91 q45 engine rpm drop

[drewbert]

For the first time yesterday I noticed that while at crusing speed (59 MPH) with a warm engine that the engine rpm's were bouncing around a little bit. It was enough of a change to feel it inside the car. The speed was held constant on a flat road, but the rpms went from 2000 to 2100 and back to 2000 in intervals of maybe 15 seconds while holding that speed. At higher freeway speeds (70 MPH) I didn't notice this happening. The car is at 191,000 miles now and the is running nearly perfect otherwise. Ideas? Something to be worried about?

[Q45tech]

You might correlate this with a Consult........sluggish O2?. The IAC functions [dirty?] to act like a rpm stop so the engine doesn't die if you suddenly let off throttle [the IAC might be at 75% supplying enough air to run at 1200-1400 rpm].........the progressive step down of rpm when you let off.

Any of these things might cause surging [the O2 is the one the manual suggests to check first].

[Q451990]

Do you think it could be the torque convertor hunting at that speed? I'd think not, but if you're cruising along and maintain the speed with some throttle, and then none, that could expalin it also... The T/C will unlock when you let off of the pedal, and then lock again under some throttle.

Heath

[maxnix]

Might try to run 60 mph in third and see if this hunting occurs. If not, perhaps the IAC valve/clogged passages can be eliminated. It is a good idea to inspect it and clean as necessary anyway.

[Q45tech]

"The speed was held constant on a flat road, but the rpms went from 2000 to 2100 and back to 2000 in intervals of maybe 15 seconds while holding that speed"

I have to trust that the owners is holding the accelerator perfectly steady.........some people unkowingly tap dance ........why you take a test drive and watch the driver and read the TPS voltage while watching the rpm........with Consult.

When you do a power balance at 1200-1400 rpms [no load a dead cylinder will only drop the rpm 60-100 rpms] engine has enough interia and power.................so at 2,000 rpm?