I did search! Please don't yell at me! Other than finding a whole bunch of miss-spelled "wondering" when searching for "wandering" and "idle wandering", I didn't find my problem. So, here goes.
1985 US 635CSiA, 177K miles. Replaced main intake hose/duct/rubber thing from the MAF to the throttle body. Checked all attaching hoses for cracks etc. Cleaned the TCV with Brakleen. Shake it and it rattles just fine. Verified resistance on pins 1-3, 1-2 and 2-3, all good.
Car starts almost from a thought. The split second you hit start with the key, it fires no matter how low the air temperature. Idles nicely, then it gets toward running temperature and the idle starts to wander. Maybe 50-100 rpm. Doesn't register on the tach, but I can feel/hear it and it seems to have a regular rhythm. The reason I replaced the main intake tube/hose/rubber thing, was because after warm up, it would idle weird and die when coming to a stop without keeping your foot on the gas after it warmed up. That seems to be fixed, but now I have this pita idle issue.
All ideas, suggestions and specially humor greatly accepted.
For fun. Pull the dip stick and see if that effects the idle at all. IF not you do have a vacuum problem. Sometimes the little seal on the dipstick can go wonky with age. This was just a shot in the dark. But My gut still thinks vacuum or a sensor.
81 Euro 635 M90 with motronic. Currently under a complete nut and bolt restoration. Pictures at
flickr.com/photos/bertsphotos.
Actually they are supposed to wander maybe 50 rpm when warm and in closed loop idle control based on the O2 sensor. The electronics on these cars are just that slow that it takes 3-5 seconds for it to over and under adjust the mixture. Maybe it gets a bit worse with age but to prove it to yourself stick a volt meter on the O2 sensor and watch it swing. Then unplug the O2 sensor and it should go rich but idle much more steadily. If it does not go rich you need to adjust the bypass screw on the AFM a bit more closed or find any small vacuum leaks.
Almost any small vacuum leak (dipstick size or smaller) can result in a more steady but maybe lean idle if the mixture is outside control range. Of course any bigger leak and idle should go kaput and it will stumble on load from being much too lean.
It was not uncommon for people to complain about this when the cars were still new and for dealer to add a resistor to the CTS to keep it in warm up mode running a bit rich but smoother.
Bert;
Thanks for the off-the-wall suggestion. I'm going to try it because I'm kind of at my wits end, so there isn't anything that's off the table.
TN-M6;
I have to ask if there are differences between US spec and EC spec cars besides horsepower and maybe emissions equipment? My euro spec 635 idles so smooth it's like a jet engine. Doesn't wander at all. I'm hoping there are differences, as I would like to just enjoy my US spec car without being paranoid that I'm causing bigger problems when driving it because it still has issues.
I imagine the euro spec car has both higher compression and different engine management/tune but someone else would need to verify that. Not at all surprised you would say they idle a bit differently and they should run differently too.
Your US spec car with catalyst is probably identical to my 86 535i which when properly tuned will run just as I described. I don' think you have anything to be concerned about. Over time my 535 seems to lope a bit more and the best thing I've ever found to make it smooth out a bit is a hard drive in the rain and occasion SeaFoam hot soak.
The more these cars are left idling or driven like grandma, the more they build up deposits on the backsides of the intake valves due to the batch fired injectors. That tends to increase the idle roughness. Back in the day BMW might have blasted the valves with walnut shells to restore it and used to recommend Ventil Sauber valve cleaner in the fuel on occasion. The hot soak followed by Italian tune up is a lot less trouble and more fun than the walnut shell blasting .
Pulled the dipstick as suggested. It almost died when I pulled it, then recovered. Idled kind of weird while it was out. I then reinstalled the dipstick, there was a very minor hesitation, then settled down into a good idle.
Don't know if this is okay, or is the sign of a problem. Please let me know.
I'm really glad it's running as it should. I didn't need more vacuum leak problems. My 850 is enough. I have to reseal both intakes due to vacuum leaks. I would rather eat glass than do that.
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