Engine bucking

I have an 1997 Thunderbird with a V-8 engine and 144,300 miles.

There is a bucking problem under a load at low speeds (around 45 mph) and at faster speeds (60 mph). At the lower speed if I hit the gas to go into passing gear, the bucking stops. At the higher speed, if I’m cruising at 65 (with or without cruise on) and let off gas then hit the gas to resume speed, the car bucks. Again if I hit gas for passing gear the bucking stops.

I’ve had the car at different engine repair shops (complete tune-up, wires, filters) fuel injectors cleaned, new catalitic converter with all 3 sensors replaced. Engine oil and filter replaced every 3000 miles. Transmission shop every year for checkup with fluid and filter replacement.

One other hint, if engine is running in very cold weather and before engine really heats up, no bucking even starting up a hill. I’ve changed gas, even using higher octane. Problem exist all year long, differnt seasons.

What else can I do and what might be the problem??

Just a wild guess: does this car have a timing chain? Very rarely they will develop some slack and give these symptoms. Really hard to diagnose. Most will think this is ignition or fuel problem since they are more prevelant.

I am going to take a wild guess miyself, and say it might be a transmission issue. This is a classic symptom of a bad mazda/ford transmission, even though I have never heard of it with the V8. Just something that shouldn’t be overlooked. In the case of the transmission issue I am talking about, I’ve seen them last for thousands of miles without getting much worse.

What does “under a load at low speeds” mean? Accelerating from a stop? Or cruising with a slightly open throttle at slow speeds? Any ideas from the mechanics? No check engine lights?

You can check for erratic timing with a timing light. Make a mark on the crankshaft pulley. Run the engine and take the rpm up to a certain stable setting. Momentarily, release the throttle, press the throttle again, all while observing the timing mark. If the timing mark varies excessively, shut down and turn the crankshaft with a wrench. Observe how much the crankshaft rotates and the camshaft doesn’t. Refer to your repair manual for the specifications for excessive wear.

Check out this Ford TSB 97-9-10 dealing with transmission “shudder”. Mercon V, or Mercon ATF replaces the older Mercon.
Just for funnsies, pull the 5A TCC fuse and have a test drive. If there is a difference, tell your mechanic.

My company’s Ford E-150 work van was recently having symptoms IDENTICAL to the ones you’re having. The problem was just worn out spark plugs. You say you had a tune up… are you sure they changed the plugs?

If the plugs and wires have been changed recently, I might suspect the coil-pack.

***oops… accidentally responded to hellokit instead of sarg. …in case it wasn’t obvious.