After removing the air-filter assembly, the manifold vacuum source on the rear throttle body will need to be capped. Unlike port EFI that places the injector in a low-pressure area, Cross-Fire utilizes no vacuum signal to the fuel-pressure regulator. Identified by GM as the Model TBI-400, it’s two four-cylinder throttle bodies attached to an intake manifold via a plenum. It has two idle-speed controls and one TPS.
Before any adjustments are made, it’s essential to verify that the ignition and engine management system are operating properly. Bill used a Snap-On Scanner to interface with the ECM and check for trouble codes. If a scanner is not available, jumping A-B on the ALDL connector will allow the Check Engine light to flash any stored problems. Remember, neither the scanner nor the ECM will be able to verify the condition of the ignition components. For this, you’ll need an oscilloscope, or you’ll have to replace them with new parts.
Fuel pressure is the key to a properly functioning EFI system. The Cross-Fire design uses a low pressure with a specification of 9-13 psi. You can use a high-pressure gauge, but we used a TBI-specific gauge for accuracy. When choosing analog instrumentation like this, the more resolution of the gauge that you can use, the more precise it usually is. The fuel pressure should be checked not only at idle, but under engine load. This will require a connection with enough length to attach it to the windshield while driving. A WOT pass in Second gear will usually reveal any flow limitations. The system should be able to maintain the idle pressure during all driving scenarios. If the pressure ramps down under load, look for a clogged fuel filter, a weak fuel pump, or low voltage supply to the fuel pump.
GM never included a Schrader valve tap on its TBI systems, and the Corvette is no exception. This adapter will take the place of the fuel filter and allow for a quick and easy single-line fuel pressure hookup. Limit the running time to testing only, since there will be no filtration beyond the sock in the fuel tank. Our Corvette had a solid 10 psi.
By using a dial-back timing light, the strobe can be adjusted to freeze the fuel spray pattern for examination. You may need to vary the engine speed and timing light adjustment to get a good look.
This represents a good conical atomized spray pattern. Both throttle bodies should be checked for injector spray before any adjustments are made. The periodic use of a good in-tank injector cleaner will keep the injectors clean and functioning for many years and miles.
Now that everything has checked out, we can start the adjustment procedures. With the engine off, use a spray carburetor cleaner to remove any varnish buildup on the throttle body.
Next we’ll clean and regasket the IAC valve. We used a special socket, Borroughs Tool Company PN BT8514A, to remove the IAC. On the front throttle body, the fuel line crossover needed to be disconnected to gain access.
The IAC has a friction compound on the threads and will offer resistance during removal.
With the IAC removed, hold it with the pintle facing down and clean the tip, spring, and shaft with spray carburetor cleaner. It’s desirable to reference it in this direction so the solvent doesn’t migrate into the stepper motor windings. Note the old gasket still attached.
GM sells the IAC gaskets in individual packages. You’ll need two, one for each throttle body.
See how nice our 17-year-old IAC cleaned up. Carefully install the new gasket without tearing or fraying it.
Before installing the IAC, use the carb spray to clean the passage that feeds the bypass air to the pintle. All of the above procedures must be performed on both throttle bodies.
Cross-Fire, like all manifolds, has a tendency for bolts to loosen up. Snug down all throttle-body, plenum, and intake-manifold bolts to ensure against vacuum leaks.
Before making any fuel system adjustments, we need to verify the ignition timing, since it will impact the idle speed. The stock specification is 6 degrees BTDC with the bypass lead disconnected. It’s located to the driver side of the distributor, near the firewall.
Some early cars did not have the previously mentioned bypass lead. Setting the timing requires Thexton jumper tool (PN 375) connected to all wires except the tan with black stripe, which is the EST bypass line. When this circuit is opened by either method, the computer has no control over the spark-advance curve and will revert back to base timing or the position of the distributor.
Dan likes to set the timing slightly advanced from the GM specification if premium fuel is used. We set ours to 10 degrees BTDC.
The rear throttle-body stop screw will be used to set the minimum air rate and throttle angle for this throttle body. From the factory, they are capped. Whenever drilling or grinding, cover the throttle body with a clean shop rag to prevent metal filings from entering the engine. Drill down at a slight angle above the Welsh plug and, with a small punch or chisel, remove the cap to allow access to the adjustment. This is usually not required on the front throttle body, since the adjustment will be made with the balance screw. In some instances, if balance cannot be achieved, this plug will also need to be removed.
This is what the plug looks like when properly removed.
The balance screw is where the throttle linkage attaches to the front throttle body; it’s held in place from the factory with a weld. This weld will need to be ground off and the screw replaced to synchronize the throttle bodies.
GM offers a replacement balance screw kit, making it adjustable. Reference the diagram in the story for proper installation procedure.
Special tools will be required to block the IAC passages to adjust the throttle plates. These professional ones expand by rotating the handle after installing. Less expensive, nonadjustable plugs are available from Thexton in most auto parts stores. They’re marketed under the name IAC adjustment plug; you’ll need two. Always lubricate the rubber with some light oil before installation, or it will stick to the throttle body. Remove the electrical connector from each IAC motor. Note the new balance screw.
GM originally suggested the use of a slack-tube manometer to adjust this system. A traditional analog manometer can be used in its place and will avoid the hassle of filling the slack tube with distilled water and dye. A regular vacuum gauge cannot be used, since it measures inches of mercury, while a manometer reads in inches of water. Water is lighter than mercury, which makes the manometer more sensitive. It will first be connected to the rear throttle body at the venturi vacuum source, which is not where the air cleaner was connected. The port next to it is the venturi vacuum and is capped from GM.
With the engine warm and idling in Drive, the brake on, and wheels chocked, adjust the minimum air rate (throttle stop) screw on the rear throttle body to obtain a reading of 6 inches of water. Then recap the port and connect the gauge to the front throttle body venturi vacuum source. Using the balance screw, adjust for 6 inches of water on this throttle body. If the reading is too high, the throttle plates need to be closed; too low, and they need to be opened. Once this is achieved, using the minimum air rate screw on the rear throttle body, adjust the engine speed for 500-550 in Drive. The factory specification is 475 rpm, but Bill has found that as the vehicle ages, it likes a little more throttle opening. Since the balance is completely out due to the screw being replaced, you may need to go back and redo the manometer procedure on both throttle bodies to bring them into specification.
With the throttle angle set properly, the last step is to adjust the TPS. With either a scan tool or a voltmeter connected to the TPS, adjust the sensor to .525 volt; .530 volt is fine. Shut the engine off. Reconnect all components and disconnect the battery ground cable for 30 seconds to erase the ECM’s memory. Start and drive the car for four to five miles, allowing the management system to teach itself the new settings. Do not be alarmed if at first the idle speed is very high; as the computer relearns, the idle will stabilize at the proper speed.
When installing the new balance screw, the throttle lever and cam will need to be supported in this manner for the screw to make contact. The cam is spring-loaded. An Allen wrench or screwdriver will work well. When adjusting the venturi vacuum signal using this screw, the cam will respond in the reverse manner of the screw. To close the throttle plate, you’ll turn the screw in instead of out.
The capped ported tube is the venturi vacuum signal. With the cap removed, you will not be able to detect any vacuum signal by touch, proving the sensitivity of the manometer. When you’re near the proper throttle angle, the throttle body will start to whistle slightly.
This is the GM-specified procedure to gain access to the throttle stop (minimum air rate) screws on both throttle bodies. Congratulations, youre now one of the few who know how to dial in this EFI system. Enjoy. Special thanks to Dan Pinto for his assistance during this photo shoot and for arranging the tuning session.