S38's Idle Still Rough

[TheE28Ranch]

This may be better off for Tech Talk, but it does involve an S38 motor, so here I am at M5 / M535i.

I have been having a problem with my M5's S38 idle. You've heard about this problem before, and I researched it here (and there) and what to do about it. Here's the ongoing problem - the car idles around 900 when it's warming up, it sounds pretty good. But after driving it, it usually goes somewhere between 1500 and 2000 RPM when the car is at rest. Obviously, it drives me crazy, and I am tired of people asking why my car idles so high and seeing mothers scoop their children off their tricycles when I cruise by with an over-revving engine. So, I removed the intake plenum etc. and accessed the ICV. Along the way I thoroughly checked every hose for leaks, they all look pretty good, with the exception of a couple getting a bit hard and one showing some cracking at a fastening ring. These questionable hoses will be replaced in the near future.

So back to the ICV, I cleaned it with brake cleaner until the fluid was clear exiting from the unit. I sprayed some silicone in the ICV, (I am not really worried about adversely affecting the O2 sensor as I plan on replacing it anyway), and then I reinstalled it.

So I get it all back together, and the car sounds great as I let it warm up and putt a bit around my neighborhood - it stays around 800-900. But then after a "spirited" drive where it runs great, it's back to the same old idle business. Again, idling at 2000 when the car is in neutral. From a stop I can get it to go down if I ease out on the clutch but realize this is a no good temporary fix. Furthermore, when I drive along at 25 mph, the car seems to be on cruise control as it powers itself along from the idle just shy of 2000 rpm on flat roads, in third gear, at 25 mph. If I start going uphill, the idle drops as well as the speed, though. Essentially, it is powering itself to a certain degree.

So, any suggestions on what to do next to rectify this problem? Buy a new ICV (I'd hate to do that and still have a problem)? Check the fuse box for something loose? Bad ECU? Bad AFM? What would a qualified shop do to rectify this problem?

In closing I thought I once read a great thread here on this same problem almost to a tee, but I can no longer find it and now wonder if it was my imagination. Thanks for reading and any help. I know that this has been discussed here before, but I've put a lot of time into this and I still seem to be at square one.

[m535is]

My first thought is that you should have also done the electrical tests outlined in the Bentley manual. I would still do this to see what you get and I would do the tests at cold and warm if possible. My next suggestion would be to get another ICV. Instead of buying a new one you can buy a tested used one. I have one you can have for $25 + shipping. That way you can see if it changes anything for little cost. The other things I can think would cause this are if you have a temp issue cause the TPS to read wrong when it is warm, either set wrong or internal fault. There is also the potential that you have a temp sensitive issue in the AFM but I doubt that seriously.

Hope this helps,

[TheE28Ranch]

My first thought is that you should have also done the electrical tests outlined in the Bentley manual. I would still do this to see what you get and I would do the tests at cold and warm if possible. My next suggestion would be to get another ICV. Instead of buying a new one you can buy a tested used one. I have one you can have for $25 + shipping. That way you can see if it changes anything for little cost. The other things I can think would cause this are if you have a temp issue cause the TPS to read wrong when it is warm, either set wrong or internal fault. There is also the potential that you have a temp sensitive issue in the AFM but I doubt that seriously.

Hope this helps,

I didn't think the Bentley would help out with an S38 issue like this. I'll have to follow the procedure it outlines. I also might have to take you up on that ICV, good idea to test a known good one (duh me, and I have a couple others, albeit installed on cars). Give me a day or two as I consider / rule out other possible points of failure. Thank You, every little bit helps.

[Coldswede]

Trey
Check the vacuum cap next to the vacuum line for the Fuel Pressure Regulator. The plug in the cap falls out.

[MicahO]

Mine behaved very similarly, though the high-idle was more intermittent and I was able to step it down with some jabs at the throttle. There were times that I thought I had it taken care of, until I took a hard romp through some hills - once it got good and hot, the bad/high idle came back.

I worked through cleaning the ICV, checked for vacuum leaks, changed a few vacuum lines, then general tune-up (cap, rotor, plugs, etc - it was new to me and needed it anyhow), then a CTS, then again making sure the linkage was all lubed and not hanging, then finally a TPS. The throttle position sensor ended up being the problem. I guess that the TPS had gone intermittent and being hot made it worse, which is why I could stab at the throttle to get it to step down. Perhaps yours is totally dead?

There is an electrical test sequence for it, though I took the easy way out and spent $42 on a new one.

[mooseheadm5]

While it is idling high, open the hood and try to close the throttle manually. If that works you either have a maladjusted TPS or your throttle is sticking. I just had to deal with that, only my throttle would stick open wherever you left it when hot (4krpm, whatever.) I pulled the nuts that connect the three throttle linkages to the main throttle shaft and found the center one was sticking. Only a tiny bit when cool, enough for the spring to overcome, but majorly bad when hot. Having already lubed the bejeezus out of it several dozen times from the outside and the butterfly side, I decided I'd have to take it apart. I took a 14 (I think) and loosened the nut on the butterfly shaft just a hair (between cyl 4 and 5) and the problem disappeared. If the throttles can't return all the way to the stop, your car will idle high. Check that out and report back (I can take pics if you need.)