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Engine Stalls, Misfire Code, & Cylinder Misfire

The engine may be stalling because it isn’t getting enough throttle opening. The cause is often a problem in the idle air control system.

In many instances, stalling ends up being an idle control motor at its limit or a failed motor.

Engine Stalls (Check Engine Light May or May Not Be On)

Stalls typically occur when the engine is idling or slowing. If the Check Engine Light comes on, you may find any of the following codes:

  • P0505 to P0509 idle control circuit codes
  • P0335, P0336, P0337, P0338, P0339 crankshaft position sensor codes
  • P0171, P0174 lean fuel condition codes
  • P0400 to P0409 EGR related codes

The engine may be stalling because it isn’t getting enough throttle opening. The cause is often a problem in the idle air control system. Other possibilities include a dirty throttle body, vacuum leak, incorrect ignition timing (retarded), bad gas (water or other contamination), an A/C compressor that is dragging, or an EVAP purge valve that is stuck open and is flooding the engine with fuel vapor. What to check: The throttle body hose connections and idle controls, also intake vacuum (check the throttle body, manifold and hose connections for leaks, also the PCV valve and hose, too). With your AutoTap Express DIY, look at engine RPM, calculated engine load, mass air flow rate, throttle position angle, short term fuel trim (STFT), and ignition timing. On some vehicles, you can also look at the idle control motor duty cycle or position, and/or idle tracking sensor (if the vehicle has one). In many instances, stalling ends up being an idle control motor at its limit or a failed motor. A vacuum leak can cause this, so don’t replace the idle control motor until you’ve evaluated the possibility of a vacuum leak.

Check Engine Light On, P0300 Random Misfire Code

A random misfire means your engine is misfiring, but that the problem is not isolated to one or two cylinders. It is jumping around in a random way from one cylinder to another. A random misfire code usually means the air/fuel mixture is running lean. But the cause might be anything from a hard-to-find vacuum leak to dirty fuel injectors, low fuel pressure, a weak ignition coil, bad plug wires, or compression problems. Even a dirty MAF sensor can cause a lean code and/or misfire to occur. The engine may be stalling because it isn’t getting enough throttle opening. The cause is often a problem in the idle air control system. The first thing to check is the intake vacuum with a vacuum gauge. On most vehicles a normal reading is 17 to 21 inches Hg. If the needle is lower, is jumping up and down or steadily dropping, you have a vacuum problem. Look for possible vacuum leaks by checking vacuum hose connections, the throttle body and manifold, and PVC valve and plumbing. An EGR valve that is leaking can also act like a vacuum leak and cause a random misfire. The next thing you should check is the fuel pressure with a gauge. If it is not within specifications (refer to a service manual for specifics because fuel pressure is critical for proper engine performance), the problem may be a weak fuel pump, low voltage to the pump (check the relay and wiring), or obstructions in the fuel line (like a plugged filter). A bad fuel pressure relay can also leak pressure and prevent an otherwise good fuel pump from delivering full pressure to the injectors. Dirty injectors can also restrict fuel delivery and cause a lean fuel condition. Many regular grades of gasoline do not contain adequate levels of detergent to keep the injectors clean. Frequent short trip driving accelerates the buildup of injector deposits. Cleaning the injectors with a good quality fuel tank additive (or having them professionally cleaned) can solve this problem. Look at short term fuel trim (STFT) and long term fuel trim (LTFT) with your AutoTap Express DIY. If the numbers are high, it tells you the engine is running lean.

Check Engine Light On, P030x Specific-Cylinder Misfire Code

One of the impressive features of OBDII systems is the ability to self-diagnose a misfire and pinpoint which cylinder has the problem. For example, P0302 indicates a misfire on cylinder #2. A cylinder specific misfire indicates that either compression, appropriate fuel mixture, or spark is missing. Start with a visual inspection, looking for a vacuum leak near the cylinder and any sign of wear on the spark plug wire. Check the compression on the target cylinder to ensure that the engine is mechanically sound. Use a stethoscope to listen to the suspect cylinder’s fuel injector. Listen for distinctive clicks and compare to another cylinder.