slowing down on the freeway a slight surge happens?

[MarauderMatt200]

so when i let off the gas and let the car coast from about 60mph and once it coast to about 45mph the car surges a little. the RPM's jump up and down from about 1200-1700. Just started doing this about 2 weeks ago. But it does not do this every single time??? Car has 93K on the clock and 4.10 gears with Mo's tune. What is going on with the car??? Any help will be greatly appreciated!!

[burt ragio]

Sounds like a vacuum leak.

[babbage]

IAC (Idle Air Control) would be my vote. Remove it, clean it, reinstall. Clean Tbody while you have the IAC out...

[MarauderMatt200]

IAC (Idle Air Control) would be my vote. Remove it, clean it, reinstall. Clean Tbody while you have the IAC out...

I have already cleaned the TB two months ago...what and where is the IAC located?

[BigM460]

Mine does this when coasting down hill. Once the converter unlocks, it will oscillate between idle and the road speed rpm.

[babbage]

I have already cleaned the TB two months ago...what and where is the IAC located?

Bolted to the throttle body.
Looks like this: http://www.doautoparts.com/html-en/image/1-omJQFRKVJEZr-1-1.jpg

https://www.youtube.com/watch?v=vFhoNykgQtc

[RF Overlord]

Mine does this when coasting down hill. Once the converter unlocks, it will oscillate between idle and the road speed rpm.^^^This^^^

[Badass_forever]

^^^This^^^

also this, but you IAC could still be sticking.

[BigM460]

also this, but you IAC could still be sticking.

I've tried this with absolutely no change. Looked perfect, swapped for another just in case, still the same.

[BLACKMARAUDER04]

You might have a bad COP.

[Blackened300a]

My car has been doing the same ever since the blower install and new tune. I'll be coasting under 60mph and the converter locks and unlocks which causes the RPM to jump up and down. I told my tuner about it, and even had me datalog it while it was happening. I tried 3 different tunes and I even went with a brand new Motorcraft cobra IAC with a tune adjustment to make it work and I still have the same issue. My tuner said he sees nothing on his end in the tune that is causing it. My friend with a lightning had the same issue and it was indeed a issue in the tune, just not sure what the fix was.
My tuner told me to come down and take him for a test drive to show him what it's doing, just haven't had a chance to. I have a 3K stall converter, but it never did this when I had previous tunes.

Here's a short video of the drop out. You can hear it with the exhaust.
http://youtu.be/A8SaVyLUXgI

[Green96]

Mine has started doing that recently too. I believe it is happening when the air conditioning compressor cycles on or off. The IAC will try to compensate for the change in load but there can be a slight delay. My charge is low causing it to cycle on and off relatively frequently. Watch your compressor. If your compressor cycles on and off while sitting still, this is probably your issue.

[RubberCtyRauder]

Mine does it when coasting, at about 2000 rpm, it will drop about 500 to 1500 then back to 2000 a couple times. According to Jeff at Mo's it is due to the converter unlocking and can be dealt with some in the tune by adjusting the IAC..Mine is better than it was after tweaking the tune a couple times....does not seem as sensitive..Not sure how far it can be tweaked. This was all last late summer when adjusting it.

[MarauderMatt200]

You might have a bad COP.

a what?

also thanks for all the feedback some really good info. Ill try to clean IAC 1st.

[RF Overlord]

COP = Coil On Plug...it's the ignition coil that sits on top of the spark plugs, and I doubt it could cause the issue you're having. Bad COPs usually show as a skip or mis-fire on acceleration, not decel.

[Green96]

Not trying to harp on this, but turn off the AC and defrost if you are using it and try to reproduce the problem. If you are not using AC or defrost then never mind.

I double checked mine tonight and the problem is gone when I turn off the AC.