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ENGINE – 4.0L OHV – CARBON KNOCK ON ACCELERATION NOISE – CARBON KNOCK ON ACCELERATION – VEHICLES EQUIPPED WITH 4.0L OHV ENGINE ONLY FORD: 1990-1997 AEROSTAR 1990-2000 EXPLORER, RANGER ISSUE Some vehicles equipped with the 4.0L OHV engine may exhibit an engine noise which may 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. ACTION 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 following Service Procedure for details. SERVICE PROCEDURE Use Motorcraft Carburetor Tune-up Cleaner PM-3. Carbon removal: a. Disconnect canister purge line from 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 Gleaner. f. Remove vacuum line and re-install canister purge line. g. Road test vehicle at 3500 RPM for 2-3 miles. h. Repeat above procedure two times for a total of three times. i. Change oil and filter.
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