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RPM's fail to drop while driving.

7K views 8 replies 4 participants last post by  chartmaker 
#1 · (Edited)
'02 24v Taurus. Runs like new, 210k. Great idle, smooth. Has the normal rpm at startup than drops to 700 rpm or so.


Problem is that cruising at 40 mph and let up on petal it will go 1/4 mile before rpm start to drop. If you pop it out of gear rpm will raise. If I plip the throttle rpm may raise to 2k and hang, but will always come back down after awhile. It doesn't do this all the time, it may be perfect for couple of months.


If I remove plug from IAC car does not do any of this, every thing is normal except rpm drops faster when petal is released.
I could live with this if winter wasn't coming.


Its been to mechanic, smoked test, etc. It has added clamp on cruise control cable, I changed IAC and TPS with Motorcraft.
pcv valve and tubing changed, vacuum harness for egr sensors changed. I've had vacuum leaks and bad pcv and that never caused this. Everything looks good. Throttle feels very free. It always returns to normal when braking.


It doesn't seem that it sure hang at 2k while relearning (did disconnect battery and turn on headlights).


With iac unplugged its good except for coming winter and engine ck light it caused. Sorry for long post, got lots of time and money in this.
 
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#3 ·
No, rpm stays constant, and if you shift to neutral rpm may raise from 1500 to 2000. Not normal, had this car 14 years so I know normal. Problem is not constant, works ok most of the time. I would expect rpm to slowly drop when petal is released. If I'm rolling in neutral and rev motor it may hang at 2000 rpm and slowly drop. If IAC is disconnected it will drop right away. Drives and ildes great with iac disconnected.
 
#4 ·
Any carbon buildup in the throttle body in and around the plate?
New PCV doesn't mean its a good one and that's what it sounds like to me that its sticking open.
IAC disconnected is fully closed and compensates for PCV passing too much air. I had a erratic issue like yours due to the PCV
 
#5 ·
Thanks, throttle body was pretty clean. Cleaned it with cloth and proper cleaner. Yes it could be the pcv valve. The first time I changed it you could look right thru and it didn't do this. I installed a motorcraft valve then. It runs perfect now with iac unplugged.
Before it could stay at 40 mph when releasing petal until you pressed and held brake in a normal way. It never strained against brake. Always returned to normal idle.
 
#6 · (Edited)
See pic, this from a used '03 DOHC. PCV stuck closed, someone replaced the elbow but left the old PCV in. I cleaned up this mess and for quick and dirty I got Autozone PCV and stuck it in. Then fast idle ~35 mph holding on level road. I ordered the MC from RA and put that in and idle is very good, good engine braking too.

The AZ PCV gives way too much air flow and the idle follows air flow. Engine can't idle fast without source of air. Air flow: TB, IAC, PCV, leaks.

-chart-
 
#7 ·
It has motorcraft pcv from Rock/Auto in it now, been awhile. I'll order another, maybe also a temperature sensor. The idle speed has been good 7/8 hundred with iac hooked up. Just trying to get it to drop from 1500 rather than hang for much longer than normal. I know its programmed to not close iac right away because of emissions but it sometimes hangs. Thanks I like all ideas and plenty to learn....


I've had bad pcv (slug inside almost gone) and sensor vac hose fell off intake manifold and it doesn't act like that. Good steady idle in/out of gear. lol, never had pcv valve so hard to change.
 
#8 ·
Thanks, it was PCV valve. Changed it, cleaned throttle body and MAF sensor. Runs perfect and rpm's drop like they should.
Tip I read says leave cables hooked to throttle body and bracket, saves time. Another is to use 3/4" i.d. plastic pipe 18" long to remove (twist) pcv valve. Use dermal tool to notch pipe so valve corners will set inside pipe. Then 1/4 turn to remove and install.
If you get engine ck codes P0113 & P0102 after your done you left MAF sensor connector off. I did...
 
#9 · (Edited)
Can't believe you left something off: :lol2:

Let me count the ways?

Pic of my home made tool, I whittled the wood peg and heated the coupling with a heat gun and formed the square. The masking tape is because mine was a bit loose and that was just enough to make it hold the PCV.

The PCV is critical for proper performance. Experience talking.

Congrats on your fix.

-chart-
 
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