Toyota Repair: Toyota Previa 1993 acceleration power loss, oil leaks, clutch pedal

Question
1993 Toyota Previa LE Van
5 Speed Manual Transmission
Rear-wheel drive.
Mfg in Japan
340000km


No unusual exhaust colours or smells. No oil leaks. Spark plug tips uniformly powdered black and dry 3 years ago. Transmission and differential oil replaced last year, no leaks detected by the shop.

This problem has been happening for the past 3 or 4 years. During acceleration (from complete stop or while moving), the engine seems to suddenly lose torque. It would pull nicely and suddenly the engine will struggle and the car jerks lightly backwards due to the sudden power loss - it continues accelerating but at a slower rate.

There is a distinct change in engine sound from a consistent smooth frequency increase to a consistent but rough lawn-mower-like sound when this happens, slightly louder but not by much.

The van accelerates normally only if it is done very lightly. Exceeding a acceleration threshold rate of approximately 2km/hour/second will cause the loss of power. During the problematic acceleration, fluctuating the throttle between its current position and the floor makes no difference. Reducing the throttle below threshold increase changes the acceleration back to normal, slightly jerking the car forward.

During the winter and a few minutes from starting a cold engine, the threshold seems to be higher. While cruising, the loss of power is encountered with strong winds, hills. The number of passengers seems to reduce the acceleration threshold. No engine light problems.

Aside, not sure if this is relevant to the main problem: During gear changes, my clutch catches on the top 20% of the clutch pedal before removing my foot. I've tried other vehicles where the clutch catches near the bottom of the pedal swing. I can still start the car from stop on 2nd gear, so I don't think the clutch is slipping. The tachometer and speedometer movements don't show signs of clutch slip. At approximately 2500-3000RPM on idle or while moving, some kind of noise similar to a couple dozen rattling empty soup cans being dragged from my bumper on the street.

Returning to subject, I figured it may be a fuel or air intake problem, but the new air filter and fuel filter replacement did not help. My van has had a previous exhaust issue where the flex hose was cracked and the shop replaced the flex hose and the catalytic converter (5 years ago). The acceleration problem did not surface until sometime after this replacement. Fuel efficiency seems to be the same from 5 years ago, fluctuating around 11-13L/100km.

I suspect that the engine block may turn too much during acceleration that it kinks the exhaust system, causing the torque loss. Maybe engine the mounts are too flexible? I can't check this while driving though.

My questions are, what do you suspect is causing this acceleration issue and how do I systematically approach this repair? I have a spare 1991 Toyota Previa LE (another 5 speed manual) to use if necessary.


Thank you for your time and effort in answering my questions!

Answer
Hello, what I'm going to recommend is that you replace both catalityc converters with the pipe and the muffler, the noise you are hearing is comong from the converters, they are a known problem, when the converters disintegrate they get blocked up an also cause the muffler baffles to break loss and the mufler to clog.