RPM drops on idle

Jesus, I can't believe there's a thread dedicated to this problem. I love this forum. I recently experienced this after filling up at a QT gas station in Georgia. I don't know if the quality of the gas is related but I just noticed a drop in RPM from ~750 to ~500 with slight jerk. I thought the engine was going to stall. I have a 2011 6MT. It's cold in ATL right now. I just crossed 100k miles. The car's been solid so far. Hope it's not the beginning of the end.

jqsmooth wrote

Jesus, I can't believe there's a thread dedicated to this problem. I love this forum. I recently experienced this after filling up at a QT gas station in Georgia. I don't know if the quality of the gas is related but I just noticed a drop in RPM from ~750 to ~500 with slight jerk. I thought the engine was going to stall. I have a 2011 6MT. It's cold in ATL right now. I just crossed 100k miles. The car's been solid so far. Hope it's not the beginning of the end.

Here in ATL with a little over half of your mileage with QT gas. I'm a consistent 550ish RPM idle with the occasional jump in idle speed.

jqsmooth wrote

Jesus, I can't believe there's a thread dedicated to this problem. I love this forum. I recently experienced this after filling up at a QT gas station in Georgia. I don't know if the quality of the gas is related but I just noticed a drop in RPM from ~750 to ~500 with slight jerk. I thought the engine was going to stall. I have a 2011 6MT. It's cold in ATL right now. I just crossed 100k miles. The car's been solid so far. Hope it's not the beginning of the end.

If it occurs after filling with gas, it is the fuel tank breather valve. Look in the DIY section. There is a very good write up.

drrust wrote

If it occurs after filling with gas, it is the fuel tank breather valve. Look in the DIY section. There is a very good write up.

Thanks for the tip.

spazzyfry123 wrote

Here in ATL with a little over half of your mileage with QT gas. I'm a consistent 550ish RPM idle with the occasional jump in idle speed.

it may have been a coincidence. I try to fill up at Costco when I can. Never had issues with their gas. This was my first time filling up at QT.

jqsmooth wrote

it may have been a coincidence. I try to fill up at Costco when I can. Never had issues with their gas. This was my first time filling up at QT.

Could be gas but I had this issue and it only occurs sporadically, not every time I filled up. I thought bad gas at first as well but ultimately it gradually started more frequently from any gas, even stalling the car a couple times. Just keep an eye on it and should happen again, you know what it likely is.

Normal every time I fill up.

serranot wrote

Normal every time I fill up.

Not normal!

http://www.m3post.com/forums/showthread.php?t=1273927

Hopefully you aren't using the phone ap so you can open the pics in a separate tab. It had the most detailed description of removing the plenum. Part is relatively cheap.

drrust wrote

Not normal!

http://www.m3post.com/forums/showthread.php?t=1273927

Hopefully you aren't using the phone ap so you can open the pics in a separate tab. It had the most detailed description of removing the plenum. Part is relatively cheap.

Mine isnt anywhere near that bad and has nothing to do with filling up.
It does it usually all the time. Idles fine but when applying the brakes at idle (at a stop light or similar) it will go down to 500rpm and cause a vibration.

I'm wondering if this idle is 6mt related. Everytime I depress the clutch the idle goes up to 7-750. Today morning with the heater on, my idle is staying around 5-500.

R3dliner wrote

I'm wondering if this idle is 6mt related. Everytime I depress the clutch the idle goes up to 7-750. Today morning with the heater on, my idle is staying around 5-500.

Mine is DCT and has the same issue

R3dliner wrote

I'm wondering if this idle is 6mt related. Everytime I depress the clutch the idle goes up to 7-750. Today morning with the heater on, my idle is staying around 5-500.

Mine definitely shoots up to that 700 range when I depress the clutch pedal. Sitting in neutral with the clutch left engaged results in my 500ish RPM idle.

spazzyfry123 wrote

Mine definitely shoots up to that 700 range when I depress the clutch pedal. Sitting in neutral with the clutch left engaged results in my 500ish RPM idle.

My car's been like this for the past 1.5 years since I picked it up at 19k miles, so I'm concluding its normal.

My last couple of e92's would drop down to around 500 RPMs with my foot on the brake pedal. Release the brake and the idle would shoot back up. I suspected a brake booster vacuum leak but without a code being thrown, BMW says no problem, normal operation. I don't think so. The other 5 e92's I had didn't behave this way at all, solid idle with a very slight increase of RPM's only when the AC was on. Now that's normal.

Denk wrote

My last couple of e92's would drop down to around 500 RPMs with my foot on the brake pedal. Release the brake and the idle would shoot back up. I suspected a brake booster vacuum leak but without a code being thrown, BMW says no problem, normal operation. I don't think so. The other 5 e92's I had didn't behave this way at all, solid idle with a very slight increase of RPM's only when the AC was on. Now that's normal.

If you suspect its a brake booster leak, wouldn't the car stall out or idle be erratic? Can you elaborate "I don't think so" with something to back it up besides you had 5 bimmers that didn't do it?

Can't speak to the cause as my sedan does not behave like this, never has. Idle is rock solid foot the brake or not....

R3dliner wrote

If you suspect its a brake booster leak, wouldn't the car stall out or idle be erratic? Can you elaborate "I don't think so" with something to back it up besides you had 5 bimmers that didn't do it?

I suspected a vacuum leak since the only time my idle dropped was with the brake pedal depressed. The harder I pushed the brake down the lower the RPM's would drop. The idle would get rough to the point of causing the car to shake a little and seemed like a stall was coming but it never did. Let off the brake and boom, RPM's would come back up and have a smooth idle. What else could it be?

Denk wrote

I suspected a vacuum leak since the only time my idle dropped was with the brake pedal depressed. The harder I pushed the brake down the lower the RPM's would drop. The idle would get rough to the point of causing the car to shake a little and seemed like a stall was coming but it never did. Let off the brake and boom, RPM's would come back up and have a smooth idle. What else could it be?

thank you sir. Interesting. I have to try this again in the morning. iirc my rpm would stay the same regardless of brake force at a stop.


Edit:
Idle stayed around 550 even after releasing the brake.............

This happens to me as well, except when I press in the clutch, idle goes back up to 750.

Mine ended up being the vent valve for the fuel tank that is found under the intake plenum.

I have unstable idle that's led to it stalling a few times. In my case, it's not when the tank is full but rather gets worse as the tank gets empty. I'm going to replace plugs, breather and both TAs since I'll be under the plenum anyways. Debating just replacing ICV while I'm at it. One of these things must be causing it and I'm okay with just doing some preventative replacements.

Has anyone heard of the breather valve being a problem with less than 1/4 full tanks?

simple2010 wrote

I have unstable idle that's led to it stalling a few times. In my case, it's not when the tank is full but rather gets worse as the tank gets empty. I'm going to replace plugs, breather and both TAs since I'll be under the plenum anyways. Debating just replacing ICV while I'm at it. One of these things must be causing it and I'm okay with just doing some preventative replacements.

Has anyone heard of the breather valve being a problem with less than 1/4 full tanks?

I don't think mine is responsive to fuel quantity but rather ambient temperatures. My guess is thermal expansion with a vent valve that says no? (That's a guess without any knowledge of how the vent actually works) Colder temps have been fine, but since it has started to warm back up a little down here it has certainly gotten worse.