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Old Ford 5.0 Mustang

Chasing Gremlins

Bench Racing

by Ryan King

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Way back when I bought the 347my first '87 Mustang LX — I attempted to improve the ignition.

It was something I'd done with the Original 351 and I still had the MSD box lying around. Along with the ignition box, I used a hotter MSD E-Core coil, brass terminal cap and rotor, Ford Racing 9mm wires, and a set of Bosch Platinum plugs.

What I got for my efforts was an intermittent rough idle.

That certainly didn't happen with the Original 351 — it smoothed right out with the MSD upgrade.

I chased that mess down for weeks. I pulled the MSD box, I replaced the coil, I checked and re-checked the computer for errors. I swapped sensors. I pulled my hair out.


So, I asked my neighbor, Mike, what his thoughts were. Mike was an ASE Master Technician at a local Chevrolet dealership and he asked me what I'd done, so I told him. He said to replace the Bosch plugs with a set of factory plugs.

I did, the idle steadied out, and she ran like a champ.

Turns out, he'd run into this problem before. It was his experience that some ignition systems that weren't engineered to use the Bosch Platinum plugs didn't run well with them.

Of course, that was a long time ago — at this point, somewhere in the neighborhood of 20 years. So, I don't know if that's still the case or if Bosch has changed their Platinum plug design, but I do know that gremlins can pop up all over the place — even from something as innocent as a spark plug swap.

From that experience — and many more like it — I've picked up quite a bit of knowledge and wisdom involving 5.0 Mustangs over the years and I'll be applying all of it when I go about restoring the Survivor and putting both the LX and the GT back on the road.

Incidents like that are also why I do so much research, like reading Fox Body Mustang Restoration: 1979-1993 even though I've already accumulated a ton of knowledge on the subject. The smallest, most unassuming thing can cause major headaches or even completely derail an otherwise well-planned project.

If you're restoring your own 5.0 Mustang — or any car — I recommend you do the same.

Just something to keep in mind before you start wrenching on your next project — or finish it and find yourself chasing some gremlins.


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