98 LTD is having serious loss of power issues. Recently gutted the cat and changed fuel filter when I initially noticed slight loss of power. Soon after a misfire appeared, not sure which cylinder I simply replaced plugs and it went away. However loss of power has continued and become worse. When engine was misfiring it would occasionally throw a CEL which would go away, usually a flicker. Put a scanner on it and no codes and misfire did not show up on scanner even though it was indeed misfiring. Buddy has repair shop and scanner is top end unit. Currently planning to replace both 02 sensors and cat. Also going to check fuel pressure possibly do compression test. My current theory is a lean condition brought on by either bad sensors/false readings from post cat sensor or weak fuel pump. Any other ideas what would cause a hesitating/surging loss of power?

Symptom is is a very rhythmic hesitation/surging with low power and engine accelerating in short rhythmic surges.

Thanks for any advice or help.

Change the Wires, they are probably original and prolly causing the misfire, the gutted CAT is prolly what's causing part of your lack of power, Gas 4runners dont have turbo's and therefore don't produce enough backpressure to sustain full power with a gutted CAT so you end up losing power and making it louder.

I guess I wasn't clear in original post, sorry. The miss went away with the new plugs. Now it's just the lack of power and surging and poor mileage. The more I research the more I'm convinced the problem was a bad 02 sensor to start with, causing lean fuel condition. Old plugs were very white. New 02 sensors and cat on the way. Read on here somewhere Amazon was best price for Denso 02 sensors, good info it's true.

I would second John's opinion that the slight power loss you're experiencing comes from the gutted cat. As explained above when you gutted the cat you've reduced back pressure AND "low end" torque in favor of higher RPM power output. This would have been a good mod for the supercharged engine but not stock.

It's not slight, and it occurred before I gutted the cat. In any event I'm putting new cat and 02 sensors in because it ran fine with that equipment before. Plenty of power and great mileage. Once that is replaced if problem continues I'll begin investigating. As I stated earlier I am pretty well convinced the original problem was faulty 02 sensor.

My brother had both cats replaced, maf, ignition coil, fuel filter, and air filter. Inspected plugs, wires, fuel pressure, and fuel trims. Every thing looks good and is within spec. Also it has a very delayed shift. Trans fluid level and quality is fair. Whatever it is, it's not throwing any codes and we are both pulling our hair out. Any other advice is appreciated. Also noticed this thread is from 2010. Any fixes?

So my brother too his truck to get smogged and they said that the rear cat is illegal for the state of California. So he went and got the correct cat put on. When they were cutting the rear cat off they must have broken loose some debris from the front cat. After a couple days of driving that debris must have come loose and clogged the rear cat. He ended up replacing the front cat with no change in performance. He took it back to the muffler shop a couple times and they told him it was a mechanical issue and that he needed a tune up. So I told him to get a second opinion from the exhaust shop next to my work and he did. They showed him the brand new rear cat was clogged from debris from the front cat. They put a straight pipe on it temporarily and this restored all his power.

I had the issue you described along with a CEL, was a bad ignition coil, but I did suspect the fuel pump/injectors.

I started checking/replacing the easiest things that probably needed it:clean the MAF sensor, replace plugs and wires, replace fuel filter, then coil and fixed.

Good luck!