It's a simple question, but one that could help with my predicament.

I recently did a 350 swap into my 84 T/A and then had the engine tuned at a dyno shop on a chassis dyno though I am afraid they didn't really know what to make of my Q-Jet. Anyway, they managed to get my idle down to around 900-1000 in park and neutral and around 600-700 when in gear. (need to call to find out exactly what they did). This fix lasted about 2 weeks. now the park idle is back up around 1200-1300 and "in gear" idle is 800-900. Every time I shut he car off, unless I leave it in gear, it'll diesel. On the upside, my Air/fuel mixture is near perfect, or so they say and I put 239hp to the wheels with AC and emissions all hooked up. Car parts include Electronic Q-jet, world heads, hooker 2055 headers, Performer INtake. Any ideas on where to start looking for problems? Thanks!

 

Idle is controlled by mechanically holding the throttle open to a certain point. There are 3 ways provisions for that on the carb: 1) The "normal" curb idle adjustment screw on the driver's side of the carb; 2) the high idle cam that is part of the choke, the green weight/lever on the passenger side of the carb behind the choke thermostat; and 3) the "kicker" solenoid on the front driver's side corner of the carb, which is electrically activated; typically when the AC is on, but also by the ECM under certain deceleration conditions (and emissions thing).

 

Could having a colder thermostat, 160* instead of 195*, have anything to do with keeping the high-idle on? Is it possible that the computer thinks the car is still "cold"?

 

No, the computer doesn't have anything to do with controlling the choke, or idle (on our cars - some CC carbs had an idle speed solenoid). It only controls the mixture and timing - which can be affected by engine temp, and which can affect idle speed, but not as much as you're talking about (50 RPMs, maybe). 110 degrees is all the warmer the coolant needs to be to kick the computer into closed loop.

The choke thermostat is heated by a 12 volt circuit. That circuit has a relay that won't engage until the alternator voltage exceeds battery voltage. If you've got a weak alternator, that could cause your problem. But, the choke light usually comes on in that situation.

Is your choke light coming on?

 

No, my choke light isn't coming on.
I did notice that last night that at random, the car would drop RPM's down to where it was tuned at, then jump back up to the high idle. When I got home from work, it was running fine, idle at 600 at a stop light in gear but when I pulled into the drive way and put it in park, the RPM's jumped all the way to 1500.

Just trying to think of anything that would cause it to do that.

o2 sensor? (that doen't make sense to me...but?)
Vacuum leak?
Bad carb idle solonoid?

On my carb I do have an idle stop solonoid (front drivers side of carb, one wire going to it)...kicks on with the AC and maybe when cold? Could tthat be getting errant voltage? Hmm...

 

Ok, it's as if my high-idle is being stuck on. Today I came home, pulled into the drive way and the car was stuck at 1600rpm. On a whim, i hit the gas and it dropped down to 1000 rpm as if the choke was stuck in the high-idle position.

Anyone else have any ideas?

 

Sounds like you need more throttle return springs. Is it always returning to the same position?

 

I put a stronger double throttle return spring on in place of the original. I'll double check to make sure it's going back to it's closed position tomorrow.