tranny question??

i was driving on the highway today and noticed some weird trany behavior when going over 50 MPH. if i take my foot off the peddal and just coast the rpm's are at a steady 3000 then all of the sudden it drops down to 1200 rpm's which slows the car down a bit then jumps back up to 3000. it will continue to do this untill 1. i excelerate or keep foot on gas. or 2. am under aprox. 50 MPH is this common for the car to do? or does it indicate a tranny problem??

That does not sound unusual. Mine has done the same thing since I have had it, (eleven years). When you take your foot off of the throttle the TPS signal drops to the idle position and this will signal the ECM you are preparing to slow down so I believe this may be the torque converter unlocking, and it also seems to reduce timing and injector pulse width for short periods. I do not know why it pulses like this, but it seems coast down has little, if any, engine braking as it tends to freewheel in overdrive. If it is a problem with something in the transaxle or ECM, it has not affected my car at all. If this is a '90 and earlier, your rpm should be less than 2000 if in high gear and the converter is locked up, more like 1500, and it will rise a few hundred rpm when the converter unlocks. It may be possible your IAC is a little slow to respond also. The IAC opening does tend to follow the accelerator somewhat, so it is prepared to let the idle down more slowly than letting the throttle snap back to closed, partly an emissions thing and partly for smoother operation.

There is a function in the ECM called "Decelleration Fuel Cut Off" (DFCO) that is intended to hold the throttle open slightly durring coast down and avoid the puff of HC that just cutting the throttle would cause. This could well be exacerbated by a sticking IAC.

Greg's car had a similar problem until I removed the DFCO entiely (manual trans).

With a stock car I would clean the IAC first.

well im glad it dousnt sound like my tranny is going out!! that is a relief, ill check out that iac and clean it our again see if it helps---thanx