hanging idle upon start up

[no1sirbutler]

everytime I start my 99 explorer the idle hangs around 2-2.5K rpms. after about 20 or so seconds, it will settle down to the normal idle. anyone know what causes this?

Mike

 

[jlsparky7]

The vehicle is supposed to have an "auto-choke" at start up which causes a rich mixture. Its controlled by I belive the IAC MAF and Coolant temp sensor. Look into those. RPMS should be slightly higher but not 2.5k. Only about 1.5k or so.

 

[no1sirbutler]

any references on what to check? ill clean the MAF but what about the coolant temp sensor? also it does this even when the vehicle is warm and i start it.

 

[jlsparky7]

any references on what to check? ill clean the MAF but what about the coolant temp sensor? also it does this even when the vehicle is warm and i start it.

If it does it when the vehicle is warm I would look into an intake or exhaust leak first.

 

[rp23g7]

everytime I start my 99 explorer the idle hangs around 2-2.5K rpms. after about 20 or so seconds, it will settle down to the normal idle. anyone know what causes this?

Mike

My 99 4.0 SOHC has always done this, since it was new, maybe not for 20 sec, but it revs really high when you first start it, cold or warm.

I thought it was normal till i rode in someone else's 99 Explorer.

 

[2000StreetRod]

Check TPS & throttle stop

If the throttle plate closed stop is adjusted incorrectly then the engine will run fast until the IAC valve has time to correct the engine speed. An easy way to check for correct adjustment is to let the engine warm up at idle and then disconnect the IAC valve electrical connector. If the engine speed increases then the throttle plate is open too far. The throttle plate should close almost completely but not stick closed when opening. Adjust the closed stop screw accordingly. Then check the TPS wiper voltage with the ignition On and the engine Off. It should be about 1.0 volts. Disconnect the battery for a few minutes to erase the stored TPS range values and the fuel trims. Reconnect the battery and the IAC valve connector and start the vehicle.

If the cold start engine speed flare exceeds 2,000 rpm after the throttle stop adjustment then there is probably an intake leak or the engine is running lean for some other reason. The PCM uses the last saved long term fuel trim (LTFT) on engine start and does not adjust the LTFT until the throttle changes from closed. If the LTFT was max rich to compensate for the lean condition then the engine may be excessively rich until the short term fuel trim (STFT) has time to compensate.