CIS detectives: I've got a challenge for you.

[911SCfanatic]

I have an idle hunt/oscillation problem. Did a search and got some good info, but I need more specific direction for my car. I've had this problem since late last year. Got it out for the season so it's time to diagnose and fix this.

Idle set at about 850 rpm. Exhaust smells very rich, like a poorly tuned carbuerated engine.

No hunt at start up and for about a minute when stone cold (I assume the time it takes to go from open to closed loop). Drove home 30 minutes and idle hunts like crazy from 200-1200 rpm at every light and coast down. Got it home and checked output from O2 sensor. Connected it's about 0.380 volts nonfluctuating. Unconnected it's about 0.560 volts nonfluctuating. Let it sit for about 45 minutes during dinner and went back and started it up. This time, no hunt, and I got the same unfluctuating voltage from the O2 sensor when connected and unconnected.

Removed the oil filler cap with the O2 sensor connected and got a slight increase in engine speed, but the O2 sensor output remained constant at 0.380 volts.

Where do I go from here? I'm getting equipment capable of measuring duty cycle this weekend. Your advice is appreciated--this thing is driving me freaking crazy!

[vash]

does it do it when the car is hot? what is your cold control pressure? bill, betcha a cup of coffee that your cold control pressure is low. at 70 degrees outside, you should see 2 bar pressure (29 psi). it can be a WUR problem, but i seriously doubt it. i think the WUR needs adjusting. unfortuanately, if i am right, then you need the brass plug to come up. therefore you need to take the WUR apart and tap the plug back up. then with the CIS gauge slowly tap the plug back down until you read 2bar. that is my best guess, but you lambda car guys always throw me curveballs.

[nostatic]

shouldn't your idle *drop* when you take the oil filler cap off? Mine does as I recall...

That being said, if I set my idle too low it will hunt when warm. If I dial it up to about 1K, it doesn't hunt at all. But I don't have an O2 sensor, so I dont' know how that fits into the equation.

[SilverPoly]

You say your exhaust smells very rich. Have you checked your CO mixture lately? I seem to recall that when I was running on the rich side my idle would hunt as you described.

[vash]

yes, low cold control pressure will also cause you to reek of extra fuel.

[RarlyL8]

You need data before you mess with anything.
A/F ratio or %CO will tell a lot. Sounds like it is simply set too rich.

[beepbeep]

Faulty mixture = idle hunt

Air leaks => unmetered air => faulty mixture => idle hunt

[911SCfanatic]

Thanks so for the info so far. To check CO, I need something like a Gunson gastester. I'll have to see if I can find a Pelican or Gruppe B'er in the Detroit area that has one. How can I get A/F data?

It definitely smells rich. Really it always has since I've owned the car. My clothes stink of exhaust after I came in from messing around with the car at the engine compartment for about 10 minutes. I've thought the gas mileage has been going in the crapper lately, but I really don't know what to expect from this car anyway.

I had a tune-up in 2002 when I bought the car. Things seemed fine for about a year. Shouldn't these setting last, or do they just drift over time?

[vash]

if the WUR cold control pressure is low, you will run rich. i guess rarly is correct, there is not enough info. but i would get a CIS gauge before buying CO tester.

[nostatic]

You should do tune and valve adjust ever year or 10K miles.

[john walker's workshop]

find a shop that can set the CO. personally i like 3.5% and leave the oxy sensor unplugged except for the smog test.

[911SCfanatic]

unfixed, what is a CIS gauge? Does this gauge tell me A/F...and what else? Do I need to make up some special pressure gauges to read some of these pressures you have referred to? It sounds like if should be doing this every year, I might as well get some of this equipment. That, and I'm a nerd that likes tools. Besides the Gunson (see them on ebay from the Florida distributor for $159), where can I get some of this stuff, or how can I make it so that I can start collecting some basic data? That way I can trouble shoot this and do what I'm supposed to in the way of tuning every year.

[speeder]

You need to read up on CIS and go through it systematically, ie., "by the book". There is no other way to correctly diagnose the system, other than eliminating causes one by one.

There is no question that your car is running extremely rich by the symptoms that you describe, that's the no-brainer part of this. See-sawing idle, car improves when you remove oil cap, (introducing unmetered air into mix), fuel smell, etc...., Ray Charles can see that. The question is why, not what, and for that you need a fuel pressure guage and a few other nifty tools plus a book or two. The Bentley manual for 911SC has a pretty good CIS diagnostic section. Once you read that and run the tests, we can help you a lot better. Good luck!

[MotoSook]

When cold, the AAR is open, not hunting ...then when the AAR starts to close, it hunts...rich!

When remove oil cap..should run lean if idle mixture is set in the desired span and idle speed should decrease or/and hunt..but your's idles faster...confirms rich idle mixture...

Com'on Bill....do I need to come there and slap you with a high HC ticket!?

Just put a 3mm allen in there and turn it 1/4 or 1/2 c-c-w.

it is possible to have that idle mixture screw to far c-w that the frequency valve can't adjust for (do you agree JW?).

A CIS fuel pressure tester is available form JC WHitney for 60 bucks or so, or get a better quality one her. It's just a guage and a 3-way valve.

I like the idea of a Gunson test, but I'd rather pay to use some's 4-gas analyzer every two years to tune for the EPA..much better read of what's going on...fuel, air and ignition will all affect the exhaust gas condition....

There is a link to Jim WIlliam's CIS Primer website from my webpage...use it.

[Rod80SC]

I have also been having idle (high) problems since buying my car. Right now I am waiting on my test gage and book from our host. Looking forward to being a self taught expert on CIS. I purchased the Bosch Understanding and Troubleshooting CIS book, but still dont have the Bently manual. What are the best books strictly for CIS. Suppose I should have asked the board this question prior to purchasing, but more is better when it comes to tech books.
Thanks

[joecrum]

911SCfanatic,
I was having symptoms similar to yours. Did lots of research, felt I was in for tons of diagnosis, parts, time, and $. About that time, the O2 sensor light came on. Replaced sensor, no more problems. This is only my experience and I only know a little about CIS.

Just my $.02

The only other thing that may have helped was that I began to drive it harder (redline every day for good health).

Good luck!

[911SCfanatic]

I wanted to start at the top of the diagnosis check list by checking the idle control valve for duty cycle. I borrowed a digital oscilliscope from a EE friend to measure duty cycle, but without the manual for it, it was too complicated for me to figure out.

Ended up using the Souk method and dialed the mixture back about 1/3 a turn ccw. Idle is perfect now and no more rich smelling exhaust!

Unless someone has any reason why it can't wait until I get a Gunson gastester and a Fluke multimeter (that measures duty cycle), I'm just going to be happy and drive. I just leaned it out until it idled smoothly so I don't expect catastrophic things to happen.

BTW, Bentley's mentions removing a plug to get at the adjustment screw and then replacing the plug when done. I don't have the plug--is this something I should be concerned about? I know that water can get in there if I'm not careful. But, I don't think it's a big deal because, as I understand it, it doesn't create an air leak. Thoughts?

[vash]

i paid a mechanic to adjust my AF mixture. i think he charged me $40. i also took my pressure measurements to him on a piece of paper so he just in case anything jumped out at him. nothing out of the ordinary. i use the ben watson CIS book. reads easier than the probst book and there are lots of pictures and blank spots for my notes. just messing with the CIS gauges taught me alot. i got bought mine at pelican, but i think i spent too much. even after my mech adjusted the mixture, when the weather warmed up, i got idle hunting. i hooked up the gauges, and found my cold pressure was way low. 13psi, versus the 29 it needed to be at (at the 70 degree weather i was seeing that weekend). i hooked up my gauge, took my WUR apart. knocked the plug out a bit, and i got 40 psi. i then just lightly tapped the plug back in until my gauge read 28 ( i went to far, but what the hell). now my car feels like it is running on electricity. it is that smooth. i even pulled up to cantdrv55, and made him drive my car! peppy little 2.7 liter that i have. the gauges are a good purchase.

[vash]

i am not a mixture expert. but if the car was running a bit rich, it could be feasible that the richer mixture will make the car run really nice when totally warmed up right? then when you lean it out, you fix the cold idle issue but you sacrifice power at the warmer temps? just wondering.

[MotoSook]

That plug would stop any debris and water, as you said, from getting inside the airbox, and it's downstream of the filter.

I bet a good number of 911 CIS cars don't have that plug. I don't have one, never did. I figure anything small enough to get in there will just get spit out the tail pipe..and I don't drive in the desert in sandstorms on a regular basis, so I'll live with a little dirty air.

If you just dialed it back toward lean just enough to get the hunting to go away, you should be fine. If you are running with the O2 sensor, it should be fine. Check your plugs for lean running if you are a fraidy-cat. Is your timing per factory spec? If so, you should be fine.

Now, if you want more fun, disconnect the O2 sensor, advance your timing and get it as rich as you can w/o hunting, then dial back an 1/8th of a turn. (do you still have your cat on?)