Idle Backwards

[Ricker]

After all the excellent advice from everybody (and greatly appreciated) on what to look for and reading other threads, here is what I did and found. The microswitch does engage when the throttle is at idle. It has continuity. Next, I checked the ECU. Pulled the center side plug and the RPMs increased and decreased when back installed. Then on to the idle speed motor. It was humming/vibrating with the key on BUT the valve did not open when the key was initially turned on. Therefore, I believe I need a new idle speed motor. Am I missing anything before I buy a new idle speed motor?

[Rich]

Therefore, I believe I need a new idle speed motor. Am I missing anything before I buy a new idle speed motor?

Sounds like your ISM is OK. It's clean inside and it hums when it should. I'd check for leaks first - see below - but will mention more ISM stuff first.

One more elec verif test for it: Unplug the connector from the top of the ISM. You can see the 3 male connectors on it. Check the DC resistance from the center male spade to each of the outer spades. It should be something like 10-20 Ohms to each outer one but don't quote me.

If you still suspect the ISM see if you can find a D owner nearby who'll lend you a known good one from their car or from their spares pile for a day or two.

Look for air leaking into your intake system. The usual culprits are already mentioned. Decel springs, throttle linkage, loose/cracked hoses or vacuum lines. Also check the injector seals, the distributor advance solenoid and the upper body of the ISM (its seal can crack near the connector). There are threads in this forum about the springs.

To find vac leaks have a mechanic run a smoke test for you if you don't have access to a good smoke machine. It takes less than half an hour.

[glockworks21]

After all the excellent advice from everybody (and greatly appreciated) on what to look for and reading other threads, here is what I did and found. The microswitch does engage when the throttle is at idle. It has continuity. Next, I checked the ECU. Pulled the center side plug and the RPMs increased and decreased when back installed. Then on to the idle speed motor. It was humming/vibrating with the key on BUT the valve did not open when the key was initially turned on. Therefore, I believe I need a new idle speed motor. Am I missing anything before I buy a new idle speed motor?

Andy mentioned the linkage issue. Were you able to check that and maybe re adjust? I too bought a new ism before i adjusted the cables and springs as i was stupidly throwing money at the problem. It did not help. To add insult to injury the new one just failed( a few days ago).. I put the original bosch back in and its perfect. That aftermarket ism was less than a year old..

[r00b]

Andy mentioned the linkage issue. Were you able to check that and maybe re adjust? I too bought a new ism before i adjusted the cables and springs as i was stupidly throwing money at the problem. It did not help. To add insult to injury the new one just failed( a few days ago).. I put the original bosch back in and its perfect. That aftermarket ism was less than a year old..

That sucks, did you get it from one of the vendors?

Ricker, if you do want to try an ISM they are $42 on Amazon https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

[Ricker]

The idle speed motor was bad and I did replace it from one from Deloreango.com. That did the trick. Car starts right away at high idle and drops down after just a little. I still think I may have to mess with the air/fuel ration along with the new injectors. We'll see. Thanks for the help!

[r00b]

[Ricker]

Well, I thought I had the idle issue solved. I don't completely. Sorry, but here is a list of questions to help me understand.

1. I thought the ECU ALWAYS controlled the RPMs at 775. If so, what does the idle throttle plate screw adjustment do? If you set it for high RPMs, the RPMs are greater than the 775. If you set it low, will the ECM maintain the 775 RPMs, or will it run at the lower RPM?
2. When does the ECM control the RPMs and when does the idle throttle plate control the RPMs?
3. The micro switch tells the car the engine should be at idle RPM, right?

Here's the problem. Engine starts fine hot or cold. RPMs are at 900 from the set screw/idle throttle plate. With the lights on, A/C on and in drive it drops to 775 RPMs. However, in neutral and not touching anything, I mean nothing, the idle will now and then go to 1,500 RPMs and after a while back to 900 RPMs. Any ideas?

I just replaced the Idle Speed Motor, Vacuum Hoses are all new. Micro switch is set and is working. Dwell is set. I will admit the engine runs a bit rough/vibrates at idle.

If you still think it might be the deacceleration valves/springs, where are they? If I take off the throttle body, will I see then and clean them with carburetor cleaner provided the issue is just grime?

Could it be the ECU or an issue with one of the inputs?

[WHO1DMC]

Well, I thought I had the idle issue solved. I don't completely. Sorry, but here is a list of questions to help me understand.

1. I thought the ECU ALWAYS controlled the RPMs at 775. If so, what does the idle throttle plate screw adjustment do? If you set it for high RPMs, the RPMs are greater than the 775. If you set it low, will the ECM maintain the 775 RPMs, or will it run at the lower RPM?
2. When does the ECM control the RPMs and when does the idle throttle plate control the RPMs?
3. The micro switch tells the car the engine should be at idle RPM, right?

Here's the problem. Engine starts fine hot or cold. RPMs are at 900 from the set screw/idle throttle plate. With the lights on, A/C on and in drive it drops to 775 RPMs. However, in neutral and not touching anything, I mean nothing, the idle will now and then go to 1,500 RPMs and after a while back to 900 RPMs. Any ideas?

I just replaced the Idle Speed Motor, Vacuum Hoses are all new. Micro switch is set and is working. Dwell is set. I will admit the engine runs a bit rough/vibrates at idle.

If you still think it might be the deacceleration valves/springs, where are they? If I take off the throttle body, will I see then and clean them with carburetor cleaner provided the issue is just grime?

Could it be the ECU or an issue with one of the inputs?

Here's a picture of what the deceleration springs look like. There on the butterfly valves of the throttle body.
Throttle body cleaner is what I used.





Dave B.

Sent from my SM-G991U1 using Tapatalk

[Bitsyncmaster]

Well, I thought I had the idle issue solved. I don't completely. Sorry, but here is a list of questions to help me understand.

1. I thought the ECU ALWAYS controlled the RPMs at 775. If so, what does the idle throttle plate screw adjustment do? If you set it for high RPMs, the RPMs are greater than the 775. If you set it low, will the ECM maintain the 775 RPMs, or will it run at the lower RPM?
2. When does the ECM control the RPMs and when does the idle throttle plate control the RPMs?
3. The micro switch tells the car the engine should be at idle RPM, right?

Here's the problem. Engine starts fine hot or cold. RPMs are at 900 from the set screw/idle throttle plate. With the lights on, A/C on and in drive it drops to 775 RPMs. However, in neutral and not touching anything, I mean nothing, the idle will now and then go to 1,500 RPMs and after a while back to 900 RPMs. Any ideas?

I just replaced the Idle Speed Motor, Vacuum Hoses are all new. Micro switch is set and is working. Dwell is set. I will admit the engine runs a bit rough/vibrates at idle.

If you still think it might be the deacceleration valves/springs, where are they? If I take off the throttle body, will I see then and clean them with carburetor cleaner provided the issue is just grime?

Could it be the ECU or an issue with one of the inputs?

Stock idle ECU always tries to hold 775 RPM. Does not matter hot or cold, idle switch on or off.

The idle switch just helps the stock idle ECU control timing when you release throttle quickly. So you don't get RPM undershoot.

The temperature sender controls how much closed the idle motor can do. When cold the idle motor can not close as much as when hot so hence you may get a "cold fast idle". It all depends on your engine oil and curb idle settings.

If you set 900 RPM cold with the curb idle screw, I would expect your 1500 RPM hot is how anyone's engines would run with those settings.

[Michael]

I suspected a ECU was faulty, there is no way to tell for sure without trying a known good one. You can however pull the ECU and look at the board and see if there are any obvious signs such as black spots where something has burned or broken pathways, melted components or damaged capacitors...not seeing these doesn't mean you have a good ecu, but seeing it sure tell you it needs to be replaced.