Car Mechanics (UK)

High tick­over

- Alan Wor­land Austria · Belarus · Iceland

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.

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