Carrera 3.2 ICV question

[Mike Young]

Hi

I've got an idle problem with my 1988 Carrera 3.2

The car normally idles fine hot or cold. Very stable at about 900rpm. But when drivig the car and you approach a junction for example and take your foot off the accelerator, sometimes the engine just drops straight past the idle speed. Sometimes it will catch itself at about 300rmp and recover, other times it just cuts out. It does this hot or cold.

I've checked and cleaned the air flow meter - that seems fine.
I'm currently cleaning the ICV but everything I've read on the net suggests that the ICV should be a kind of proportinal solenoid valve with a spring return and two wires connected to it.
Mine has three wires and when disconnected from the engine and moved by hand, the valve body will stay at any position I place it, i.e closed, open, half way. There appears to be no spring return.

Can anyone explain ? Is it faulty ?

Thanks Mike

[IROC]

I'm going from memory here, but I think the valve you're talking about is supposed to "rattle" when you remove it. That is, the piston should move back and forth freely. Your's appears to be sticking. May be a problem. I think you can clean these and re-install them.

I hope I'm not thinking of something else...

Mike

[ChrisBennet]

The ISV has 3 wires going to it and no spring - it's floppy when not in use. Clean it out with spray cleaner and see if that helps. They do eventually wear out and need replacement.
-Chris

[Mike Young]

Thanks for your help both.

I'll give it another try later.

Mike

[Rot 911]

According to the Bentley manual with the car off remove the hose that runs from the ICV to the rubber boot on the throttle body housing. disconnect the electrical connector on the ICV. Turn ignition to on position. When you reconnect the connector to the ICV you should see the metal plate (it is curved like the housing) move about 1/2 way across the opening. Mine does. You might need a flashlight to see in there.

[Steve W]

There are some factors that contibute to the sub idle dip on deceleration. Worn ICV (or AIC) valves where the brushes contact the motor inside wear out grooves over time on the aluminum slip ring. There are three inside. Where the groove is most worn, there is no contact. Second, when the idle switch is closed, the DME is programmed on deceleration to turn off fuel to the injectors until the rpm drop back to 1080 rpm, at which point, the injectors turn back on. This is done primarily for fuel economy and emissions reasons. The unfortunate side effect with older ICV valves is that the rapid drop of rpms when the fuel is cut off, makes it harder for an older valve to catch the idle and lock onto it, so thus a sub idle dip. The problem is also exacerbated with lightweight flywheels or when the AC is turned on. There are couple of ways to address this problem. First, adjust the base idle speed correctly first, and if necessary, adjust the idle fuel mixture. Clean out the ICV valve, and if you can get a hold of a good one, swap them out and see if it make a difference. The other solution that works extremely well is to reprogram the decelleration fuel cut, from 1080 to about 1440-1520. This allows the rpms to drop normally like a rock on throttle closure until about 1000 rpms, at which point, it floats down to 880 rpm idle in about a second, giving the valve time to catch and sync the rpms.

[Mike Young]

Thanks for your suggestion Steve but does this make any sense to you ?

I've cleaned out the ICV put it back on the engine. Its made no difference but there is something I have noticed that is probably a good pointer to the problem. When the engine speed is anywhere above about 2000rpm and you take your foot of the accelerator the engine speed drops quicklyto to about 1100 rpm and then something obviously kicks in and causes the car to idle perfectly at abot 950 rpm. If though the engine speed is less than about 2000 rpm when the throttle is shut off the engine fall in engine speed is not arrested at 1100rpm but continues to fall down until either the DME recovers it and pulls it back up to idle or the engine cuts out.

Also you mentioned the "idle switch" ...

Is the idle switch part of the same housing as the Wide Open Throttle switch. There is another switch in the same housing and I can hear it clicking when I move the throttle just off idle. The complete switch housing has 3 connections, the WOT switch deffinately works but there is no closed circut when the "idle switch" clicks.
This would obviously lead me to think the idle switch is faulty - but there is no wire going to it. Just two wires to the plug and these are deffinately normally open, and close when the throttle is opened fully.

Is the "idle switch" you refer to in a seperate housing ?

Mike

[Steve W]

the idle switch connector is a separate plug with two wires. make sure the switch operates freely and the throttle linkage has the proper slack to engage the switch. A rotting rubber seal in the microswitch could be holding back the activation and creating an intermittent connection. However, from what you describe, it sounds a lot like the AIC valve is worn out and needs to be replaced. re; at higher rpms, the valve is in a different position where brush contact is provided. Below 2k, the valve may be in a different postion where there is poor brush contact.

[Mike Young]

Thanks Steve

I really apreciate your help, Ill take a look tomorrow.

Mike