I have a 91 Bronco with 5.0 V-8 that is exhibiting a miss at 1200 to 1500 rpm while driving. The problem has been looked at by a reputable drivability problem expert and has "seen" the miss when engine is connected to a diagnostics scope. It shows up on the scope as one of the plugs firing out of order or firing more than once and out of sync with the normal firing order pattern. Anyone have any ideas?

 

I had a similar problem that drove me nuts for months. It turned out to be the spark plug wires. The 5.0 will get a miss if the spark plug wires are too close together. I think the #7 8 wires are known to give problems. It turned out I had my AC serviced and the mechanic must have moved my wires looking for a leak and after that I developed the miss. I changed every part wires,distributer cap ect. with no change I even had a local garage look at it. Who said it was definitly ignition and recommended the dealer fix it.$80 later the dealer told my wires where routed wrong.

 

Joe,
Thanks for the response. I was aware of the crossfire situation with the Ford 302's and new wires, rotor and cap have been installed. The wires were routed exactly as prescribed by the latest Ford tech. bulletin. The missfire is still apparent and does still feel like a crossfire situation.

Thanks Again

 

Do you have a tach? If so,does the needle drop to zero when the mis-fire happens? You may be having an interuption of the PIP signal which could no more than a faulty connection or damaged/corroded wire.
Phillip

 

Yes, it has a tach. and no it does not go to zero during the miss.

Thanks for responding

 

I have an 87 bronco with the 302. I`ve done the wires and the cap and the miss still occurs. In the case (as mentioned in other mail on this subject)that the PIP signal is flaky, what is a PIP signal and where does it come from? Thanks.

mike

 

The PIP,Profile Ign. Pick-up, is a signal that comes from the pick-up coil,through the module, and on to the proccessor(computer). The computer uses this signal to judge where #1 is at so it can predict when the banks of injectors are to be fired, and also to monitor RPM's ,and probably other things that I haven't associated with a driveability problem yet.The PIP wire is in the # 56 cavity at the computer,and the top cavity of the module with locating tab to the right.Keep in mind that the PIP is another term for the pick-up coil we so commonly refer to as a good component to change every time the module is changed.I have never experienced a bad wire between the module & computer, but plenty of corroded wires from the pick-up to the module which requires a new part,plenty of bad modules even when the engine runs,and only once,a defective computer(as a last resort,i installed a reman. computer and PIP was restored).
Phillip

 

Phillip, thanks for the response. You noted in an earlier message the RPM`s dropping to zero during the miss. I`m noticing the exact opposite, sometimes jumping as high as 4500rpm. Do you still suggest the PIP. If so, is this an expensive fix !!!

 

Oh man,you are starting to make me remember more & more now! I forgot to mention earlier that a faulty tach could also affect engine performance!Now this comes from experience on Chevy engines in airboats with aftermarket marine tachs,but all the principals are the same for tachometers.Back to Ford,the tach signal should still be off the - side of the coil for an aftermarket install,so simply remove the wire for a day's drive to verify.For an O.E. install,the tach signal may come from the IDM circuit(ign. diag. module)which is the first to last wire from the bottom of the module w/ lock tab facing right,leading to the computer with a 22k ohm resistor in line.Could be a bad resistor ? This IDM circuit is also the - side of the coil circuit as well! To disconnect the O.E. coil you will have to carefully study the wire diagrams for your model in order to decide the proper place/wire to snip the wire for a test drive.After that,simplt butt splice the wire back together.
Phillip

 

>well! To disconnect the O.E. coil
>you will have to carefully study
>the wire diagrams for your model

What I meant to write was; "To disconnect the O.E. Tach",not coil.
Phillip