RPM dip, stumbling

Post by **jblackburn** » 15 Jun 2010, 18:04

I have been trying to get a good video of this for the past week, but my car just won't do it when it knows I'm watching.

For the past month or two, I've noticed that when I let off the gas ~ 50 mph to coast on a flat road or downhill, the RPMs will fall a bit below what they should, the engine feels like it stumbles, and then the needle pops back up slightly, then repeats. This ONLY happens noticeably with the air conditioning off. My car runs flawlessly otherwise, no stumbling ever, plugs are good, no codes, no vacuum leaks, and everything appears to be hunky dory.

IAC valve is clean, and it's not the transmission I'm fairly sure of. MAF is new as of last March. It's almost like it's the ECU doing it, and I'm not sure why. I ran into this discussion on Volvospeed which sounds similar, but my car idles perfectly fine:
http://volvospeed.com/vs_forum/index.ph ... -rpm-drop/

Does anyone else's '98 do this? Or even those of you with 850's? Anyone think it could be part of the EVAP system? No typical purge valve issues, but it is original as far as I know. I also have an ongoing fight with highway gas mileage, which I suspect an old O2 sensor, but I don't know whether or not that is related to this.

Thought I'd throw it out to some of you, I'm out of ideas of things to look for.

Sorry about the music, I had to catch this thing in action before it disappeared again.

Post by **E Showell** » 16 Jun 2010, 16:32

Have you tried cleaning the throttle body? Might help.

Post by **jblackburn** » 16 Jun 2010, 22:18

I actually hadn't thought of that. I'll haveta get a gasket and give that a try; I sprayed some carb cleaner through it when I had it apart for the PCV job but it probably didn't clean it very well.

Good call.

whoa · Post by **whoa** » 17 Jun 2010, 08:35

A few months back my 850 did something similar, except it was at 65mph, and the RPMs would drop from 2500 to 2000, with accompanying loss of power, so that I'd have to give it more gas, which got it back up to 2500, and so I'd ease off the gas, when it would drop back to 2000 . . . Annoying! The feeling was a like the AC going on (the AC was off), but a bit more noticeable.

I fixed it by ignoring it until it went away.

Enid Puceflange · Post by **Enid Puceflange** » 17 Jun 2010, 09:02

Well, perhaps my experience might help - I had an issue where the real problem was the front O2 sensor went bad, but as part of the fault-finding process I replaced the MAF sensor with an aftermarket one (EBay special). The car ran, didn't throw any codes BUT the gas mileage was awful (around 16mpg round town) and it had a stumble like you describe. The MAF worked (the ECU saw the air flow change with throttle position) but when out of desperation I changed it for the old Bosch sensor I got the miles-per-gallon back and the stumble went away.

Post by **jblackburn** » 17 Jun 2010, 20:25

Thanks guys. I'm hoping it's not the MAF again, but I couldn't get the car to do it at all again today.

I've got a huge maintenance day planned some time in the near future...during that time I'm going to try changing the O2 sensor and exhaust gasket over 4th of July weekend...or sometime next month, so we'll see what happens.