Cuts Out/Misses

Definition: Steady pulsation or jerking that follows engine speed, usually more pronounced as engine load increases. Not normally felt above 1500 rpm or 30 mph. The exhaust has a steady spitting sound at idle or low speed.

^ Perform careful visual check as described at start of Symptoms. - Visual/Physical Underhood Inspection
^ Perform PFI System Check. Testing and Inspection
^ Check for intermittent injector operation. See Chart A-5. Chart A-5 Injector System Diagnosis
^ Perform an injector balance test:
^ Select ECM override ES04 "injector."
^ While in the injector override select ECM Data ED11 "RPM."
^ Run engine at 1500 RPM with steady throttle.
^ Select each injector (1-8) and disable fuel (Push "Cooler" button) while observing ECM parameter ED11.
^ Check any injector that does not decrease engine RPM at least 100.
^ Perform a cylinder balance test as follows:
^ Disconnect and plug vacuum hose to EGR valve. Start the engine. With the engine at its slow (warm) idle speed, disconnect the four terminal connector to the ISC. Ground one spark plug circuit at a time using an engine analyzer or scope. As each plug circuit is grounded, record the drop in engine RPM. NOTE: Do not disconnect spark plug wires as an arc will occur elsewhere in the system - in the cap, coil or rotor. Damage to cap or coil may result.
^ If there is an equal RPM drop on all cylinders (plus or minus 50 RPM) go to "Rough, Unstable, or incorrect idle, Stalling" symptom chart. Rough, Unstable, or Incorrect Idle, Stalling
^ On cylinders with no RPM drop or excessively small RPM drop, perform the following:
^ Check for sufficient spark using J-26792 (ST 125 HEI Tester) or equivalent. if no spark, see Ignition System.
^ Check spark plug wires by connecting an ohmmeter to the ends of the plug wires in question. if the meter reads greater than 50, 000 ohms, replace the wire(s).
^ if the spark and the wire(s) check OK, remove plugs in suspected cylinders and check for cracks or carbon tracking, wear, improper gap, burned electrodes and plug fouling with fuel (sooted) or oil.
^ Check ignition secondary scope pattern with the vehicle in drive, brakes firmly applied, drive wheels blocked, loading the engine against the brake to obtain a 90 kPa MAP reading. Look for spark plugs, wires or ignition coil, cap and/or rotor breaking down under load. A problem such as a cracked plug may cause a severe miss on a light acceleration, yet be difficult to detect on a scope. Replace any suspect spark plugs, ignition wires and inspect the cap for arcing before proceeding with diagnosis.


NOTICE: Total running time during which engine is loaded against brake must not exceed 20 seconds.


CAUTION: Brakes must be applied at all times.


^ Perform compression check on questionable cylinder(s) found above. if compression is low, repair as necessary.
^ Check ignition timing and idle speed per emission control information label.
^ Check EST operation.
^ Visually check distributor cap and rotor for moisture, dust, cracks, burns, etc. Spray cap and plug wires with fine water mist to check for shorts. Check in a dark room if possible.
^ Check for restricted fuel filter. Fuel pressure should be 40 to 50 psi (276 to 345 kPa) at key "ON", engine "OFF". With the engine running, fuel pressure should be 32-38 psi and should increase with engine load. Also check fuel tank for water. if fuel pressure is not in this range, see Chart A-4B. A-4B Dynamic Fuel Pressure Check
^ Remove rocker covers. Check intake manifold bolt torque. Check for bent push rods, broken or weak valve springs. Check cam lift. Repair as necessary.
^ Decarbon the engine using top engine cleaner. Follow the instructions on the can. if top engine cleaner improves or eliminates the condition then diagnose for cause of flow restricting deposits in the heads.