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

[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?15...umpiness/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