jolt212

Hi all, I'm new to the forum and so I'm not sure how to navigate my way around the site as yet. Apologies if I have not followed the correct procedure by not introducing myself etc. I am in desperate need of help. My 2005 Lexus RX 330 has been at the mechanic for some time now and he is at a loss to diagnose the problem. Unfortunately no error codes are being displayed. The symptoms are rough idling and hesitation. The RPM seems to dip like its about to stall and then jumps back up, it hesitates and surges upon acceleration, especially when I put my foot down on the accelerator or am going up a hill. However, if I am driving at a constant speed on a flat surface it's fine and there seems to be no problem. The mechanic has thrown a whole lot of parts at it but to no avail. I have noticed of late that when it gets started and is left to idle, especially when the air-conditioning is put on to place the engine under further load, the gas usage on the dash screen seems to jump to limits I have never seem before. If anyone could please offer some advice with this I would be extremely grateful, I miss my Lexus and want her back. Thanks