RPM's unsteady while coasting

[BrentS]

I have an intermittent occurance of the RPM's dipping while coasting at highway speeds (i.e. 50-70 mph). It kinda feels like someone's lightly applying the brakes for short periods (i.e. 1/2 second or so).

Any ideas on this?

To detail; I would take my foot off accelerator pedal at 65mph, and instead of rpm's slowly dropping with speed, they abruptly go from say... 1600 to 1200 for 1/2 to 1 second, then back to the more normal 1600-ish.

RPM's at idle or when foot is on accelerator is otherwise very constant.

[NCReatta]

Have you done anything to the exhaust system lately?

[padgett]

Dirty IAC not responding to DFCO command ?

[2seater]

Is this a new occurance? The speed is high for the deceleration fuel cutoff to be activated but it sounds like the same feeling. The other possibility is the torque converter is unlocking, which is normal.

[Guest Mc_Reatta]

EGR valve reacting to TPS going to idle value or TCC doing same?

[ductune]

Is this a new occurance? The speed is high for the deceleration fuel cutoff to be activated but it sounds like the same feeling. The other possibility is the torque converter is unlocking, which is normal.

This is a strange one but I think you're on to something with the TCC but it sounds like the TCC is staying engaged instead of unlocking, as you said is normal.

The TCC unlocking would raise the idle instead of it dropping.

Maybe it is engaging and disengaging sporadically. Mine normally unlocks on deceleration and reengage if I step back into it very slightly. If I step into it harder it stays disengaged longer.

I would try going into diagnostics and watch the status light for the TCC to see if it coincides with the rpm change.

[ductune]

I have an intermittent occurance of the RPM's dipping while coasting at highway speeds (i.e. 50-70 mph). It kinda feels like someone's lightly applying the brakes for short periods (i.e. 1/2 second or so).

Any ideas on this?

To detail; I would take my foot off accelerator pedal at 65mph, and instead of rpm's slowly dropping with speed, they abruptly go from say... 1600 to 1200 for 1/2 to 1 second, then back to the more normal 1600-ish.

RPM's at idle or when foot is on accelerator is otherwise very constant.

Are you sure the rpm isn't dropping when you slightly step back into the throttle? That would be normal. If you step into it harder it should keep the torque converter clutch disengaged but if you slightly rest your foot on the gas after letting off the TCC will unlock but reengage causing the rpm to drop.

[BrentS]

I also have the intermittent "high idle" issue that I (and others) have not been able to solve. For some reason, the ECM commands the IAC to be a little more open, giving me 900 rpm idle; but if I shut/re-start engine, idle is normal......weird. But, that's probably a different problem, as this "coast RPM" issue is recent. PS pressure switch seems fine, too.

IAC was replaced when trying to trouble shoot above high idle (although not surprisingly, it wasn't the issue, since ECM was commanding the more open position).

I guess I can try to watch the IAC command, when it happens; although it would be nice to see RPM at the same time...sometimes it's tough to feel the drop.

CAT was replaced a while ago; nothing in exhaust has been done lately.

RPM's are definitely dropping (briefly) when foot is completely off pedal; you can feel the increased deceleration when the RPM's dip, as I described above.

I'll take a look at TCC status light.

Should I just change EGR valve, anyway? Are they easy? Never did one.

[padgett]

Have seen a bad coolant sensor (reading low) cause a high idle and poor MPG but should be able to see that.