I have a 1997 Ford Escort LX sedan with a 2.0L, SOHC engine with split-port induction. I have 137,763 miles on the engine. In the past, I've had certified mechanics replace the water pump and timing belt (on time), and they aren't due to be changed again for another 60k miles or so; I've also had certified mechanics completely overhaul the brake system recently. I've been diligent about changing the oil every 3-5k miles, keeping the filters clean, etc. I've replaced the battery, and had all new tires put on the car. I drive the car, on average, 500 miles per week at highway speeds. The majority of the above mileage has been on highways.
The issue is as follows: at speeds up to about 50-55mph, the engine is fine; I can accelerate and coast without issue. Over 55mph, however, I can keep accelerating - or, indeed, keep traveling at any given speed as long as I keep the accelerator pressed - without incident; however, if I attempt to coast (taking my foot off the accelerator) the engine makes a horrid "rattling, chattering, metallic vibrating" sound until the speed drops back below 50 or I slightly push the accelerator. As long as I keep the flow of fuel and air moving into the engine, even if it's only a little bit, it's happy enough at any speed; if I attempt to make the engine pull fuel and air on its own, it has a problem.
I asked a friend, who knows engines, what it might be; he thought that it might be the crankarm (or rod) bearings, and said that I should change them out. I did, and while the engine sounds smoother than it has in years, changing the bearings did not take care of the issue above. I examined the oil that I drained out of the engine; there were no specks of metal in it. Nor was there any residue in the oil filter in the engine.
Might anyone know of something else that should be checked? I can't really afford a new car right now, and am willing to get the tools I need to have to do the job unless we're talking about wholesale engine rebuilding.
Thanks in advance for any and all pertinent assistance.
Edited to include: the car is an automatic 5-speed, and if I have any part of the fan/ventilation system on the rattling/chattering is much more noticeable, even at lower speeds. If the AC is on, it's a constant.
Monday, January 9th, 2017 AT 3:32 PM