Bringing a Dead Mustang Back to Life

Jan. 1, 2020
It came into the dealership on the hook as a no-start, as yellow as a lemon and actually in pretty good shape, with what seemed to be a good track record. The speedo clock registered enough miles to have circled the planet four times as the crow flie

This dead Mustang had received a brain transplant under the care of an unsuccessful practitioner who had placed his trust in a sack full of 'plain white box' parts

It came into the dealership on the hook as a no-start, as yellow as a lemon and actually in pretty good shape, with what seemed to be a good track record. The speedo clock registered enough miles to have circled the planet four times as the crow flies.

Details on exactly how the Mustang had died were a bit sketchy, but with a minimal amount of vehicular forensics it became evident that someone had performed a brain transplant. The first thing my partner noticed was an aftermarket PCM plugged in and swinging by the harness. When the key was switched on, the engine cooling fan was running constantly. Connecting the New Generation Star Tester to the EEC IV VIP test connector under the hood, he checked for codes and instantly snatched every DTC in the library relating to automatic transmission hard faults. It didn't take a rocket scientist to note that automatic transmission codes were out of place in a Mustang with a five-speed gear stick protruding from the console. First and foremost, the dead yellow horse would need the right PCM.

Resurrection

The PCM was ordered and came in the next day. When the new brain box was installed, the Mustang roared to life and seemed to run remarkably well. Case closed? Not quite! An experimental test run down the highway went well at first, but the hotter the horse got, the worse it ran. At 3,000 rpm and above, the Mustang was bucking, jumping and backfiring out both ends.

Thick film ignition and 'Signature PIP'

The distributor on the '95 Mustang 5.0L is what is referred to as a 'closed bowl' design, with the Profile Ignition Pickup (PIP) sensor receiving voltages from and feeding signals out to a remotely mounted heat-sink-protected ignition module on the fender well under the air cleaner. Like most Thick Film Ignition (TFI) systems, one vane on the 5.0L Mustang is slightly narrower, and the PCM reads this signal to determine which cylinder is number 1, a practice referred to by Ford as 'Signature PIP.' Many, if not most TFI-equipped Fords had the TFI module mounted on the side of the distributor and plugged directly into the PIP sensor, but there were heat transfer problems associated with putting it there. Subsequently, Ford began mounting some TFI modules in cooler places to improve reliability of this hard-working little part. Like the PCM we had already replaced on the yellow Mustang, the TFI module was brand-new and an unfamiliar aftermarket brand. Whoever the erstwhile horse doctor might have been, he or she also had replaced the distributor cap and rotor, but the spark plug wires were original and properly routed.

Being careful ...

Let me digress for a moment to say that, about a decade ago, I worked for about four hours on and test-drove a car that had exhibited symptoms similar to the ones we were experiencing on the Mustang. Without dropping any name brands here (I've found that some are actually equal to or better than OEM), I can categorically say that the new TFI ignition module on that particular car hadn't come from Ford. Because the module was new, I trusted it, to my own detriment. The end of that story was that until I had replaced the off-brand TFI module, nothing I did made any difference as to how that particular vehicle ran. Replacing the module first would have been wise. Rather than putting myself through that torture again, I wrote a new rule in my personal policy book that day: Never trust a strange brand when it comes to electronic parts. This rule includes Hall-effect switches. This is not to say that name-brand parts manufacturers don't ever make duds. However, in a pinch I'm personally more comfortable with OEM electronics or, at the very least, a well-known aftermarket line, particularly since labor time runs well over a dollar a minute in most 21st century auto shops. This Mustang would get a new TFI module before we did anything else.

No cigar

The TFI module was replaced, but the bucking and jumping never changed. Furthermore, with a warm engine we found that it was quite easy to duplicate the problem in the service bay. Holding the throttle up to about 3,000 rpm at no-load produced the same sort of snorting and backfiring experienced during the test-drive. It only took a moment to pop the oscilloscope on, and what we saw was particularly interesting.

At idle, the scope pattern looked just fine, but as the throttle was opened and the engine tached its way into the thousands, a peculiar pattern began to emerge on the left side of the screen. The firing order on this 5.0L is 1-3-7-2-6-5-4-8. At first, the spikes for 3 and 7 would distort while the misfiring was underway, but as speed increased, the pattern became even more interesting. We began losing cylinders 3 and 2 altogether at 3,000 rpm and at the same time those cylinders were gone, cylinder number 7 would exhibit an enormous spike.

Pulling up the memory banks

I saw a problem like this once several years ago on a '93 2.3L Tempo that had come to my service bay through the body shop after having rear-ended another car. Our body shop had repaired the body damage and had subsequently replaced the distributor, which had been damaged in the collision. The vehicle ran well at idle but would drop cylinder #3 as the engine was throttled up. It took a few minutes to figure that one out, and this Mustang reminded me of the Tempo. Tempos went SEFI in 1992, received a cam position sensor to time the fuel injection cycle, and because they immediately lost the one narrow vane on the Hall-effect cup, 'Signature PIP' became a thing of the past for the Tempo. What had happened to the body shop guys was that in those days, the Ford Authorized Parts Remanufacturer listed the same replacement distributor for all Tempos. The result was that the body shop had installed a 'Signature PIP' distributor in a system that was programmed to expect all the Hall-effect vanes to be alike. This anomaly produced a disappearing spark on cylinder #3 exactly like we were seeing here. Since #3 follows cylinder #1 in the Tempo's 4 cylinder firing order, as the timing was advanced by the PCM during the increase in engine speed, # 3 would drop out because of the unexpected duty cycle in one part of the square wave signal. This set of circumstances didn't apply to the Mustang 5.0L, since its calibration still called for 'Signature PIP.'

Back to the Mustang

The secondary scope pattern on the Mustang remained virtually normal at idle, but was consistently snaggle-toothed at higher rpms. Shutting the engine down and yanking the distributor cap for a cursory exam revealed nothing out of the ordinary there. Pulling the spark plugs in question, we noticed that the number 3 plug had some peculiar stains on one side of the ceramic and the electrodes, but that in and of itself meant very little. For the last several years, I have engaged in a practice that may seem peculiar to some techs. I like to measure the resistance between the center electrode of the spark plug and the snap terminal where the wire connects to the plug. The readings I get are usually anywhere from 3K to 6K ohms. If I see a reading higher than 7K, I replace the spark plug, and I have repaired more than a few vehicles that way. In this case, the plugs I examined measured about 3.5K ohms. With the plugs reinstalled and the spark plug wire routing rechecked for possible parallel induction crossfire problems, we restarted the engine and put a spark tester on number 3. Sure enough, the number 3 tower was going dead and dark at 3,000 rpm.

Connecting the primary trigger lead to the tach side of the coil, we reran our 3,000-rpm test again to carry the diagnosis further upstream. What we saw was a virtual copy of the secondary pattern on the primary ignition screen. The coil wasn't even getting signal to fire number 3. This was getting weird. The problem absolutely had to be in the distributor.

Connecting the scope leads to the green PIP signal wire leaving the distributor, we tested again while watching the PIP signal. Instead of a nice even square wave pattern, we noticed that one wave was suddenly becoming twice as wide as all the others at the exact moment that the engine began to skip at 3,000 rpm. We were rapidly closing in on our problem.

A look under the cap with the cup removed

We popped the cap back off, disabled the ignition system, felt the shaft for wear and watched while somebody spun the engine with the starter. The shaft wasn't warped and there was no noticeable wear in the bushings. On this particular distributor, the vane cup that interrupts the magnetic Hall-effect field is easily removed from the top of the shaft, and in a moment I was holding it in my hand. There it was. What I saw turned out to be some pretty nasty signature marks where the Hall-effect magnet had been contacting the vanes, but the vane cup itself wasn't noticeably bent. Looking very carefully at the Hall-effect magnet, it became evident that my previous experience with aftermarket electronic parts applied in this case as well.

The shop foreman had become involved in the diagnosis by this time and made a trip to the parts counter to fetch a name-brand PIP sensor. Comparing it to the existing part revealed a noticeable difference. The magnet was protruding slightly farther into the path of the vane cup on the aftermarket sensor than on the new unit. The mildly egg-shaped vane cup would never contact a properly built sensor, but the aftermarket part was out of line enough to strike the cup and create the misfire we were experiencing.

Parting shot

Let me conclude by saying that there are some extremely high-quality aftermarket parts on the market, and I'm not advising against using those. The hitch is that you get what you pay for. This is particularly true with electronic components where one brand may look exactly like another except for the packaging. And any knowledgeable technician knows to stick with reputable manufacturers.

If the part appears to cost too little or if it comes in a plain white box with no manufacturer name, beware. You may be taking a dreadful chance, especially if the failure of the component in question can disable the customer's vehicle.

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