HELP: '87 runs super rich and dies if it idles lower then 1,900 rpms

Check this out:

Help me create the "Surging Idle Checklist" - Mustang Forums at StangNet

Some helpful ideas in there. I had a problem with mine running rich and not idling very well. As it turns out I had a pretty massive vacuum leak.

Pull the vacuum line off of the fuel pressure regulator and check to see if the line has gasoline in it. If so you need a new fuel pressure regulator.

There are some vacuum lines hidden under the upper intake, you might have to pull the upper intake to see them (not hard to do). If you remove the upper you might as well replace the lines while you are at it.

check the vacuum line to the map sensor. after 2 years you could have stuck sensors or wires that have been chewed on.

BlackGT89 said:

Pull the vacuum line off of the fuel pressure regulator and check to see if the line has gasoline in it. If so you need a new fuel pressure regulator.

There are some vacuum lines hidden under the upper intake, you might have to pull the upper intake to see them (not hard to do). If you remove the upper you might as well replace the lines while you are at it.

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Very good advice, do it first.


You got no codes or an 11? The 11 indicates that the computer passed it's internal diadnostics and can dump codes. No code 11 and you have computer or wiring problems.

87fromMI said:

its had other computers in it to see if thats the problem. Also It was just switched to mass air and it ran even worse, so it when back to speed density.

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You have not answered my question. In order for me to narrow down the list of possible problems, I need good answers. Without them there is very little I can do to try and help you. Please answer: did you get a code 11 or not?

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 voltage drop in the ground cable will cause incorrect resistance readings.

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
which 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. 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.

See http://www.stangnet.com/mustang-forums/749974-computer-issue.html#post7490537 for Joel5.0’s fix for the computer internal signal ground.

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 - Wiring & Engine Info

I agree with jrichker completely, bad grounds can wreak havoc with these cars. I would take them all apart and clean them down to shiny metal ! Also look for any grounds that might be missing.

Next I would check for any stuck injectors. It's not common on these cars but it happens and could be pouring gas down the intake. Check out this thread in order to do that. http://www.stangnet.com/mustang-forums/786225-86-5-0-injectors-not-opening.html

Check to make sure you have the proper injectors. http://www.stangnet.com/mustang-forums/799352-injector-identification.html

If the computer and other parts have been swapped around I would attempt to varify that you have the proper computer for your application. This link might help with that. Your Mustang's Computer Code (1987-2011+) - The Basics at AmericanMuscle.com

In your case I would be tempted to start over with mass air parts and make sure you have the proper computer to match. If you have speed density and you already have a cam in the car, that could be part of your problem. From what I've heard not all cams are speed density friendly.

The mass air computer swap reqires the installation of a Mass air harness pigtail and moving of several pins in the computer harness. Did you add in the mass air harness pigtail or swap the entire harness out for an 89 or later 5.0 Mustang?

See MASS air conversion instructions from Mustang Mass Air Conversion « for more details.

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.

You say there is an exhaust leak.. is that before or after the 02 sensors?