2000 S70 RPM undershoot/conk when stopping

[Brucebo]

Others have had this issue but the last one was started six years ago. I also added some new information.

When coming to a stop, idle dips to below 500 rpm, then recovers (except when it occasionally conks.) Feels like the transmission is dragging the engine down until it "disengages" and the idle recovers. I can "compensate" by lightly pushing the gas pedal while braking.

I don't think it's the transmission though because if I put the car in neutral, race the engine, then suddenly let go of the accelerator, the rpms will drop all the way down below 500 rpms, and more often then not, the engine will conk. If it doesn't conk it sputters then immediately recovers and idles fine. Similarly if I'm coasting, with foot off of the accelerator, and put the transmission in neutral, the rpms will drop and more often then not the engine will conk. If it doesn't conk it recovers.

I did some more diagnostic style driving with this result. On a level road, letting the idling engine drive the car, with foot off the brake and gas, the car will roll along at about 1-2 mph and the rpms will change state every 5 seconds or so between normal idle at about 800 rpm, and lugging drag at < 500 rpm. This is NOT a bouncing "hunting" style change like is typical with a bad ETM. It is more like a square wave in the RPM speed, 5 sec normal, then 5 sec slow.

As mentioned before, this issue only occurs (or at least is only relevant) at very low (but not stopped) rolling speeds with foot off the gas, or in neutral, coming down from a high rpm rev.

[Brucebo]

I found this AW 55-50 technote while searching around. So this explains the "transmission drag" as you come to a stop. The transmission is acting normal. Something in the engine control loop is not functioning right. There is no "thud." Everything is good here, but this explains when the transmission disengages the rpms go back to 800 or so. The engine control is not providing "enough gas" (metaphorically) during very low speed transmission engagement.

– Harsh Neutral Control Engagement/Disengagement: After coming to a complete stop in Drive, with your foot on the brake the TCM waits 2 seconds and then disengages drive to reduce emissions and minimize idle vibrations. This disengagement is normally smooth and isn’t felt by the driver. If adaptation is not complete then a “thud” can be felt in the car 2 seconds after coming to a complete stop. When the brake pedal is released, the re-engagement of Drive should be relatively smooth. A harsh re-engagement can be felt if the adaptation is not complete. Neutral Control is present on all 2001 MY S60s and V70s. It is also present on non-turbo 2002 S60s and V70s produced before January 2002.

[HurricaneLuis]

If your car is a non turbo, its your ICV. I had your same exact issue on my V70 and cleaned my MAF and my ETM and nothing would fix it until I found out the car had an Idle Control Valve replaced it and all is well now.

[abscate]

It does sound like the transmission issue more than ETM. Did you recently disconnect the battery for anything - maybe it isn't adapting correctly?

[Brucebo]

If your car is a non turbo, its your ICV. I had your same exact issue on my V70 and cleaned my MAF and my ETM and nothing would fix it until I found out the car had an Idle Control Valve replaced it and all is well now.

It is a non-turbo and I'm inclined to think it's the ICV too. That's the next think I'm going to check.

-B

[HurricaneLuis]

I would definitely say it's that, I did all those other things first because honestly I thought the ETM took care of the idle, I had no idea non-turbos also had an ICV.

[Brucebo]

Well it's NOT the ICV. I swapped it with one from another S70 and no change. I did recognize a new symptom though, and it's repeatable. The problem does NOT occur when the engine is cold. If I start and then race a cold engine, then let go of the accelerator the RPMs drop back to normal idle. Nor do the RPMs undershoot when I come to a stop. Drive for three minutes and warm up the engine and the engine will conk if I race the motor in neutral then let go of the accelerator.

In web searches I've seen other people complain of this problem in this and other forums, but there is never a definitive "fix" in the thread. Seems like a real puzzler. No OBD codes and all OBD live data telemetry looks good.

[Brucebo]

Well I finally broke down and swapped the ETM and that eliminated the problem. Feh!! I was reluctant because the symptoms were different than the ETM symptoms I had experienced before.