Hello all,

Long story here. Here we go...

1981 Stock w/ 4-speed. Im having trouble with the car idling way too fast after it has warmed up. So much so to the point that the engine will diesel after I shut it off. I just had the carb rebuilt by all american carburetor in Florida. It runs better than it ever did before. Prior to rebuilding the carb, the car did had the same issue, but I passed it off thinking it was a carb issue and the rebuild would take care of it. I got the carb back, bolted it on and it ran great except it still idles too fast when its warmed up. I know it is not the fast idle being stuck on because it drops right down when you tap the gas pedal like it should. Also the throttle does close all the way. I replaced some vacuum lines and put on a new carb gasket when I replaced the carb itself. I dont believe the car has a vac leak because I sprayed all over the engine and carb with carb cleaner, and nothing happened. I did however unplug the vac line that goes to the MAP sensor and the idle dropped right down. It went right back up after I hooked it back up to the carb. I then put the end of the hose in my mouth and sucked on it to imitate vacuum and the idle shot right back up again. I am wondering if this is a timing issue because I know that sensor controls the timing via the ECM. Anybody think I should check the timing or have any suggestions on where I should look next? Any help or suggestions are greatly appreciated.

-Thanks

 

You don't use timing to set idle speed - you should have full control over idle speed, to the point of killing the engine, regardless of where the timing is set. If the throttle blades are closed and you are still idling too high, you absolutely, positively have a vacuum leak. Either through a source in the carb, or external to the carb. The engine has to get air to run from someplace, and if the throttles are closed, it's sucking that air from a false source.

Check your secondary idle speed and make sure the secondaries aren't cracked open too far...

Lars

 

There's 3 notches on that fast idle and no messed up timing is going to keep you from adjusting the idle down. Is the carb linkages resting on the idle screw or is it stuck up off of it?

 

I had a similar issue - it was an incorrect carb base gasket. Lars suggested the same test: If it won't idle down and die, air must be getting in somewhere. You have a vacuum leak.

 

I would check the electric choke first. The butterfly's might be open but the hi speed idle cam might be sitting on the first notch. That's about 1000-1200 rpm notch.
Loosen the 3 screw on the round black circle thingy on the passenger side and it will rotate. If you have good eyes, you can read the writing stating which way is more choke and which way is less. You obviously want less. You can see the choke idle screw behind it also and the hi idle cam it rides on.

 

With a Q-Jet, you can verify whether or not it is on fast idle by visually inspecting the slow idle speed screw. Is the primary throttle stop resting against it? If so, fast idle isn't a part of the equation...

 

The throttle rests against the slow idle stop screw just fine. The engine only does this when its warmed up completely. It idles fine at 700 right off of fast idle just fine. After I drive it say around the block or so is when it does this. And when I unplug the map sensor the idle drops down. I'm stumped lol. I know it's something stupid but what it is is the question...

 

The computer uses the MCS to change the amount of air coming in at idle. It should be idling a bit lower with the MCS disconnected, however. Take a look at the emissions sticker.

What is the idle speed it reaches when warm?

 

Update: Solved the problem

Went to harbor freight tools and bought a cheap timing light to check my timing. Timing was spot on at 6 degrees BTDC with the ECM connector unhooked and the idle where it should be. I plugged the connector back in and the idle shot back up and the timing was off the scale. I thought about it and then I remembered that I installed a Hypertech chip in the ECM that changes the timing curves. I popped the Hypertech chip out, put the old PROM chip back in and bam, problem solved. Idles right where it should. No more dieseling upon shutdown either. Im going to rock out with the stock PROM because in all honesty, I feel no difference in performance with that or the HT. Just a gimmick that I fell for thinking that I would get performance in a box! Maybe on a newer car, but not an ol' 81. Anyway, Ill run a can of seafoam through her one of these days to try and clean out the carbon out of the motor. That was probably the ignition source for the dieseling that I was having. Anyway she runs like she should now and im happy