Technical Low idle speed on 1.6 Marea

[MVirta]

A couple of weeks ago I had a MOT test for my 1.6 Marea. I noticed there that the idle speed was ”only” 703 rpm. Earlier years they used to give me the OBD test print but not any more. However, I have some of those prints, and they tell similar values: 700, 704 and 690 rpm. The oldest one (690) is from 2006.


Haynes states that the idle speed should be 800 [FONT=Times New Roman, serif]± [/FONT]30 rpm. Is there something wrong on my Marea?

 

[bpetersb]

The idle speed is set by the engine management ECU, from information from one or more of the engine sensors.

If the idle is rough or lumpy then perhaps there may be a problem, if your car idles relatively smoothly and does not stall at idle then I wouldn't worry too much about it.

If you are concerned then you could get the engine idle speed checked on a diagnostic machine which will probably take the RPM information from a plug lead. You may find that the idle speed is fine and it is your rev gauge that is under-reading.

Good luck with it

 

[franglais]

if you don't have access to diagnostic equipment you can still find the idle speed with a guitar tuner. you hold it near the engine and let it detect a note, normally it will be around 60hz, you then multiply this by 60, to find the Hz per minute, and then divide by the number of cylinders you have (4 in the 1.6).
for example, my marea runs at about 65hz, so (65*60)/4=975rpm
if you dont have a guitar tuner, there are free tuners available for android https://play.google.com/store/apps/details?id=org.cohortor.gstrings&hl=en and iphone https://itunes.apple.com/us/app/gibson-learn-master-guitar/id355744218?mt=8
Allan

 

[MVirta]

Thanks for the replies. The engine runs smoothly and does not stall. About a couple of years ago I had a problem with the crank sensor which was dirty, but after having it cleaned it does not bother anymore. (To have the engine management light stop blinking and to get rid off the fault code I had to do ”the phonic wheel adaptation”.)

Those rpm figures I gave are specifically from a diagnostic machine which is used at the MOT station to measure emissions and fault codes. Mareas own meter is quite close to 800 rpm – a bit below but not close to 700 rpm.

I can't think this is serious as it has already lasted about seven years.

Another thing: As the rpm is controlled by the ECU, why the rpm is not exactly 800 rpm? Why there is this [FONT=Times New Roman, serif]± 30[/FONT][FONT=Times New Roman, serif] tolerance?[/FONT] It couldn't be too diffucult to keep the rpm exactly at 800 rev.

 

[bpetersb]

just about every measurement on a car has a tolerance, yes it is probably possible to build a car without but we would not be able to afford them!

For instance, when they build an ECU do they specify gold band electronic components (plus or minus 5%) or will silver band (plus or minus 10%) be acceptable. The gold band resistors will be more expensive adding to the cost of building the component. Multiply the savings by the number of electrical and mechanical components on the car and the numbers get very high very quickly

If for example the maker can save £50 per car and they are building a million cars a year that means they are making fifty million pounds worth of savings. Any accountant will tell you that savings on expenses will transfer directly to bottom line profit.

Regarding your problem, if your rev gauge is reading around 800 RPM and the engine is idling smoothly you have nothing to worry about.

 

[MVirta]

Someone suggested that the timing between the crankshaft and the camshaft could be wrong by one tooth causing this low idling. Could it be so? And would there be other symptoms?

Once I had a Uno Fire on which the timing belt once jumped by one tooth causing poor idling and performance. Well, the timing belt can't jump on Marea's engine, so if there is a wrong timing it has been caused by a wrong installation. The belt has been changed twice in 2005 and in 2011, and at an authorized garage. The oldest OBD test print I have is from 2006, so basically it could be possible that an error was made in 2005 when the belt was changed the first time, and the error was repeated in 2011. Well, this is only speculation...

 

[bpetersb]

No, if your timing was one tooth out the idle would be lumpy!!

Stop worrying about a non-fault and enjoy your car!