Engine hunts at idle

[airfireman1]

Found a vacuum hose at the back of the engine that was an incorrect size (too big) and was loose.It connects at the back of the left hand intake manifold then to a solenoid. Now that it has been replaced with the correct size hose and does not leak my idle is hunting from 750 to 1000 rpm. Can someone tell me what needs adjusting to get it to idle smoothly. Just a note that it idles smoothly if the hose is disconnected completely.
As I am in Australia there is not much knowledge base here.
Looking forwards to some answers.

[Bitsyncmaster]

Check that your solenoid is powered via the idle switch. It's common that solenoid connector falls off and that give you ignition advance at idle.

[airfireman1]

Check that your solenoid is powered via the idle switch. It's common that solenoid connector falls off and that give you ignition advance at idle.

The solenoid appears to have the wires connected. Should it have voltage at the female plug end?. Aslo what else to check,
Thanks Bitsyncmaster

[Bitsyncmaster]

The solenoid appears to have the wires connected. Should it have voltage at the female plug end?. Aslo what else to check,
Thanks Bitsyncmaster

It should have voltage at idle.

Note, one pin has +12 with the key on and the other pin is grounded via the micro switch. So read voltage over both pins.

You also should not get vacuum on a cold engine so even with a failure of idle hunt, it would only do that with a warm engine.

[David T]

Hunting is caused by 1 or more cylinders not working equally to the others. The motor speeds up and slows down as it hits the uneven cylinder(s). The first thing to check is cylinder pressures by doing a compression test. If they are all within 5% of each other then clean and regap the plugs and put them back. Now you have to find the vacuum leak that is causing 1 or more cylinders to be lean or you could have 1 or more bad injectors. Timing would affect all cylinders equally so it is NOT a timing or timing advance problem.

[Josh]

Hunting is caused by 1 or more cylinders not working equally to the others. The motor speeds up and slows down as it hits the uneven cylinder(s). The first thing to check is cylinder pressures by doing a compression test. If they are all within 5% of each other then clean and regap the plugs and put them back. Now you have to find the vacuum leak that is causing 1 or more cylinders to be lean or you could have 1 or more bad injectors. Timing would affect all cylinders equally so it is NOT a timing or timing advance problem.

What are you going on about....?

[Horsebox]

If you have fixed a vacuum leak then the engine is probably running somewhat richer than it was before. So leaning the mixture via the 3mm adjustment screw would probably sort your hunting.

But you probably shouldn't touch this screw unless you have some direct measurement (exhaust CO) of what is happening to the mixture.

In my experience engine hunting is caused by a rich mixture.

Bear in mind even seemingly small leaks, leaving the bung off the mixture adjustment screw for example, will have a big impact on exhaust CO. For example on my car replacing a "forgotten" mixture bung will increase exhaust CO by 2.0%.

[David T]

If you have fixed a vacuum leak then the engine is probably running somewhat richer than it was before. So leaning the mixture via the 3mm adjustment screw would probably sort your hunting.

But you probably shouldn't touch this screw unless you have some direct measurement (exhaust CO) of what is happening to the mixture.

In my experience engine hunting is caused by a rich mixture.

Bear in mind even seemingly small leaks, leaving the bung off the mixture adjustment screw for example, will have a big impact on exhaust CO. For example on my car replacing a "forgotten" mixture bung will increase exhaust CO by 2.0%.

Vacuum leaks are "sneaky" meaning that they will lean out the cylinder or cylinders closest to the leak while the others are running well. Because the cylinders are not all firing evenly you get the hunting or surging at idle. Adjusting the mixture screw is a "global" adjustment meaning it will affect all of the cylinders equally but if some were firing well and others lean, by making it richer the ones that were OK are now too rich and the lean ones may be OK. Not the way to fix it. You don't usually fix anything by turning that mixture screw but everyone tries to anyway. In fact, that screw is the LAST thing you do once you know EVERYTHING else is 100% right. An example of a vacuum leak is an injector seal. If the ones you have are rock hard and you can pull an injector with no drag, you KNOW that seal is leaking! Very common. A temporary fix is to wrap it with a little teflon tape and see what happens. Other common leaks are the W pipe, the air tube to the idle motor in the bottom of the mixture unit, a missing plug over the mixture screw, old, hard, cracked and split hoses to the intake manifold. "Sneaky" ones include a bolt hole, a plug at the end of the intake runner, the idle motor on top, the hose going into the left pontoon, bad decel valve springs, the list can get long. The best way I know to find them is to use a smoke machine.

[Horsebox]

The best way I know to find them is to use a smoke machine.

I've been to several garages looking for ideas to do this but nobody knows.

I guess the idea is to pressurise the intake via the airflow sensor and watch for leaks everywhere and anywhere.

Any ideas for equipment to help with this?

Mark

[David T]

I've been to several garages looking for ideas to do this but nobody knows.

I guess the idea is to pressurise the intake via the airflow sensor and watch for leaks everywhere and anywhere.

Any ideas for equipment to help with this?

Mark

Any decent garage should have a smoke machine. It is the only way to find problems on newer cars with evap systems on the fuel tanks. I remove the air filter, seal the intake where the air filter fits onto the mixture unit, and pump the smoke in the hose going to the oil filler.

[Rich_NYS]

The original paperwork that came with my car states it's normal for it to hunt....ever hear that one?

[Horsebox]

The original paperwork that came with my car states it's normal for it to hunt....ever hear that one?

Never heard that before but interesting! Who wrote that, DMC?

I think I remember from the Pennebaker documentary that they were moving cars about on the boat and one of them sounded as if it was hunting.

Maybe first generation idlespeed control really was running at its limits even 36 years ago.

[opethmike]

No, it is not normal for them to hunt. I had the factory K-Jet system on my car for the first five years of my ownership, and had no issues with hunting.

[David T]

Hunting or surging is a relative term. By the specs the idle is 775 RPM + - 50 so even if it fluctuates from 725 to 825 it is still within tolerance. A well tuned motor shouldn't fluctuate that much. One thing you will notice is the frequency valve. It makes a lot of noise and it's pitch changes as the RPM's change which makes the hunting more noticeable.

[airfireman1]

I think it is running too rich now because it has no vacuum leak. It certainly is more responsive now, but it hunts at idle between 600 and 1000 rpm. I do have bad hearing but I don't think I can hear the frequency valve buzzing. Would I be able to feel it if I put my hand on it? It idled rock steady with the vacuum line not connected, ( open to atmosphere ).

[opethmike]

Yes, you should feel the valve buzz if you put your hand on it.

[airfireman1]

Finally getting some time took look at my car. I have replaced all the vacuum lines that I can get to but not the ones hidden under the intake manifold. I can't see how you can change them without taking the manifold off. I can definitely say that I cannot feel the frequency valve buzzing. Engine is at operating temp.
At the moment it can idle nicely at 750 and other times it will idle between 500 and 1100 rpm. So my question is what causes the frequency valve to operate and what would make it fail?. I cannot get a decent dwell reading, It just pegs out to one end of the scale. I have connected it to the Orange wire at the bulk head as I don't have the diagnostic wire at the back of the engine.
I know that the car has never had the O2 sensor changed and it has never had a Cat converter fitted as it is Euro spec.
It has only recently had new injector seals and fuel pump and filter. I have had to unplug the cold start injector because when the engine is warmed up then shut off it will flood the engine when re starting.
When the car is at operating temp it drive very well, its just the crappy idle that annoys me.
Any input would be appreciated.
Cheers

[Bitsyncmaster]

The most common cause of the FV not working is someone plugged the wrong relay into the lambda relay position. That relay position is wired differently for some unknown reason.

[airfireman1]

OK will check that in the morning,
Thanks again

[David T]

Bunch of "common" reasons the frequency valve stops buzzing.
Wrong or bad Lambda relay (it is different from all of the others so sometimes they get switched around and the wrong one winds up in the Lambda spot)
One of the connector terminals in the block "backed out" and is not making contact with the relay
The connector plug on the frequency valve has some corrosion. Plug and unplug it a couple of times
Someone disconnected the ground wire to the frequency valve. It has to be connected to ground on the engine.

Next bunch of problems with the Lambda system:
O2 sensor missing, disconnected, contaminated, or just old
Mixture screw WAY off/plug missing
Vacuum leaks.
Bad/misadjusted micro switches on throttle spool

The Lambda system can not work correctly if the motor is not in "good tune". All of the cylinders must be balanced and operating as equally as you can get them to for the Lambda system to function correctly. The mixture screw is the LAST thing you touch after you get everything else working good.

[NckT]

In addition, missing earth connection on pin5 of the fuel comport (earth on pin 16 is not enough) or just a bad quad op amp IC (LM2901N) in the fuel computer itself.

52390

[airfireman1]

Checked that the correct relay is in the correct spot. That's all Ok. I might get a new oxy sensor and try that. Can anybody please let me know the correct Bosch part number as I am hoping to get one from my local auto parts store here in Australia.
Thanks again.

[NckT]

Checked that the correct relay is in the correct spot. That's all Ok. I might get a new oxy sensor and try that. Can anybody please let me know the correct Bosch part number as I am hoping to get one from my local auto parts store here in Australia.
Thanks again.

Just disconnect the electrical connector for the oxygen sensor thata located at left hand drive drivers side under the rear wooden board behind the grey rubber bung. Disconnect this blade electrical connector and the fuel ecu should default the switching off the frequency valve to 50% duty cycle (45 deg on dwell meter on 4 cylinder song).

If the frequency valve does not buzz, then replacing the oxygen sensor won't fix the problem.

If you want to change the oxygen sensor disregarding the simple test, then the bosch code is 11032

The universal product code is UPC 28851110327

Or you could just get a universal single wire narrow band 0.0 - 1.0v zirconia type m18 x 1.5 thread.

[NckT]

Should read:

(45 deg on dwell meter on 4 cylinder setting).

[airfireman1]

Put the car up on my new hoist today. First time I have used it since it got installed this week. Such a joy to be able to get under the car when standing. Anyway on to more serious stuff. I had a brand new single wire oxy sensor in my garage so I decided to change it anyway. The old one undid without too much effort and I swapped it over.
Then I noticed that the wire had rubbed on the chassis and taken off the insulation so it was shorting out all the time.
With the new sensor installed it seems to idle and run so much better.

[NckT]

Glad it was a cheap fix in the end