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Diagnosing Engine Noise Under The Hood Of A Ford F-150

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This article is about diagnosing a mysterious engine noise in a 1989 Ford F-150 pickup equipped with the venerable 5.0L or 302 cubic-inch V8 engine following a rebuild.

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Most of us working in the automotive service trades are very familiar with Murphy’s Law and how it affects our shop’s cost of doing business.

Murphy’s Law says, “Everything that can go wrong usually will go wrong.”

When Murphy’s Law takes affect, a single defective part can cost us our day’s profit. A series of defective parts can cost us our week’s profit, especially on a large job like an engine replacement.

If our long-term parts warranty times exceed, let’s say, 5% of our total shop revenues, we find ourselves headed toward bankruptcy.

This article is about diagnosing a mysterious engine noise in a 1989 Ford F-150 pickup equipped with the venerable 5.0L or 302 cubic-inch V8 engine.

It’s not often that I’m called upon to diagnose internal engine problems and, in this case, I wasn’t called as much as I volunteered.

I taught heavy-duty line mechanics during the early 1970s and worked heavy line during my time as a dealership auto mechanic. When I opened my shop in 1977, I did internal engine repairs for over 10 years. So diagnosing odd engine noises is a process that I’ve generally mastered on most applications.

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Engine Number One
Several years ago, the engine in the F-150 was removed at approximately 130,000 miles to address cold-engine piston slap and minor oil consumption. Like many caught up in the current economy, the owner felt that his money was better spent on an engine rebuild than on payment books and hefty registration fees.

Immediately after the newly rebuilt engine was started, it developed a half engine-speed tapping noise. I wasn’t present, but the installer faithfully followed the rebuilder’s instructions by installing a zinc-based oil supplement and keeping the engine above 2,000 rpm for at least 30 minutes to help the new flat-tappet camshaft and lifters break in correctly.

The installer rightfully assumed that one new lifter had some debris stuck in the lifter check valve and that the noise might go away when the debris eventually dislodged itself. As Murphy’s Law might dictate, the noise didn’t go away and, in fact, became worse. A few weeks later, my opinion was asked.

Sure enough, when the engine was accelerated over 1,000 rpm, a somewhat irregular half-speed tapping noise emanated from the passenger-side valve cover. Because the upper intake manifold plenum must be removed to access the right-hand valve cover, we couldn’t physically inspect the valve train.

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Working in good faith, the rebuilder agreed to replace the valve lifters, since that was the most likely cause of the noise. The valve guides, springs, rocker arms and pushrods were also inspected when the lifters were replaced.

As Murphy’s Law would have it, the noise remained in both frequency and intensity.

Several thousand miles later, the intensity of the noise worsened. Because the right-hand valve cover couldn’t be removed without disabling the engine, I used my old Snap-on model 1665 ignition scope to disable individual cylinders.

I immediately discovered an open-circuit spark plug wire on the #3 cylinder, which was then replaced. By using the non-intrusive cylinder cancellation feature, I hoped to eliminate or change the intensity of the half-speed engine noise. Here again, the noise persisted despite canceling individual cylinders.

At this point, I’m speculating that one of the valve seat inserts had come loose or that the engine had developed a valve guide problem. While a compression test revealed normal cranking pressure, a cylinder leak test indicated about 10% on six cylinders. Number 1 cylinder on the right bank came in at 15% and #4 cylinder came in at 20%.

While leakage on cylinder numbers 1 and 4 were higher than they should be for a rebuilt engine, a high of 20% certainly wouldn’t normally be indicative of the damage caused by a loose valve seat insert after 3,000-plus miles of operation.

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The next step was to analyze intake manifold vacuum by attaching a pressure transducer to my lab scope. Here again, I didn’t discover any definitive issues other than a very slight anomaly in the vacuum waveform pattern on #4 cylinder.

All in all, the engine ran perfectly, except for the annoying half-speed tapping noise. Looking into the right-hand cylinders with a borescope indicated that the piston rings had just seated and that no foreign materials were imbedded in the pistons.

See Photo 1.

At this point, I had exhausted my bag of tricks for externally diagnosing the cause of the mysterious half-speed tapping noise.

Failure Probabilities
Nearly any diagnosis involves the probable failure rates of various parts. For example, it’s highly unlikely that two sets of valve lifters would create an erratic half-speed engine noise at 1,500 rpm.

Not that I haven’t seen a half-speed tapping noise be caused by a spun connecting rod bearing or a severely worn valve guide, but it was highly unlikely on a freshly rebuilt engine. So the decision was made to remove the engine and send it back to our local rebuilder.

Fortunately, I visited our rebuilder just as he discovered that the new pistons on #1 and #4 cylinders were badly scored on the thrust skirt, with the non-thrust skirt appearing nearly normal. This wear pattern indicated that the piston had adequate skirt-to-wall oil clearance. This wear pattern also might indicate that insufficient wrist pin oil clearance might be causing the wrist pins to seize, which would contribute to the scoring pattern.

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See Photo 2.

As the situation developed, the rebuilder re-checked the wrist pin bore diameters on the connecting rods and actually added a few ten-thousandths of an inch in diameter to the wrist pin bores on the new pistons. This was done to eliminate the possibility of a warranty return.

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Engine Number Two

In short, the second engine developed the same half-speed tapping noise within 700 miles of operation. At this time, I knew that we were experiencing the full effects of Murphy’s Law. In addition, the truck owner was becoming extremely frustrated with this sequence of events.

I also knew that the engine rebuilder wouldn’t be receptive to a second warranty claim and rightly so.

Because most of the old EEC-IV Ford engine management systems didn’t have data streams, I attached a 60-pin breakout box (Never toss out those old tools!) and performed a complete pin-out test on all sensors and actuators. Everything passed muster except for a minor wiring problem with the air injection diverter solenoid on the air injection system.

At this point, the original installer removed the engine and placed it on a rotating rebuild stand.

See Photo 3.

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