M271: momentary dip in idle at stop when electrical load applied
Thread Starter
Super Member
Joined: Mar 2012
Posts: 968
Likes: 62
From: San Francisco, CA
2005 c230SS
M271: momentary dip in idle at stop when electrical load applied
This may be normal behavior, but ...
I have the M271, 4cyl engine. Just replaced the motor mounts and spark plugs, tranny fluid done last year, removed and cleaned throttle actuator, the car is running really smooth and quiet now.
When the engine is fully warmed up and I come to a stop, the idle is perfectly stable and quiet with no vibration. But, if I apply a significant electrical load such as turning on the rear defroster, I get a momentary dip in idle speed and a bit of a rumble in the engine before it corrects itself and brings the idle back to normal.
Ii have an Ultra-gauge hooked up permanently to the car so I've been able to monitor rpm, throttle position, and ECU voltage during this process.
At normal idle with foot on the brake and off the gas:
* 650 rpm
* throttle position ~13.3%
* 13.75v
When I turn on the rear defroster:
* rpm drops as low as 615 rpm before it recovers back to 650
* throttle position bumps up to ~13.7%
* voltage drops to 13.45v
So this process takes only a moment, but it's enough for me to notice and want to fix if possible. At 650 rpm the engine is silky smooth and quiet, but under 625 it rumbles.
Is this normal for these engines, or is there something I can correct? I understand that an electrical load is going to cause back pressure on the alternator, but I'm a bit surprised that it's enough of pull to drop the engine rpm that easily. Might the engine already be under excessive load from another belt accessory? Or will an alternator at the end of it's life create excessive backpressure like this?
And suggestions on what to investigate?
Thanks. John
I have the M271, 4cyl engine. Just replaced the motor mounts and spark plugs, tranny fluid done last year, removed and cleaned throttle actuator, the car is running really smooth and quiet now.
When the engine is fully warmed up and I come to a stop, the idle is perfectly stable and quiet with no vibration. But, if I apply a significant electrical load such as turning on the rear defroster, I get a momentary dip in idle speed and a bit of a rumble in the engine before it corrects itself and brings the idle back to normal.
Ii have an Ultra-gauge hooked up permanently to the car so I've been able to monitor rpm, throttle position, and ECU voltage during this process.
At normal idle with foot on the brake and off the gas:
* 650 rpm
* throttle position ~13.3%
* 13.75v
When I turn on the rear defroster:
* rpm drops as low as 615 rpm before it recovers back to 650
* throttle position bumps up to ~13.7%
* voltage drops to 13.45v
So this process takes only a moment, but it's enough for me to notice and want to fix if possible. At 650 rpm the engine is silky smooth and quiet, but under 625 it rumbles.
Is this normal for these engines, or is there something I can correct? I understand that an electrical load is going to cause back pressure on the alternator, but I'm a bit surprised that it's enough of pull to drop the engine rpm that easily. Might the engine already be under excessive load from another belt accessory? Or will an alternator at the end of it's life create excessive backpressure like this?
And suggestions on what to investigate?
Thanks. John
Last edited by jkowtko; Dec 4, 2014 at 02:38 PM.
Thread Starter
Super Member
Joined: Mar 2012
Posts: 968
Likes: 62
From: San Francisco, CA
2005 c230SS
Oops, forgot to mention -- new battery around 6 months ago (one dead cell) ... the charging system tested out fine. My voltage ranges between low 13v and low 14v depending on rpm. It definitely never drops down into the 11 range and rarely even down into the 12 range.
Last edited by jkowtko; Dec 4, 2014 at 02:39 PM.
