what causes the idle hunting?

[John U]

Just refurbished everything in the VOD. I still have the idle hunting but not as bad as before. I have heard that this is fairly normal with the Kjet. Can anyone explain in easy terms what causes it?
thanks
John

[David T]

You have several systems (idle speed motor and Lambda and ignition) indirectly interconnected all operating at different frequencies and time delays. If all 6 cylinders are not firing EXACTLY equally the engine will tend to speed up and slow down as it goes through the firing order. Any imbalance (dirty injectors, air leaks, etc,) will tend to exacerbate the imbalances. Just the Lambda system all by itself has a big delay built in. Any adjustment arrives way too late to correct the mixture in real time so it is always "running behind". Makes the engine "hunt" or oscillate because the mixture is always changing which affects the idle speed so the idle motor is always trying to correct the speed. That just impresses another harmonic over the Lambda's oscillations.
David Teitelbaum

[Mike C.]

Question: What causes the idle hunting?
Answer: Idle over-population.

sorry, bored at work.

[SamHill]

Question: What causes the idle hunting?
Answer: Idle over-population.

sorry, bored at work.

It's not your fault, Will. It's not your fault. Look at me. It's not your fault.

[Bitsyncmaster]

How technical do you want the answer?

When a closed loop system (output is adjusted according to and input) has delays it has to be in perfect timing to keep things stable.

Not much delay in reading RPM. RPM is computed by the time between one (in our case they use two) ignition pulses.

The idle controller reads the RPM and moves the idle motor to compensate for a deviated idle RPM. The idle motor it self has quite a bit of delay to get the mass of the motor internals moving.

The engine itself has the most delay from air fuel volume changes to resulting RPM changes.

So say your engine is at 600 RPM and the idle controller makes a change to the ISM to increase the RPM. If it does not wait for all those delays and makes another RPM reading then it will still see a low RPM and keep increasing the ISM to compensate. Then the engine RPM finally catches up to all those ISM movements and since the controller moved it to much the RPM is now high........So it moves it back down and the whole thing repeats.

Now when the Lambda system is running it is changing the mixture. A mixture change does change the RPM. The lambda system is another closed loop system and can do the same as the idle controller......= hunting idle.

If you slow down those two system (longer wait for the delays) you will stop hunting idle. BUT then they are slow to respond to load changes on the engine. Alternator loads and AC kicking on and off are the high speed load changes. When your AC kicked on the idle may drop from 775 to 600 RPM and take a few seconds to come back to 775.

[82DMC12]

I have noticed that my car only hunts for a few seconds (less than 60) if it quite cold out. It does not hunt when it is hot outside. I believe it is more related to OPEN-LOOP operation. There are two fixed-lambda modes the car goes through if the ambient air is very cold before the O2 sensor warms up enough to give a good reading. The engine should not hunt if the engine is warm or hot. That's my experience along with what I have heard from other owners.

Andy

[Bitsyncmaster]

I have noticed that my car only hunts for a few seconds (less than 60) if it quite cold out. It does not hunt when it is hot outside. I believe it is more related to OPEN-LOOP operation. There are two fixed-lambda modes the car goes through if the ambient air is very cold before the O2 sensor warms up enough to give a good reading. The engine should not hunt if the engine is warm or hot. That's my experience along with what I have heard from other owners.

Andy

Your correct that is another case of hunting. Definitely caused by idle ECU. The OEM idle ECU it timed right near the point of causing hunting as I stated in the last post. Now that quick changing mixture sometimes drives the idle ECU into hunting. They made it fast so it would respond faster to a condition of driving down a hill in gear.

Your foot is off the gas so the OEM ECU will close the ISM because the RPM is above 775. Now when you step on the clutch the idle motor is closed (almost fully) and engine RPM dips down to the curb idle setting. Since the ECU is timmed for fast recovery that dip is only a split second long and the engine does not stall.

[John U]

i'm almost sorry I asked....

so it is normal to hunt a little when the engine is cold, but not when it is warmed up?

[tiger38117]

Or, to give the guy the simple answer he asked for:

The most common cause is vacuum leaks. Make sure everything is tight before you start messing with the more complicated features of our engines.

1905's engine will hunt just barely (maybe 200 rpm) when it is dead cold. Depending on ambient temp, that will last 30 sec-2 min until the o2 sensor warms up from the exhaust. After that, it does not hunt at all.

[John U]

I just replaced all the smaller diameter vac hoses, and triple checked all connections. the hunting is pretty minor compared to what I had before tearing into the VOD so I will live with it for now.

thanks for all the answers....I need a tylenol