Idle hunting... why? How to fix?

[Quicksilver]

Most of the big things I'd check have been mentioned:
- Intake bolts to 18 ft/lbs (the gaskets under them can be cracked so not a sure fix.)
- Cracked hoses. After this many years there is almost certainly toasty hoses.
- ICV is possible. A number of articles on checking that.

- One I haven't seen is the cylinder head temp sensor.

Most important is TEST not REPLACE. (Always drives me crazy to watch someone dueling with their car blindfolded with shotguns.)

A good test for the intake is to use an infrared thermometer gun to read the exhaust port temps after startup. A vacuum leak in the port can cause a port to read cold. Full throttle it will run fine.

-------
Helpful hints for anyone that needs to replace their intake gaskets...
- There are 2 gaskets for each port. One below and one above the plastic spacer.
- The plastic spacer will need to be replaced if you open it up. At this age they are crumbling.

------------
Oh yeah...
And as mentioned before: Ignore the advice from people referring to cars 1983 or before, or referring to "SC" or CIS". Their cars are completely different and are not electronically managed.

[Discseven]

Originally Posted by GH85Carrera

The ones the hold the intake runners down. There are two on each cylinder sealed with a individual gasket.

Ahhh... Those! TY

[GH85Carrera]

Yea, it was a weird mystery how two of the bolts that hold the intake down could be loose. The others all came off with that nice snap. Two were just finger tight.

[Discseven]

Originally Posted by aoncurly

My mechanic adjusted the mixture, so I am not really sure what they did. I did have to recently take to a mechanic to adjust the mixture again, as it was stalling and bucking between 2000-3000 RPM. I watched him - he took off the airbox cover, and used a ratchet at something near the fuel distributor. He also adjusted the idle screw and it ran great. Took him 5 minutes and it passed smog too!

I remember that move... you're talking CIS. Has allen set screw in fuel distrib. My situation is oddball... '80 car with '85 engine. So Motronic fuel injection.

Looks like you caught this before I posted. No worries.

[Discseven]

Originally Posted by techman1

I would bet the Idle Speed Valve, or ISV. Others referred to it as the blending valve above. On a cold engine, the DME tells it to open, adds more air to the engine. as the car warms up, the idle starts to raise, the DME tells the ISV to close some. If it doesn't, the idle gets higher, car gets warmer, until it hits the DME limit of about 1200 rpm. DME cuts the signal, maybe to the injectors, I am not sure. The idle drops, signal is restored, idle increases cause the ISV is still stuck open, repeat.

Could be other things, like a vacuum / intake leak. What is happening is the idle gets too high because of the extra air, and the ISV is closed all the way, so it can't help. Pulsing starts.

Since you said it will eventually stop pulsing, and a vacuum leak would stay leaking, I am pretty sure it is a sticky ISV.

Looks like ISV and ICV are one and the same... ?

Nice deduction at the end Tech. Suggests where to start. TY for input.

[Discseven]

Originally Posted by wrxnofx

LOL. Well, I guess not THAT "recklessly" As far as I can ascertain, the ICV has a coil of some kind on the left end that the connector goes into. The valve is coaxial to the coil. So when the system opens and closes the valve by applying voltage to the coil, the valve is actually rotating. If you remove the ICV and look at the air intake part you'll see what I mean.

To be more specific, if you rotate the ICV in your hand like it's a screwdriver and you're quickly turning it, you should see the valve rotate in there. Mine seemed pretty sticky, and there was a good amount of black "soot" in there. So I sprayed some carb cleaner to clean it out. I could then rotate it, and the valve was way more free and "floppy" in there.

So in my mind, the DME was trying to move the ICV during idle, but the ICV was slow to react due to the valve being sticky, which was responsible for my hunting.

The nice thing about checking the ICV is it's free

OK... will stir ICV martini XNO, not shake. TY for added explanation. Credit to "Sox" for photo below. That looks to be "spinner."




Evidence is building for sticky ICV.

[Discseven]

Originally Posted by JAR0023

85 3.2 with the factory chip should idle around 800 rpm. If you are idling at 1k and haven't made any adjustments I would put my money on a vacuum leak. Could be as simple as a cracked hose or loose intake runner. Hopefully you don't have to replace intake runner gaskets.

Lots of info if you search 'idle bounce'.

Idle was last set nearly 2 years ago to 800 JAR. Has crept up over time. Good point --- will bring it down. TY for search info.

[Discseven]

Originally Posted by Quicksilver

Most of the big things I'd check have been mentioned:
- Intake bolts to 18 ft/lbs (the gaskets under them can be cracked so not a sure fix.)
- Cracked hoses. After this many years there is almost certainly toasty hoses.
- ICV is possible. A number of articles on checking that.

- One I haven't seen is the cylinder head temp sensor.

Most important is TEST not REPLACE. (Always drives me crazy to watch someone dueling with their car blindfolded with shotguns.)

A good test for the intake is to use an infrared thermometer gun to read the exhaust port temps after startup. A vacuum leak in the port can cause a port to read cold. Full throttle it will run fine.

-------
Helpful hints for anyone that needs to replace their intake gaskets...
- There are 2 gaskets for each port. One below and one above the plastic spacer.
- The plastic spacer will need to be replaced if you open it up. At this age they are crumbling.

------------
Oh yeah...
And as mentioned before: Ignore the advice from people referring to cars 1983 or before, or referring to "SC" or CIS". Their cars are completely different and are not electronically managed.

Wayne... some good points. Will update menu with this juice. Would very much like dirty ICV as opposed to shot intake gasket(s).

[Discseven]

Originally Posted by PabloX

Agree, mostly. I replaced my O2 sensor and that made it better but didn't cure it. I cleaned the idle valve and that almost completely cured it, but it still happens occasionally when the car is hot. Going off the throttle quickly will often cause the symptom where coming off the throttle gently will usually make it settle down.

Going to test your "coming off throttle" Pablo to see what I get.

[Discseven]

Updated Checklist:

To keep track of info, am listing suggested causes, tests, recommended fixes, and relative links. Will move this forward as it's updated. Please advise correction(s) if you see anything wrong.

O2 sensor - Test (how?)
Air blending valve - remove and clean (if hunting exists when engine is cold... 02 sensor can be ruled out) - searched for Air Blending Valve and did not find fuel related info. Are we talking ICV/ISV here?
Idle mixture too rich - lean out idle mixture - Pelican Technical Article: The Motronic Engine Management System - 911 (1984-89) -
ICV - remove and test / clean - Idle Valve
Intake manifold bolt tightness - check tightness (18 ftlbs --- not sure what years this applies to). Good test for the intake is to use an infrared thermometer gun to read the exhaust port temps after startup. A vacuum leak in the port can cause a port to read cold. Full throttle it will run fine.
Intake manifold gaskets - Helpful hints for anyone that needs to replace their intake gaskets...
- There are 2 gaskets for each port. One below and one above the plastic spacer.
- The plastic spacer will need to be replaced if you open it up. At this age they are crumbling.

TY for input guys.

[Discseven]

Tomorrow am into ICV/ISV and will check intake bolts.

[porschetub]

Worked with DME before,ICV failure is rare and yes they can be cleaned and will work again,check for a buzzing noise with the key on if not don't dive in and buy a new one cause they are $$$$ new,check the plug on to it first,they do get old and don't connect properly,the result of this is a rolling idle from say 900 to 1200rpm or stalling after coming off throttle.
If you have air leaks the ICV gets out of running range and your idle will increase past what it will compensate for,as mentioned check the inlet manifolds for leaks.
O2 sensor normally won't effect idle but will cause a rich (default) condition after normal driving and increased fuel consumption,make sure your replacement one is exactly speced for your model and not a cheap universal one,ask me how I know
Think there should be a new tech book called "101 places for air leaks on EFI systems" because IMO this is the primary cause of issues.

[Quicksilver]

Originally Posted by Discseven

Wayne... some good points. Will update menu with this juice. Would very much like dirty ICV as opposed to shot intake gasket(s).

I had 3 ports with cracked intake gaskets. Best I can figure is that when the intake bolts are loose the gasket isn't supported and can crack,

The bad intake gaskets were kind of a blessing in disguise. I had to take off the intake and I discovered a couple cracked vacuum lines, some lines hardened to the point where they cracked on reassembly, and the biggie was that the crimped connections in the fuel lines would spin like they were bearings. Glad I caught that before the Car-B-Que.

An intake teardown to replace rubber parts is a good preventative measure on cars of this age.

[rick-l]

Originally Posted by Discseven

Idle was last set nearly 2 years ago to 800 JAR. Has crept up over time. Good point --- will bring it down. TY for search info.

I don't think you can set the idle speed on these cars. Jumpering those pins deactivates the idle control and you set the bypass air to the RPM the DME wants to see steady state with no correction. I think to give it linearity and control authority.

It does have a fast idle when cold.

Originally Posted by Discseven

Intake manifold gaskets - Helpful hints for anyone that needs to replace their intake gaskets...
- There are 2 gaskets for each port. One below and one above the plastic spacer.
- The plastic spacer will need to be replaced if you open it up. At this age they are crumbling.

TY for input guys.

BTDT.... helpful hint #1 might be to drop the engine. I did it with the engine in the car and the lower gaskets are cooked onto the head. You can't get access to the edge of the gaskets due to the shroud.


Sal is the expert on all of this.

[Discseven]

Originally Posted by Quicksilver

I had 3 ports with cracked intake gaskets. Best I can figure is that when the intake bolts are loose the gasket isn't supported and can crack,

The bad intake gaskets were kind of a blessing in disguise. I had to take off the intake and I discovered a couple cracked vacuum lines, some lines hardened to the point where they cracked on reassembly, and the biggie was that the crimped connections in the fuel lines would spin like they were bearings. Glad I caught that before the Car-B-Que.

An intake teardown to replace rubber parts is a good preventative measure on cars of this age.

Yeah... typical to chase one thing and find a few other things to work on.

Car-B-Que. Saw one some years ago on highway. 911 engine fire off to side of road and guy obviously had no extinguisher---just standing back watching it burn. Sinking feeling.

With you on rubber replacement Wayne.

[Discseven]

Originally Posted by rick-l

I don't think you can set the idle speed on these cars. Jumpering those pins deactivates the idle control and you set the bypass air to the RPM the DME wants to see steady state with no correction. I think to give it linearity and control authority.

It does have a fast idle when cold.

BTDT.... helpful hint #1 might be to drop the engine. I did it with the engine in the car and the lower gaskets are cooked onto the head. You can't get access to the edge of the gaskets due to the shroud.


Sal is the expert on all of this.

Now that I think of it... I've seen that pin jump thing done. Does work. Notes on procedure are in Motronic link in "Check List."

Good note on gasket access Rick. Am questioning whether I want to go that far at this time. Have other projects lined up so am hoping this idle hunt will be a quick resolve.

[Discseven]

Today...

Pulled ICV and removed case to check guts. Rotation doesn't seem sticky but not sure how delicate the movement of valve is. NOTE: Valve does not go round and round. It opens in one direction... and reverses to close. Here's looking into valve... open then closed. A bit of gunk can be seen on closed valve.




Brake cleaned valve area. O-ring is in good, pliable condition. So are hoses. Now am going to clean crud off electricals...




Inside housing are 3 contacts. Will bend them in slightly towards center. Beyond contacts there's floating plastic shield. That can fall out if not careful.




Obvious weak link in this area (in my engine) is top hose connection to intake plenum. That connection came apart with very little effort. Am going to replace the 2 crimped hose clamps there with screw clamps asap.




Went for 20 mile city drive afterwards. Did a couple of "Pablo variations" ...no more hunting. RPM is now near 800.




Had planned to check intake manifold bolts today but ran out of time. Is on tomorrow's menu.

[Discseven]

Fix = Clean ICV valve passageway with brake cleaner, slightly bend in 3 internal contacts, clean o-ring.

Also replaced 2 old crimp clamps that make connection at air intake plenum seen below (because this connection pulled out far to easy.) This was done after hunting was corrected so not necessary at moment but no need to leave it to loosen further.






Installed screw clamps but they don't fit visually compared to (solid) stock clamps. Will sort this out later.




Considered checking intake manifold bolt tightness but to access all of them... it's no small undertaking. So, sampled #4's intake bolts. Definitely under 18 ft lbs. Left bolts where they were. Will deal with this come next year's case split.

TY for input guys.

[techman1]

Great news, good luck with your alignment work!

[CCM911]

I would also add that you really don't want to mess with the mixture. All that will do is mask the real problem. As stated before, ICV, and then vacuum leaks. I tightened down all my manifold bolts, chased down all my leaks and then bought a used(know good) ICV, and replaced my high pressure fuel lines. Now my idle is fine.

I think too many people want to dive in and start monkeying with the mixture settings. Not the best way to go in my humble opinion.