Misfire at high rpm...help...

I got a faulty cap and rotor from a parts place once for a 5.0. Looked perfect, nothing wrong visually, but I had a high RPM (3k+) miss I couldn't figure out. Replaced the cap and rotor with Ford parts, and the miss went away.

Did the miss start after you changed something?

Have you checked for codes? Just because the check engine light isn't on doesn't mean there aren't stored codes in there...

Good luck!

Jeff

Had the exact same problem. Replaced everything you said, plus the dizzy. Turned out to be a bad coil, and a bad replacement coil from the parts store. Spark should be nice a BLUE, not orangish yellow.

bad ground?

swap the MSD with a stock coil that you have. See if that changes anything,

The same problem that keeps you from pulling codes probably is causing your miss. The signal ground for all the sensors is the same wire that is used to help dump the codes.

Computer will not go into diagnostic mode

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. 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 it 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: 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 connector.


Diagram courtesy of Tmoss & Stang&2birds



See the following website for some help from Tmoss (diagram designer) & Stang&2Birds
(website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine

mine did the same thing at 4000 rpm bad fuel pump and egr valve worth a shot

I corrected the title spelling for search index purposes.

Great information in here. Great work, guys.

my289eatsrice said:

Sorry, it won't let me...

Click to expand...

No worries, I meant that I changed it.