87 idle reving

[bubba8]

my 87 carrera when cold ,and also at idle when hot, fluxuates the idle about 400 rpms
in a pulsating manner.
any thoughts?

[86 911 Targa]

Originally Posted by bubba8

my 87 carrera when cold ,and also at idle when hot, fluxuates the idle about 400 rpms
in a pulsating manner.
any thoughts?

Either your ICV, or a vacuum leak are strong possibilities.

[DRACO A5OG]

+1 what Master Gerry wrote

To test, when she HUNTS, open the lid and tap on the ICV, if it clears up then you need to remove and service it by taking a carburator cleaner and clean the innereds until clear fluid comes out, shake until it rattles, clean again then if you wish spray a bit of silicon based lube into it. Replace and test. While you have it off check the OHMs per bentley manual.

Vacuum leak, check all the hoses for cracks or disconnection. Especially behind the throttle body, there is a U connection that tends to break or dislodge.

If I were you, I would service the ICV just for extra measure.

Hope this helps,

Jim

[stlrj]

Reducing the base idle speed usually fixes it, at least it did on mine. Something to try after all the complicated solutions fail.

Joe
86 3.2

[Chuck.H]

This can also indicate a 'rich' condition (car, not you). Is the motor 100% stock with respect to DME chip, airbox, exhaust etc?

Chuck.H
'89 TurboLookTarga, 332k miles

[hcoles]

I have auto-revving when cold and has been sitting for awhile, but doesn't happen every time

[stlrj]

Originally Posted by stlrj

Reducing the base idle speed usually fixes it, at least it did on mine. Something to try after all the complicated solutions fail.

Joe
86 3.2

If like me, and just want to get it fixed without too much techno-mumbo-jumbo: locate idle air bypass screw (the one in the middle of the throttle body), turn clockwise 1/4 to 1/2 turn, problem solved!

If still not satisfied, you could always return it to its original settings and no harm done.

[nesslar]

Originally Posted by stlrj

Reducing the base idle speed usually fixes it, at least it did on mine. Something to try after all the complicated solutions fail.

Joe
86 3.2

Yep. Here, too. You can usually (at least I can) dispell the searching by just letting the clutch out a bit while at a standstill (just a bit), for just a second, and it will settle.
I believe the lower idle setting is still the best bet; personally I like the idle at 850-900, though, which brings the "problem" to life....sometimes....
Easy enough to sit at idle with a lower idle RPM (800-820) and just "goose" it a little as needed so as not to drive you crazy with a low 800ish idle (which I do NOT like). Lar

[db_cooper]

Be sure you don't have any intake air leaks. Our engines like to suck the intake gaskets. The air leak(s) will make the engine run rich as the O2 sensor tries to compensate. Till the O2 sensor warms up and kicks in..you will have adjusted the engine rich to compensate too.

Very difficult to set the idle and CO and keep it stable.

It is worth checking.

[Nine9six]

Do a search for SURGING or HUNTING idle. As you can gather from these posts, there are several possibilities for the issue you describe.
HTH

[avale]

I went through the same thing on an '87 not too long ago. You are getting good advice here. Check out my thread here. Mine turned out to be an ICV that also fried the ECM. When you read my thread focus on the posts by Ingo (Ischmitz) and Loren (Lorenfb) as they provided the steps to debug. Feel feel to PM me to compare notes.

Alan

[Lorenfb]

"Be sure you don't have any intake air leaks. Our engines like to suck the intake gaskets. The air leak(s) will make the engine run rich as the O2 sensor tries to compensate."

- db_cooper -

Nice guess, but not correct!

1. Only an air leak around the throttle body, i.e. thru the AFM, will cause a high idle.
2. An air leak at the intake gaskets will cause a poor lower idle, i.e. lean condition.
3. In most cases an air leak rarely causes the O2 system to over-correct with a rich
condition.

"Very difficult to set the idle and CO and keep it stable."

- db_cooper -

Not really!

Bottom line: Let's not confuse the poster with mis-information resulting in potentially
wasted time & money!

[ancashion]

Originally Posted by Lorenfb

Bottom line: Let's not confuse the poster with mis-information resulting in potentially
wasted time & money!

Well there's a breath of fresh air in a sea of "money is no object so I'll just start throwing parts at it til something fixes it.... or doesn't."

[Nine9six]

After cleaning and checking my ICV, my surging idle issue was rectified with a new O2 sensor.
What Loren may lack in tact, he makes up for in knowledge about what he does for a living. The guy knows his business.

[kidrock]

"3. In most cases an air leak rarely causes the O2 system to over-correct with a rich
condition."

- Lorenfb -

Would you care to expand on this?

[Lorenfb]

"In most cases an air leak rarely causes the O2 system to over-correct with a rich
condition."

The O2 sensor will attempt to correct but never over-correct the result of its integration
within the feedback system and its overall limitations.

[kidrock]

thank you, sir.

[stlrj]

Originally Posted by Lorenfb

"In most cases an air leak rarely causes the O2 system to over-correct with a rich
condition."

The O2 sensor will attempt to correct but never over-correct the result of its integration
within the feedback system and its overall limitations.

True.

A vacuum leak is nothing more than unmetered air, which would be the same as backing off the CO adjustment screw on the AFM. However, the 02 sensor is only able to compensate for slightly over lean or rich conditions within a very narrow window which is why it so important to set the base mixtures to factory spec to stay within that window for best performance and smog.


Cheers,

Joe
86 3.2

[stlrj]

Originally Posted by Lorenfb

"In most cases an air leak rarely causes the O2 system to over-correct with a rich
condition."

The O2 sensor will attempt to correct but never over-correct the result of its integration
within the feedback system and its overall limitations.

True.

A vacuum leak is nothing more than unmetered air, which would be the same as backing off the CO adjustment screw on the AFM. However, the 02 sensor is only able to compensate for slightly over lean or rich conditions within a very narrow window which is why it so important to set the base mixtures to factory spec to stay within that window (I call it the sweet spot) for best performance and smog.


Cheers,

Joe
86 3.2

[db_cooper]

Originally Posted by Lorenfb

"Be sure you don't have any intake air leaks. Our engines like to suck the intake gaskets. The air leak(s) will make the engine run rich as the O2 sensor tries to compensate."

- db_cooper -

Nice guess, but not correct!

1. Only an air leak around the throttle body, i.e. thru the AFM, will cause a high idle.
2. An air leak at the intake gaskets will cause a poor lower idle, i.e. lean condition.
3. In most cases an air leak rarely causes the O2 system to over-correct with a rich
condition.

"Very difficult to set the idle and CO and keep it stable."

- db_cooper -

Not really!

Bottom line: Let's not confuse the poster with mis-information resulting in potentially
wasted time & money!

Loren, How would checking for air leaks cost money and more than a little time???...you are a total jerk. Do not critique me..make a positive suggestion or stay silent. No dollars spent if there is not a problem.

Because the air leak(s) are on only one or two cylinders..it will confuse the O2 sensor.



This engine had an unstable/surging idle cold and "piff" out the exhaust at idle hot. The leaking intake gasket was only on one cylinder.

$30 in gaskets and the problem is repaired.

I repaired 911s for a living, enjoyed factory training and an excellent reputation.

Air leaks are a problem and should be addressed first when sorting out an idle/mixture problem.