Engine stumble

palacios289 said:

My 1993 5.0 has a pretty bad stumble. I pulled for codes. Got 94 and 44 which is expected as I have GT40p heads. Did the cylinder balance test and got code 60. Checked fuel pressure and noticed my pressure was low and would not hold after key off. Changed fuel pump. Fuel pressure is at 42PSI. checked all cables and plugs and their all good. Cap and rotor is good. Pulled for codes again and I got code 90 but engine still stumbles. Took for test drive and during acceleration on top of the stumble it sounds like a ping. Re ran for codes and I got code 60 again. I am thinking a bad injector on number 6. Anywhere else I should check.

Click to expand...

Replace the spark plug and the plug wire on #6 cylinder before you replace the injector. It's a lot simpler and cheaper to do.

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.

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.


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 $25.

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.



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.

See http://oldfuelinjection.com/?p=85 for some help on cleaning and tightening up the 10 pin connectors.

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.

palacios289 said:

tested the injectors. they all had about 14ohms resistance. All injectors had 12v and tested with NOID light. I still have a stumble and miss. reran for codes and got nothing. Did cylinder balance test and came back with #6. It seems that #6 pings when accelarator is pressed. What is weird is that is started all of a sudden. Can a malfuction PIP cause one cylinder to act up?

Click to expand...

Not unless the PIP shutter wheel is bent. PIP malfunctions usually affect all cylinders and get worse as the engine warms up.