Idling issues, checked MAF etc

[JimBee]

Some of your erratic up-down idling sounds familiar. I have a '96 850 that has had similar symptoms but is now cleared up, hopefully for good.
A couple of things I've discovered.
1. when the engine rpms are fluctuating if I try to keep it running by jiggling the accelerator I might be defeating the computer from resetting. This did happen on mine. I could barely keep it running by rapidly jiggling the accelerator, but then I just let it alone for once, it dropped to about 300 jumped up to 900 and 1200 back down, then magically straightened out and started running perfectly, which it still does.
Why would it straighten out and then act up again? When the computer is reset it straightens out. HOWEVER, something might be dyregulating it again.
Two possibilities. I've read on this forum that the MAF connector on some models can move around enough to break connection then reestablish connection. My theory on that is, if it breaks connection, you're back to dysregulation and the need for the computer to reset. If the connection is reestablished and the computer resets, all is good—until the next time the connector breaks connection. Then you have to go through the reset again (up-down-up-down, etc.). If you jiggle while it's doing that, you keep it dysregulated.
But there's one more thing in that loop (tighter than I used to think, according to a Volvo indy).
2. Your O2 sensor. After I'd replaced my MAF with one that worked well on my other 850 I still had the same erratic idle. I cleaned things up, looked for air leaks, etc., but it was still doing it. I bought a new O2 sensor and was about to install it, when I discovered the contacts on the O2 plug looked dirty. I cleaned them up, dabbed on some dialectric grease, replugged the original O2 sensor (i.e., didn't replace it with the new one) and THEN next time I started it, is when I sort of accidentally left my foot off the accelerator when idle was jumping around and it straightened itself out.
So there are a couple of things you can try.
Let us know what works!
QUICK EDIT: the poster who mentioned the MAF connector issue said he'd heard it was somewhat common on some models. The fix is zip tie it somehow to it can't get loose. I didn't do that but you might try it.

[Raycharles90]

JImBee,

Thanks for your suggestions. I will look at the O2 contacts tonight. I also noticed that the car wont die but it takes a lot of restraint to not tap the gas when it is shaking and rattling at 400 rpms. Feels like the hole front end will shake apart. I figure it has got to be a connection or leak since there have been two times after replacing the MAF and rerouting the PCV vacuum line that it ran perfectly.

Hopefully I can get it resolved this weekend. I actually was looking at used cars this morning (the horror), knowing that I would have a hard time finding something to replace the S70.

[JimBee]

Exactly. I had been failing to restrain myself and, if my theory is right, continuing to defeat the computer.

Every time you jiggle the accelerator, you are also jiggling our electronic equivalent of the old time accelerator pump—in our case, the throttle position sensor. When you open the throttle, that sensor sends an electrical signal to the computer that more gas needs to be injected (and I assume the spark advance gets a message too), then that increase in fuel injected along with spark advance sets the engine in a "rev posture" awaiting the signal for more air as rpms increase. But if the MAF signal isn't properly calibrated the main computer doesn't get the correct message about how much air is actually coming in. So it could be flooding one second, leaning out the next as it tries to get in sync with the MAF. But all that accelerator jiggling further confuses it, because it's calling for more fuel, etc. etc. while it's trying to get right with the MAF.

Meanwhile, downstream, the O2 sensor is trying to determine if the fuel/air mix is correct and if not, make some adjustments based on that (the computer is expecting this input). But if there's no signal (or a confusing/weak signal) coming from the O2 sensor, then I assume it's all the harder for the computer to get the fuel/air mix right. But it keeps trying—too much, too little, too much too little as you jiggle the accelerator.

I'm not 100% confident in this theory, but complicated and mysterious to us as they are, these are still rational systems. Anyway, this is my best understanding of how mine got straightened out and I have to say I was giddy with amazement when it happened.

BTW if you have a turbo, you might need to pull the air hose to get enough room to two-hand the O2 sensor plug. I worked around that but it's frustrating.

[Raycharles90]

Ok,

Found a few other issues and corrected them but still the same issues. Saw that the air filter was collapsed on side which would cause a leak. I replaced the air filter and cleaned and greased the MAF and O2 connections. So at this point I am really scratching my head. I think I have checked all the vacuum lines and gone over everything. I did pull off the battery bracket and noticed the contraption attached to the bottom. Anyone know what that is for? There are a couple of vaccum lines attached to it and one runs back behind the air filter.

Could my issues be a bad new MAF? I purchased it new from FCP. The frustrating thing is I may have to bring it in somewhere that I will pay someone else to scratch their head over it.

[Raycharles90]

Ok, so I took the car in to a local shop because I had needed a fresh set of eyes on the problem. Different shop than the first one. They called me at the end of the day and said that it was the Idle Air Control valve. I cleaned this out pretty well and it seemed to help out the problem but not fix it. I notice that these are quite pricey new (~$240). Question, do these go bad or is it usually the connections? Second do they go bad rarely enough that I could pull one off a junker and be ok?

[erikv11]

These rarely go bad. There are others who have been around longer than me, but I have actually never heard of a dead IAC valve. Just a clogged one.

[Raycharles90]

Talked to the mechanic this morning and he said that the motor in the IAC was bad. The strange this is that it seemed to help my problems a little when I cleaned it out real good. The local pull-a-part sells these for $7 a piece so I will go there and pull a couple and see if that helps. For $7 I can't go wrong. What is the best way to tell if they are working? I have read other posts that mention a high pitched hum or should I be able to feel it turning if I place my hand on it?

[Raycharles90]

Ok, well not quite fixed but driveable and not worried about it dying on me. So the IAC was bad and went out to the pull a part and grabbed two. Stuck the newer one in everything worked ok but after a couple of blocks it started to stumble so I jumped out and wiggled the wires around a bit and it popped back up. So I then got a couple of zip ties and tied up the connector. Everything looked clean and and no loose solder points. This did the trick, I still think that it loses connection at some times. The one thing that it is doing now is that it seems to be without power sometimes. It kinda feels like it felt when I had some bad vacuum lines. But I also wonder if the bad connection is causing the computer to go haywire. Anyone know of a place that sells that connection? Or, slightly off topic, can I get someone to measure my fuel pressure at one of the chain stores?

[erikv11]

Are you sure the vacuum hoses to the IAC are well connected?

I don't think the chain stores have a loaner fuel pressure gauge but wouldn't hurt to ask.

[Raycharles90]

The air hoses are well connected and secured with clamps. Just pulled some codes from the car. Had P0130, P0172, P0171, P0455, P0133P; in that order. Thoughts? It is still running fine but acts like it has a vacuum leak every now and then. It will get sluggish sometimes. I have searched high and low and haven't found a bad hose. Sometimes at a stop sign I can see the rpms dropping slightly. At that point I tap the gas slightly then it levels out and stays steady. I am not sure on my gas consumption (waiting to refuel) but it looks like I am running a bit rich. There is certainly something wrong if I get codes that I am running rich and lean.

If I was to find a fuel pressure gauge, do I just put that on the nipple at the end of the rail?

Thanks