Hunting Cold Idle

[yelcab1]

I did a search and found a couple of threads describing the problems but no answers. So here goes again.

1. Cold engine, starts right up, idle hunting between 500-1200 RPM, for about 1 minute, then steady out at 1500 RPM until the car is fully warmed up, and rock steady at 1000 RPM. No issues after that.

2. Warm or hot engine, starts right up, rock steady idle, no issues.

I could start changing things but these CSV and WUR are expensive and I do not want to throw money away without a decent diagnostic first.

Any good guess? WUR?

[NY65912]

I've got a similar problem. 85 Carrera The colder the worse it is. Upon cold start up...she hunts from 900 - 1200 rpm after about a min she goes to a high idle,,,,1000rpm and then settles to 950 rpm and then again to 900 and runs great from there out. I have no clue, but I guess I can live with it. I'll do some research and if I come up with anything I'll let you know.

Cheers, Mike

[pbs911]

Check the fuel mixture before you do anything. A rich fuel mixture can cause hunting.

[yelcab1]

Originally posted by NY65912
I've got a similar problem. 85 Carrera The colder the worse it is. Upon cold start up...she hunts from 900 - 1200 rpm after about a min she goes to a high idle,,,,1000rpm and then settles to 950 rpm and then again to 900 and runs great from there out. I have no clue, but I guess I can live with it. I'll do some research and if I come up with anything I'll let you know.

Cheers, Mike

Yours is a DME, which is much better than CIS.

I really do not want to live with this. As it hunts up and down the idle for 60 seconds, the tension in the chain is giving way and I can hear the timing chain slaps. I am afraid that one of these days it is going to spit out a valve because of this.

FYI, I have just updated to carrera tensioners (problem was before that), new fuel filter, new spark plugs, new wires, new dizzy cap, and new rotor.

[yelcab1]

bump

[cowtown]

I don't think you should be hearing the timing chain slap around just because of an erattic idle. You might want to pop off the covers and look at the phenolic ramps and tensioners one more time.

If mixture and pressure are OK, then you're pretty much looking at a CSV problem or your cold control pressure being off. If CSV tests good, move on to checking the cold control pressure - if rich, too low, if lean, too high. There's a mod you can make to the WUR that allows you to adjust cold (but not hot) control pressure.

EDIT - one more thing - there is a graph in the shop manuals that indicates what your control pressure should be for given engine temps. Before you do any parts replacing, try to check against that graph. I can probably send you a copy if you need one later tonight. There are also some air-control valves to check out, but I'm not familiar with them because I don't have them (AAV, etc). Good luck.

[makaio]

Whatever you do, don't just throw parts at it. Check the mixture and adjust if needed. If that doesn't solve it, run a pressure check.

[dbraunsr]

I've got the EXACT same problem.
'80 911 SC with CIS.

This may confuse the issue, but I can list a few things that did NOT help.

- new WUR, no improvement
- check dead cold to operating temp fuel pressure, in spec.
- found bad Temp switch (15 deg C), thought I found the problem but did not change or improve anything.
- Thermo-time switch is new also

I hope somebody knowledgeable steps in and helps us.
I'm slowing learning more about Bosch FI than I ever wanted to.

[yelcab1]

pulled the Aux Air Regulator (AAV) last night and it was open at room temp, so it seems that is also working, and not a culprit. I don't have a pressure tester, but I guess I will have to get one.

[targa80]

Originally posted by dbraunsr
I've got the EXACT same problem.
'80 911 SC with CIS.

This may confuse the issue, but I can list a few things that did NOT help.

- new WUR, no improvement
- check dead cold to operating temp fuel pressure, in spec.
- found bad Temp switch (15 deg C), thought I found the problem but did not change or improve anything.
- Thermo-time switch is new also

I hope somebody knowledgeable steps in and helps us.
I'm slowing learning more about Bosch FI than I ever wanted to.

I had the same problem as you and it turned out to be nothing more than fuel mixture adjustment. I used a dwell meter connected to the test jack and adjusted the mixture until I had the meter needle no longer hunting. Do you have your O2 sensor connected and when was the last time it was replaced? There is a procedure for testing using a dwell meter in the Bosch fuel management book that I own.

[yelcab1]

Pat,

you are suggesting that we connect the meter up before the engine is even started in the morning, fire her up, adjust the mixture during the first 60 seconds until the hunting went away?

All the mixture adjusting procedures I have read said to adjust it only when the engine is warmed up, at that point, there is no idle hunting problem.

I am confused with your suggestion then.

BTW, the O2 sensor is connected and I don't know when it was replaced. I have not tested its operation, but the car just passed Cal Smog (which is pretty damn stringent now with a dyno) so if the O2 sensor died, it probably would never have passed.

[stlrj]

I have read said to adjust it only when the engine is warmed up, at that point, there is no idle hunting problem.

But an over rich mixture when warm will also affect cold start mixture and cause the cold idle hunt. It is also common practice to run richer than factory spec mixtures for best performance which is most likey what a P-wrench would have done on a previous tune up.

Unfortunately, if you set the mixture back to spec you may loose some performance in exchange for a smooth cold start idle so you may not be eating your cake too.

Cheers,

Joe

[JP911]

On the advice of a former Andial mechanic who now works for Porsche Motorsport rebuilding motors, replacing fuel injector seals (if the injectors are loose) can eliminate a hunting idle. According to him, aging injector seals are a common source of vacuum leak and idle problems. He replaced these seals on my brother-in-law's SC and the idle hunt dissapeared. This is by no means a magic bullet, but it is something to consider.

[targa80]

Sorry about not claifying my statement. I did not mean that you should make the adjustment while it is cold. Set the mixture when the O2 sensor is heated enough to provide a reliable signal to the lambda control box in the closed loop mode. After starting the engine the lambda circuit runs in the open loop mode(thermoswitch is closed) which provides ~ 60% duty cycle to the frequency valve. After warm up The thermoswitch opens and the lambda control box sends a 50% duty cycle signal to the frequency valve. When the O2 sensor has warmed up and provides a reliable signal the lambda box goes into closed loop providing a duty cycle between 45~ 55%. That is when I adjust the mixture to maintain the minimal deflection(cycling) of the dwell meter. this would put the duty cycle of the frequency valve as close to ~50% in the closed loop operation. Using a dwell meter you can see how the system is operating on a % duty cycle basis during open loop and closed loop. If you are not in the 60% duty cycle at startup you will run rough and have driveability problems.

In my own case I traced the signal line to the lambda box and hooked my dwell meter directly to it that way I was able to monitor the duty cycle signal from cold start, warm-up and closed loop operation while sitting in the drivers seat and driving down the highway.

[dbraunsr]

On the subject of O2 connection - yes, it's connected and the sensor is brand new.

I'll need to read the test procedure about 6 times to fully understand it.
Thanks, I'll give it a whirl...

[VaSteve]

Originally posted by yelcab1
I did a search and found a couple of threads describing the problems but no answers. So here goes again.

1. Cold engine, starts right up, idle hunting between 500-1200 RPM, for about 1 minute, then steady out at 1500 RPM until the car is fully warmed up, and rock steady at 1000 RPM. No issues after that.

2. Warm or hot engine, starts right up, rock steady idle, no issues.

Mine does the exact same thing. Did it again tonight after 1.5 weeks of sitting in the cold. I know the mixture is rich, because the wrench helped me set it that way.

After it's warm, it runs great. Is this really a problem other than passing smog, which mine won't in the current set up.

[dbraunsr]

I passed Calif smog just a few months ago with flying colors, so all the emissions must be correct and not too rich.

It's not a huge problem, but I know when it left Germany it didn't have this pulsing idle problem.

It's just a machine and must be fixable.
It's getting the best of me so far, but I WILL prevail....

[stlrj]

Just becaiuse your car passed emissions you are assuming that the mixture is correct when in fact your catalytic converter may be in good enough condition to process the excess HC and CO to make it seem like your mixture is correct when a true test measurement taken ahead of the cat might show it to be too rich.

Joe

[tsuter]

I would just insert my hex "T" and give it a 1/4 turn counter clockwise. Let the car sit over night and try again in the AM. If it is worse then I would go richer instead of leaner. I think you can get rich surging as well as lean surging on cold start.
Then reset the idle speed when it is hot - long handle screw driver.

[Gunter]

Tsuter: Where does the long handle screw driver come in? For the large slotted screw left side which also can be turned by hand?
Anybody: With the O2 disconnected (No CAT) the LAMBDA still works sending signals to the frequency valve. What differences in mix adjust has anyone experienced? How does the O2 sensor-disconnect affect the settings? What exactly happens to the duty cycle when the O2 sensor is just disconnected? Thanks.