My lil TBI has always had a weird habit of revving down slowly when the throttle is closed. It'd rev up fine but when you let go it sounds different (almost muffled from the intake), the revs would stay up for a second or two, and slowly drop back down to idle over the next three seconds or so. I always thought it was a vacuum leak I couldn't track down... Today I let it idle, disconnected the IAC while running, and found it to rev much smoother and came back down to idle quickly (well quicker) like it should. Obviously the IAC is doing funny things when the throttle blades are closed. Sound like a flakey IAC motor? I have a spare TPS I'm going to try (looks like they're fairly closely linked, although the old one tested fine a few weeks ago). Any comments or suggestions?

I also found that the line to the charcoal canister didn't seem to have any vacuum. Must be plugged up at the TBI...

 

The system is designed to do that. The ecm will litereally follow the throttle opening with the IAC to ensure smooth transitions. The obvous side effect is the motor will hang. On top of this, 700-R4s can overrun the engine in OD so without the throttle follower on all models, the engine would be at idle at closed throttle at lower speeds below 50 mph. Even worse with a manual as letting out the clutch with the engine at idle while moving will cause the car to jerk or lock the rear tires up in bad weather. This may be undesirable from a factory standpoint.

 

You've just discovered the dreaded throttle follower. It's built into the GM code to ....... well actually I'm not real sure what that engineer was smoking.

It makes sense on a manual tranny car, it would allow you time to shift without the revs dropping too fast. if you drive like grandma. On an auto it really has no use other than to keep the car from lurching forward when you lift off the throttle. This throttle follower can some times be so severe that it can feel like you didn't even lift off the throttle.

I've spent a good bit of time tuning on my IAC routines in my chip. I've been able to peacfully coexist with it. Some other guys have gone as far as to just unplug it and have a different chip for colder months where it's really necesary on startup.

edit: demented beat me to it

 

Thing is a high throttle follower setting will help your high end power and air flow to an extent. If the IAC is wide open at wide open throttle that means there is just that much air going in.

That routine also helps keep the engine from stalling when you get on the gas and back off in rapid undecisiveness. Basically you nail the gas toward an opening in traffic, then have to let of real quick. In some cases the engine would die if the IAC was disconnected. When you went to mash the GO pedal again your engine would be konked out dead. Basically what would happen if you did the same thing with a double pumper carb. GM used something called a Decel valve on their carbed cars to get rid of this as it did the same thing.

 

So this is normal? Damn, it acts like a vacuum leak! My ol' Honda C70 Passport scooter acts the same way (another issue I never did manage to track down). It's really annoying when you're looking right at it.

The only engine I can compare it to is my TPI 305, and it doesn't beyhave like the TBI engine at all (if memory serves, that one is still in storage...)

 

Like fast said, there are some reasons to keep it. Thats why I didn't completely disable it in my chip, but I have subdued it heavily and I'm very pleased how everything works now. This is just another reason to get into chip tuning.

 

I got rid of it entirely in my code. Ive never had any stalls but the engine goes right to ~800 rpm as soon as I lift my foot off the gas when its in OD. The plus side is I have much better control of wheelspin, and the car in general. I crashed a mustang due to that throttle follower type of routine. The rear tires lost traction around a turn so I naturally let off the gas but the stupid IAC took forever to close so the rear tires jsut kept spinning. Ended up hitting the median.

 

The throttle follower steps are there for emissions. When you lift the fuel on the walls of the intake get pulled off from the vacuum, this can be seen as a really rich spike when you quickly lift. That's the real reason for the IAC throttle follower. The IAC is still very useful when you figure how to tune it. On rather "hot" engines I disable it all together so that the tuning gets done faster, on mild engines I just limit the TF steps.
You can adjust the rate at which the IAC closes depending on mph.
I don't know what the mph values are but during the limited time I spent tuning these areas the low mph is like 5mph, high mph I don't know, never bothered with it. You get the idea though. So you can have the TF decay go FAST when you're not moving to give you better control.

 

A sudden throttle closure, produces a rich spike. The EPA just hates that.

 

Some cars like to stall after a burn out, with it disabled.

 

Ah, the sudden grab of traction pulls the engine speed down fast...makes sence. My rather loose (and yes annoying) torque converter probably prevents this.

 

I almost did that to my van, right after I went to TBI. It was raining and I was making a right turn onto a 2 lane street. Their was a car coming from the opposite direction. I touched the gas and wasn't use to the throttle response of the TBI and the tires started spinning. Well being in a corner swung the rear end almost into the other lane. As soon as the rear end started to come out from under it, I let off the gas. Well the throttle follower kept the tires spinning. I narrowly avoided wiping out by stabing the brakes real quick.