Engine lopes when cold
filed in 1994 Ford F150
Question: I have a 1994 Ford F150 with the 351 Windsor engine. The engine lopes when it is cold. The rpm’s fluctuate from 1000 to 800 in about 5 – 10 second intervals. I recently tuned up the engine, it has new plugs, wires, distributor cap, rotor, fuel filter, air filter, and PCV valve. I checked the resistance of the Idle Air Control Valve and the Throttle Position Sensor, They both check out. When the engine has reached normal operating temperature the rpm’s basically level out to around 750, there is a slight variation but not much. I am completely stumped by this issue. Its driving me crazier than I already am. I would appreciate any assistance. Thank you for your time.
Answer: Possibly a cold start sensor, purging or vacuum advance. Speaking for most cars, the cold start sensor boost power and raises the rpm to about 1200 or so, when cold. The vacuum advance (tied to a coolant sensing device) advances the timing for extra horsepower when cold. You may have a cold sensing problem, or may need to purge air from the coolants for a steady, continuous sensing. Trapped air commonly fluctuates the rpm from 1000 to 800 in about 5 – 10 second intervals. A minor leak in the air pipe, between the MAF sensor and throttle body can, paradoxically, cause a rich condition when cold, without setting a diagnostic code. It can also cause a lean condition. The system can “hunt” for a good mapping, and mixture will be unstable.
TIP: Use propane gas or carburetor cleaner to find air leaks. When the flammable agent hits the leak, conditions will improve. Never use ether (starting fluid). Get a can of Berryman’s B-12 Chemtool, or Gumout, or similar product. It’s what the smog inspection techs use to pinpoint air leaks. 
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