I recently bought a FiTech Go EFI fuel injection system for my big-block Chevelle. The engine is a 468ci big-block with a mild hydraulic roller cam, a TH400, a warmed-over converter, and 3.55:1 gears in the 12-bolt. I used to run an 850 Holley. It ran great but always was rich. We installed the system and it runs pretty good except that the engine idles too fast, like 1,025 rpm instead of the 850 rpm we put in. We tried lowering the idle rpm on the hand-held but it really didn’t do much and it should idle where we tell it to, right? Can you help?



Jeff Smith: We’ve seen this issue on several self-learning throttle body EFI systems and from what we’re reading on many of the forums, this is a common problem. The good news is there is an easy fix. If you’ve been reading my column for any length of time, you know that we always start with the basics, so here goes…

Before we even get into how to tune the idle speed correctly, we have to also first look at the mechanical side of the engine to make sure there are no vacuum leaks. An engine with a vacuum leak can run at a higher than commanded idle speed and will tend to idle erratically. So make sure all of the intake manifold and throttle body bolts are tight and there is no uncovered vacuum port. It might also be worthwhile to spray a little brake clean or carburetor cleaner around the intake with the engine idling as a test for vacuum leaks. This is a very common problem and any leak must be eliminated before proper idle speed tuning can be accomplished.

When any of the EFI companies build a product like a self-learning TBI system, they are forced to design it to accommodate an incredibly broad range of engine sizes and combinations. Your Go EFI FiTech is designed to control fuel for engines that make up to 600 horsepower. That’s a bunch of power but it also could be called upon to control a very mild engine. This is where the variables begin to stack up. Typically, a large displacement engine will demand more air than a small displacement engine. It’s also important that the engine start and run the first time you turn the key. So the EFI companies dial in a tune that is guaranteed (assuming you set all the initial inputs properly) to run the first time.

Now let’s talk about one of the crucial systems on any EFI system: the idle air control (IAC) motor. This is a small stepper motor that controls the amount of air that is by-passed around the throttle blades. This is done to control idle speed electronically. With a carburetor, if you need more idle speed you just turn the curb idle screw clockwise and this opens the throttle blades to allow more air into the engine. That’s simple and very effective.

But an EFI system also must be able to control the idle speed with different load dumps like dropping the engine into gear with an automatic or when you switch on the air conditioning compressor. An EFI system controls idle speed by using a combination of the air allowed past the mechanical curb idle setting combined with a measured amount of air bypassed by the IAC. The ideal combination is where the throttle blades flow a majority of the idle air with a small contribution from the IAC motor setting.

The ECU establishes the final commanded idle speed by opening or closing the bypass air using the IAC motor. The amount of bypassed air is determined by a tapered cone positioned by the IAC motor. It operates by turning a screw that positions the tapered cone. These are sometimes referred to as steps and one complete revolution of the screw is referred to as one step. The ECU then uses rpm feedback to position the IAC.

If the engine idle speed needs to increase, the IAC will open a few steps, allowing more air into the engine. Or, the IAC motor will move close to fully closed if the idle speed is too high. With any aftermarket EFI throttle body system, the first thing that must be done is to ensure that the throttle position sensor (TPS) actually reads 0 with the throttle at curb idle. Other EFI systems require you to perform certain steps to make this happen, but the FiTech does this electronically. So if you change the position of the throttle plates with the curb idle screw, all you have to do is cycle the key off – wait a few seconds for the numbers on the display to disappear from the screen and then turn the key back on and the TPS will reset to zero.

Now that we know how this system operates, you may be able to see how some engines might not experience accurate idle speeds given that the factory must establish a given position and that may not be perfect for all engines. So with a big block Chevy that idles too fast, the solution is to follow a simple procedure. This should only be attempted with the engine at normal operating temperature – which for the FiTech is above 160 degrees F.

The first step is to ensure the TPS reads 0 with the key on and engine idling. The TPS position is found on the dashboard screen. With that established, we need to find what FiTech calls IAC steps. This can also be found on the main dashboard by scrolling down roughly about a third of the way. With the engine idling, monitor both the IAC steps and idle speed. In the case of this big block Chevy, it appears that the throttle blades are open too far which means the IAC steps will be a very low number – perhaps 0. In order to reduce the engine speed, the first thing we need to do is to close the curb idle position of the butterflies. Do this by slightly by turning the primary idle speed screw out (counter-clockwise) about a half turn. This should slow the engine down. Then shut the engine off long enough for the TPS to reset. Restarting the engine should reveal an increase in the IAC steps to perhaps around 8-10. If the IAC steps are more than this, you will need to turn the curb idle speed screw in slightly and then repeat the shut-down procedure.

This is the dashboard screen on the FiTech that indicates the IAC steps that will help you dial in the idle speed on the system. It may seem a hassle but it’s not difficult, but it can require 10 or 15 minutes of effort to get it dialed in properly.

Conversely, for example, if your engine idles at close to your commanded rpm but the IAC steps are higher than the preferred 3-10 steps and instead are up around 25 to 50 steps or higher, this would indicate the curb idle position of the throttle blades is not open enough, not allowing sufficient air in and forcing the IAC to open a greater amount than desired. You might notice this as a gushing or a noisy air inlet sound coming from the top of the throttle body. This noise is a large amount of air being bypassed by the IAC. This reduces the total authority of IAC movement which could affect drivability, so it’s best to open the throttle blades by a half-turn clockwise, shut down the engine, allow the numbers on the handheld to go blank and then restart the engine. If this change achieves IAC steps at idle of between 3 and 10 then you’re where you need to be.

All this should only take a few minutes to establish, but these small efforts pay off with enormous benefits. Notice that the idea is to make very small changes — never more than a half-turn on the idle speed screw. These small changes will make it easier to sneak up on the proper position for the throttle blades and the IAC steps.

We’ve done some around-town driving experiments in L.A. traffic and depending upon where you live and how you drive, it’s entirely possible that your engine spends 70 percent or more of its time at idle. So spending some quality time dialing in a repeatable and stable idle adjustment is really very important. And your friends will think you’re a tuning genius!

Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.