NorfolkBird,

Firstly I have a 1.5 SX Sirion so I have some insight into the nature of things, although mine is not an automatic transmission as yours is. I have had a 1.3 automatic in a previous life though. Both are awesome cars. I also apologise in advance if this is long-winded but you did ask, and I am honestly trying to keep it at a less complex level (but it is complex) too, but at the same time making it hopefully enjoyable to read. I have an engineering background and I have done all my own car maintenance since birth.

When I bought my 1.5 SX second-hand it idled low from start-up, as you say of yours, and frankly it seemed generally “unhappy” in the engine running department. Now being a man who likes his car to run well and efficiently I embarked upon some putting right - there was nothing basically wrong with the car, except missing some tender loving care and basic servicing, and giving it a good thrash (an Italian tune-up). Anyway my motor is how it should be now – it goes like the clappers, runs very smoothly, and it is spot on for fuel efficiency (in fact very good). It does everything it should very well.

My engine now idles at 1400 rpm on a cold start and it takes quite a while for it to accept when the temperature is hot enough to begin using the normal idle condition. There is no temperature gauge in this car and temperature indication is via a little green dashboard light - if it is green the engine is cold; no light and the engine is within the correct running temperature range; if it is red it is too hot (never seen that one).

I am headed somewhere with this is. It is quite a dumb system for actually gauging what the engine temperature is but my experience is that even when the green light extinguishes it is still some way off having reached full running temperature and a few more miles are required before it is at proper running temperature. I have found both my Sirions slow to warm up.

So now to rpm. Modern engines are controlled by a computer and that includes control of the idle speed. It is sorted out by sensors feeding back information to the computer, and devices receiving information from the computer to control the actual idle speed for varying conditions. In short there are a few conditions which affect a range of idle speeds.

My car does the following:
• Car stationary and cold – 1400 rpm “ish”
• Car stationary and at proper running temperature – normal idle 600rpm “ish”
• Car rolling in neutral and cold – 1400 rpm
• Car rolling in neutral with proper running temperature – 1000 -900 rpm
• Car going from a rolling state to a stationary state – the rpm is initially at 900–1000 rpm and then decays under control of the computer to 600 rpm. This can take a few moments.
• With electrical current draw on the alternator (lights on, fan etc) the stationary idle rpm will rise slightly to compensate for the load imposed on the engine. (the computer is doing its job)

You will note I mention the car being stationary and rolling. Even in neutral if the car is rolling (even slightly) it maintains a higher rpm from normal – not quite sure why it has this feature, but it seems to be related to softening the throttle action from full open to full close and subsequently dampening any tendency to jerk or lurch the engine in an engine braking condition (over-run).

I will add a caveat to all of the above soon due to your car being an automatic, but to summarise the engine idle is in a constant state of alteration due to a bunch of changing conditions, and for what it is worth I would say your car is doing exactly what it should be doing. The low idle condition you have been used to was actually the wrong condition due to needing a servicing, which it has just had.

The caveat I mentioned is of course your car is an automatic, which just throws in another one of those ever changing conditions for the computer to sort out. Indeed automatics generally idle higher than there manual equivalents to compensate for the drag of the gearbox at a stationary idle condition. So you have to use the information I have given to suit your own needs but the basis is the same.

In short it is my belief your car is working exactly as designed. It just feels different because it has been serviced and things are now as they should be. I say that because the rpm you quoted before the servicing was 500 rpm – that is low. The rate of slowing down you mention after the servicing is entirely like my 1.3 automatic I had – automatics don’t engine brake as well as manual cars. And my manual 1.5 SX now is also reluctant to slow down. I honestly think all is well with your car.

Incidentally you have a very rare car – there are only 350 Sirion 1.5 SX in UK, and an automatic is even rarer. I have made other posts here regarding my car and you may want to have a read especially about the rear axle thing.

Regards