CIS issue... oscillating idle right after startup

[wowzer911]

Subscribing for the entertainment value.

[Mysterytrain]

Wow, I think this is going well. I love pulling politics into a CIS tune up. Typically, the high/low hunt is a rich condition. I agree that if the only tool you have available is a 3mm wrench then WTF, try to lean it out. What do you have to lose. Now if the issue is to then tweek the screw one more time is a CW direction to maintain some tension in the threads..well yeah sure ok. Thats how you should tune a guitar too you pull the string up to proper pitch by going sharp... but of course guitar tuning is a bit more critical then a gummed up CIS mixture adjustment screw.

[wowzer911]

I am having the same problem with my CIS, but I know for sure my airbox is cracked, and assuming there is an air leak issue. But haven't gotten around to putting it on. Should I even bother trying to tune my CIS until I'm sure there is no air leaks?

[Groovy911SC]

I will absoutely try to adjust the mixture but... here is my next question... the mixture is rich as my mechanic who did the tune did so after the rebuild to keep the engine running cooler on hot days. My "home track" is No Problem Raceway in south Louisiana. The temps, starting now, get really warm. Is this a condition I should attempt to live with or should I have this condition looked at and corrected cause something bad will happen one day while making the Lotus Elise drivers cry while wondering why a 25 year old car caught him again?

[psalt]

Groovy911SC,

If your engine is stock and the lambda system is working, you can see what the mixture correction factor is by hooking up a dwell meter to the test plug under the plastic cover on the left hand side of the engine compartment (green/white wire). If your mixture is substantially off, the duty cycle will read outside the range I gave you.

My advice is that if this is a track car, you should forget about idle mixture and concentrate on mixture under load at WOT. Beg, borrow or steal a wide band meter like an LM-1 and test it during your track time.

[psalt]

wowzer911

It is pointless to tune a leaking airbox, also pointless to speculate about a leak. Swapping the airbox is no picnic, some people insist on a partial engine drop to do it. I've done it both ways and neither is fun. If you are attempting it in place, loosen the box mounts as soon as you can get to them.

You may want to use a propane torch and check for leaks around the airbox and injectors. You will hear the engine note change when it finds the leak. Porsche chose an inferior method of installing the injector sleeves and their seals typically are toast before the injector seals. After 34 years you can get wiggly injectors and the sleeves can pull out and destroy themselves over the stakes when you pull the injectors. You want to consider replacing the runner boots, gaskets and oil switch while you are at it. When its back in, check the bell crank bushings and for full throttle using an assistant. I am always surprised a how many 911's with performance parts have a throttle that only opens 3/4.

[wowzer911]

Hmmm, I won't be dropping the engine as I just did an engine swap a couple of years ago but kept my existing CIS (w/ good boots and runners). Recently cracked box w/ a backfire as there was not a pop-up valve in it (my bad). I was looking forward to installing the SS airbox (till now). Also my injectors are somewhat wiggly, is this bad, or is a little movement acceptable? I will check for leaks now that you mention it. Throttle linkage has already been adjusted, no problems there.

[psalt]

Also my injectors are somewhat wiggly, is this bad, or is a little movement acceptable?

The leak test will tell you.

[Paulporsche]

Other than the fact that we can't seem to agree on what going "from lean to rich" means, psalt has given sound advice. What mysterytrain and others (incl me) are saying is that it sounds like you might be just slightly rich. While running too lean will make temps rise, a 1/16 or 1/8 turn leaner to cure your idle hunt probably won't change your temps, especially if your mix was too rich to begin with. Also, if this is exclusively a track car, I agree that you should tune for your race conditions, which is not the same as real world conditions.

[clevy70911T]

I personally make a small mixture adjustment when warm weather arrives with a 1/16 leaner and yes the idle occilates at start up and stops after the adjustment. I am curious too why the final turn of the mixture screw should be clockwise and if anyone knows the techical/mechanical answer, please advise us If you just put on new carbs or a new FI system and had no idea where you are set, then I can see tuning rich to lean. But binding or ruining something with an allen wrench removal after a 1/16 CC turn is interesting to me.

I've been through the airleak deal too. Think you found it, fix it and then it still idles high. Finally I done a another partial drop and went through the complete CIS to finally correct the leak. It wasnt too bad, but I was proficient in the partial drop by the time I got it fixed.

Man teaches chimp to remove CIS in 90 minutes with partial drop.

I made a small writeup including non-Porsche procedures like using JB Weld to seal the airbox No duct tape though.

[psalt]

Scott,

It has nothing to do with the concept of tuning from rich to lean, like when you are programming an EFI fuel curve. It has to do with the design of the CIS air flow sensor. The basic fuel metering of the system is controlled by the mechanical relationship between the air flow sensor and the control plunger. The air flow sensor is hinged at one end, with a second floating lever that contains the adjustment screw and makes the actual contact with the control plunger. The reason Bosch and Porsche have advised to always make the final adjustment in the clockwise direction is that when you back out the screw, you remove the tension from the screw threads and the adjustment is not reliable. The screw is also more likely to move from vibration when left in this position. It is a mechanical issue, not a "rich or lean" issue. This has be known since the 1970's, mentioned in all the publications, and one of the first thing taught in K-jet 101. Unless you have your head on the desk.

If your 81SC CIS is working correctly, you can use the lambda test port and a dwell meter to make repeatable adjustments with more accuracy than "1/16 of a turn", especially when you make the adjustment using the proper procedure.

[safe]

I'm putting down a dollar on the WUR heat element.
If the electrical connection is disconnected the idle will oscillate until the heat from the engine heat up the WUR.

[clevy70911T]

Been there and done that by testing the FV with the dwell before and after the 1/16 turn. All your help is appreciated, but we're all not primative Pete's without procedure manuals. I think the confusion arose when you called it tuning rich-2-lean, when it has nothing to do with the mixture. Phrasing it as "always make the last turn clockwise to lock the setting" would have been suffice. Thanks gain

Originally posted by psalt
Scott,
Unless you have your head on the desk.
If your 81SC CIS is working correctly, you can use the lambda test port and a dwell meter to make repeatable adjustments with more accuracy than "1/16 of a turn", especially when you make the adjustment using the proper procedure.

[psalt]

Yes, reading is a skill. It is lean to rich, not rich and lean, and that is how it is written in all of the Porsche factory manuals. Reread the posts, you are the only one calling it rich to lean. Then read the manual.

[JFairman]

My 930 does that sometimes too, and it is a symptom of being a little rich at warm idle.
But the reason mine is doing it is something is causing the sensor plate to hang a tiny bit.
Sometimes when the engine is off the fuel pumps will come on when turning on the ignition.
That is also because the sensor plate is not fully returning to the rested position it should be in when the motor is off.
Because it only does it occasionally I havn't done anything about it.

[psalt]

Sounds like you need to reset the sensor plate Zero position. There is a good description of how to do this in Probst's book on Bosch fuel injection.

[clevy70911T]

Originally posted by psalt
By following the factory's procedure:

"always adjust CO level from rich to lean" (Porsche AG)

Yes, reading is a skill. It is lean to rich, not rich and lean, and that is how it is written in all of the Porsche factory manuals. Reread the posts, you are the only one calling it rich to lean. Then read the manual.

LOL. You are a piece of work my fellow pelican. See below

If YOU read your personal post again, you quoted the Porsche manual as rich to lean. Please read carefully and correct your mistake before you write another condescending post. You seem to have alot of knowlegde and experience. It would be well received and usefull if you could leave out the off color comments.

condescending: To deal with people in a patronizingly superior manner.

[dougcl]

Originally posted by safe
I'm putting down a dollar on the WUR heat element.
If the electrical connection is disconnected the idle will oscillate until the heat from the engine heat up the WUR.

Safe, this is not true. If the heat element has failed, or does not receive power, the car will run well after a warm start because the WUR has heat soaked. After a few minutes of running, the top of the engine will cool sufficiently, and the WUR will cool to the point of richening the mixture again. I suppose a hot running engine may behave as you described, but if all else is in order, no.

Doug
75 911S

[Mysterytrain]

I'm convinced 'lean to rich' is refering to the mechanical direction of the final adjustment of the mixture screw. Isn't this the same concept as torquing a bolt? If you over torque you back off past the setting and then bring it backup. I still think we are spliting hairs. Lets not forget that this is the same system that requires a hammer to adjust the cold pressures.
Speaking of the WUR, I'm not sure the WUR or the AAR will heat soak from the engine and be able to operate correctly without the heating elements functioning. Remember the elements don't timeout..they remain hot when the ignition is on.

[psalt]

Ron,

Yes, the "why" is a mechanical issue. Porsche chose to write it as lean to rich in the manual, but like many other instructions, it does not explain the reason.