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Another Hesitation Topic By A New Owner

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Car: 1990 LS400 with TC

Miles: 236,XXX

Dash lights: CEL (right side o2) and TRAC OFF stays on (computer control malfunction)

Amb temp when problem: 80-85 degrees

Problem: Under more than 5% throttle or so the car "bucks" or hesitates. Also noticed the eng cooling fan only operates when I give the engine throttle in Park and comes on proportionate to the amount of throttle I give it. maybe in Drive (cant hear it while Im driving)

Where do I start diagnosing?

The full filler cap gasket is bad - hisses when I unscrew it, can this have anything to do with the problem? I guess I will replace it anyway.

It has a newer alternator but I just read the rectifier could be fused causing problems if the guy that put it in screwed it up?

I am taking it to a friends tonight who is a very experienced tech, I guess we will figure something out, just wanted to see if you "senior" members have experienced this problem and could point me in the right direction.

BTW- Not to start my relationship with this forum off on the wrong foot, but I came from S2ki (owned one) and there are some VERY detailed FAQ and DIY stickies in the "under the hood" section of their boards. Stuff that would help you diagnose issues and a terrific culmination of "common problems". I am yet to find anything other than Prospective buyers stickies on this site. You gotta figure a guy buying a 10-20 year old Lexus is going to plan on getting his hands dirty considering the cost of labor at a stealership?

Thx for reading.

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With the age and mileage of the car, there are multiple issues you may want to consider. Has the car had proper maintenance performed lately? You may have fuel system dirt issues and/or ignition problems. How long ago was the fuel system cleaned? When was the last time the plugs, rotors, caps, wires were replaced? You might start by examining some fundamental routine service.

Also, I am not sure what you mean by the gas cap gasket being bad because it hisses when unscrewed. The hissing is an indication that it IS sealing effectively.

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Clean the throttle plate, tune the motor up (plugs, distributor cap, rotor and plug wires) and replace the pre-cat oxygen sensors then come back here if any drivability problems persist.

You can't reasonbly expect a 200K+ Toyota motor to run well if these essential tune up items have been neglected for years.

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