Idle fluctuation, Rpm between 1000 and 2000 Options

[SWH]

Completed rebuild on 1974 914 2.0 L. All fuel and vacuum lines are also new. Engine idle varies between 1000 and 2000 rpm every few seconds what could cause this?

[brant]

Did you rebuild the throttle body?

Most of them are worn creating a vacuum leak

Also worth servicing the distributor
Or just replacing with 123

[ctc911ctc]

Did you rebuild the throttle body?

Most of them are worn creating a vacuum leak

Also worth servicing the distributor
Or just replacing with 123

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Start here - this is the foundation of all things DJet

[emerygt350]

That is interesting, there are not any feedback mechanisms in the djet, so idle fluctuation is a weird one to me. How much time are you talking about between fluctuations?

[VaccaRabite]

I'm going to go with throttle body as the prime suspect.

Build a smoke machine and test your intake tract! Or buy a cheap one off Amazon. They are incredibly helpful at finding air leaks.

Zach

[rhodyguy]

Known as ‘hunting’? Up and down? Examine the stacked elbow. Cracks can hide.

[914sgofast2]

You have an uncontrolled vacuum leak somewhere downstream of the throttle body. That's why it has a "hunting" idle. Check the rubber elbow for cracks, as well as check the plenum seams for leaks using propane or carb cleaner.

[ctc911ctc]

You have an uncontrolled vacuum leak somewhere downstream of the throttle body. That's why it has a "hunting" idle. Check the rubber elbow for cracks, as well as check the plenum seams for leaks using propane or carb cleaner.

(IMG:style_emoticons/default/agree.gif)

Plenum cloth connectors got me before.

[TJB/914]

[quote name='brant' date='Feb 6 2025, 08:34 AM' post='3190560']
Did you rebuild the throttle body?

Most of them are worn creating a vacuum leak

Also worth servicing the distributor
Or just replacing with 123
[/quote/

I agree.
When I rebuilt my Throttle Body & added 123 my engine issues are gone.
Tom

[worn]

Did you rebuild the throttle body?

Most of them are worn creating a vacuum leak

Also worth servicing the distributor
Or just replacing with 123

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Start here - this is the foundation of all things DJet

I hope that you will expand on your answer. How do the dizzy and throttle body feed into this? Anything that I can learn about the little fellers is a bonus. Thanks.

[emerygt350]

Hunting normally isn't the result of a vacuum leak. A djet doesn't hunt if you crack the throttle or the aar is open. It might be due to your timing or a temp sensor sending strange signals. Running from 1k to 2k is a very large amount of air. You can hear the aar and that only brings you to 1.4k.

[brant]

Did you rebuild the throttle body?

Most of them are worn creating a vacuum leak

Also worth servicing the distributor
Or just replacing with 123

(IMG:style_emoticons/default/agree.gif) (IMG:style_emoticons/default/agree.gif) (IMG:style_emoticons/default/agree.gif) (IMG:style_emoticons/default/agree.gif) (IMG:style_emoticons/default/agree.gif)

Start here - this is the foundation of all things DJet

I hope that you will expand on your answer. How do the dizzy and throttle body feed into this? Anything that I can learn about the little fellers is a bonus. Thanks.

I'll take a stab at an answer Worn....


so the TB has the TPS which is how it is calibrated.
the TPS knows when the butterfly is closed, and thus how much air should be entering the intake ... when the butterfly is worn out... air enters arounds the sides of it.

and also... the calibration becomes wrong. as the units of air it is expecting (very little, just the hole in the butterfly at idle) when in reality on a worn out TB, you are getting more units of air than the TPS thinks... that is an unmettered vacuum leak

regarding the dizzy. it has the points plates which slide or rotate and change the timing. those plates need to slide easily. easily enough for the vacuum canister to advance or retard the timing slightly as the centifugal weights pull it in a direction with rpm.

some of these stick... also I've had very worn plates on cars. with a worn or stuck plate they are not returning or opening smoothly ... any time timing is changed, so does rpm.


short answer.
but 2 of the most common causes of surging idle
and not surprising as these parts are 50 years old.

2K is a pretty high idle. I would want to verify the accurate and correct timing. and then I'd expect to find more vacuum leaks for such a high idle.

brant

[brant]

Dup

[76-914]

Did you rebuild the throttle body?

Most of them are worn creating a vacuum leak

Also worth servicing the distributor
Or just replacing with 123

(IMG:style_emoticons/default/agree.gif) (IMG:style_emoticons/default/agree.gif) (IMG:style_emoticons/default/agree.gif) (IMG:style_emoticons/default/agree.gif) (IMG:style_emoticons/default/agree.gif)

Start here - this is the foundation of all things DJet

I hope that you will expand on your answer. How do the dizzy and throttle body feed into this? Anything that I can learn about the little fellers is a bonus. Thanks.

I'll take a stab at an answer Worn....


so the TB has the TPS which is how it is calibrated.
the TPS knows when the butterfly is closed, and thus how much air should be entering the intake ... when the butterfly is worn out... air enters arounds the sides of it.

and also... the calibration becomes wrong. as the units of air it is expecting (very little, just the hole in the butterfly at idle) when in reality on a worn out TB, you are getting more units of air than the TPS things... that is an unmettered vacuum leak

regarding the dizzy. it has the points plates which slide or rotate and change the timing. those plates need to slide easily. easily enough for the vacuum canister to advance or retard the timing slightly as the centrifugal weights pull it in a direction with rpm.

some of these stick... also I've had very worn plates on cars. with a worn or stuck plate they are not returning or opening smoothly ... any time timing is changed, so does rpm.


short answer.
but 2 of the most common causes of surging idle
and not surprising as these parts are 50 years old.

2K is a pretty high idle. I would want to verify the accurate and correct timing. and then I'd expect to find more vacuum leaks for such a high idle.

brant

add to that the butterfly shaft and bushings. (IMG:style_emoticons/default/beerchug.gif)

[brant]

I hope you’re doing GREAT. Kent
It’s been a while
Hope to see you soon

[emerygt350]

I would go with a leak like brant described if it were not fluctuating, the op still hasn't cleared up the interval and whether it is hunting or just random. If it's just "I came to a stop and this time it's at 2k" or if it is bouncing between 1k and 2k. Maybe I missed that post.

[saigon71]

A hunting idle can also be caused by a lean idle mixture.

Try rotating the mixture knob on the ECU full clockwise and see if it makes a difference.

[emerygt350]

A hunting idle can also be caused by a lean idle mixture.

Try rotating the mixture knob on the ECU full clockwise and see if it makes a difference.

Ahh yes, now that would do it.

[SWH]

Thank for all the feedback on the idle searching problem. The searching starts as the cold idle ends. The fluctuation is back and forth every few seconds. I have not driven since rebuild. Only did the cam break in. I did clean the throttle body and distributor, but did not do any rebuilds on these items. I did just complete brake system rebuild and should be able to drive soon. Some had mentioned the rubber vacuum which is new. I also tried adjusting the ECU, but not sure of the proper technique

[emerygt350]

remind me, was it modded at all? I bet you are just running lean after you warm up. After my 2056 I had to retune the MPS and change the ECU idle mix. A quick test is as mention above, just turn the knob on the ECU to richen the idle mix, if it goes away then you know that is your problem and you better take a look at your mix across the board before driving it much. Don't want to drop a seat/burn a valve etc.