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. :hihi2:

sorry, bored at work.

[SamHill]

Question: What causes the idle hunting?
Answer: Idle over-population. :hihi2:

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 :bawling:

[DMCMW Dave]

Make sure you have the throttle stop set (engine warm) as far open as you can without speeding up the engine. This insures that the idle motor has the most range and is not operating near-open.

Check your CO adjustment, within the normal range often setting it just a touch rich will get rid of the hunting.

[tiger38117]

Check your CO adjustment, within the normal range often setting it just a touch rich will get rid of the hunting.

I suggest this as well, 1905 was running VERY rich before I set the CO this spring, and it had a hunt even when warm. Bringing the mixture within spec solved it.

[David T]

Vacuum leaks are a big cause of exaggerating the hunting. The idle motor doesn't have enough power to overcome a lot of "False Air" or air leaking into the induction system after the air sensor plate. A big and common source of leakage is old injector seals. Another is the "O" ring seal on the air tube sticking into the bottom of the mixture unit. I also saw a couple of cars with no or an improper seal over the mixture screw. I used a smoke machine on a bunch of volunteers at the DMA Spring Tech session to look for leaks. After wrapping some teflon tape as a quick fix on some leaky injector seals there was a noticeable improvement.
David Teitelbaum

[DeLorean03]

Can anyone explain in easy terms what causes it?
thanks
John

My two answers for 1:30 in the morning:

1. What "DOESN'T" cause it is more like it =p.
2. It's a DeLorean

[sdg3205]

My car will hunt for about 5 - 10 minutes until the car gets up to operating temperature. My CPR is also currently suspicious so Im not sure if it's related or not. The hunt disappears when the car warms up but reappears under load - like when the fans come on or the head lights are on.

Any ideas?

[Bitsyncmaster]

Since the idle ECU is designed very close to always causing a hunting condition, any component can make it go over the edge. You have to have a good idle motor that is not sticky. If the idle motor is good, just a small adjustment in the curb idle screw could stop your problem. That adjustment will make the idle motor run in a different position to hold the preset 775 RPM.

[John U]

I'm pretty sure i have no vac leaks....replaced every seal and triple checked it all.

How do you adjust the CO? (where is the adjustment?)

[DCUK Martin]

WOW! Such an overdose of information!

John, the basic answer to your original question is: When it runs rich.

I always refer to "hunting" as being the rhythmic up/down oscillation of the engine as if someone's pressing the gas pedal every 2-3 seconds. This is common when the engine is cold and the lambda system is in open loop. It's caused by the reaction time of the engine being outside that which the idle system was designed for. If everything is set up spot on, there will be a moment about 1-2 minutes in from a stone cold startup that the engine suddenly stops hunting. This is the moment the lambda system switches to closed loop (ie feeding back a signal saying what's coming out the tailpipe) and leans out the mixture. If you then press the full throttle switch, you'll probably find the engine will start hunting again. Again, this is good. With the engine idling in front of you hot, the lambda system may swing slowly back and forth causing a much longer and smaller oscillation in engine speed that people sometimes also refer to as hunting. The cycle length is maybe 7-10 seconds and you can hear the frequency valve buzzing away changing pitch in time with this behaviour. This too is normal, though not ideal.

Vacuum leaks cause the engine to run WRONG. They cause things to go LEAN unless someone's been playing with it. Let's say you have a leaky injector seal causing one cylinder to run lean and misfire. Someone might try increasing the mixture screw to compensate and fix the misfire. What they've done is increased the fuelling on the lean cylinder to the point where it fires properly, but then the other 5 are running rich and this causes hunting.

My way to set up the mixture screw at idle is as follows:

On a hot engine with the frequency valve buzzing and regularly changing pitch, turn the screw clockwise (richen) until the frequency valve stops buzzing, then back off slightly. If it isn't buzzing when you start, turn it anti-clockwise till the frequency valve starts buzzing. Either way you want to get it to the point where the valve occasionally buzzes, maybe once or twice every 10 seconds but not constantly. Then replace the bung in the mixture screw hole - this causes a slight richening of the mixture by itself so you may wish to lean it off just a hair more so that with the bung in place, the frequency valve does that all important occasional buzz.

[DCUK Martin]

How do you adjust the CO? (where is the adjustment?)

Directly in front of the metering head as you look at it there should be a hole plugged with a removable bung of some sort. Drop a 3mm allen key into this hole and it finds a screw which adjusts the offset on the see-saw between the flap and the metering head. This bung may never have been removed and must be pushed up from underneath, which can be tricky!

[John U]

ok...that's what I thought you were referring to...yes it has not been removed.

[kings1527]

On a hot engine with the frequency valve buzzing and regularly changing pitch, turn the screw clockwise (richen) until the frequency valve stops buzzing, then back off slightly. If it isn't buzzing when you start, turn it anti-clockwise till the frequency valve starts buzzing. Either way you want to get it to the point where the valve occasionally buzzes, maybe once or twice every 10 seconds but not constantly. Then replace the bung in the mixture screw hole - this causes a slight richening of the mixture by itself so you may wish to lean it off just a hair more so that with the bung in place, the frequency valve does that all important occasional buzz.

Great explanation here but I do have a question with this. If I hook up my dwell meter and it's reading, then I push my wide open throttle switch (WOT), it causes my reading to go higher on the scale...past 45-50 or so. And that makes sense because it'll want to enrichen the mixture while at wide open throttle for better performance.

But if I'm adjusting my idle mixture, if I turn to the left (counter-clockwise) that makes the number go higher, or richer, (towards 45-50) and in the same direction of the WOT switch if I were to press it. Martin said turning CLOCKWISE will make the system go more rich.

So which is it to go richer? Clockwise or counterclockwise?

I like Dave's recommendation to adjust to normal reading plus just a tad richer for the best idle but I want to make sure I'm doing that right. I thought I knew this!

[thirdmanj]

Man. I'm gonna be all over this thread. Lots of good info all in one place. My girl has a pretty dramatic hunt after warm up. It's also a Stage ll, and I've been told the cam exaggerates this issue.

[ccurzio]

Man. I'm gonna be all over this thread. Lots of good info all in one place. My girl has a pretty dramatic hunt after warm up. It's also a Stage ll, and I've been told the cam exaggerates this issue.

So your girl gets revving after she's warmed up, and she gets really hot and heavy thanks to the cam.

You can make money on the internet with a line like that.

[thirdmanj]

So your girl gets revving after she's warmed up, and she gets really hot and heavy thanks to the cam.

You can make money on the internet with a line like that.

To continue this conversation, we'd need an "uncensored" forum. Too bad there isn't one nearby.

[kings1527]

Anyone have info on this? Thanks.

[DMCMW Dave]

Great explanation here but I do have a question with this. If I hook up my dwell meter and it's reading, then I push my wide open throttle switch (WOT), it causes my reading to go higher on the scale...past 45-50 or so. And that makes sense because it'll want to enrichen the mixture while at wide open throttle for better performance.

But if I'm adjusting my idle mixture, if I turn to the left (counter-clockwise) that makes the number go higher, or richer, (towards 45-50) and in the same direction of the WOT switch if I were to press it. Martin said turning CLOCKWISE will make the system go more rich.

So which is it to go richer? Clockwise or counterclockwise?

I like Dave's recommendation to adjust to normal reading plus just a tad richer for the best idle but I want to make sure I'm doing that right. I thought I knew this!

Counterclockwise is leaner (mechanically). You see the numbers go higher because the ECU is trying to drive it back to normal (i.e. richer to get back to stoich). The "dwell" reading (actually pulse duration) is showing you what the ECU is trying to do, not what it is reading. So it seems backwards, as you turn it lean the numbers to rich and vice versa. But that's what it is doing.

[hmcelraft]

A vacuum leak(s) is the usual cause for a rich fuel mixture - the O sensor reads too much O and thus more fuel. More fuel - less fuel ...... you get the hunt. The best method to check for leaks is a smoke machine. A manifold vacuum gauge might help with a big leak but, I would bet you have numerous little leaks. This is especially true of cold to hot engine and aging injector and idle system seals. There are, of course, numerous other possibilities but checking for vacuum leaks would be my first test.

[kings1527]

Counterclockwise is leaner (mechanically). You see the numbers go higher because the ECU is trying to drive it back to normal (i.e. richer to get back to stoich). The "dwell" reading (actually pulse duration) is showing you what the ECU is trying to do, not what it is reading. So it seems backwards, as you turn it lean the numbers to rich and vice versa. But that's what it is doing.

Thanks Dave. It does seem backwards but it makes total sense.

I had a mixture problem that I initially thought was a rest pressure problem. I'd let my car sit after being driven for awhile and it'd give me a hot start problem. I hooked my car up to my dwell (Actron analog CP7605) and I was reading 26-32 while idling. I read that a mixture that was too lean could cause the same problem so I adjusted my mixture screw counterclockwise and back to spec and it's been perfectly fine since.

Was I not too lean but actually too rich? And by adjusting the screw counterclockwise I actually leaned it out back to spec?

Thanks!

[Elvis]

A vacuum leak(s) is the usual cause for a rich fuel mixture - ...


wow, everyday I learn a bit more...:what_the:

[DMCMW Dave]

Was I not too lean but actually too rich? And by adjusting the screw counterclockwise I actually leaned it out back to spec?

Thanks!

Could be, excessive rich can cause a hot start issue. AKA flooding.

[kings1527]

wow, everyday I learn a bit more...:what_the:

Yes, I know. I think he means that a vacuum leak will cause a lean mixture. Clarifying to avoid confusion.

[DMC5180]

Yes, I know. I think he means that a vacuum leak will cause a lean mixture. Clarifying to avoid confusion.

You would think that, But I believe Harold was looking at it from the perspective of the O2 sensor reading the presence of more Oxy (lean from Vacuum leaks) telling the lambda system to add (Richen) fuel mixture. Which it does and this forces the the RPM to increase and Over shoot. Meanwhile the Idle speed system is trying to counteract the sudden RPM increase by closing the by-pass air valve. Two independent systems that don't play well together. What Harold is saying, fix all the potential vacuum leaks and Lambda system will behave itself by not seeking to react to the Higher O condition. This does make perfect sense.

[kings1527]

You would think that, But I believe Harold was looking at it from the perspective of the O2 sensor reading the presence of more Oxy (lean from Vacuum leaks) telling the lambda system to add (Richen) fuel mixture. Which it does and this forces the the RPM to increase and Over shoot. Meanwhile the Idle speed system is trying to counteract the sudden RPM increase by closing the by-pass air valve. Two independent systems that don't play well together. What Harold is saying, fix all the potential vacuum leaks and Lambda system will behave itself by not seeking to react to the Higher O condition. This does make perfect sense.

I see. So I guess it really depends on what angle you look at it from. You could look at it from the reality of the situation (lean) or how the Lambda system is trying to compensate (by making it rich). Semantics, kind of.

[Jonathan]

Make sure you have the throttle stop set (engine warm) as far open as you can without speeding up the engine. This insures that the idle motor has the most range and is not operating near-open.

Check your CO adjustment, within the normal range often setting it just a touch rich will get rid of the hunting.

Just to clarify, are we talking about setting the lower (curb) screw such that it is lengthened enough to push the lever arm and pivoting butterfly valves open a small amount, only up until you hear the engine speed up? Is that the idea? (with the top screw still engaging the idle microswitch while the throttle is at rest, of course.)

[DMCMW Dave]

Just to clarify, are we talking about setting the lower (curb) screw such that it is lengthened enough to push the lever arm and pivoting butterfly valves open a small amount, only up until you hear the engine speed up? Is that the idea? (with the top screw still engaging the idle microswitch while the throttle is at rest, of course.)

Yes. That gives the ISM the most range. Set fully warmed up.


Sent from phone

[DMC5180]

Yes. That gives the ISM the most range. Set fully warmed up.

Does adjusting the slotted brass air screw effectively do the same thing?

[DMC-81]

I was able to adjust 108516 a small amount to solve mine. However, I knew that I disassembled the throttle stop and the throttle rod, but not the idle screws during my engine refresh, so I tried that first. I still have to monitor it, so if it returns, I'll try adjusting the idle screws.

When a read post 2, 5, and and 7 above about the two engine systems trying to work together and sometimes interfering with each other to produce one cause of the idle hunt, that made sense to me as it sounds like a confused engine. It also reminded me of the need to separate the set points on an HVAC system by a few degrees to avoid conflict between the heating and cooling processes.

[DMCMW Dave]

Does adjusting the slotted brass air screw effectively do the same thing?

I've never set those anywhere but fully closed. Seems like it would, my reluctance is that they also can (un) balance the two sides of the engine which is pretty hard to measure without opening up the test ports on the exhaust manifolds. I also don't like how easy it is for them to move on their own, i.e. they won't lock anywhere but closed.

[David T]

You would think that, But I believe Harold was looking at it from the perspective of the O2 sensor reading the presence of more Oxy (lean from Vacuum leaks) telling the lambda system to add (Richen) fuel mixture. Which it does and this forces the the RPM to increase and Over shoot. Meanwhile the Idle speed system is trying to counteract the sudden RPM increase by closing the by-pass air valve. Two independent systems that don't play well together. What Harold is saying, fix all the potential vacuum leaks and Lambda system will behave itself by not seeking to react to the Higher O condition. This does make perfect sense.

This assumes all of the cylinders are perfectly balanced to each other ( they almost never are). The other problem is the systems are too slow to correct in real time, they are correcting AFTER the event so they overcompensate. Bottom line, some idle hunting is normal. The goal is to minimize it to the extent possible. Vacuum leaks are the biggest problem, especially on an older car where the rubber has dried out and shrunk and cracked. The next biggest problem is most of the time nothing is adjusted correctly. It takes a lot of "futzing" (a technical term) to get it all right. And you usually have to do ALL of the adjustments because the PO before you figured he could just adjust things to make it run right. Often turns into a major tune up replacing a lot of old, worn parts.

[Shark Pilot]

Man. I'm gonna be all over this thread. Lots of good info all in one place. My girl has a pretty dramatic hunt after warm up. It's also a Stage ll, and I've been told the cam exaggerates this issue.

I was told this as well. I have Isky Cams in my DMC with a full DPI exhaust and the engine was not properly adjusted by Bauerle Auto when installed. When I picked the car up it was idling around 1500-1700 rpm. That was annoyingly high. He told me it was "normal for those cams to have a high idle" which it turns out was bullsh*$. There were at least 5 vacuum leaks discovered and the CO was adjusted WAY too high to compensate for the bad idle and engine performance so my fuel mileage was AWFUL. To add to this, whenever I used the AC it would hunt at idle at least 400 rpm which again he told me was normal (and was not). :banghead:

Took it to Rob Grady and he fixed all of these issues. The car idles at about 900 rpm (my choice, my adjustment) with NO hunting at all warm or cold and when the AC is engaged at idle it might hunt 50 rpm or so if at all and the AC is blowing at 36C. :worship:

Long and short is that it's about the tune/mechanical adjustments, vacumm leaks and not the cams.

[David T]

I was told this as well. I have Isky Cams in my DMC with a full DPI exhaust and the engine was not properly adjusted by Bauerle Auto when installed. When I picked the car up it was idling around 1500-1700 rpm. That was annoyingly high. He told me it was "normal for those cams to have a high idle" which it turns out was bullsh*$. There were at least 5 vacuum leaks discovered and the CO was adjusted WAY too high to compensate for the bad idle and engine performance so my fuel mileage was AWFUL. To add to this, whenever I used the AC it would hunt at idle at least 400 rpm which again he told me was normal (and was not). :banghead:

Took it to Rob Grady and he fixed all of these issues. The car idles at about 900 rpm (my choice, my adjustment) with NO hunting at all warm or cold and when the AC is engaged at idle it might hunt 50 rpm or so if at all and the AC is blowing at 36C. :worship:

Long and short is that it's about the tune/mechanical adjustments, vacumm leaks and not the cams.

The cams have a lot to do with the quality of the idle. As the cams get more and more radical they have more and more overlap. Overlap meaning the intake and exhaust valves are both open at the same time. Great at high RPM's but at idle you don't have a lot of manifold vacuum so it is difficult to keep the motor running smoothly the slower you go. The typical "fix" is to just live with the higher idle. I guess in your case the cams don't have all that much overlap. It isn't who makes the cams it is all about the timing of the lobes, when the valves open, close and how far they open.

[Horsebox]

My own experience with idle hunting was that it was caused by the mixture being too rich. I replaced leaking air hoses going from the cold start valve tube to the charcoal canister and on the next start-up it hunted badly, exhaust smelled of petrol and CO measured 7%. Leaning the mixture counterclockwise with the mixture screw fixed the hunt. I also took the idle valve off the car and sprayed half a can of cleaner inside it and jiggled it about to flush out any crap so that it rotated freely.

I'm no mechanic so if I can stop my car hunting then anybody can. It may be common but I wouldn't see it as normal.

Interesting thread, the hunting problem is a bit of an enigma just like the car itself!