75 Twinspark idle speed

[Evan Bottcher]

Almost afraid to ask a question on here, with all the bickering that's going on here   But anyway...

Our 75 Twinspark idles high - about 1050-1100 rpm on the gauge.  It's not really a problem I s'pose but a bit annoying when stopped in traffic.

I'm told that the idle control valve is okay, although I haven't tried swapping it for a known good one.  I just changed the leads.

So what should I check? 

thanks,
Evan.

[shiny_car]

I'd be checking for any air leaks along the air intake, like the plastic/rubbery intake pipe which can crack. You can use some water-detergent in a spray bottle; with the engine running, spray it along suspected leaky areas, and you should see the detergent foam.

[Alan Hopla]

Evan,

I had a play with the 75 TwinSpark this morning and found the following, as measured on an external tuneup tacho.

Cold started the car - idle speed 1200.
After a period this settled down to a speed of 700-800. Coolant gauge read ~45 degrees.

Went to disconnect the ECU coolant temperature sensor lead. (Found a write up from GM that says that this varies the mixture and ignition timing).
Found it was already disconnected, so reconnected it.
This made no difference to the idle speed, still 700-800.
As discussed at Sandown there are two coolant temperature sensors located in the rear of the head.
One for the gauge which is located at the intake edge an has a standard male spade connector facing the fire wall.
One for the ECU which is located more towards the centre of the head and has a narrow spade connector which is facing upwards.

Disconnected the Idle Control Solenoid - idle speed 1200.
Plugged it back in while running and went back to 700-800.
Unplugged while running and the car stalled.
Restarted and again 1200.

So to me looks like you could have an issue in this area.
Either the solenoid is not activating, (bad solenoid or ECU not triggering, or bad loom).
Or these is another air leak into the plenum area.

Hope this helps.

Alan.