2007 Ranger Hesitates under Load when Warm

[matteo_g]

I searched all the threads on hesitation, but I don't think I found any that pertain to my situation, so I decided to start a new thread. Please excuse me if I missed one. Here is my situation.

2007 Ford Ranger Sport 3.0L
Manual transmission.
78k miles.
Excellent shape. No known issues.

Last Saturday, I filled up with mid-grade fuel and then went across the street to the car wash. After that, the truck started hesitating significantly, much more than what I would consider normal "lugging", under load in 3rd or 4th gear, at 1750 RPM and below, with the accelerator pressed to the floor. Once the speed and engine rev up to 2000 RPM and higher, there is no more hesitation. I can reproduce this behavior in just about any gear if the RPM is brought low enough. Higher RPM operation at all speeds, startup, and idle are still flawless. Not a single miss or abnormality, even when revving in neutral. Also, the problem doesn't appear to be quite as bad immediately after cold startup. However, long before the engine reaches normal operating temperature, the problem does start happening again. Therefore, I will describe it as being more prevalent when "warm" and "hot".

I first suspected water intrusion into the ignition module or plug wires, so I drive the truck for a day to thoroughly dry it out, then inspected the wires. There was no sign of water anyway, and the dielectric grease was still in the rubber boot. There were no visible cracks or anything in the coil pack. All looked very good. The fact that there is no missing at idle speed (or all speeds for that matter) I felt supports that the ignition is not the cause of this problem.

I then suspected it was the gas I purchased. I bought and added some STP gas treatment which also claims to remove water. That didn't help either.

Then, I suspected it was a clogged fuel filter since this is a common cause of loss of fuel pressure under heavy load. I don't know when it was last changed, and it was a bit rusty, so I went ahead and did that job per the proper procedures (including depressurizing the fuel system via the bleed on the fuel rail, etc.) All went magnificently well during this Christmas Day fuel filter replacement, but the subsequent test drive then showed no improvement again in the hesitation at low-to-mid RPMs under load.

Now, I'm back again to perhaps the fuel being the problem. It'll take me a while to burn through this tank since I don't put on a lot of miles regularly, so I won't know for a few more days if a fresh, full tank (from another gas station) resolve the issue.

I'm still thinking the cause has to be some sort of fuel delivery issue. Now that the fuel filter is new, I know the fuel pump and its associated pressure regulator could also be a cause. I don't know how if Rangers from my year frequently have such problems though. I hope that is not the case since the fuel pump replacement looks very difficult. (I haven't researched it thoroughly yet.)

Beyond fuel delivery, I'm wondering what else could possibly cause this behavior. I believe that I should have already eliminated ignition. I don't think I have an issue with air intake since my air filter is clean and the mass air flow sensor also appears clean. It seems logical that air intake or MAF sensor issues would cause problems outside of just the RPM and load conditions I am experiencing anyway, but I am not certain of that.

Thanks in advance for any suggestions or ideas.

Merry Christmas to everyone!

 

[matteo_g]

UPDATE: Problem Resolved

Since changing the fuel filter didn't provide any improvement and before going overboard on investigating other fuel system causes, I decided to eliminate several other causes and perform a bunch of regular maintenance tasks since some were probably way overdue. I replaced the spark plugs, spark plug wires, air filter, and I gave the MAF sensor a thorough cleaning with the recommended spray cleaner. I used all Motorcraft parts. I also added dielectric grease to all the spark plug wire connections.

The old spark plug wires, old air filter, and MAF sensor all looked okay. However, the spark plugs themselves were definitely worn and needed replacement. The top portion showed the most wear. The average gap for each old plug was around 0.055". Spec for the 3.0L of my year is 0.044". This was very likely the root cause.

Sure enough, after completing all the work, the test drive showed that the hesitation problem was completely gone. I even drove down the street a short distance at about 20MPH, in 5th gear, with the parking brake on, and the pedal to the floor. The motor lugged like a champ and didn't skip a beat. Note that the same tank of fuel was still in use. Obviously, the fuel fill-up and car wash events from last Saturday were not related to the cause as originally thought. It was just a coincidence that I happened to notice the hesitation issue immediately afterward.