The Misfiring Ranger Mystery

The Misfiring Ranger Mystery
by Rick Layton – Diagnostics on Wheels

I received a call early one morning from a service manager I had dealt with many times in the past who had just moved to another shop that I hadn’t dealt with before. It turns out that when he switched shops he brought one of the top technicians with him who is a very thorough diagnostician. The new boss greeted them with a mystery misfire that was plaguing a fleet of 2005 Ford Rangers that this shop serviced and it had stumped all of his previous technicians. Knowing how most of the fleet vehicles in our area are maintained; I gritted my teeth and agreed to schedule a visit.
Upon arrival, I was pleasantly surprised to find a clean and well-maintained 2005 Ford Ranger with a 3.0l V6. commented on the condition of the vehicle and the technician explained that the fleet manager for this company got a brand new fleet of trucks all at once on the condition that they make them last. To accomplish this the fleet manager serviced all of the fleet on the severe duty service schedule and only used the best quality parts for repairs. That is why he was so perplexed when the Rangers started developing misfires that no one could solve.

I reviewed the service history of the vehicle, which had 119,000 miles, and spoke with the technician about the tests he had performed on the vehicle. The vehicle had started with an intermittent misfire on cylinder 6 (P0306) about 2,000 miles ago which had progressively gotten worse. Since the vehicle was almost due for its second tune up the fleet manager brought it in and had a full tune up along with a decarbon and injector clean performed. This had no effect on the intermittent misfire at all. Then two other trucks in the fleet started developing the same symptoms on different cylinders. The technician had checked the TSBs and had found that Ford had issued a bulletin 05-26-03 for misfires being caused by valve seat recession. Ford recommend performing a compression test looking for any cylinders whose compression was outside of spec and replacing the cylinder head with a revised unit if a problem was found. The technician had performed a compression test and found all cylinders within 10 PSI of each other with the misfiring cylinder being the lowest on compression. Even though the difference was less than 10% between cylinders, the fact that lowest cylinder was number 6 led him to follow up with a leakdown test. The leakdown test showed all cylinders to be within 1% of each other and the best cylinder was the problem cylinder. Convinced that the cylinder integrity was intact the technician proceeded to check for a vacuum leak, swap spark plugs, spark plug wires, ignition coils and injectors with other cylinders to try to locate the problem; with no luck.

I started by verifying that the P0306 was present and by test driving the truck to duplicate the problem. The miss got better when the vehicle warmed up but was still noticeable at idle. I then decided to verify cylinder integrity quickly by using my ATS EScope Pro with a 300 PSI pressure transducer installed in place of the spark plug to record a waveform of the pressure within the cylinder. This technology takes the dynamic (also known as running) com-pression test to the next level. I started with a known good cylinder and recorded this pattern (figure 1).

I then proceeded to our problem cylinder and recorded another pattern (figure 2).

Right away I noticed that the compression peak in the problem cylinder was lower than the good cylinder and was also below the 50 PSI minimum pressure that Bernie Thompson, founder of ATS, says is necessary for combustion. I was expecting to see something in the 60 to 80 PSI range as is shown in figure 1. How is it possible to have a good compression test and a good leakdown test with a bad dynamic compression result? If you look at figures 3 and 4 you will see that I have blown up the same section of the pattern from figures 1 and 2 for comparison. If you notice in our good pattern (figure 3) that the intake and exhaust pressure pulses are almost even while on our bad cylinder the exhaust pulse is much lower than the intake pulse indicating that we have an issue with the exhaust event on cylinder 6. I recommended that the shop remove the cylinder head for inspection and they found that the valve seat had receded into the cylinder head but was still sealing. A removal of the heads to replace the valve seats cured the misfires on all three vehicles.