrow down the possibilities a bunch.

There are quite a few potential causes for this problem. First and foremost is a vacuum leak. Check all the vacuum lines, and make sure they are in good shape and connected properly. Check the PCV hose and lines also. In addition, check the large air intake hose from the Air Flow Meter to the intake manifold for cracks and leaks. They often crack in the valleys and are hard to spot.

Another good possibility is the EGR valve is stuck open or the EGR control is allowing vacuum to get to the EGR valve. Try unplugging the vacuum line from the EGR valve. If the idle smooths out, you have an EGR valve control problem.

If not, try tapping the EGR valve and see if it shuts. You can reach under the EGR valve and manually push the diaphragm up and down to see if it makes a difference. You may have to remove the EGR valve completely to check it.

Another good possibility is a bad Coolant Temperature Sensor (CTS). If it is bad, it will send a wrong signal to the computer and lean out the fuel mixture.

Other possibilities are clogged, leaking or inoperative fuel injectors. Listen to each injector to see if they click. A dull sounding click indicates a partially or totally clogged injector. You can use a noid light, available at parts stores, to see if the electrical circuit is working. You can also measure the resistance of the injectors. The resistance should be approximately 13.8 ohms at 68 F.

Other possibilities: Weak fuel pressure. You can check this with a fuel pressure gauge. The AFM may have a stuck measuring core. There is a spring-loaded measuring core, connected to a potentiometer, that moves in relation to incoming air volume. The Idle Air Control Valve/Bypass Air Control (IACV/BAC) may be stuck or bad. You can try cleaning the IACV/BAC. That will often fix a bad idle problem.