Idle Problems While Driving

[GreenStang302]

94 GT. AODE. When you let off the gas going down the road, it does not slow down on its own. The rpms stay where they were. It will do this for at least a mile. Probably more but I have not tried. It is like having cruise control on but it is not. When applying the brakes, it feels like the car does not want to stop. But, when I do stop, the idle is where it is supposed to be. I have a slightly higher than stock stall (2700) and before this issue, when I would be in gear and let off the brakes, the car would barely move forward by itself. Now, it will move fairly quickly without help.

I have replaced IAC, TPS, ECT, IAT, VSS, O2's, MAF, and did a full tune up. (All of that was done before this idle issue.) I even got a 2nd IAC to see if that was the cause. TPS is at .97V at idle. The only code I have is 327. I also tried the base idle reset.

Any help is greatly appreciated.

 

[Three50won]

Might be something with the throttle return spring, the throttle cable, or the throttle blade. Also, I know you said you replaced the IAC but try running the engine and then unplug the IAC while the engine is running. See if there is any change to the idle quality.

And maybe there is something wrong with the ECU.

 

[GreenStang302]

I have unplugged the IAC when the car was warm and running. When I unplug it, the car's idle stumbles pretty bad, usually dies but not always. Both new IAC's did this. I also have a BBK cold air intake that makes noise when going down the road. But when I let off the pedal, the noise gets extremely loud like it is sucking in tons of air.

I have two ECU's and the car does the same thing with both.

How would I go about checking the throttle return spring, cable, and/or blade?

 

[Three50won]

First go under the hood and open the throttle manually and release it. See if it snaps back immediately. If it does then check for slack/tension at the actual cable. If the tension is appropriate then go in the car and press the gas pedal...release it and listen to hear if the throttle blade closes immediately. If the blade closes appropriately after all this then you have to look somewhere else for your problem.

As far as the BBK CAI, did you connect the hoses appropriately? I think there are vacuum or crank case hose that the CAI should have provisions for. You would simply place the hoses from the OEM intake setup onto the CAI in the same place.

From what you wrote it sounds like the engine is getting extra metered air from somewhere. Like there is air going through the MAF but somehow bypassing the IAC and getting into the intake. How is the IAC gasket looking? Is there any type of defect in the throttle blade? Maybe it's chipped or opentoo much? Maybe you have a hose from the CAI going to the wrong place? Take some pics of the engine bay and post them.

 

[GreenStang302]

The throttle seems to be fine. The CAI is hooked up correctly. I haven't been able to get pictures. Crappy weather in FL the past couple of days.

I turned the idle air screw, drove the car, and it slowed down better when I let off of the gas but now I have a slight jumping (or miss) when coming to a stop. I found an article on another forum that explains how to properly adjust that screw. When I took it to Ford for another problem (idle surging/dying), the mechanic messed with my idle. I guess he threw it out of whack.

I will update as soon as this rain clears and I get a chance to work on it. Thanks for the suggestions so far Three50won.

 

[Three50won]

Ok, see when you said your TPS voltage was at 0.97 I assumed you had already adjusted the idle screw. That's where the extra metered air is getting into the engine, because the throttle blade is open too far. The Ford mechanic probably opened it either because it was not open enough or he did it as a bandaid type solution to whatever the problem really was. But yea, you're on the right track...

 

[GreenStang302]

Well, I did not realized that is had been messed with but I talked to him on Friday and he did say that he "adjusted" it. So, I am going to try and adjust it correctly. But, the TPS was within the acceptable range so like you, I did not think anything of it.

 

[toyman]

I turned the idle air screw, drove the car, and it slowed down better when I let off of the gas but now I have a slight jumping (or miss) when coming to a stop. I found an article on another forum that explains how to properly adjust that screw. When I took it to Ford for another problem (idle surging/dying), the mechanic messed with my idle. I guess he threw it out of whack.

Ok, see when you said your TPS voltage was at 0.97 I assumed you had already adjusted the idle screw. That's where the extra metered air is getting into the engine, because the throttle blade is open too far. The Ford mechanic probably opened it either because it was not open enough or he did it as a bandaid type solution to whatever the problem really was. But yea, you're on the right track...

I think you and Three50won are talking about two different screws. You appear to be referencing the idle air bleed screw on the TB and Three50won is talking about the idle stop screw on the throttle linkage. The idle air bleed screw fine tunes the air flow to the IAC and the idle stop screw alters the throttle valve setting which is connected to the TPS.

 

[GreenStang302]

Oh, well, I am talking about the idle air bleed screw on the throttle body. I assumed that when you messed with anything to do with the idle, the TPS could be affected.

Thank you for the clarification though, toyman.