4.0L Jeep MPI Engine Stumble or Stall During Warm-Up Solved

[Moses Ludel]

No codes, just an engine stumble as the coolant temperature reached around 140-degrees F.  With the vehicle at a standstill or parked, the engine would die during this transition.  Dissecting the problem ruled out the TPS, crankshaft position sensor, ignition misfire, injector flow, heated O2 sensor and the IAC valve.  What was left?  The MAP sensor.  This 1999 XJ Cherokee engine (identical to a '91-up YJ or TJ Wrangler 4.0L MPI engine for troubleshooting purposes) had 190,000 miles on the original MAP sensor.  The TPS, O2 sensor, all ignition tune-up parts and an injector cleaning with flow test had taken place within the previous 5,000 miles.

So how could the MAP sensor cause the stumble during warm-up?  The cold engine runs in open loop until the PCM gets the temperature sensor signal to go into closed loop.  The engine started cold or hot and ran flawlessly in open loop or closed loop once the system fully transitioned into closed loop.  The TPS, IAC and other devices worked well in both open loop and closed loop.  Considering each sensor other than MAP, there would be no reason for the engine to stumble only on the transition to closed loop at the same precise coolant temperature each time.  There would be misfires, stumbles or stalling under other operating conditions as well. 

The MAP sensor receives a vacuum signal that translates to an output voltage. That voltage is read by the PCM.  When the MAP sensor is defective, the voltage signal can be out of normal parameters during the transition to closed loop.  Here, all other devices were sending accurate voltage signals to the PCM. 

A quick check for a vacuum leak to the MAP sensor showed no issues.  Electrical connections were not faulty, either.  A MAP output voltage test while applying vacuum from a vacuum pump revealed a defective MAP sensor.  A new MAP sensor cured the problem.

Moses