RPM's drop down when stopping...
Senior Member
Joined: Sep 2004
Posts: 483
Likes: 0
From: AZ
One day I noticed that my PCV was making a buzzing noise. I replaced it and my truck acted exactly like what you are describing.97 4.6L
I think the PCV must have been going bad for quite a while before the spring got weak enough that the buzzing could be heard.
I knew the PCM would take a while to adapt to the new air fuel mixture. I did not want to wait so I applied the "Microsoft solution"- disconnect the battery for two minutes. Brought the in gear idle back to its steady 600rpm, where it has always been for the last 9 years
I think the PCV must have been going bad for quite a while before the spring got weak enough that the buzzing could be heard.
I knew the PCM would take a while to adapt to the new air fuel mixture. I did not want to wait so I applied the "Microsoft solution"- disconnect the battery for two minutes. Brought the in gear idle back to its steady 600rpm, where it has always been for the last 9 years
Member
Joined: Mar 2011
Posts: 10
Likes: 0
I also seem to notice, more when engine is warm that when i start from a stop there is a clunk type noise, i almost thought a u-joint, but it doesnt do it all the time, and not always under load, maybe related also to this
Senior Member
Joined: Nov 2009
Posts: 113
Likes: 0
How is the PCV? You might try and clean it out or replace it. Also the IAC could need to be cleaned. And already mentioned in this thread, is the PCV hose collapsing under high vacuum.
Hope it all works out for you.
Hope it all works out for you.
Junior Member
Joined: Jul 2008
Posts: 4
Likes: 0
From: Atlanta, GA
For those of you still having the problem with RPM's dropping when stopped. Try turning off your air conditioner.....the problem should go away. Turn on your air conditioner.....the problem returns. If that's the case, pick up a can of Arctic Freeze R-134a refrigerant from your local auto parts store (the one with the gage) and follow the instructions to charge the system. Harbor Freights carry's the 18oz can with the gage for $25.
Member
Joined: Sep 2010
Posts: 82
Likes: 0
havent used the a/c until today and it does it when its on or off, so i dont think the a/c has much to do with it. I went by the junk yard today and pulled the entire intake tube mass air flow and all off a 5.4 navi engine for 25 bucks. Im going to convert my intake back to stock and see if that changes anything which im thinking it will. If it does im going to take the intake i bought back to where i got it from.
Junior Member
Joined: Mar 2016
Posts: 2
Likes: 0
RPMs drop when decelerating. 97 F150 XLT 4x4 4.6L
If the converter lockup was the problem you should be able to distingish that by the way it feels with the engine trying to pull the truck with no throttle applied.
When you brake, the converter disengages as a normal function from the contacts on the brake switch signalling the PCM.
On decelleration, the PCM shuts down fuel to a low level and closes the IAC as a normal action and updates tables but is supposed to recover the idle so stalling never happens.
Something is causing this action to fail.
The 500 rpm you see is the base idle throttle stop setting without the IAC open.
Dirty/corroded connector, sticky IAC, vacuum leak under high intake vacuum conditions or no speed feed back to the PCM is a cause.
Does it do it setting still and jazzing the throttle?
When you brake, the converter disengages as a normal function from the contacts on the brake switch signalling the PCM.
On decelleration, the PCM shuts down fuel to a low level and closes the IAC as a normal action and updates tables but is supposed to recover the idle so stalling never happens.
Something is causing this action to fail.
The 500 rpm you see is the base idle throttle stop setting without the IAC open.
Dirty/corroded connector, sticky IAC, vacuum leak under high intake vacuum conditions or no speed feed back to the PCM is a cause.
Does it do it setting still and jazzing the throttle?
I'm not sure if it's related but I get this code: P0174 - Powertrain System too Lean (Bank 2).
I replaced the IAC and nothing changed. I read someone else's post that maybe the PCV has a vacuum leak or is collapsing. I'm hoping it's not the torque converter.