Fuel injectors inoperative, one or more injectors either on all the time or will not squirt.
Revised 27-Jan-2008 to include expanded testing for proper power to injector wiring
Tools needed: Noid light, Multimeter (volts & ohms), 10 MM socket &
extension, & ratchet.
Note: Do all of the steps and do them in order. The results of the
subsequent tests are based on the prior tests being successfully passed.
1.) Each injector has a red power wire to provide power to the injector. Turn the ignition switch to Run and remove
each injector electrical connector and use the multimeter to check for 12 volts on the red wire. Each injector should
have 12 volts +/- .5 volt. More voltage is always better than less voltage. No 12 volts on a singe injector and the wiring
for that injector is broken inside the engine fuel injector harness.
No 12 volts on all injectors:
A.) Check for a bad connection at the 10 pin connector. Check for 12 volts at red wire on the MAF or TAD/TAB
solenoids mounted on the aft side of the passenger strut tower. Good 12 volts
there and you have a wiring problem with the 10 pin connectors or associated wiring.
Diagram courtesy of Tmoss & Stang&2birds
See the graphic for the location of the 10 pin connectors:
See the graphic for the 10 pin connector circuit layout.
The injector power pin is the VPWR pin in the black 10 pin connector.
B.) Bad ECC power relay. The relay is on top of the computer, it provides power to the fuel injector system.
It is somewhat difficult to get to, since it requires you to remove the computer from its 2 bolt mounting.. If the relay or
socket is bad, you will not have 12 volts on any of the red wires in the engine compartment or to pins
37/57 on the computer. The pins 37/57 are the main power feed to the computer.
Diagram courtesy of Tmoss & Stang&2birds
C.) Blown fuse link – The blue fuse link for the computer is up near the starter solenoid. Check for no 12 volts
on the ECC relay socket or computer black/orange wires. No 12 volts and the fuse link is blown open. If the fuse link blows,
there is no power for any of the computer functions.
D.) Bad wiring. Broken or damaged red wire to the chassis side of the 10 pin connectors.
Some basics about the computer:
Remember that the computer does not supply power for any
actuator or relay. It provides the ground necessary to complete the
circuit. That means one side of the circuit will always be hot, and the
other side will go to ground or below 1 volt as the computer
switches on that circuit.
2.) Get a noid light form AutoZone or other auto parts store, or even better
a set of them
This set is from
http://www.toolking.com/ProductInfo.aspx?productid=12492 and costs about $12-$14.
3.) Use the noid light to determine that the injector pulses and isn’t stuck in the on position.
It you have a set of them install all of them and compare the pulse intensity. Install the noid lights,
turn the ignition switch to Run and crank the engine. A light that doesn’t pulse and stays on has a
short to ground in the computer side of the circuit. That can be either a wiring fault or a failed computer.
If this is the case, remove the passenger side lick panel and disconnect the computer connector.
There is one 10 MM bolt holding it in place. Pull the connector all the way out of the
computer so that you can see the computer side pins.
Use the list from the graphic below to find the fuel injector pins for the injectors that
didn’t turn the noid light off.
Diagram courtesy of Tmoss & Stang&2birds
4.) Set the multimeter to low scale Ohms and measure between the
computer ground located below the computer and the suspect fuel injector pins. You should
see greater than 100 K Ohms resistance. If you see less than 100 Ohms, the wiring
between the injector and the computer has a short to ground and needs service.
Check the harness and look for damage, kinks or frayed spots.
5.) A single noid light that never turns on is either a wiring fault, or a
failed computer. Either the injector has no DC power or the computer has
failed and cannot switch the injector circuit to ground. Determine if the
injector has power by using the multimeter to check for 12 volts on the red
wire on the suspect injector connector. No 12 volts and you have a wiring fault.
Check the harness and look for damage, kinks or frayed spots. Check the 10
pin salt & pepper shaker connectors for bent pins, corrosion and damage.
If none of the noid lights flash and you have 12 volts at each injector, check
to see that you have good spark. Before you even think about replacing the
computer, see step 6.
Next Check the fuel injector wiring end to end. Each fuel injector has a red
wire (power) and an non-read wire (computer controlled ground). Set the
multimeter to low ohms and measure each non red fuel injector wire from
the fuel injector connector to the matching pin on the computer connector.
You should see less than 2 Ohms. More than that means a bad connection
or bad wiring.
6.) Use an ohmmeter set on the low resistance scale and measure the resistance of
each injector across the two contacts inside the electrical connector. You should
see between 11-16 ohms. More or less than that is a bad injector. Next measure
between either one of the contacts and the metal on the injector body. You should
see greater than 100,000 ohms. Don’t hold the metal probe tips with your bare
hands when you make this measurement. It will give incorrect results if you do.
Once you have determined that the suspect injectors have good power and
good wiring, the computer is the likely suspect, since a ground is required
to complete a circuit and make it function. The computer provides the
ground: if doesn’t, then the noid light will not flash.
7) If you have gotten this far, then the problem is likely ignition
related. Remember the noid test using all of the noid lights? All of them
were supposed to be equally bright. Since you have already tested all the
electrical side of the fuel injector circuit, the one remaining common item is
the pip sensor inside the distributor. A failing pip sensor, damaged shutter
wheel or bent distributor shaft could all cause the pulse delivered to the
injectors to be faulty. A bad pip sensor will cause all the injectors not to fire
and you will have no spark. Dumping the codes will usually show a code 14.
8) Spark plugs indicate one or more cylinders not firing: use the
multimeter to measure the resistance of the spark plug wires. The wires
should measure 2000 ohms per foot of length. A 2 foot wire would be 4000
ohms and a 3 foot wire would be 6000 ohms. Some Taylor and Accel wires
have metal cores and will measure much less: that’s OK.
Next examine the spark plug wires very carefully for burn spots, cracks and
damaged insulation. One good thing to try is to start the engine while the
car is a very dark area, open the hood and look for sparks or blue glow.
They indicate the electricity is leaking out of the spark plug wires.
Thanks to Tmoss & Stang&2birds at
www.veryuseful.com/mustang/tech/engine/ for some of the graphics
Thanks also to
www.oldfuelinjection.com for some of the graphics.
Cylinder balance test: use this to find dead or weak cylinders:
Revised 25 March 2012 to add necessity allowing the KOEO tests to finish before starting the engine and the need for a properly functioning IAB/IAC to run the cylinder balance test.
The computer has a cylinder balance test that helps locate cylinder with low power output. You’ll need to dump the codes out of the computer and make sure that you have the A/C off, clutch depressed to the floor and the transmission in neutral. Fail to do this and you can’t do the engine running dump codes test that allows you to do the cylinder balance test.
Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.
Be sure to turn off the A/C clutch depressed to the floor, and put the transmission in neutral when dumping the codes. Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.
Here's how to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.
If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.
The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.
89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.
The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.
WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.
What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems.
This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and drivability problems
Some codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.
Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, clutch depressed to the floor and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.
Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.
Cylinder balance test
If you have idle or IAC/IAB problems and the engine will not idle on its own without mechanically adjusting the base idle speed above 625-750 RPM, this test will fail with random cylinders pointed out every time it runs. The IAC/IAB
must be capable of controlling the engine speed to run in the 1400-1600 RPM range. Playing with the base idle speed by adjusting it upwards will not work, the computer has to be able to control the engine speed using the IAC/IAB.
Warm the car's engine up to normal operating temperature. Use a jumper wire or paper clip to put the computer into test mode. Let it finish the Key On Engine Off (KOEO) code dump. Start the engine and let it go through the normal diagnostic tests, then quickly press the throttle to the floor. Remember to keep the clutch pedal (5 speed) depressed to the floor during the test. The engine RPM should exceed 2500 RPM's for a brief second. The engine RPM's will increase to about 1450-1600 RPM and hold steady. The engine will shut off power to each injector, one at a time. When it has sequenced through all 8 injectors, it will flash 9 for everything OK, or the number of the failing cylinder such as 2 for cylinder #2. Quickly pressing the throttle again up to 2500 RPM’s will cause the test to re-run with smaller qualifying figures.
Do it a third time, and if the same cylinder shows up, the cylinder is weak and isn’t putting out power like it should. See the Chilton’s Shop manual for the complete test procedure
Do a compression test on all the cylinders.
Take special note of any cylinder that shows up as weak in the cylinder balance test. Low compression on one of these cylinders rules out the injectors as being the most likely cause of the problem. Look at cylinders that fail the cylinder balance test but have good compression. These cylinders either have a bad injector, bad spark plug or spark plug wire. Move the wire and then the spark plug to another cylinder and run the cylinder balance test again. If it follows the moved wire or spark plug, you have found the problem. If the same cylinder fails the test again, the injector is bad. If different cylinders fail the cylinder balance test, you have ignition problems or wiring problems in the 10 pin black & white electrical connectors located by the EGR.
How to do a compression test:
Only use a compression tester with a screw in adapter for the spark plug hole. The other type leaks too much to get an accurate reading. Your local auto parts store may have a compression tester to rent. If you do mechanic work on your own car on a regular basis, it would be a good tool to add to your collection.
With the engine warmed up, remove all spark plugs and prop the throttle wide open, crank the engine until it the gage reading stops increasing. On a cold engine, it will be hard to tell what's good & what's not. Some of the recent posts have numbers ranging from 140-170 psi. If the compression is low, squirt some oil in the cylinder and do it again – if it comes up, the rings are worn. There should be no more than 10% difference between cylinders. Use a blow down leak test (puts compressed air inside cylinders) on cylinders that have more than 10% difference.
See the link to my site for details on how to build your own blow down type compression tester.
