Hi everyone, new to the forums and to the pistoned powered Mazda's, but been around the rotary powered ones for some time. I am trying to help out a family member with the 929 they recently picked up and I am at a loss as to the problem with the engine in the car. Put simply, the engine has an extremely bad misfire and hesitation problem. The idle hunts between 1000-1500rpm when not in gear, and when in gear it hunts between 400-800rpm. The part that is confusing is that on initial inspection, there are no vacuum leaks that are easily seen or any issues with the intake hose. Spark plugs were changed with no improvement in the way the car runs.

To top off the problem, the check engine light IS NOT on in the car, which was a surprise to me with how badly the car runs. The light does in fact work as it lights when the engine is not running. So far the fuel pump has been checked, the spark plugs replaced, the air filter and intake hose checked, vacuum lines checked, and general inspection of the parts of the engine for anything obvious. Any suggestions where to go next would be appreciated.

 

Has the distributor cap been removed and inspected for carbon tracking?
Have the plugs been removed and inspected since they were installed? If there's an issue with air-fuel ratio, the plugs may be fouling. However, that alone would've caused short-term improvement when new plugs were installed, hmmm... Still, not a bad idea to check them, see if the car is running rich or lean, or just right.
Has the engine been compression tested?

What year is the car and how many miles does it have?

 

To answer a few questions, the spark plugs were replaced with no improvement at all. The old plugs were obviously in there for a long time, took some work to remove without pulling out the threads with them. The old plugs were black though, a clear sign of overfuelling I think. The distributor cap hasn't been checked, will have to give that a check next time I look at the car. No compression test has been done yet.

The car is a 92 with about 250,000km. The engine ran fine about 2 weeks ago when it suddenly started running horribly, to the point where it can't really be driven anymore. This leads me to believe a sensor or part of the fuel system or ignition system has failed.

 

In my experience it is best to replace the plugs wires, distributor cap and rotor all at once. The coil(pack) could be at fault too. A coil pack can have an internal short.
Give this a try: when it is really dark run the engine and use a spray bottle to spray water on the engine. You may see sparks flying from the wires to the valve cover or between wires.
The throttle body may benefit from cleaning inside and out. The IAC valve may benefit from cleaning, so will the EGR valve and the MAF sensor. A new air filter could help as well.
Also run a fuel system cleaner and may be go to at least medium octane gas.
If all of that does not help you may have to measure the fuel pressure.