Before you do anything else, verify that the air filter is good and not clogged. Verify that the throttle cable is adjusted properly and opening the throttlebody to 100%. If not fix throttle cable or replace until you do get throttlebody to open all the way.

Then check spark plugs, if they have more than 25,000 miles replace. If the gap is excessive also replace. Replace them with the correct ones and nothing but the correct ones, Wires also. Go by the Toyota ones as the wires already come with the right clips in the right spots.

Compression test hot and cold while you’re there with the throttle open and all the plugs removed. Is there any deviation in the numbers from cylinder to cylinder and hot cold that’s out of spec? If there is check valve clearances and or cylinder leak down.

You very well could have a transmission problem. Until you verify that the engine is running like it supposed to you’re wasting your time in my opinion.

My 99 had miserable drivability initially due to previous owner neglect. New plugs, wires, and and an adjusted throttle body to the correct position created roughly an 87% improvement. So much so that I had to readjust the transmission kickdown cable as the shifts were way too firm.

Check plugs and compression, don’t assume it’s good. Also swap out the transmission fluid it has been a while. Once those are done then you can begin chasing transmission issues.

Both of you have my same problem. Pressing the pedal to floor doesn't change RPMs or the noise the engine should be making when you are trying to accelerate.

Your comment about the O2 sensors are interesting since I've been chasing a P0420 issue for about 2 years. Have replaced both front and rear sensors and still have the CEL light on. My exhaust actually split in half between the 1st and 2nd cat when I was driving to the beach and was laying on the frame cross member's due to rust. Had a shop weld it back in place and they defitnely missed a spot because I hear a slight hiss that sounds like gas escaping through a hole between the weld. Maybe installing that computer mod that spoofs the O2 readings would do the trick.


Air filter was replaced at 176k. Spark plugs at 171k. I'll take it to a shop for a compression test.

The oxygen sensor simulator you are talking about is only for the rear oxygen sensor. Rear oxygen sensor is strictly for monitoring catalytic converter efficiency. Any exhaust leaks before or shortly after that sensor will affect its readings.

Your due for another set of plugs.That’s an every 30k thing. You need to swap tranny oil also. Lastly the air filter. That’s a long time on an air filter….. seriously. They’re 16$. Takes 20 seconds to change……

If you do any amount of of road, your way past due.

I helped to clean out an air filter on a 99’ powerstroke about 20 years ago. It wouldn’t even do 55 mph on the interstate. The owner, a guy who I really looked up to at the time, kept cussing it. Damn governor…… yada yada. I finally said let’s check the filter minder. Pulled all the way up. Banged out the air filter on the off-ramp. Left a hilariously large pile of dirt. Boom, power restored. 75 mph uphills with ease then.

All in trying to say it’s that pulling the well I changed this at X miles doesn’t mean anything. You Could have a nest full of leaves and garbage in front of the air box from rodents for all you know. Go verify, mileage doesn’t mean squat.

The P0420 will kill your gas mileage and power, pretty much every Check Engine Light will. It's seeing air in the exhaust and trying to tune it out. Have a shop weld it up or replace the offending section and power should start to come back.

There might be a dead spot on you tps if you are not getting any response from the throttle. When you are manully down shifting into 2nd, the transmission is already in 2nd and that's why you don't get a rpm spike when you move the automatic shifter from D to 2.
In my experience the TC locks, and the rpms will drop. Once you push the pedal to the floor the TC unlocks and the rpms go up. With the power loss at that altitude specially Floyd hill, Eisenhower pass there is just not enough hp in the motor. My truck down shifts to 2nd the rpms go up and doing about 60 mph even on 7% incline rpms at about 3500 to 4000rpms. That's just normal for 20 year old motors.

Well but see everybody keeps saying that that's normal, it's only 180 horsepower, etc... But I'm starting to wonder if some of that is just maybe people getting used to the lower amount of power and just thinking that it's normal?

I mean I've literally been passed by another third gym pulling a big trailer uphill in the mountains, there was literally no way I could keep up with him. I mean I actually try because I was thinking well hey if he can do it I can too!

Nope. Pedal to the floor RPM's up to 4000+... Not even close, I just watched to see drove off into the distance uphill.

I mean maybe he was supercharged, but again I just have this theory that we're all actually missing a lot of horses then just don't realize. I test drove a lot of these when I was trying to find one to buy years ago and I recall noting that they all seem to have varying amounts of power. Some of them were a lot quicker than others.

I've actually been trying to find a place that could Dyno it so we could literally see the exact number of horses she still has. I feel like that would be the ultimate deciding factor.

Another factor here, one for sure worth discussing is do you have E load rated truck tires on? If you do what are they inflated to?

Because I have to tell you guys, I’m pretty sure my 4runner has more miles than almost all of yours, and mine still averages 19mpg with a cruise set 70 mile an hour on the eight hour haul to Moab. It will do 80, but it’s not happy at 80, it’s super happy at 70 mph.

BUT, my tires are never less than 38 psi for a road haul. At less than 35, say 28-30, the 4runner literally can’t even get to interstate speed. In fact at the nice cushy 28 to 30 for the around town stuff, it can’t hardly even crack 60 to 65 mph without straining to death.

Are your tires inflated to the proper psi for road travel? Not what’s “comfortable” but the actual needed psi? Because I’m here to tell you, with load range E tires you will absolutely slaughter the drivable speed of these machines. Go test it for yourself, find a country Road and run a low pressure and a high pressure run. Seeing is believing on that one. It feels like a difference of 50 to 100 hp.

I have a fresh set of loads C general grabbers just to go further test that one out. My theory is is that these machines absolutely don’t need an E load tire. Took a little while but I found a C rated tire with a 3ply sidewall that seems to be durable enough to survive abuse on the rocks.

Don’t discount tires on this one. My engine pulls 145 psi compression at almost 4000 feet elevation. Under inflated tires will absolutely bring her to her knees on the highway.

P0420 is for catalytic converter low efficiency. It uses the 2nd (downstream) oxygen sensor and does nothing to fuel trims at all. The first oxygen sensor (or air/fuel sensor is what the ECM uses to make changes to fuel trims.

TPS is another possibility. Dead zones do happen. Easy to diagnose also. Plug in your flavor of OBD2 and verify.

Will this work with electrics on but engine off? That would be a bit safer.. haha

I confirmed with my 98 that the rear O2 sensor does adjust LTFT. I have an aftermarket wideband AFR sensor and the drive after I replaced my original rear O2 sensor the wideband was reading a full 1.0 AFR leaner than before I had changed it 30 minutes earlier. That meant my rear O2 sensor was skewing the trims rich. I too was under the impression it was just a cat sniffer but it does more than that.

Yes, key on but engine off. Slowly press the throttle and the percentage should move in a linear manner. Some OBD readers will say something stupid like you're at 140% throttle but just ignore the actual value and watch to see if it freezes at a certain point. Could also try to view in voltage and not percentage.

Also the throttle cable must be near tight when fully closed. A photo of where it sits right now would be super helpful in telling if it's tight enough. Some cables stretch beyond the point of adjustment - mine did that so I couldn't get it fully tight anymore.

I'll stick with what the FSM says.

Yes. Any diagnostic tool or obs2 gauge worth its salt will provide voltage and percentage of the throttle position sensor in key on engine running or key on engine off states.