High Idle When Warm

[JRNY13]

My car idles fine when cold. However when it warms up the idle sits just below 1000rpms consistently. Snapping the throttle does nothing. Is this a vacuum issue? A vacuum leak would be present on a cold engine as well wouldn't it? Would getting vacuum on the ignition distributor cause a slightly high idle? If my decel springs were the culprit, wouldn't I get a high idle as soon as I pressed the throttle?

[David T]

My car idles fine when cold. However when it warms up the idle sits just below 1000rpms consistently. Snapping the throttle does nothing. Is this a vacuum issue? A vacuum leak would be present on a cold engine as well wouldn't it? Would getting vacuum on the ignition distributor cause a slightly high idle? If my decel springs were the culprit, wouldn't I get a high idle as soon as I pressed the throttle?

Verify the idle motor is running when you are at idle. Vacuum leaks will reduce the ability of the idle motor to control the idle. Make sure the vacuum hoses for the vacuum advance are correctly hooked up and not leaking. The decel springs should be checked.

[Jonathan]

Check to see that all three of those brass screws just behind the W-pipe and threaded in all the way and tight. Their design purpose was to act as bypasses for air around the butterfly valves, but they were deemed unnecessary to be open at all with the engine set-up in our cars. If they are opened slightly, they will in effect create an offset from the normal design idle speed of 775 RPM to something slightly higher (depending on how far open they are and which one, or ones, of the three are open). You can see a good diagram explanation of how that air pathway works in the workshop manual.

[Bitsyncmaster]

Yes ignition advance on at idle will increase the RPM if your idle system can not compensate.

My car had the same problem and it was the springs on the deceleration valves were bad. You can see your deceleration valves when you remove the W pipe. If the spacing on the springs is not even then your springs are bad.

[JRNY13]

Yes ignition advance on at idle will increase the RPM if your idle system can not compensate.

My car had the same problem and it was the springs on the deceleration valves were bad. You can see your deceleration valves when you remove the W pipe. If the spacing on the springs is not even then your springs are bad.

How would I go about replacing them if they're bad? Is this a part that is even sold?

[Bitsyncmaster]

How would I go about replacing them if they're bad? Is this a part that is even sold?

I just got another used throttle assembly. I guess you could find some springs but you would need to test and calibrate the unit. I would guess they open around 20 inches/Hg

[David T]

I just got another used throttle assembly. I guess you could find some springs but you would need to test and calibrate the unit. I would guess they open around 20 inches/Hg

Not a critical setting but they should be closed at idle. You can use the springs from old pens, cut them shorter.

[Bitsyncmaster]

Not a critical setting but they should be closed at idle. You can use the springs from old pens, cut them shorter.

Have you used springs from old pens?

I think it is critical for the calibration. You get 15 to 16 inches/Hg at idle so you just want them to open when you take your foot off the gas.

[MikeWard]

Check to see that all three of those brass screws just behind the W-pipe and threaded in all the way and tight
They most definitely should not be tight. They are shear screws and the heads will snap off if you tighten them up, leaving you SOL if you need to adjust them.

[Jonathan]

They most definitely should not be tight. They are shear screws and the heads will snap off if you tighten them up, leaving you SOL if you need to adjust them.

Snug? Is snug a better choice of words? I hadn't meant to imply them being torqued, but rather not loose. If they were valves, I'd have said closed as opposed to open.

Out of curiosity, when would you want to adjust these screws anyway? Page D:01:13 of the Workshop manual (fuel and emissions section) states that this single air adjusting screw and the two air balancing screws are not to be used on the DeLorean since we have an electronic idle speed control system. If they get adjusted, isn't that only trying to correct a problem elsewhere (per the design that is)?

[NckT]

They were originally used to balance air flow and idle speed on non electronic idle motor systems. (The engines with the auxiliary air valve where the electing idle speed control valve should be).

I personally use this to an advantage to get rid of the idle and off idle hunting. I described this in Post No.19 in:

http://dmctalk.org/showthread.php?15633-Attempting-to-destroy-my-idle-lumpiness/page2

As far as the air admittance deceleration springs, you could either replace the throttle assembly in its entirety, replace the throttle plate discs, or solder then shut.

Good luck

NickT

[PJ Grady Inc.]

Snug? Is snug a better choice of words? I hadn't meant to imply them being torqued, but rather not loose. If they were valves, I'd have said closed as opposed to open.

Out of curiosity, when would you want to adjust these screws anyway? Page D:01:13 of the Workshop manual (fuel and emissions section) states that this single air adjusting screw and the two air balancing screws are not to be used on the DeLorean since we have an electronic idle speed control system. If they get adjusted, isn't that only trying to correct a problem elsewhere (per the design that is)?

Generally from the factory the CO (Shear style heads) were left a few turns open and the shorter slotted screw was closed which blocked off both CO bank screws. This allowed you to raise the idle for CO adjustments without fiddling with the CO screws. It also allows you to temporarely raise the idle for some other yet to be solved fault causing low idle/stalling conditions. It is usefull to leave the screws set to the factory positions for the above reasons.
Rob

[JRNY13]

Ok, so today I drove the car again and it was idling pretty much fine cold and warm. When I noticed the high idle before, it was after a period of the car sitting for about 3 weeks. Now I'm thinking it may have been throttle cable related. Could the throttle linkages have loosened up after driving it several times this week?

[Rich]

Ok, so today I drove the car again and it was idling pretty much fine cold and warm. When I noticed the high idle before, it was after a period of the car sitting for about 3 weeks. Now I'm thinking it may have been throttle cable related. Could the throttle linkages have loosened up after driving it several times this week?

Yes, that's possible.

Next time you get a high idle see if you can push the throttle linkage closed with a dowel or a screwdriver at the arm closest to the throttle body just to verify the problem is a mechanical one. See if you get normal idle right away.

From there the root causes vary. The cable is one of them.

[TristanC]

Sorry to revive an old thread.

I’m tracing a high idle issue when 11477 warms up.
My checks have led me to suspect the decel springs.

I removed the W-pipe and this is what I saw (zoom in to see the left spring.... it seems out of position....)
http://newdelorean.co.uk/wp-content/uploads/2018/09/C19E9A6F-C40F-495D-88B8-A81664784B3C.jpeg

She has a brand new idle motor, a refurbished idle ecu, no vacuum leaks....
The idle microswitch only reduces idle when the car is cold.......

The curb idle screw is all the way out and not touching anything (previous owner).
I’ve disconnected the throttle arm and the butterflies can’t be closed any more to bring the idle down.

I’m hoping the decel springs are the culprit as I’ve been chasing this issue for a months!!!

Thanks guys
Tris

[Bitsyncmaster]

That left spring looks funny. Mine both looked like some coils were close and other coils looked wide. You can probably feel with your finger if that left spring has the same pressure as the right spring.

[TristanC]

That left spring looks funny. Mine both looked like some coils were close and other coils looked wide. You can probably feel with your finger if that left spring has the same pressure as the right spring.

I saw some of the picks with back spring coils. Mine look ok, except the left one looks not be seated correctly.
Could this potentially cause a high idle when the car warms?

She idles nicely when cold. As she warms, the idle gradually increases.

I’ve also been suspecting the thermistor......

Tris

[David T]

The left spring just looks like it is not seated properly. They look nice and shiny and they do not appear to be distorted from heat. If they snap the valve plates shut and hold some tension on them when closed, they are probably OK.

[Bitsyncmaster]

I saw some of the picks with back spring coils. Mine look ok, except the left one looks not be seated correctly.
Could this potentially cause a high idle when the car warms?

She idles nicely when cold. As she warms, the idle gradually increases.

I’ve also been suspecting the thermistor......

Tris

I fought the same problem many years. Cold was fine but when hot the idle climbed to about 1000 RPM with the curb idle screw all the way out. My deceleration springs were the cause of that problem. No one has the correct springs I've seen. Even if you get them I would think you need to calibrate them. The spring caps screw to adjust the spring pressure.

[Ron]

+1 on the left spring...but it looks like the valves are shut to me.

How high is the RPM when warmed up?
I'm wondering if the ISM could be sitting in a cold position and not moving since it idles nicely cold, then gradually increases (assuming it never goes down at all). I'd try letting it cool all night (below 15C/69F). Then start it up, let it run a few seconds to settle, disconnect the ISM and note the RPM. [Should be in cold position.] Then let it warm until the fans cycle once, note the RPM and reconnect the ISM. If the RPM then falls but not to idle, I'd guess the ISM system was ok and look elsewhere. If the RPM does not change, I'd suspect the ISM system has a problem.
You could record the thermistor's resistance at the beginning and end of the test to see if it is changing/compare the values to those found posted here somewhere...(Disconnect Idle Speed Control Unit, harness side pins 9 & 11, BS/BY).