Idle Hunting -- The problem continues!!
 

My 84 has had an "idle hunting" problem. Based upon "conventional wisdom," I've replaced the idle control valve, the O2 sensor, and the cylinder head temp sensor. Yet, the problem persists. :(

Does anyone have any ideas on what could be wrong? Help would be appreciated.

Scott

Scott,

Sounds like you have replaced the key items I would check.

One additional item you may wish to check is the mixture level. I had some unstable idle issues and engine surging when my mixture was set to rich. I leaned the mixture (with the use of a CO meter) and my idle issues went away. This may not be your problem, but might be worth a shot.

Bruce
'84 Carrera
'77 911 (Euro spec)

With ignition timing and CIS system functioning properly the engine RPM will hunt when the mixture is not set properly. I would suggest adjusting the mixture to spec using one of two methods using an Co2 sensor up the tail pipe or adjust using a dwell meter on the O2 test jack. I used a Dwell meter to adjust my mixture and was able to eliminate the idle hunt and pass the Calfornia smog check at the DMV test station. My car was selected to be tested at the DMV test station and not just the local smog check shop. I believe this was required due to the age of the car and possibility of being a gross polluter.

The car runs great -- very strong, and the gas mileage is good. It also passed the California smog test when I registered it. If the idle is too rich and I adjust it, what will that do to performance and smog issues?

Thanks for the help, guys.

Scott

Scott,

I would think leaning the mixture would reduce emissions, but I don't need smog testing on my car, so I do not speak from experience.

Bruce

Scott,

Had a similar problem when I got my car back in December.

Started it up one morning and it was fine for a few seconds and then it would start hunting.

The hunting would get progressively worse, rpms would go slightly higher and lower with each cycle, and it would eventually die.

Had all of the same stuff checked and it turned out to be the DME had gone bad.

My wrench swapped it and all was fine.

The problem "seemed" to come out of no where but in retrospect the cars driveability was poor before the hunting started. Might have been a sign of trouble but as I'd only had the car 2 weeks I had no prior experience with how it behaved.

You might try to find someone and swap out your DME as a test.

Good Luck

Scott

When you say the DME, are you referring to the DME relay? If so, mine was replaced within the last 3000 miles.

Thanks!

I'd suggest checking for unmetered air leaks. My '84 [rip] had a serious hunt issue. I found that a few of the intake manifold nuts were finger-tight, which later proved to be due to gasket deterioration. I pulled the injection system off the engine and replaced all the gaskets, had the injectors refurbed while I was there, and all was well, afterward.

regards,

mark hald

Scott

Not the relay but the brain itself.

Scott

Drive down to your local shop and have them swap one (DME) from a car that runs great. If yours then runs great-problem solved, buy a new or rebuilt one. Or it could be an air leak issue.

My 84 is driving me nuts :mad:

Wroummm.....rrrrrr.....wroummm......rrrrrr....wrou mmm....rrrrrrrr......wroumm.......rrrrrr etc.etc.

Mechanic tried to fiddle with a contact at the side of the throttle housing where the gas cable attaches - made things worse. :(

If your car has cruise control, you might want to check the cable at the throttle to the left of the idle adjustment screw. If it is adjusted slightly too tight (i.e. no slack at all), then it could cause the throttle to not fully close, which results in a "surge" effect- 600rpm up to 1100rpm down to 600 rpm and so on. The cable end has a black plastic adjustment, similar to a bicycle's handbrake, and can be adjusted by hand- clockwise loosens the cable to provide slack.

Doug: My problem is intermittent. It usually happens at the end of a spirited drive, and usually when I'm above sea level. I'm guessing the mixture advice will cure the problem, but will the cruise control cable cause an intermittent issue?

Thanks for your help.

How's your idle speed? I've read here that lowering it sometimes solves the problem. I've also read that vaccuum leaks can cause it, and a mechanic told me to slowly spray carb cleaner on the hose joints; when the car stalls, then you just found the leak. Dunno if it works.

BTW, I'm having the same intermittent problem. My idle is at 1000 rpm and I think that may be part of the problem.

Are you sure it doesn't do it below sea level sometimes? :D
-Wiseass

Wiseass, er, Chris:

Here in Los Angeles, most of my driving is at sea level or within 1,000 feet of sea level. When I get up in the hills, however, and the car is warm, I have the problem. Based on what I'm hearing, it sounds more and more like a mixture issue.

Scott

I wonder if the barometric pressure thingy beside the DME could be disconnected or not working?
-The Poster Formerly Know As Wiseass :D

Well, if the idle hunts when the air is thinner, such as at altitude, then your car is running richer up there - so perhaps you need to adjust your idle mixture at the air flow meter. This is adjustable via something like a long 3 mm allen wrench up under a plugged hole in the air flow meter. The aluminum plug needs to be removed to access the adjustment screw. Best way to remove it is to drill two small holes in the plug and insert a pair of needle nose pliers into the holes and pull it out. Without complex air/fuel measuring equipment, disconnect your O2 sensor and measure the voltage fluctuation off the sensor with a DVM. Properly calibrated, it should fluctuate evenly between 0.2 to 0.8 volts at running temperature. Turn the screw clockwise to richen, counterclockwise to lean. You basically are adjusting the amount of air bypassing the measuring flap in the air flow meter to set the idle mixture. Attached is a pic of the plug location on the meter.

http://forums.pelicanparts.com/uploads/afmplug.jpg

Here's a typical voltage output of a healthy Bosch sensor:

http://forums.pelicanparts.com/uploa...or voltage.gif

Check the Altitude Correction switch under the driver's seat. Changes the injection pulse width at altitudes > 3300 feet. Without this compensation the car will run richer as Steve mentioned.
Good luck, (and post the findings!)

Steve
Where do you read the voltage from? Will an ordinary digital multi-meter do the trick?

Regards

David

Scott- yes, it will do it intermittently, especially when warm during spirited driving! I will confess to my knowledge of this- during my 1st year of ownership I thought that cable was the idle adjustment...what an idiot I was!!!! Anyway, I have lots of experience with adjustments of this and its' effect on my idle. You might as well eliminate this as a possible cause- it costs nothing and takes 2 minutes to do. Just adjust the cable clockwise until there is a 1/4"-1/2" of slack at the throttle and drive the hell out of the car--if the idle is better, you've fixed the problem for free! If not, my next step would be to adjust the REAL idle screw, which is just above the ICV that you replaced on a 3.2, usually covered by a yellow cap. Clockwise raises the idle. I think it is a 7mm long socket. After that, my guess is vacuum leaks, which are beyond my scope of knowledge.

hope this helps:http://forums.pelicanparts.com/uploads/pelifan.jpg


If you look above the idle control valve, you can see the yellow cap which covers the 7mm idle adj. screw


If you look to the left of the idle control valve, you can see a black cable ending at a black plastic adjustment nut (like a bicycle handbrake).

Voltage 0.2 to 0.8 CO mix
 

Steve:
I get no reading with a meter at to o2 plug, conected or not the idle is the same. Is that right? I put new o2 on and did the splice, solid conection, heat shrunk. Tried to tweek air flow screw but it is awful tight so I didn't move it. I think I'm running a bit rich, it smells that way.
Thanks in advance.
Still getting to know her before I buy your Chip.
Bob

Ugh i bother with this for 2 month. And after i took off AFM and cleaned it, sprayed all contacts with some contact spray the engine was idling as it should. Before i checked for vacum leaks, cleaned ICV and spent many hours watching engine ( im sure this didnt help) and even bothered with multi meter ( which i didnt even know how to use )
I also have S.Wong chip waiting to get into the car as soon as i get use to it.
Last thing i would do is adjusting mixture or idle speed.
Oh ya gotta check for click from idle switch

Make sure you are measuring off the black wire off the O2 sensor, disconnected from the car's harness. Also, make sure your DVM is set to read DC volts, and if not autoranging, set it to the 2 volt scale. The red lead of your meter should be on the O2 sensor's black wire, and the black lead should be any ground point of your motor such as your intake manifold. If you are getting 0 to almost no voltage, perhaps your mixture is lean. If your AFM mixture screw is tight, you should figure out why it's binding and fix it. Normally these screws are loose. It may be bottomed all the way clockwise, and if so, just try to remove it by turning it counterclockwise - it won't lock up in that direction. Then start with a base setting of turning the screw full clockwise into the meter, and backing it out 1.5 turns, and analyze your sensor voltage.

The meter has
DCV 2.5, 50, 250,1000
DCmA 10, 250
Conected the
red to male pin on 02 lead (black wire) that goes to sensor not dme.
blk to ground.
no readings.
if the AFM screw is all the way in, would this create the need to open the idle bypass screw on the throttle body more then normal to off set lean air flow setting to get idle solid? if so that's where I think I am, the idle cold start goes to 1200 then down to 500/600 for awile till temp gets up, then it sits pretty good at 750/825.
At idle it seem the fumes are eye burning, more at cold start and strong after fully warm.
What do you think?

It sounds like it's very rich - the eye burning is the high HC component of a rich mixture. If such a case, the O2 sensor should be reading over 0.8 volts dc. If you get no reading, either your sensor is defective, or your meter is not working properly. It's best to diagnose and fix the simple problems first - in this case getting a signal from the O2 sensor, then adjusting your mixture. To lean out the mixture, turn your mixture screw counterclockwise out to allow more air to bypass the sensor's flapper door. Note, turning the screw all the way in does not lean the mixture, it richens it, which typically increases the idle speed. The only reason you readjust the base idle speed on the throttle body, after adjusting the idle mixture, is to get the base idle speed within the range that the idle control valve can control it under. Out of it's expected range, it will cause an oscillating hunt or if set too low, sub idle dipping.

I ajusted the air by-pass screw to match idle after jumping ISV. That brought my idle up and more solid then before always around 500 and hunting up to 650. It seems to be soild now as said in prior post.
This is the reading from my emissions back in August.
Readings / Limits State of MA. 87 3.2l 177,000 mi.
grams per mile
HC 1.89/2.00
CO 15.58/30.00
Nox 2.56/3.00
Co2 411.15/ N/A
I have no hesitation when warm, seems a bit fickel when cold first 15min.
My conserns are if I am running fat am I deluting oil on cylinder wall causing hot head temps and consuming oil in the combustion chambers. No blue smoke when running, just the puff after a couple of days sitting.
The new o2 sensor could be faulty, but that dosen't seem right. My meter dosen't have a setting on the dial below 2.5 DCV so maybe that why it can't read 0.2 to even 2
Thanks for working with me here.

The other thing if i unplug o2 while running at idle no change, does that mean i'm at the 14.72 mark or abit higher if the o2 is good?

The 2.5 v dc should work fine at measuring anything below 2.5 volts. You can use something like a 1.5 volt battery to check it. If the O2 is connected, there may be a discernable 50 rpm hunt of the idle while the O2 sensor is modulating the mixture back and forth around the stochiometric ratio. If you're getting no difference connected or disconnected, either your sensor is not working, the DME is not recognizing the sensor, or you really do have the idle mixture closely adjusted to 14.72. Are you sure you spliced the connector to the sensor correctly? There are three wires from the sensor, two white, and one black. The two white are for 12 volts to preheat the sensor when it is cold. Only the black one puts out a voltage signal.

The whites might be fliped, black to black.

Just checked the meter w/ battery it works.

You can try blowing unlit propane into the intake and see if the sensor goes rich while the car is running, or safer yet, remove the sensor from your exhaust, and blow the propane gas over the tip and see if it induces a rich voltage.

When I tested idle with meter on I also increased the rpm to see if that would give me a reading on meter, Nothing!

Would increasing rpm up and down show some activity on the meter?
I still have my old o2 i'll check that one w/ the propane and see what goes on, if I get a reading then i'll check the new one.

Might not be till next week.
Thanks again for your help.

Just checked old o2, + on black wire - on threaded out side of sensor. no reading with propane. heated sensor tip and threaded area with propane pretty hot then raw propane no reading.
Couldn't wait.

You should get a reading of 0.6 volts or higher with tip hot, moving the flame back an forth should show fluctuation swings. Sounds like your meter doesn't have enough resolution to read under 1 volt.

Re-tested old sensor off car, heated it up pretty hot with torch got reading near .5 pulled flame away and she droped, passed heat over she would rise. Looks like I could of left old on.
Now as far as the one thats on the car, with the 12v heat leads disconected maybe the sensor just doesn't have the temp to get a reading.
I tested after a hour drive, the out side air temp was about 48 degrees, running at idle. Is it advisable to put jumpers between leads so 12v are heating sensor then test off the black wire?
Would HC and NOx be high if running to lean?
my readings
HC 1.8/2.oo limit
NOx2.56/3.00 limit
I think I saw a post that said NOx is related to high head temps.
Whats your thoughts?
Thanks

question about the o2 sensor and it's wires
there is a 3 prong plug coming from the o2 sensor,
is that were i check the voltage from?

Sub'd