Idle hunting, Where do I start? Options

[steuspeed]

I'm working on fixing my idle search. All new vacuum lines, plugs and wires. Car starts and runs smooth for about 1-2minutes, then the idle dumps and starts hunting up and down. My ECU knob was turned full clockwise by the PO. I turned it back to the factory setting.

Stock 2.0L 1973 car with Pertronix. Where should I look next?

[Bleyseng]

Do you have the resistor installed?

[rjames]

Are you sure no vacuum leaks?
Does changing the position of the ECU knob do anything?
Are all of the FI parts correct for your car?
Has the MPS been rebuilt or futzed with?

[wonkipop]

not sure i can help as have a 1.8 L jet.
but have had an idle hunting issue and i think i cracked it....maybe.
but perhaps not the same symptoms as yours.

the cars are getting old, that maybe new issues arise in relation to materials etc.
who knows. its fun sorting them out.

see here what i found.
don't read the whole thing. just go to my last post.
http://www.914world.com/bbs2/index.php?showtopic=352145

hope you crack the problem. (IMG:style_emoticons/default/beerchug.gif)

[steuspeed]

I see a white wire/connector/black wire to cylinder head temp sensor. I do not see a resistor. Can I probe the white wire back to the ECU?

[steuspeed]

Are you sure no vacuum leaks?
Does changing the position of the ECU knob do anything?
Are all of the FI parts correct for your car?
Has the MPS been rebuilt or futzed with?

Pretty sure no vacuum leaks.
Car start and runs nice. I can adjust the idle to say 1100 for warm up. After about 1 min the idle drops and starts hunting. It will hunt and die with the ECU knob in the factory location. If I turn the knob fully clockwise it will still hunt but barley stay alive.

The MPS epoxy is gone, so someone has at least turned the outer screw. I tried to get it out, but it only turns about 2x back and forth. The coils read 87.6 ohms and 339.7 ohms. No continuity between coils.

I believe all the FI parts are correct and there except Decel valve has been removed. Caps on air box.

[BeatNavy]

I think you're running lean. You need to troubleshoot the things that can cause that condition:

1. Low fuel pressure
2. Clogged injectors
3. Incorrect TPS calibration (although it seems like your idle knob IS making a difference)
4. Bad or poorly tuned MPS
5. Missing (or bad) D-Jet component, e.g., as Bleyseng says, ballast resistor.
6. etc. etc.

Try disconnecting the TI (ambient air temp) sensor. That will enrichen things a tad. See what impact that has, if any.

Are you seeing any symptoms outside of idle? Poor power or hesitation, bucking, etc.?

I assume you don't have an O2 sensor / gauge.

[914werke]

(IMG:style_emoticons/default/agree.gif) hunting idle is usually a symptom of lean AFR

[mgphoto]

Most times it’s the idle screw on the throttle body, its screwed out to far.

[cary]

Concur with Rob & Rich, too lean.

[steuspeed]

(IMG:style_emoticons/default/agree.gif) hunting idle is usually a symptom of lean AFR

What should the start idle be set at?

[steuspeed]

Most times it’s the idle screw on the throttle body, its screwed out to far.

what should the start idle be set at?

[BeatNavy]

I wouldn't worry too much about the idle bypass screw at this point. You can try adjusting it to see what happens, but if you are lean it isn't going to have that much impact. That's actually another symptom of a lean idle condition -- idle bypass not having much impact on idle speed.

Confirm you are running lean and then figure out why.

Again, any symptoms under driving conditions, or only idle?

[steuspeed]

I think you're running lean. You need to troubleshoot the things that can cause that condition:

1. Low fuel pressure
2. Clogged injectors
3. Incorrect TPS calibration (although it seems like your idle knob IS making a difference)
4. Bad or poorly tuned MPS
5. Missing (or bad) D-Jet component, e.g., as Bleyseng says, ballast resistor.
6. etc. etc.

Try disconnecting the TI (ambient air temp) sensor. That will enrichen things a tad. See what impact that has, if any.

Are you seeing any symptoms outside of idle? Poor power or hesitation, bucking, etc.?

I assume you don't have an O2 sensor / gauge.

I checked in my 74 car for a resistor. None in that car either. I swapped the MPS and no change. I disconnected the air temp sensor. No change either.

The car runs great. Lots of power. No hesitation or bucking. No O2 sensor. Hands free key starts. Idles nice for about 1-2 minutes before going into oscillation.

[steuspeed]

I wouldn't worry too much about the idle bypass screw at this point. You can try adjusting it to see what happens, but if you are lean it isn't going to have that much impact. That's actually another symptom of a lean idle condition -- idle bypass not having much impact on idle speed.

Confirm you are running lean and then figure out why.

Again, any symptoms under driving conditions, or only idle?

The idle screw is working well. I can dial up any idle I choose. I found 1500-1800 cold on another thread.

[saigon71]

Agreed that you are running lean at idle.

Start by turning the idle mixture knob on your ECU clockwise all the way - this is full rich. If this smooths out your idle, turn the knob counterclockwise a click at a time until the engine idle starts to hunt, then back off one click. Give the engine a little time to adjust between each adjustment.

If that doesn't work:

The 1973 model year used a ballast resistor of 270 ohms from the factory - it seems you are missing this. Pick up a resistor of this value and wire it in series with the CHT temp sender. This resistor effectively richens the mixture accross all operating conditions.

Keep us posted on what you find. Good luck!

[JamesM]

Agreed that you are running lean at idle.

Start by turning the idle mixture knob on your ECU clockwise all the way - this is full rich. If this smooths out your idle, turn the knob counterclockwise a click at a time until the engine idle starts to hunt, then back off one click. Give the engine a little time to adjust between each adjustment.

If that doesn't work:

The 1973 model year used a ballast resistor of 270 ohms from the factory - it seems you are missing this. Pick up a resistor of this value and wire it in series with the CHT temp sender. This resistor effectively richens the mixture accross all operating conditions.

Keep us posted on what you find. Good luck!

That^^

Also should add though that the resistor was only used on some of the 73 cars, this (and a few other things) changed through the 73 production run and the later 73 cars use the same setup as the 74 cars. Need to verify your ECU and MPS part numbers to see what is in there. If you search you may be able to find the cutover date/vin as i know that information is out there, just dont recall off the top of my head, and it isnt reliable anyways as a lot of cars have swapped parts at this point.

But, if it is running fine everywhere else, start with the idle mixture knob on the ECU as stated.

[rjames]

Agreed that you are running lean at idle.

Start by turning the idle mixture knob on your ECU clockwise all the way - this is full rich. If this smooths out your idle, turn the knob counterclockwise a click at a time until the engine idle starts to hunt, then back off one click. Give the engine a little time to adjust between each adjustment.

If that doesn't work:

The 1973 model year used a ballast resistor of 270 ohms from the factory - it seems you are missing this. Pick up a resistor of this value and wire it in series with the CHT temp sender. This resistor effectively richens the mixture accross all operating conditions.

Keep us posted on what you find. Good luck!

That^^

Also should add though that the resistor was only used on some of the 73 cars, this (and a few other things) changed through the 73 production run and the later 73 cars use the same setup as the 74 cars. Need to verify your ECU and MPS part numbers to see what is in there. If you search you may be able to find the cutover date/vin as i know that information is out there, just dont recall off the top of my head, and it isnt reliable anyways as a lot of cars have swapped parts at this point.

But, if it is running fine everywhere else, start with the idle mixture knob on the ECU as stated.

Agree. Go slow with the adjustment knob. Each click or two should make a noticeable difference.

[914_7T3]

My ‘73 had the same symptoms and it was indeed the ECU adjustment. Turned out to be an easy fix.