Toyota Repair: low idle and tranny clunk, late model cars, mass air flow

Question
I have a 99 avalon, 130k. I have been getting a gradually worsening low idle especially when I come to a sudden stop. Now it runs rough as soon as I take my foot off the gas at most speeds. Also, when i take my foot off the gas at 30-40mph and then put it on again the tranny clunks in hard. This does not happen during the first 1-2 miles from a cold start. Both problems disappear when I disconnect the top wire harness or the vacuum hose off the egr valve. The check engine light only comes on when I start disconnecting things. I have replaced the egr valve, no difference. I had a place look at it today and all the parts he checked seemed to test out ok, ( mass air flow, egr, low idle adjuster etc). The return tube to the egr is not plugged at all. Gas mileage all together or egr disconnected is poor, 20 around town, 23 on the highway. thanks, Bradd

Answer
On older model cars, there are transmission "kick down" linkage that's attached to the throttle linkage. When the driver accelerates, the linkage to the transmission will kick the trans down to a lower passing gear. Late model cars do not have this type of set up. Late model cars have ECT (electronically controlled) transmissions. In your case, the Throttle Posision Sensor sends a signal to the ECU how far the throttle is open and how much to drop the transmission passing gear to gain acceleration. If this did not occur, your transmission would burn up and a rebuild would be necessary. In your case, it is possible that the Auxilary Air Valve inside the intake manifold is dirty and clogged or your transmission is not downshifting properly. Worst case scenario is that your ECU needs to be flashed or is going bad. However, a bad ECU is not that common but are very costly...between $1,000-$1,500 depending on year/make/model...