Intermittent stumble with no code

I've been living with this for a couple of months - seems to come and go every few days. My truck is an '02 Sport with 150 K miles. Had it about three years - only modifications, done by the previous owner, are a hole in the airbox and the noise baffle is removed from the intake tube. I mostly use it on the street.

I got a code this summer to replace the front oxygen sensor (CEL + VSC light)
and had that fixed - this current problem started after that, but not right away, so maybe they're unrelated. The truck starts and runs well and gets decent mileage, but stumbles on light acceleration or on just maintaining speed, but not over 65mph. Usually most noticeable between 20 and 30, or between 50 and 60. Fine under hard acceleration or at 70-75 on the highway. Clicking out of overdrive seems to have no effect.
It happens every few days, and after three visits to my mechanic, he's yet to see it happen, so I'm sort of stuck with it. We changed the plugs and wires, and then the fuel filter, but no change. When it happens it's repetitive - several misses until I speed up. The tach drops about 100 rpm with each - feels like driving into a headwind.
Thanks for any help.

 

My guess, is a bad throttle body. As in either the Throttle Position Sensor (TPS) side or the Pedal Position Sensor side has gone bad. I had some of these symptoms at 200k miles. A new throttle body ($650) which also included both sensors, rectified a MASSIVE number of driveability issues for me. The two sensors are not available from TOYOTA but the TPS is now available from alternate avenues. What happens with positional sensors like these is a wear spot is created where it's most used. For me, the computer couldn't tell where the sensors were so it had hesitation in acceleration from idle. It also had bad surging with the AC on.

HOWEVER! Try the simple stuff. Clean the throttle body! Clean the MAF!

Also, what kind of "hole" in the air box? Is it bypassing the filter? That would be very bad...but I've seen dumb things.

Why people mess with the air intake, is beyond me. Toyota engineers know their shit. And on a low hp 3400...you simply aren't going to make improvements by wrecking your air intake.

 

Thanks for the quick reply. I bought the truck from a younger enthusiast who sent me a link to one of the forums, which I didn't save, where everyone seemed to agree these two mods were effective, and harmless. I couldn't say one way or the other. There's a six inch diameter hole at the front of the filter box with a screened velocity stack about an inch tall out the front - in front of the filter. I cleaned the throttle body and MAF about a year ago because it looked a bit sooty in there - didn't have the issue then. I'll be out there with a can of cleaner tomorrow.
So you have to buy a throttle body to get both sensors? So be it, but can the sensors be tested or inspected for wear? And if I buy a new throttle body, is it just bolt on and go (fingers crossed) or is there reprogramming etc. that needs to be done?

 

Here is a thread I made on another site a while back, pulling this Throttle Body apart.

You can buy the TB and bolt it on and let the car run for 20 min to relearn. Don't forget to get new intake manifold gaskets.

No adjustments needed.

For me, it was worth the money. It was like "BOOM" new car.

 

I looked up TPS replacement - the sensor that is available separately - and may give that a try. Seems it's a common high mileage failure causing several driveability issues.
Why did you decide to replace the whole assembly instead of the $50 TPS? Did you have any failure codes to go by?
Thanks again for the advice.

 

At the time the TPS was not available or I would have tried it.

 

I replaced the TPS with the Aisan one - $42 at Partsgeek. Same as the oem. Cleaned the MAF sensor and throttle body/plate, which was clean. It's been almost two weeks and a couple hundred miles, and the problem hasn't happened since. An easy job, several videos on Youtube.

 

Just wondering if your TPS replacement was a long term fix for your problem, Calal, Thank you!