Pickup truck loses power when accelerating onto the highway

Q. My son’s 2014 Ford F-150 3.5L EcoBoost with 135,000 miles develops a severe misfire only when accelerating onto the highway. The truck loses power and can barely be driven until it is shut off and restarted, after which it runs normally. Two repair shops found no trouble codes, replaced spark plugs, serviced the fuel system, and found nothing obvious. Could this be an ignition, fuel, engine, or transmission problem?

A. I would start with another scan of the computer, looking for pending trouble codes. Using a function called Mode $06, a technician can view misfire data to determine whether one or more cylinders are misfiring, even when no codes are stored. The most common causes are a faulty ignition coil, fuel injector, or wiring issue. If the Mode $06 data shows one cylinder with a high misfire count, I would closely inspect that cylinder’s coil, injector, and related wiring. If the misfires are spread across multiple cylinders, it could indicate a fuel delivery issue or, in some cases, the beginning of a timing chain problem. On these EcoBoost engines, I would also want to check for signs of moisture buildup in the charge-air cooler, which can occasionally cause a misfire under hard acceleration. Finally, if the coolant level is dropping with no visible leaks, it may be worth checking for the early stages of a cylinder head gasket problem.

Q. I purchased a used car that now has 75,000 miles on and, I believe, the original battery.  No starting or running problems as yet but I was just curious about something.  When you ask a mechanic or a retail auto parts store to check a battery, what exactly do they do, or more importantly, what should they be doing? 

A. There are many types of testers, such as load testers and impedance testers. The most common and easiest to use are impedance testers which are used by most parts stores and repair shops. At AAA we use a tester made by B2Q which uses a combination of conductance and dynamic load testing. With any battery test the preferred method is to remove the battery cables and clean the posts, then attach the tester directly to the battery. This will provide the most accurate test. In reality, typically most shops and parts stores will just attach the tester to the cables (some will clean the outside of the connection) and perform the test. The test will typically give you the battery state of charge, overall health and cold cranking amperage. Some of the better testing equipment will compensate for temperature. Most of the testers will give the user a result of good, marginal or replace. If you are a AAA member, as part of your membership AAA will come out and test your battery. 

Q. I had an issue recently with the check engine light activating. Shortly thereafter, the light turned off. I dropped the car off at my mechanic, and he found the engine was low on oil. Upon further investigation, the oil had turned extremely viscous, almost like molasses. The mechanic applied an oil treatment and refilled the engine with oil. The mystery is that I had recently had the oil and filter changed at a large chain tire store. I supplied a Nissan oil filter and Mobil 1 0W-20 oil that I had purchased at a local wholesale club. What could have caused this? Could it have been defective oil? Your expert opinion is greatly appreciated.

A. I have purchased oil by the gallon for years and have never encountered defective oil. I would say that defective, name-brand motor oil is extraordinarily rare. If the previous oil change interval was significantly overdue, the fresh Mobil 1 may have cleaned some accumulated deposits from the engine, but that alone would not typically turn the oil into a molasses-like substance. This makes me wonder if the oil was actually changed or filled to the proper level. Another possibility is contamination of the oil from an engine-related issue, although that is less common and something I would have thought your regular mechanic would have looked for. I would change the oil and filter again in 500 to 750 miles, or about 30 days, to help remove any remaining contaminants and the added oil treatment. I would also check the oil level weekly out of an abundance of caution and monitor it for any unusual changes in color or consumption.

Q. I own a 1999 Jeep Cherokee with a 4.0L engine and automatic transmission, 4-wheel drive. It has always been well maintained and a very reliable vehicle. Here is the problem. Just recently, as I drive and accelerate normally, the Jeep actually “stalls,” for lack of a better term, for a fraction of a second between each transmission shift. Then all the dashboard warning lights illuminate, again for that fraction of a second, and then it shifts to the next gear. Strangely, after driving for a few minutes, the problem seems to resolve itself. Of course, I have checked the transmission fluid. I have been told this might be the torque converter or transmission, but no actual diagnosis has been done. I am not the most mechanically inclined, but I am thinking this problem might be electrical. Sorry if this description is a bit vague. Any thoughts would be helpful?

A. I don’t believe the transmission is the most likely cause. The way you describe the problem sounds more like the engine is momentarily cutting out rather than a transmission issue. A failing crankshaft position sensor is a common cause of intermittent stalling and loss of power on the 4.0L Jeep engine and is where I would begin my diagnosis. I would also inspect the wiring connector to the sensor for corrosion or damage. Since the dashboard warning lights come on when the problem occurs, I would also want to check the battery connections, engine grounds, and ignition switch for any intermittent electrical issues. Even if the check engine light is not currently on, I would scan the computer for stored or pending fault codes. Based on your description, I am leaning toward a crankshaft sensor or related wiring problem rather than an internal transmission or torque converter issue.

John Paul is AAA Northeast’s Car Doctor. He has over 40 years of experience in the automotive business and is an ASE-certified master technician. E-mail your Car Doctor question to [email protected]. Listen to the Car Doctor Podcast on Apple Podcasts or other popular podcast sites.