Carbon Knock Noise On Acceleration for Ford Vehicles with 4.0L Engines - Engine Builder Magazine

Carbon Knock Noise On Acceleration for Ford Vehicles with 4.0L Engines

Some vehicies equipped with the 4.0L OHV engine may exhibit an engine noise which might be perceived by the customer as a piston/connecting rod bearing knock. This carbon knock is heard only under load during the drive cycle. Carbon knock is a customer drive-duty-cycle phenomenon that cannot be repaired with an engine exchange. This may be caused by carbon build-up within the combustion chamber.


Verify condition. If normal diagnostics fail to correct the condition, de-carbon the combustion chamber to help quiet the carbon knock noise. Refer to the appropriate service manual for details.

Service Procedure for Installers

1. Use Motorcraft carburetor Tune-up cleaner PM-3.

2. Carbon removal:

  a. Disconnect canister purge linefrom throttle body.

  b. Attach a vacuum line to the canister purge port.

  c. At hot engine idle, allow the engine to ingest 1/2 to 2/3 of a can of Motorcraft Carburetor Tune-up Cleaner. Use caution not to ingest too quickly due to potential hydro-lock issues.

  d. Shut engine off and allow it to soak for one hour.

  e. Start engine, allow engine to ingest the remainder of the Motorcraft Carburetor Tune-up Cleaner.

  f. Remove vacuum line and re-install canister purge line.

  g. Road test vehicle at 3,500 rpm for 2-3 miles.

  h. Repeat above procedure two times for a total of three times.

  i. Change oil and filter.

3. Review’ the customers’ driving habits. The recommended drive cycle should include daily periods of engine operation above 3,000 RPM, such as a brisk acceleration from a stop position. This will break/burn the carbon off the piston head.

4. Ford recommends customer use regular unleaded 87 octane fuel. Mid-grade and premium fuels may increase the probability of carbon buildup, leading to a knock noise.

Some or all of this information was provided by the Automotive
Parts Remanufacturers Association (APRA). For more information on
technical bulletins available through APRA call 703-968-2772 or visit
Here is an example of carbon buildup. Note: not 4.0L engine shown.

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