High tickover

Upon starting my 1998 Astra 1.8 CDX estate from cold, it starts immediately with revs at about 1500rpm and this drops within seconds to 850rpm and then to its normal 750rpm. When starting from hot, the revs start at 850rpm and drop almost immediately to 750rpm. Leaving a hot engine for two or three hours, upon starting the revs are about 1700/1900, then drop over the next few seconds to normal, but you can encourage them to drop more quickly by blipping the throttle.

I wondered if the temperature sender was causing this, so I replaced it, but the new sender read lower than the original. I have checked the original sender in a can of water and, when heated, confirmed that it was accurate, so I refitted it.

I have removed the throttle body and the cover over the idle control motor assembly and it all looks clean and free, although I did give the potentiometer track a squirt of Servisol contact cleaner.

Do you think I am looking at a problem that isn’t there? I am used to engines starting at lower revs – my Cavalier 2.0 with no cat only seems to tick over at the normal speed. I wondered if the increased revs have something to do with getting the cat up to temperature?

The circumstances you describe are pretty standard running conditions. The ECU does take over control of the engine tickover speed and, given the information available to it from the air and coolant temperature, regulates the tickover speed accordingly. This is calculated to give the best warm-up cycle for the temperatures. Autodata does warn of one possible failing with the system, which is a poor connection from the ECU to the vehicle earth – although, when this problem occurs, the engine will run erratically, which does not really apply to your vehicle.