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2003 Tacoma Summer Mystery

My 03 Tacoma 3.4V6 (5vz-fe) 4x4, manual transmission loses power and has difficulty starting from idle when there is high ambient air temperature 80+ degrees–I live in Albany, NY so that only happens in the summer. It never stalls out and always idles fine, but when you give it gas in 1st gear it often stumbles and hesitates and it continues intermittantly through subsequent gears. If you are driving at speed on the highway and are trying to maintain a steady speed, it will spontaneously lose power and the tac will start dropping toward 1000 RPM. Giving it gas at this time doesn’t do anything. If you put it in neutral, the car will coast and the engine idles fine but hitting the gas in neutral does not usually increase the tac. After the tac/speed decrease significantly you can give it gas and the truck will respond and pick up speed, usually until you try to level off the acceleration when the whole thing will start over again.

EXCEPTION: If you rev the engine high in first gear and start it as if you were going to peel out, it won’t hesitate. As you go through the gears, if you are always accelerating, it will usually be fine. On the highway, if I keep it in 4th gear and keep the rpms up or the truck is generally accelerating, I can prevent the problem from happening as frequently, but this does not work all the time.

The key here seems to be the fact that it only happens when there are high ambient temperatures–it won’t happen early in the morning or in the evening or at all fall through spring.

FACTS and REPAIRS:
It happens regardless of whether the A/C is running.
My mechanic has hooked the truck up to an OBD scanner while the truck was experiencing the problem and NO CODES come up.
The Air/Fuel Ratio sensor was slightly out of range and it was replaced.
Just changed plugs, wires, coils, fuel filter.
Checked the PCV and it is not stuck (though the vacuum hose was a bit cracked at the fitting and I will replace it today).
Cleaned MAF.
Checked Air Cleaner hose, no cracks, leaks, no blockages.
Throttle Body is clean.

Catalytic converters (TWCs)? Some online forums seemed to point to this as a possible cause. The three main parts of the exhaust system after the down pipe from the manifold are rusted at the flanges and bolts/nuts, but the pipes are generally solid, no holes, all aluminum parts of the TWCs are in-tact, heat shields look good. First question: Would a faulty/clogged TWC manifest itself only during hot weather? Second question: Could the TWC be faulty/clogged if the pipes around it are all in-tact?

Thanks for the underwhelming response… Figured it out. http://www.arfc.org/complaints/2003/toyota/tacoma/vehicle_speed_control/problem.aspx

Needed a new TPS, which meant a new throttle body.