General Idle Revs

[yuri]

Hi all,

today i found out my Marea is making more noise idling than normal. Looking at the rev meter, i saw it was doing 1100 rpm instead of around 900.
After waiting a few seconds all is well again.
But when i push the gas pedal while in neutral, it revs up good, but it seems more "slow" when the revs are returning to their idle state.
I'm not really sure if it's possible but it looks like the engine is holding back a correct rev "countdown"

It's a Marea ELX 105 JTD and it's about to have it's cam belt changed, so that might be the problem too.
I just wanted to check it out, because if it is the cam belt, i'm going to have it replaced sooner...

Thanks in advance for any replies

edit:
offcourse, i looked under the bonnet and the cam belt looked fine, but i'm no car mechanic.
Also, I continued reading topics which have similair subject and read something about a rattle around the alternator. I think i heard that too, might that be a problem?

 

[jug]

the timing belt cant have anything to do with it unless it has become slack and slipped a tooth, which i doubt has happened because you'd really notice it.

when you release the accelerator the injectors close which starves the engine of fuel and brings the rpm down to idle. when the rpm reaches idle the injectors begin to work again as the ecu controls the idle (based mostly on the coolant temp sensor and rpm sensor readings).

i cant think of anything that could slow the rate at which the revs will drop to idle unless the idle is constantly 'on' in the ecu, meaning the injectors do not completely stop fueling when you take your foot off the accelerator. the fact that the idle rpm has changed could indicate a fault in the idle management, which could explain this. however the only way this could happen is if the signal getting sent to the ecu would make it constantly maintain the idle fueling, and thats not really possible unless the rpm sensor was duff, and if it was you could not drive it, or it would at least having more serious problems.

the other more likely option is that you only noticed this problem when the engine was not warmed up, in which case it is normal. when the coolant temp sensor has a low reading (below operating temp) it will cause the injectors to maintian fueling even when you lift off the accelerator. this is done to stop the engine stalling in the cold. in an older pre-ecu car you had to accelerate and stop at the same time! the ecu does this for you on modern cars so they can be driven when cold. its part of the cold running management strategy. and one of the best things about ecu's.

 

[John Mc]

My 115 Weekend does the same. Taking it back to Grays tomorrow.

It's the plug going into the throttle potentiometer.

The cable broke off inside the plug 2 years ago and was found and fixed by them.

Had a service 2 weeks ago, changed the throttle pot at the same time and has been getting worse ever since. Stsrted it this morning and the nrevs went down to 500 and uo to over 2k, and cycled. Looked tonight and the plug is knackered again.

Basically the ECU can't read the throttle position, so it "guesses".



John.

 

[jug]

the revs went down to 500 and up to over 2k, and cycled.

yes an erratic idle is almost always a throttle position sensor fault, but that is very different to the fault described by yuri