1991 Toyota Previa

Engine Mechanical problem
1991 Toyota Previa 4 cyl Two Wheel Drive Automatic 205000 miles

Van is well maintained. Wife was driving car with 800 lbs cargo, following me in UHaul truck from Tucson to St George, UT. After 3 hours, running 65-75, wife called me and asked to slow down to 65-75 as engine was struggling. Stopped at 4 hours for gas, van was " tapping" at idle, like a stick had gotten into fan belt, or a loose part. Noise stopped soon as off idle, accelerated fine. 3 hours later, no more noise, running fine at 65 mph. 2 days later picked up UHaul trailer to drive to Salt Lake City. After 30 miles at 70 mph, saw smoke behind van, stopped and so did smoke, but tapping noise returned. Canceled trip, returned home with no trouble. Wife drove to store next day, and reported tapping sound at slow speed in parking lot. We are 600 miles from our great Toyota repair folks, and know no one in St George. Any idea what this might be? Oil level is still full.
August 30, 2009.

Does the noise seem to be coming from the top of the engine? Is the oil clean? When you hear the noise, was the engine temp normal or running hot? What color was the smoke from the exhaust, white, black, gray? Was the oil pressure good when this was happening?

The smoke was gray, stopped when I stopped and idled. Have not seen more smoke. Oil was clean and 2000 miles old. Unable to decide where noise was from. Never over heated; no oil pressure gauge. Today I followed a solution of possible sludge in oil, possible as van was unused most of last 3 months, and driven only once/twice a month for 20 mile shopping trip last 2 years. Drained oil, filled with ATF, ran one hour, drained and refilled with oil. The only noise I heard sounded like a rattle from the exhaust pipe or header.

Aug 31, 2009.
I never heard of using ATF. That is interesting. As far as the engine, has it made any noise since? Also, the " gray" smoke concerns me. Have you had to add coolant to the engine?

Very odd. I think the van needs to be retired to just around town. No further problems, but wife is driving 20 miles today to get new registration, so let's hope all is well.

Thanks for your help.

Sep 1, 2009.