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Fuel Injectors

Leaking fuel injectors can cause hard starting when the engine is hot, a lumpy idle, or high emissions (unburned hydrocarbons).

TBI engines (non-turbo 2.2 and 2.5, some V8s) have a single fuel injector. Some four cylinder engines used in Mexico had multiple-port fuel injection (MPI; 4 injectors), a far better system for economy, power, and idling quality. Multiple point fuel injection was phased in starting with the turbocharged engines in the 1980s; the Neon arrived in late 1993 with sequential multiple-point fuel injection, a key reason why the new 2.0 liter engine outperformed the older 2.2 liter engine. (AMC 2.5 liter engines also used multiple point injection.)

How fuel injectors work (Bob O’Neill and Bob Lincoln)

The logic module controls the fuel injectors and the amount of fuel supplied to the engine. Single-point injection is used in Throttle Body Injection (TBI) applications. This uses a single large injector mounted in the center of the bore of the throttle body.  This injector has a spray pattern of 6 orifices which spray fuel into the air flow. This flow is deflected off a nozzle to deliver a 45 degree spray pattern.

Similar to a carburetor, the throttle body provides air and fuel through a bore where the butterfly valve regulates the amount of air flowing into the manifold. This butterfly valve is moved using a throttle cable. As the engine speed increases, the logic module cycles the injector on and off at the proper rate to provide the right amount of fuel to the engine. As the engine speed increases, the logic module provides more fuel into the air flow. (At idle, airflow is controlled by the idle speed motor).

For turbocharged engines and other engines which use a multi-port injection (MPI), there is an injector for each cylinder.  The requirement of the injector is reduced because it doesn’t have to supply fuel to the entire engine, so the MPI injectors are smaller but have a similar spray pattern. Since in MPI engines there is one injector per cylinder, the injector sprays fuel directly into the cylinder’s air stream, through the manifold port connected to the intake port of the head; more fuel should stay in suspension this way.

Engines controlled with logic modules other than the SBEC use a batch-fired injector. This means that the injectors are fired in pairs rather than one at a time (sequential). In batch-fired systems, fuel is injected behind the intake valve on the power stroke. This fuel is pulled into the cylinder on the intake stroke as additional fuel is injected during this intake stroke.  The total amount of fuel injected on both sprays ends up being correct, as calculated by the logic module. This system cannot react as quickly or precisely, and some fuel comes out of suspension, but it is nearly as effective.

The auto shutdown (ASD) relay supplies 12 volts DC to the fuel injectors. To activate the spray, the power module provides a ground path, based on a signal from the logic module. The length of this signal is controlled by the logic module on 1984 models, and uses the fuel injector sync signal and ignition reference signal from the Hall Effect Pickup (HEP) to determine which injector should fire. In later models, the power module uses the logic module for the signals to determine when and which injector to fire.

As the logic module turns an injector on or off, it checks the output of the injector driver circuit to ensure that it is working properly. In 1984 models, if it detects something other than zero volts when the circuit is open or twelve volts when the circuit is off it triggers a code 42. In later turbocharged engines, if the logic sensor detects a problem with injectors in circuit 1 (injector one and two) it triggers a code 26 and a code 27 for circuit 2 (injector three and four). For TBI engines a code 27 is triggered if the circuit is open or shorted and if the driver output current is not high enough it triggers a code 26.

Leaky fuel injector diagnosis

Open the air cleaner and locate the fuel injector (that big mechanical thing in the middle. Turn the key to the ON/RUNNING position (not START); this activates the fuel pump. Place a paper towel or some paper underneath the injector and wait a few minutes; if gas drips onto the paper towel, the injector is probably leaky. (There is not yet a consensus on whether the injector is leaky if it drips onto the paper when you first put the engine to the running position).

Alternative. (Tim Boisvert). Check the pressure at the fuel rail to see if the pressure is correct when the car is running. A weak fuel pump can cause hard starting when the motors hot. If the pressure is good, turn off the car and see if the pressure drops. It should not drop or at least not much. Check a service manual... to see what the pressure should be.

Notes (Tim Boisvert). The computer always assumes good pressure. That's why on any fuel injection system, fuel pressure should be checked before troubleshooting starting, idle , performance, or injector problems.

Let us think about what's happening. As you crank the motor, the injectors are opening for the duration set by the computer. When the injectors do open, a very fine mist of gas is sprayed in to the intake manifold just in front of the intake valve(if it's not a thottle body). As this mist is drawn into the cylinder it is mixed with the air and becomes very flammable as it becomes compressed. But, if we take the same amount of fuel and squirt (because the fuel pressure is low) it into the manifold, we don't get that good flammable mixture. It may take longer to get the pressure (if we had a weak pump or leak somewhere) up to where it should be because we're bleeding it off as the injectors open. Now the pressure comes up but there is already a small pool of gas sitting in the manifold. This makes the mixture very rich and makes the vehicle hard to start.

Try this: Turn the key on but don't crank the motor. Let the pump run (you can hear it if you listen). It should ran for several seconds then turn off the key. Count the seconds that the pump runs until the pump turns off. If it runs for 5 seconds, the next time you start the car, turn the key, count 1..2..3, then crank the motor. You will have to do this every time you start the car. If you have cranked the motor over before counting, this will not work. If it starts every time, I don't think the injectors are leaky.

It really comes down to measuring the fuel pressure and your time may be worth the money to have someone measure it if you can't. You are at least armed with some idea of whats going on now. ...

I had a starting problem on my 84 Chevy. It would start when it was cold but never if it was hot and just turned off. The fuel pressure was just low enough that it would not open the fuel regulator to push any vaporized fuel out (vapor lock) but high enough to let the motor run as long as it didn't get too hot. Sometimes it would run poorly or stall on really hot days and had to cool down before it would start. The pressure was 10 to 12 psi. The regulator was set to 13 psi.

Fuel injector cleaning

Tim Boisvert wrote: Before you decide to replace the injectors you may want to have them cleaned. Whomever you have do this, make sure to ask what the procedure is. They should pull the injectors out to run a cleaner through them and then check to see if they hold pressure without leaking (guess the cleaner they use to clean the injectors is to harsh for the rest of the fuel system and that is why the injectors are pulled). At least try adding some good (expensive) injector cleaner to your fuel tank. Don't use the stuff that says add at each fill up, but the stuff that says use only every 6 months (or something like that). If it helps great, if not it only costs 7 or 8 bucks for the cleaner.

Replacing the fuel injector (Ed Hennessy)

It's a rather simple procedure. The following is for 2.2/2.5s with TBI (i.e., non-turbos).

Take the cover off the air cleaner. Looking into the throttle body, you will see a black cap with a Torx head screw (T20 I think), with wires leading from it. That is the injector cap. Remove the screw and pop the cap off by gently prying up with a small screwdriver.

Attach a jumper to one of the terminals on the injector, and ground the end for not more than 10 seconds (more than that, and you damage the injector, for those replacing the same one). This de-pressurizes the fuel system. Do not proceed without depressurizing the system. You will spray gas all over--at either 14 PSI (up to '88) or 39 PSI ('89-up).

Once the system is depressurized, you can simply pop out the injector. Insert a narrow screwdriver into the slot just below the top of the injector, and pry upwards. The injector will pop up and out. It may be a little stubborn--note that there are two holes and you can pry from both sides if it doesn't come right out. Use steady, slow pressure.

Apply a little vaseline to the new injector's o-ring, and insert it into the orifice in the throttle body. Be sure to orient it right--the cap is keyed to only go on one way, so be sure to line up the notches on the injector with the indentation in the cap (or else the wires won't reach). Press the injector in firmly--it will distinctly pop into the groove at the bottom of the orifice. (To make inserting it easier, you can put the cap on the injector and press down on the cap. This prevents damage to the injector pins.)

Install the cap back on the injector. Be sure that the cap fits tightly on the top of the orifice--there should be no gap between the cap and the orifice. If there is only a small gap, tightening the screw will probably seat the cap properly. If there is a large gap, don't force the cap down--the injector is not fully seated. Re-seat it before tightening the screw.

Once the injector is fully seated, be sure the cap is aligned properly with the shape of the orifice (this will be obvious as you look at it. If it needs to be re-aligned, you can twist the cap and injector by moving the cap's 'tail'--the vaseline will make moving the injector easy. Once it's lined up, tighten the screw. Not too tight, or you'll strip the head of the screw (yes, you can round a Torx head. It's not easy, but it can happen. I know.)

Test the car by starting it with the airbox cover off. Watch the injector--if it is not fully inserted, it will tend to spritz gas droplets out of the throttle body. If the car runs fine (and it will unless the injector is not properly seated), put the cover back on the air box and put away your tools--you have finished.


Ed Kelly wrote:

Cost is the big issue.  A new injector is ~$75-100.  As far as replacing it, pretty straight forward.  The Haynes manual covers the procedure pretty well. Just make sure you get the old O-rings out and that you lube the new ones before installing.  It is about a 15 minute job...

As for the benefits: it will improve fuel economy, may help with starting and throttle transitions.  The big concern is that with the engine running rich it will tend to clog the catalytic converter and thus further reduce power...

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