Idles high, maybe a bad idle motor or wiring?

[nullset]

Here's the symptoms:

It usually idles around 1.5k-2k RPM. Sometimes it goes as high as 2.5k. This annoys me.

If I shake the electrical connector that sits on top of the idle speed motor, it sometimes drops back to about 1k.

This behavior happens even if the lower idle adjust screw is completely out and the switch is depressed, which makes me suspect the motor or wiring, not the idle screws.

What do you think? Any other tests I should run?

--buddy

[outatym2001]

Below is the thread where I describe my similar problem and my solution. Please read my post number 7.
It was an expensive fix. I have over 130,000 miles on my D and I had to replace the Idle Speed E.C.U. at $275.00 and the Idle/Low Speed Motor at $295.00.

Idle speed suddenly high
http://dmctalk.org/showthread.php?541-Idle-speed-suddenly-high&highlight=idle

I honestly can’t think of any tests. All you can do is replace both and see if that fixes it.
You can also read my post number 13 in the same thread for cleaning the Idle/Low Speed Motor.

[Kevin]

You could try testing the thermistor. It's 2 of the pins that connect to the idle ECU, there's a diagram of those pins and what goes where on page D:05:03 of the workshop manual. I'm not sure what the resistance value should be, but seeing if it even completes a circuit is a good start.

[jorge figueroa]

Hello
I had a similar problem with my car
and I found the switch on the throttle damage
in many cars this piece already has 30 years
to test this piece is bad
I just put a test cable between the contacts of the switch
and immediately the engine worked perfectly

[nullset]

Hello
I had a similar problem with my car
and I found the switch on the throttle damage
in many cars this piece already has 30 years
to test this piece is bad
I just put a test cable between the contacts of the switch
and immediately the engine worked perfectly

It definitely behaves differently when I press the switch, so I'm pretty sure that's not it.

I'll check the ECU connections and diode(s) tomorrow.

--buddy

[TTait]

Not trying to hijack, but I have had a chronic high speed idle that I have been working on, and I may have a clue to my problem - but not a solution.

Background: I have had a high idle problem only in warm weather for a couple years. It started just after some engine work was done on the car by a reputable vendor (no blame intended) but there was significant tusseling around with the wiring harness.

I have swapped out the idle ecu and idle speed motor from a known good car, the problem continued so I put the originals back. The problem continued but the swapover parts work fine on my wife's car. I have checked all throttle plates and linkages etc. The microswitch is working properly (switch is open at idle, and closes when you apply throttle - right?)

I have replaced the thermal switch in the head by cyl 1. I checked the diode in the black/lt green wire. All seem ok.

The car consistently gets a high idle on hot days, anywhare from 2000-3000rpms.

What I discovered tonight is that (with diode disconnected) I have continuity between ground and the black/lt green wire from the ecu to the microswitch. When I unplug the white bulkhead connector and check the engine harness portion of the wire the short to ground becomes intermittent.

I note that the black/lt green wire is spliced to a blue/black wire at the bulkhead connector - this splice seems to be the source of the intermittent ground. That blue/black wire runs to a resistor (somewhere - but where?) and on to the vacuum solenoid according to the wiring diagram.

I assume the vacuum solenoid is the one that hooks up to the forward portion of the air meter assembly - right?

The possibilities that come to mind (though I'm a bit outside my comfort zone) are:

That even though this connection to ground reads as intermittent, this is a trick of the digital meter I'm using and that an attenuated ground is provided via that hidden resistor - and its fine.

That the ground should be there consistently, and that the fact that it sometimes falls away is part of my problem...

Or that there should be no continuity to ground from the black/lt green wire - if this is the case I either have a gap in the insulation somewhere, the resistor or something else (vacuum solenoid?) in the blue/black wire circuit is faulty.

By the way, there does not appear to be any continuity to ground on the half of the wire which is inside the car - once the white bulkhead connector is unhooked, that wire has good continuity end to end but no continuity to ground, the grounding of this wire seems to only happen in the engine compartment area.

Dave, or anyone, what should I check next - any suggestions?

Any help appreciated - this one has been vexing me for a while.

Tom

[Elvis]

Have you used the SEARCH function ?

How to test the idle motor - I've described that about 10 times in the last ten years.

You do have a schematic ?

You do know how the idle system works ?
And what that switch does ?

Did you measure and bridge the thermistor ?

What kind of tools and measuring equipment do you have ?

You know that the idle motor is humming with 100Hz ? Does it do that ?

[WelmoedJ]

I note that the black/lt green wire is spliced to a blue/black wire at the bulkhead connector - this splice seems to be the source of the intermittent ground. That blue/black wire runs to a resistor (somewhere - but where?) and on to the vacuum solenoid according to the wiring diagram.
Tom

I've not been able to locate a resistor in that circuit.
However there is a capacitor in the circuit from a splice to the vacuum solenoid.
It could be you have been measuring that capacitor's resistance and considered it to be a resistor.

If the capacitor is bad, it can influence the system.

Welmoed.

[Bitsyncmaster]

Here's the symptoms:

It usually idles around 1.5k-2k RPM. Sometimes it goes as high as 2.5k. This annoys me.

If I shake the electrical connector that sits on top of the idle speed motor, it sometimes drops back to about 1k.

This behavior happens even if the lower idle adjust screw is completely out and the switch is depressed, which makes me suspect the motor or wiring, not the idle screws.

What do you think? Any other tests I should run?

--buddy

If you move the connector and it seems to work then you most likely have bad connections at the connector. The idle motor itself has no brushes so is pretty solid.

[Bitsyncmaster]

I've not been able to locate a resistor in that circuit.
However there is a capacitor in the circuit from a splice to the vacuum solenoid.
It could be you have been measuring that capacitor's resistance and considered it to be a resistor.

If the capacitor is bad, it can influence the system.

Welmoed.

That capacitor only cleans up electrical noise when the solenoid turns off. I've found it to help with my circuits if you have problems when you step on the throttle. It will not affect a constant idle condition.

[Bitsyncmaster]

If the thermistor going to the idle ECU is open, my car (with the OEM ECU) would idle about 2500 RPM. If the advance solenoid is not on, the idle jumps to about 1500 RPM.

If you suspect the thermistor, just jumper it to ground at the idle ECU. That could affect some engine startup conditions so you may not want to leave it shorted but that will tell you where to look for the problem.

I've found many bad connections on crimped pins so buying components may get expensive and not fix the problem. The wires in the engine bay get marginal after all the heat they see.

[DeLorean03]

When first restoring 3299 back in 2004, I had a similar problem as the original poster. My car would immediately idle at 2500 rpm, and NOTHING would stop it. Turned out the idle speed thermister had failed. To fix it, there are two wires that run from the idle speed ecu that need to be "jumped" - similar to what Kevin said. Since the idle speed thermister is such a pain to get to, I snipped the two wires, stripped the ends and crimped them together. What that did is completely bypassed the idle speed thermister.

Once I get 3299 up here in about 2 weeks, I'll be able to take a picture for you. You DONT have to snip them like I did - to test - you can simply make a jumper wire (piece of 16 gauge wire with alligator clips, one clip on each end), and jump the two wires together. Thankfully, the wiring leading to the idle speed ECU actually has breaks in the insulation for this kind of testing, so it's honestly a VERY convenient way to check this. If you jump them and your idle goes down, you found the problem.

[TTait]

Thanks guys...

Yes Welmoed, I guess I meant capacitor - its good to have smart friends...

D03 and Dave, I think you have identified my problem - will work out a jumper & switch for my next hot day drive... Follow up info when I have it. Fingers crossed - it is a TRULY annoying problem, and it really diminishes my enjoyment of the car in the summer. Thanks again!

[David T]

There are a LOT of things that can mess up the proper operation of the idle system. There are also some things that are rare. It is rare for the idle ECU to be bad. It is not rare but it is uncommon for the idle motor to be bad. Most common is vacuum leaks. They reduce the ability of the idle motor to control the air to the motor so it can't accurately hold the RPM's. Next is a problem with the throttle linkage and the micro switch. Any play or looseness in the throttle linkage and you don't "set" the micro switch. It is also very common for someone to start "adjusting" things to "fix" the problem and by the time they are finished nothing is right. It is not rare but it is uncommon for the micro to be bad. Usually because it got adjusted badly. Finally, wiring problems, dirty connections, bad grounds, etc can mess up the electrical portion of the idle system. Other things that affect the idle include:
Dirty injectors
The brass balance and speed screws open
Misadjusted impulse coil reluctor, stuck mechanical advance
Weak battery
Worn spark plugs, bad ignition wires
Sticking plunger in mixture unit
Valve adjustment
cylinder imbalance
Improper routing of the vacuum system. If the vacuum solenoid is not hooked up right you can't get it to idle.
To make the idle system run as it should the Lambda system must also be working as it should and all systems that operate during warm-up.
As you can see, the idle is affected by just about everything if it is not "just so". It can take a lot of work to get it to idle right.
David Teitelbaum

[Bitsyncmaster]

There are a LOT of things that can mess up the proper operation of the idle system. There are also some things that are rare. It is rare for the idle ECU to be bad. It is not rare but it is uncommon for the idle motor to be bad. Most common is vacuum leaks. They reduce the ability of the idle motor to control the air to the motor so it can't accurately hold the RPM's. Next is a problem with the throttle linkage and the micro switch. Any play or looseness in the throttle linkage and you don't "set" the micro switch. It is also very common for someone to start "adjusting" things to "fix" the problem and by the time they are finished nothing is right. It is not rare but it is uncommon for the micro to be bad. Usually because it got adjusted badly. Finally, wiring problems, dirty connections, bad grounds, etc can mess up the electrical portion of the idle system. Other things that affect the idle include:
Dirty injectors
The brass balance and speed screws open
Misadjusted impulse coil reluctor, stuck mechanical advance
Weak battery
Worn spark plugs, bad ignition wires
Sticking plunger in mixture unit
Valve adjustment
cylinder imbalance
Improper routing of the vacuum system. If the vacuum solenoid is not hooked up right you can't get it to idle.
To make the idle system run as it should the Lambda system must also be working as it should and all systems that operate during warm-up.
As you can see, the idle is affected by just about everything if it is not "just so". It can take a lot of work to get it to idle right.
David Teitelbaum

You forgot the bad deceleration valve springs in the throttle plates. That was my problem and a few others had it also.

[David T]

You just can't think of ALL causes. The main point here is if there is ANY problem it seems to show up most acutely as affecting the quality and speed of the idle. Yes, the decel valves will also affect the idle. On one car I found a tapped hole, the bolt was missing and the hole was drilled all the way through so it was a big vacuum leak. On another the owner didn't know the plug was missing over the mixture screw. On one car the idle was always "loppy". I guessed and it turned out to be that someone had replaced the impulse coil. It was probably bad. The problem was they did not gap the reluctor so the ignition signal was changing and not firing evenly causing a bad idle. On another car the mechanical advance was stuck. On another the vacuum solenoid was hooked up backwards. I can go on and on but you can't take ANYTHING for granted and must examine everything. On many automatics the vacuum line is disconnected causing a large vacuum leak.
David Teitelbaum




You forgot the bad deceleration valve springs in the throttle plates. That was my problem and a few others had it also.

[nullset]

What does the Idle ECU look like? I know it's behind the driver seat. There's a black box there and a silver one. Which one should I be messing with?

Is there a simple way to test the idle speed motor on the bench? I do have test equipment (signal generators, scopes, etc) on hand and some experience.

This weekend I'm going to try first pulling the motor and seeing if it's stuck. I'll also test the resistance and continuity of the wiring.

I do have a second car I could pull an idle motor and ECU from if necessary.

--buddy

[Bitsyncmaster]

What does the Idle ECU look like? I know it's behind the driver seat. There's a black box there and a silver one. Which one should I be messing with?

Is there a simple way to test the idle speed motor on the bench? I do have test equipment (signal generators, scopes, etc) on hand and some experience.

This weekend I'm going to try first pulling the motor and seeing if it's stuck. I'll also test the resistance and continuity of the wiring.

I do have a second car I could pull an idle motor and ECU from if necessary.

--buddy

The black box with the two connectors is the idle ECU. Center pin on the idle motor goes to 12 volts. Then one of the outer two pins is driven to ground. The ECU will drive each outer pin with a pulse (to ground) opposite of the other pin at a 100 Hz rate.

[TTait]

On another the vacuum solenoid was hooked up backwards.

You lost me on that one, is the connector keyed in some way? I didn't recall that being the case. How does one know its hooked up correctly versus backwards?

[nullset]

The black box with the two connectors is the idle ECU. Center pin on the idle motor goes to 12 volts. Then one of the outer two pins is driven to ground. The ECU will drive each outer pin with a pulse (to ground) opposite of the other pin at a 100 Hz rate.

Thanks Dave. I'm going to try to flex my 555 timer muscles to make a 'tester' circuit for this that anyone can build with parts from ye olde radio shack.

--buddy

[Bitsyncmaster]

Thanks Dave. I'm going to try to flex my 555 timer muscles to make a 'tester' circuit for this that anyone can build with parts from ye olde radio shack.

--buddy

The motor will draw about 1 amp so you will need a pair of drivers (transistor or MOSFET). My new ECU has a test mode that slowly moves the idle motor from full closed to full open and repeats. It's useful to see how bad our idle motors are for non-linear motion. The Volvo motors are much better.

[nullset]

The motor will draw about 1 amp so you will need a pair of drivers (transistor or MOSFET). My new ECU has a test mode that slowly moves the idle motor from full closed to full open and repeats. It's useful to see how bad our idle motors are for non-linear motion. The Volvo motors are much better.

Are the side pins alternating between +12 and GND, or floating and ground?

Thanks,

--buddy

[David T]

Electrically it doesn't matter but it is very important the right vacuum hose goes to the correct port on the vacuum solenoid.
David Teitelbaum




You lost me on that one, is the connector keyed in some way? I didn't recall that being the case. How does one know its hooked up correctly versus backwards?

[TTait]

Okay, I know I got that right. Thanks!

[Bitsyncmaster]

Are the side pins alternating between +12 and GND, or floating and ground?

Thanks,

--buddy

The ECU just drives the side pins with an NPN transistor to ground and then it floats the pin to shut if off.

[nullset]

Alright, I've got my car back together and it's still idling high, even with the idle switch shorted and the lower screw removed, and the throttle plate pushed all the way in.

I've tried the Idle ECU from my other car, and that didn't seem to make a difference. I think my next step is to check for vacuum leaks. Is there a good "Vacuum leak finding" HOWTO somewhere?

Are there any other electronics that affect the idle? Other parts I should try from my project car to see if it improves?

Does the RPM relay affect idle at all?

--buddy

[ccurzio]

I think my next step is to check for vacuum leaks. Is there a good "Vacuum leak finding" HOWTO somewhere?

One method I can recommend that I've had success with is to blow smoke into the system and look for where it exits.

[David T]

Out of all the things I have tried to find vacuum leaks, pressurizing with smoke is by far the best. It helps you find the smallest leaks, and quickly!. On some of the cars I checked among the most common leaks are:
Injector seals
Idle pipe under mixture unit
Idle motor top
Dipstick
On one car smoke was POURING out of the left pontoon. Someone took everything out and left the vacuum hose just dangling there.
I would also get a timing light and make sure BOTH the vacuum AND the mechanical advance systems are functioning properly. Recheck the vacuum hose routing to the CPR also.
David Teitelbaum



Alright, I've got my car back together and it's still idling high, even with the idle switch shorted and the lower screw removed, and the throttle plate pushed all the way in.

I've tried the Idle ECU from my other car, and that didn't seem to make a difference. I think my next step is to check for vacuum leaks. Is there a good "Vacuum leak finding" HOWTO somewhere?

Are there any other electronics that affect the idle? Other parts I should try from my project car to see if it improves?

Does the RPM relay affect idle at all?

--buddy

[Bitsyncmaster]

Alright, I've got my car back together and it's still idling high, even with the idle switch shorted and the lower screw removed, and the throttle plate pushed all the way in.

I've tried the Idle ECU from my other car, and that didn't seem to make a difference. I think my next step is to check for vacuum leaks. Is there a good "Vacuum leak finding" HOWTO somewhere?

Are there any other electronics that affect the idle? Other parts I should try from my project car to see if it improves?

Does the RPM relay affect idle at all?

--buddy

It sounds like my car before I replaced the deceleration springs. My car would idle great until it warmed up (but I had to keep that lower screw backed out). Then the lowest I could get the RPM was 1000 to 1100. Can you get a photo of your decel springs. You have to pull the W pipe to look at them.

[nullset]

It sounds like my car before I replaced the deceleration springs. My car would idle great until it warmed up (but I had to keep that lower screw backed out). Then the lowest I could get the RPM was 1000 to 1100. Can you get a photo of your decel springs. You have to pull the W pipe to look at them.

I did inspect when I had it apart, and they at least looked fine. I'll take some pictures.

--buddy

[kings1527]

Buddy, did you ever locate the problem? I'm going through something similar right now.

Alex
#6575

[nullset]

Buddy, did you ever locate the problem? I'm going through something similar right now.

Alex
#6575

Not yet. I've been busy chasing other problems.

How is yours behaving?

[kings1527]

Not yet. I've been busy chasing other problems.

How is yours behaving?

I have a high idle at about 1k once I'm warmed up. At start up, it's about right on at 750. I'm running super rich and failed smog so I need to figure this out. I might just bone up and take it to DMC Garden Grove and let them take care of it. $$$$$$

Also, I have a little bit of a cold start problem and I'm wondering if it's the CSV or accumulator. Once I crank, it takes about 3-5 sec at the most while turning the key to get it to start. Once it's warmed up, it fires right up in a heartbeat. I'm going to try some tests today.

Alex
#6575

[nullset]

I have a high idle at about 1k once I'm warmed up. At start up, it's about right on at 750. I'm running super rich and failed smog so I need to figure this out. I might just bone up and take it to DMC Garden Grove and let them take care of it. $$$$$$

Also, I have a little bit of a cold start problem and I'm wondering if it's the CSV or accumulator. Once I crank, it takes about 3-5 sec at the most while turning the key to get it to start. Once it's warmed up, it fires right up in a heartbeat. I'm going to try some tests today.

Alex
#6575

That's not really all that high. Mine idles at 1800-2k, sometimes 2500 all the time.

If I were you, I'd pull the injectors and check for leaks, and connect a dwell meter to check your mixture settings. Replacing the O2 sensor probably wouldn't hurt as well. How old is your cat?

You have a lot of issues to look over for emissions.

--buddy

[deloumis]

I have a high idle at about 1k once I'm warmed up. At start up, it's about right on at 750. I'm running super rich and failed smog so I need to figure this out. I might just bone up and take it to DMC Garden Grove and let them take care of it. $$$$$$

Also, I have a little bit of a cold start problem and I'm wondering if it's the CSV or accumulator. Once I crank, it takes about 3-5 sec at the most while turning the key to get it to start. Once it's warmed up, it fires right up in a heartbeat. I'm going to try some tests today.

Alex
#6575

Check your CSV make sure it isn't leaking (I don't know if it is a common problem). Check your injectors make sure they aren't leaking when the car is off. Also check for gas in the oil, that's a sign of leaky injectors.

[kings1527]

That's not really all that high. Mine idles at 1800-2k, sometimes 2500 all the time.

If I were you, I'd pull the injectors and check for leaks, and connect a dwell meter to check your mixture settings. Replacing the O2 sensor probably wouldn't hurt as well. How old is your cat?

You have a lot of issues to look over for emissions.

--buddy

Yeah, I feel the same way. The injectors were replaced by the PO by DMC CA back in '07 but only driven about 70 miles between now and then. The O2 sensor was replaced back then, too. So not a lot of use but a lot of sitting time. My HC and CO are very high right now.



Check your CSV make sure it isn't leaking (I don't know if it is a common problem). Check your injectors make sure they aren't leaking when the car is off. Also check for gas in the oil, that's a sign of leaky injectors.

I'll give the oil another check, thanks. My CSV functions a lot better if I jump it from the grey electrical connection from the CPR. So that's gotta be something there. And my frequency valve is sometimes intermittent at start up. Buzzes for a bit, then stops, then goes again and of course that messes with the idle. I'm thinking either bad connection or bad Lambda ECU.

Alex
#6575