Sylvie Willis
Testing the fuel injectors on a Chevy 350 TBI engine can be a complex process, but it's essential to ensure optimal performance and fuel efficiency. The Throttle Body Injection (TBI) system relies on proper fuel injector functioning to deliver the right amount of fuel to the engine. Various factors, such as vacuum leaks, clogged injectors, or electrical issues, can impact the performance of the fuel injectors. In this guide, we will explore common issues, troubleshooting steps, and solutions to help you effectively test and maintain the fuel injectors on your Chevy 350 TBI engine.
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What You'll Learn
Check the injector's wires for power and ground
To check the injector wires for power and ground, you will need to use a noid light. This will allow you to check for power and a pulsing ground on the injector wires.
First, pull the connector from the pick-up coil off of the ignition module. Then, connect the noid light to the battery on the clip end. After that, you can alternate touching the leads on the ignition module with the noid light.
If you have power to both sides of the injector, but no ground, this is normal when the ECM isn't supplying the ground. When the module "sees" the distributor firing, the ECM gets this information and then grounds the injectors to fire.
You can also check for power on the injector wires with the key on. When cranking, you should have power to the red and white injector wires, while the blue and green wires will not fire.
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Ensure the injector is receiving a signal to spray
To ensure that the injector is receiving a signal to spray, you can perform the following steps:
Firstly, put on the appropriate safety gear, such as eye protection, to safeguard yourself from any potential debris. Then, turn the car on by turning the key to the second "ON" position. This will ensure battery power is flowing, but the engine is not running. Set up a DVOM to measure DC voltage, selecting the lowest range if the meter doesn't auto-range. Touch the negative lead of the DVOM to a ground source, typically an unpainted piece of the car's frame under the hood.
Now, bring your ear close to the end of a metal rod or screwdriver that is opposite the injector. As the engine runs, listen for an audible clicking sound, indicating the injector is being activated. Repeat this process for each injector, ensuring your hair is tied back to prevent any accidents. If you find an injector that is not clicking, there may be an issue with the injector or the electronic control transmitting to it.
Another method is to look for flashing or flickering lights. With the engine idling, a test light should flicker dimly, and as the throttle is applied by pressing the gas pedal, the light should flicker more brightly. This light indicates the signal transmitted by the ECU to the injector to spray fuel. If the test light is not lighting up, the injector may be faulty or there could be an issue with the ECU.
Additionally, you can check the wiring by disconnecting the wiring clips to each injector and repeating the test. With no injectors connected, the flickering pulse should transmit through all wires without issues. If there is still no pulse, there may be a problem with the ECU.
Finally, you can check the voltage by ensuring there is power on both sides of the injector. If there is power on both sides, but no ground, this is normal when the ECM isn't supplying the ground.
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Check the colouring of the plugs
When checking the colouring of the plugs, you should be aware that the colouring should match. If the colouring does not match, this is most often a bad injector, although it could also be a faulty chip. If both plugs are black, this indicates that there is too much fuel. If you have dark plugs and the BLM numbers are high, you have an o2 sensor problem. If the numbers are low and the plugs are black, then the vacuum level is good, but either the programming on the chip is off, the fuel pressure is too high, or the injectors are stuck.
The plugs should be spark plugs, and AC Delco gives the most consistent results. R44TS for stock 1/2″ iron heads or R44LTS 3/4″ reach for Vortec iron heads or Aluminum heads. Most Dart heads now use the 3/4″ reach, but if you are using an aftermarket head, find out which reach plug you need because running a 1/2″ reach plug in a 3/4″ reach hole will run terribly and be difficult to diagnose.
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Examine the MAP sensor and rpm readings
To examine the MAP sensor and rpm readings of a Chevy 350 TBI fuel injector, you need to understand the role of the MAP sensor in the TBI system. The MAP sensor measures the vacuum in the engine and sends this information to the computer. The ignition module then provides the RPM, and the computer uses a fuel table on the chip to determine how much fuel to spray for that specific vacuum level and rpm.
The MAP sensor system is not the most accurate method of metering fuel into an engine, but it is functional. More advanced OBD2 systems use a MAP sensor as a backup and instead meter their fuel based on a MAF Mass Air Flow sensor, which measures the incoming air.
If the MAP sensor does not receive a good vacuum signal, the engine will run rich. This is because the computer will look in a different part of the programming as the throttle opens and air rushes into the motor, causing a drop in the vacuum level. A broken vacuum line, faulty MAP sensor, or an engine problem can all lead to this issue.
To diagnose a potential problem with the MAP sensor, you can use a vacuum gauge. Hook the gauge to all ports to check the vacuum level. If the vacuum level is good, but you still have black plugs and low BLM numbers, the issue may lie elsewhere. For example, the programming on the chip may be off, the fuel pressure may be too high, or the injectors could be stuck.
It is important to note that the MAP sensor is just one component of the TBI system, and other factors can also contribute to issues with fuel injection. For instance, ignition problems and electrical issues can also lead to rough idling and other performance problems.
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Look for vacuum leaks
A high idle is one of the most common issues with TBI systems. This is often caused by a vacuum leak. A MAP sensor-based system will see the leak as opening the throttle blades, which will then feed the leak and cause a high idle.
A vacuum leak can be caused by a leak in the base gasket between the throttle body and the intake, which is common. It can also be caused by a worn TBI shaft that sticks a little open when close to closed/idle. If you restart the engine when this happens, the idle may return to normal.
To test for a vacuum leak, you can use a propane soldering torch, unlit, around all possible vacuum leak spots. You can also check the linkage back to the TBI unit to ensure it is returning all the way closed.
If you have a vacuum gauge, you can take a reading when the idle is high. If the vacuum is steady, floating slowly between 17 and 18 inHg, the truck will likely run very well with a 750 RPM idle.
If you have a high idle, you can also "force" the IAC motor closed and then see what the RPMs are. First, jumper the A and B terminals of the ALDL connector. Second, turn the key on, but do not start the engine, as this can blow the ECM. Third, wait 30 seconds, then disconnect the IAC wire connector at the TBI IAC motor. Fourth, remove the jumper wire from the ALDL connector. Fifth, start the engine and observe the engine RPM.
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Frequently asked questions
You can test the fuel injectors in your Chevy 350 TBI by checking the voltage of the wires. If you have power to both sides of the injector, you have no ground. This is normal when the ECM isn't supplying the ground.
If your fuel injectors are not spraying, it could be due to a clog, pinched line, or plugged filter. It is also worth checking if you have power to the injector wires.
Idle surge can be caused by a vacuum leak, too lean or too rich fuel-air mixture, or jumpy timing. It is recommended to check for any changes since the idle was correct.
Used injectors for the Chevy 350 TBI can be found in junkyards, online forums, or from specialized vendors such as Cruzinperformance.com.
