Still having idle troubles

SCORPIO72 said:

i have tryed running self test but when i hook up light nothing happens. check eningine light only comes on when you try to start car. no blinks no nothing with self test with engine light or test light?


thanks to all who may reply......

Click to expand...

My opinion, the above bold needs to checked out first. You can try all the surging idle stuff in the world, but if the ECM is correcting for a certain fault, your pissing in the wind.

So, you need codes first and it sounds like there is a problem in the code return circuit.

Go back to the test light method. Except this time, instead of running the small jumper from the STI to SIGRTN, just run the STI(small grey plug) directly to a body ground. Then try and run the test.

Just do the KOEO test and see what codes show up. Leave the running test alone for now. Sounds like it may be an ECT sensor circuit open. Of course it could be a few other things also

So you know

KOEO = Key On Engine Off

KOER = Key On Engine Running

No info about your car's model year, EFI or carb, no mods listed. Makes it hard to troubleshoot when we don't know what you have.

Therefore, no clue as to which wiring diagram you need. The wiring changed several times between 79 & 95.

That's why it is a good idea to use the sig under the user control panel (User CP) option. It allows you to post your car year & mods, which help greatly when troubleshooting things. No, it is not there for us to snoop and see any "Secrets" you have hidden away under the hood. Be a good stangnetter and update you sig for future reference & don't keep us guessing.

What Diagram you going to give him, thats going to help a 3000 rpm problem while running a Code test?

Computer will not go into diagnostic mode on 86-90 model 5.0 Mustangs

Disconnect the battery positive terminal before making any resistance checks.
The small voltage drop in the battery cables will cause incorrect resistance readings.

Computer diagnostic connector:



How it is supposed to work:
The black/white wire (pin 46) is signal ground for the computer. It provides a dedicated
ground for the EGR, Baro, ACT, ECT, & TPS sensors as well as the ground to put the
computer into self test mode. If this ground is bad, none of the sensors mentioned will work
properly. That will severely affect the car's performance. You will have hard starting, low power
and drivability problems. Since it is a dedicated ground, it passes through the computer on its
way to the computer main power ground that terminates at the battery pigtail ground. It should
read less than 1.5 ohms when measured from anyplace on the engine harness with the battery
pigtail ground as the other reference point for the ohmmeter probe.

What sometimes happens is that the test connector black/white wire gets jumpered to power.
There is a dark brown connector with a black/orange wire near the diagnostic connector. It is
the 12 volt power to the under the hood light. If this happens, it either burns up the wiring or
burns the trace off the pc board inside the computer. That trace connects pins 46 to pins 40 &
60. Only an experienced electronics technician can open the computer up & repair the trace if
it burns up and creates an open circuit.

The STI (Self Test Input ) is jumpered to ground to put the computer into test mode. The STI
has a gray connector shell and a white/red wire Jumpering it to power can produce unknown
results, including damage to the computer. The ohm test simply verifies that there are no breaks
in the wiring between the test connector and the computer input.

How to test the wiring :
With the power off, measure the resistance between the computer test ground
(black/white wire) on the self test connector and battery ground. You should see less than
1.5 ohms.



If that check fails, remove the passenger side kick panel and disconnect the computer
connector. There is a 10 MM bolt that holds it in place. Measure the resistance between
the black/white wire and pin 46 on the computer wiring connector: it should be less than
1.5 ohms. More that 1.5 ohms is a wiring problem. If it reads 1.5 ohms or less, then the
computer is suspect. On the computer, measure the resistance between pin 46 and pins
40 & 60: it should be less than 1.5 ohms. More that that and the computer’s internal
ground has failed, and the computer needs to be replaced.

If the first ground check was good, there are other wires to check. Measure the
resistance between the STI computer self test connector (red/white wire) and pin 48 on
the computer main connector: it should be less than 1.5 ohms. More that 1.5 ohms is a
wiring problem

The following is a view from the computer side of the computer wiring connector.


Diagram courtesy of Tmoss & Stang&2birds

Check out the diagram and notice all the places the black/white wire goes. Almost every
sensor on the engine except the MAF is connected to it.


See the following website for some help from Tmoss (diagram designer) & Stang&2Birds
(website host) for help on 88-95 wiring Mustang FAQ - Engine Information



See Computer issue? - Mustang Forums at StangNet for more help on fixing the computer innards.

If, what J said has happened, and the SIGRTN circuit was blasted with 12v for some reason. Just Turn key on and check the SIGRTN for voltage. It shouldnt have any. If its been blasted it can show up to 5v. And its easily fixable by just soldering a wire in its place and checking the connections.

Only thing I can think of, is that somewhere you had a bad electrical connection, and while you where messing with it, it kinda corrected itself.

Ive seen some intermittent problems where the electrolytic capacitors inside the ECM have went bad and caused all sorts of problems. Let us know if it comes back.

EFI can be nerve racking