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I was checking all the vac. tubing in my z and found a bad line from the carbon canister to distributer vac advance to what i think is a deccel valve on the bottom of the throttle body, replaced it all, and now the idle won't hold, ether 800RPM or it jumps to 1500RPM i know deccel keeps the revs up to 1500 after a long high RPM run and that along pisses me off. any ideas? also my cold start valve is malfunctioning. could that contribute to the problem? kind of makes me want to take the whole emmission system out no more bleeding EGR, BCCD ect. :)Thanks again. Stan.
 

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> I was checking all the vac. tubing in my z
> and found a bad line from the carbon
> canister to distributer vac advance to what
> i think is a deccel valve on the bottom of
> the throttle body, replaced it all, and now
> the idle won't hold, ether 800RPM or it
> jumps to 1500RPM i know deccel keeps the
> revs up to 1500 after a long high RPM run
> and that along pisses me off. any ideas?
> also my cold start valve is malfunctioning.
> could that contribute to the problem? kind
> of makes me want to take the whole emmission
> system out no more bleeding EGR, BCCD ect.
> :)Thanks again. Stan.

Stan,
YOur '76 has a myriad of devices that affect the idle - especially when it is cold.
1) EGR - open only when engine is cold - reach under the top of the unit - you will feel the diaphragm - push on it - it should move like it is spring loaded. If it doesn't budge or is loose - replace it.
2) BCDD - boost controlled decelleration device. This only works when you are decellerating. Shoul have no effect on your idle. Check the hard line coming from the back of your engine to the bottom of it. Make sure it isn't plugged or broken off. Do the checks identified in the Haynes manual.
3) Air regulator - this device sits on your intake manifold right by your cold start injector. Has a 15mm ID hose running in and out of it plus an electrical connection that goes to your ECU. This device allows extra air to pass when the engine is cold and your are running rich for warm-up. If it sticks closed you will have a very rough idle - if it will run at all - and will belch huge clouds of black smoke out your tailpipe. If it sticks open you will start fine but run lean once the car warms up and the engine will have no power and just quit on you. Check the plate right under it. It has radiator water that runs through it so the air regulator wwill know when the engine has warmed up and do its thing properly. If the lines are plugged it will lean you out.
4) Thermotime switch - this is the larger of the two-wire sensors on the front of your thermostat housing. It tells the cold start injector to fire for cold start-up.
5) Engine temperature sensor - this is the smaller of the two-wire sensors on the front of your thermostat housing - this unit senses coolant temperature - feeds a signal to the ECU and the ECU varies the duration of the fuel pulse on your main injectors. This baby can be a real nightmare if it has corroded connections at the sensor or about 12-16 away where it is connected to your wiring harnes with shrik wrapped bullet connectors. When they corrode the ECU thinks major cold engine as floods it with fuel - again huge clouds of black smoke and eventually gas fouling of your plugs and car doesn't run.
6) Engine temp gauge sensor - this is the one wire sensor on your thermostat housing - it does nothing but feed coolant temperature to the temp gauge in your car.

Your varying RPM sounds like you still have a vacuum leak somewhere or a ruptured diaphagm. Have you tried UNdoing what you just did and see if the idle goes back to normal? That would be my first move. If it does, then focus your attention there.
Phantom
 
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