Odd idle behavior - Mercedes Benz SLK Forum

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#1 Old 08-23-2016
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Odd idle behavior

Car did something weird yesterday and today.

• Start car, idle goes up to ~800 RPM
• After around 30 or so seconds, RPM drops to 500, all normal to this point
• While driving in traffic, (maybe 12 mins from start) RPM went back up to 800 RPM or greater, which made for a pretty fast “foot off the gas” speed, maybe almost 10 mph?
• After a few minutes, idle went back down to 500 RPM and the car behaved fine from there

Any one else experience this?
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#2 Old 08-23-2016
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did in my r171 when i first got it

they did a transmission update that was out then


then i also did it myself, accidentally 'slapping' the shifter and it went into 'manual' mode. tach would go up on acceleration but car would not go because i was still in 1st gear


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#3 Old 08-23-2016
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Originally Posted by sleebus View Post
Car did something weird yesterday and today.

• Start car, idle goes up to ~800 RPM
• After around 30 or so seconds, RPM drops to 500, all normal to this point
• While driving in traffic, (maybe 12 mins from start) RPM went back up to 800 RPM or greater, which made for a pretty fast “foot off the gas” speed, maybe almost 10 mph?
• After a few minutes, idle went back down to 500 RPM and the car behaved fine from there

Any one else experience this?
I had an intermittent irregular idle problem on my previous vehicle that turned out to be the IAC (Idle Air Control) valve. Not sure if these have one or not but that would be the first place I would look.
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#4 Old 08-24-2016
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So a bit of an update on this. I took it into the dealer as I was having a problem with a roof leak (they replace the entire roof!!!) and had them look at it at the same time. They said there were unable to replicate and the car had thrown no codes. Said it might be related to cylinder management (unlikely IMHO) or A/C compressor.

Well, it was doing it again this morning when I parked. Turned off ECO mode, no change. Turned off A/C, idle drops down to ~500 RPM. Yay! Turn A/C back on, idle stays at ~500 RPM (wt Heck?) There must be some sort of weird algorithm going on here, because in Texas, the A/C never goes off in the summer. Car starts up with the A/C on, and it drops to 500 RPM just like it's supposed to. Beats me what's going on.
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#5 Old 08-25-2016
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Originally Posted by sleebus View Post
So a bit of an update on this. I took it into the dealer as I was having a problem with a roof leak (they replace the entire roof!!!) and had them look at it at the same time. They said there were unable to replicate and the car had thrown no codes. Said it might be related to cylinder management (unlikely IMHO) or A/C compressor.

Well, it was doing it again this morning when I parked. Turned off ECO mode, no change. Turned off A/C, idle drops down to ~500 RPM. Yay! Turn A/C back on, idle stays at ~500 RPM (wt Heck?) There must be some sort of weird algorithm going on here, because in Texas, the A/C never goes off in the summer. Car starts up with the A/C on, and it drops to 500 RPM just like it's supposed to. Beats me what's going on.
My car idles at 600RPM and bumps to 800RPM when AC compressor engages. Is your AC working? Blowing cold? I would like to add that I always turn off AC and let cold air blow out of system b4 shutting vehicle down. The reason I blow cold air out of system is because the cold temp in it will cause condensation to build up which in time can contribute to a musty moldy AC system. Also its good idea to start car and let it go through choke and idle stage b4 putting additional load on engine right from start.
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#6 Old 08-25-2016
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Yep, A/C working fine, blowing ice cubes. I do let it do its thing when it starts; I wait for the idle to drop before putting it in gear. With older cars, I used to do the same with the A/C, newer ones automatically switch to "fresh air" mode when you shut down (although I really don't use the recirc) and that dries the coil so it doesn't get all musty.
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#7 Old 08-25-2016
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Yep, A/C working fine, blowing ice cubes. I do let it do its thing when it starts; I wait for the idle to drop before putting it in gear. With older cars, I used to do the same with the A/C, newer ones automatically switch to "fresh air" mode when you shut down (although I really don't use the recirc) and that dries the coil so it doesn't get all musty.
When you say odd idle, does the RPM fluctuate any or it just stays pretty steady at 500RPM?
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#8 Old 08-25-2016
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Idle is steady. The odd bit is when it decides to run at ~800 rpm instead of ~500 rpm.
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#9 Old 08-25-2016
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Originally Posted by sleebus View Post
Idle is steady. The odd bit is when it decides to run at ~800 rpm instead of ~500 rpm.
Does this happen when AC is on or off? When the AC is on, it will idle up when compressor kiks in (based on your temp setting) to compensate for the load and idle down when no load which I'm sure you know that already. I have found that even with AC switch off and fan blower on, the compressor will still cycle if temp is set to a low setting (had top down, need to reconfirm but thought it was odd to be still blowing cold). I may be misunderstanding, but it shouldn't be a problem unless it is doing it without AC or fan running.
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