Q: I have a 2002 Jeep Grand Cherokee with a 4.7-liter, eight-cylinder engine and 69,000 miles on the odometer. At speeds above 70 miles per hour under load, the engine starts pinging. For most local driving it is not a problem, but on the highway it is very annoying. I have tried higher octane fuel, but that didn’t fix the problem. I have just had the Jeep serviced, cleaning fuel injectors, throttle body, intake valves and ports, which also removes combustion chamber deposits, thinking that might fix the problem. It didn’t. What do you suggest for the next attempt to solve this problem?
A: What has to be done is a road test with diagnostic recording gear connected. When the pinging begins on the car, the technician only has to hit the record button on the diagnostic tool. After the tech gets back from the road test, he can review the recording frame by frame and find the root cause of the problem. The tech will be looking at items like fuel pressure, map sensor readings, timing advance, EGR percentage, and knock sensor action. Another very important reading is going to be short-term and long-term fuel trim as well as the oxygen sensor readings. A trained technician will need a few hours to perform these tests, but your results will be accurate. I do find it curious that you never mentioned a check-engine light being on. The Jeep computer should have picked up the malfunction if the knock sensor is not working. Just so you know, you have a very popular Jeep, and it is capable of giving you 200,000 miles. I am sure you know the pinging is harming your engine. Therefore, I encourage you to stay with the pursuit of having this problem professionally repaired.
Car Care Tip: When a repair shop asks to keep your vehicle overnight, be glad they are taking the time to check and recheck the repairs.
Larry Rubenstein is a master technician who owns a North Shore service station. His column appears most Tuesdays. Write to Larry at the Gloucester Daily Times, c/o Auto Scanner, 36 Whittemore St., Gloucester, MA 01930, or send an email to email@example.com.