I have noticed lately that when my car is first warmed up the idle settles in at a nice steady 900RPM. However, if after it's warmed up I sit idling for a few minutes at a traffic light or at a drive through the idle will dip to 400-500RPM. The idle will stay low until I spend some time at higher speed (+50MPH?). When I stop from higher speed it will be back at 900RPM until I sit in one spot for a while and then I am back to low idle.

The car runs great. No overheating and no stumbling at all hot or cold. It's not a big deal I'm just wondering if I'm the only one with these symptoms or if all 240Z's do the same.

Car details: 3.0 liter, stock E88 top end, Ztherapy round tops, 180F thermostat, 3 core aluminum radiator, mechanical fan, shroud, insulated fuel lines, extended carb heat shield, mechanical and electric fuel pump, Mallory distributor, MSD ignition

Edited March 23, 201113 yr by steve91tt

Is the idle control valve still in place? Does it function?

My Mikuni-carbed Z actually goes the other way. Cold, it farts and stumbles and idles at ~600. Warmed up it settles in at ~900. I figure it's the carbs needing the heat from the headers to properly mist the fuel. No change in idle speed at stop lights.

Frank

Is the idle control valve still in place? Does it function?

No, the idle control valve and other emission gadgets are sitting comfortably in a box under my work bench.

My Mikuni-carbed Z actually goes the other way. Cold, it farts and stumbles and idles at ~600. Warmed up it settles in at ~900. I figure it's the carbs needing the heat from the headers to properly mist the fuel. No change in idle speed at stop lights.

Frank

I wonder if I'm over-misting the fuel when hot and therefore going rich. I guess I could try leaning it out a couple of clicks and see it that changes things.

I am not a carb guy but this might help. Let me know what you think.

http://www.teglerizer.com/sucarbs/garrettech.html

BGM

That's basically the way I tuned them a couple of months ago but I guess it wouldn't hurt to go back and make sure nothing has changed.

I forgot to mention that I don't have a lower splash shield (the one that goes under the front of the engine). I know that splash shields can change the way air moves in the engine bay. Could it be that the lack of a splash shield is causing my under hood temperatures to be too high?

I, too, lack the lower splash shield. But at idle, would that lack have any effect?

I don't know if the splash shield would have an effect but I thought I'd throw it out there to see if anyone had any experience with it.

No, the idle control valve and other emission gadgets are sitting comfortably in a box under my work bench.

While most of us defeat the emissions system, most do keep the idle control valve in place because unlike the air pump system, it actually serves a purpose.

Correct me if I'm wrong but I thought the idle control valve keeps the butterflies open as your quickly let of the throttle and has no effect at idle. No?

The throttle opener system is operated by the servo diaphragm and control. It is activated when there is negative pressure in the intake manifold. From my understanding of the system, it gives the throttle linkage a "kick" under deceleration and at the moment right before the engine dies due to a low idle.