Mystery power loss - 2RZ-FE

This is my first post. After too many diagnostic and repair efforts to count, we've run into a frustrating head-scratcher that even the auto shop couldn't solve. This is my hail mary to revive the truck. If it doesn't work, we part ways at a loss, which would be sad.

Some background specs on the truck:

• 1997 Toyota Tacoma Base 2.4L 4 Cyl GAS
• 2RZ-FE Naturally Aspirated
• I have been the owner since ~65k

Some recent work:

• 193k - Transmission repair (leak)
• 203k - New spark plugs, wires, distributor cap
• 204k - New fuel injectors, deep clean of throttle body
• 210k - New cylinder head, replaced all related gaskets, changed oil and radiator fluid

Since purchase, the car has had two transmission repairs (most recent in 2023). My father in law and I do basically all the maintenance, repairs, and upgrades. The truck saw a couple minor accidents around 130-180k, but no longstanding issues have been noted since that time. We are diligent about changing out liquids and parts, following the recommended schedules.

In July 2021, the car overheated (long desert drive with AC on). I put the wrong coolant in (american formula, not japanese), and kept driving it. In July 2024, I was having trouble in 1st gear/idle. I tried a few things but eventually took it to a couple shops and confirmed that the transmission was fine, but that the head gasket was compromised (coolant in cylinder 1). At that point (July 2024) we bought a refurbished head from CylinderHeads.com (part# CH65115; confirmed correct), and installed it along with top of line gaskets. The car drove fine for a while, but power and fuel economy dropped progressively. Eventually, by May 2025, I had to floor it to go 60mph. White exhaust and tailpipe carbon deposits were observed. The engine developed a medium loud rattle on acceleration.

We took the car in again and confirmed that the replacement head was the source of the issues. We disassembled our work, successfully filed a warranty claim for the bad head (cams were bad), then installed the replacement head they provided, with all new gaskets and fluids all over again.

Within two weeks of the repair (early July 2025), the rattle was back and the power was low again. When the car is cold, it won’t upshift from 3rd to 4th until it warms up. I have to floor it just to go 60 on flat terrain. The truck is okay around town, but it feels like I’m driving a lawnmower when I’m on the freeway. Hills are very challenging. In late July 2025, I overheated after dropping down the Grapevine, likely because the engine has to go full throttle just to go 45-50mph on a hill like that. Totally… gutless.

Took it into the shop the other day, after 6 hours of diagnostic work, this is what they found (sorry for the caps):

• CONFIRMED HUGE LOSS OF POWER.
  
  
• STRUGGLING TO ACCELERATE, ALL FLUIDS ARE FULL AND CLEAN. FOUND NO CODES FOUND IN THE COMPUTER.
  
  
• VALVE TRAIN IS VERY NOISY AND RATTLING. ESPECIALLY AT START UP.
  
  
• FUEL FILTER HAS BEEN REPLACED.
  
  
• CHECKED THE EXHAUST FOR POSSIBLE RESTRICTION. NONE FOUND.
  
  
• CLEANED AND ADJUSTED THE MASS AIR FLOW SENSOR.
  
  
• THE THROTTLE POSITION SENSOR ONLY REGISTERS 76% AT WIDE OPEN THROTTLE. TRIED TO ADJUST IT. NO CHANGE. INSTALLED A NEW ONE AND THERE IS NO CHANGE. REINSTALLED THE ORIGINAL.
  
  
• COMPRESSION TEST SHOWS 180 PDS ACROSS ALL CYLINDERS.
  
  
• ACCORDING TO THE CUSTOMER THE VALVE ADJUSTMENT WAS MADE BY THE COMPANY THAT SUPPLIED THE REPLACEMENT CYLINDER HEAD.
  
  
• WE REMOVED CRANK PULLY TO MAKE SURE IT WAS IN THE CORRECT TIMING POSITION. WITH NUMBER 1 CYLINDER ON TOP DEAD CENTER. IT IS CORRECT. THE VALVE ADJUSTMENT AND TIMING CHAIN TIMING SHOULD BE CORRECT BECAUSE OF THE COMPRESSION BEING SO GOOD, BUT THEY HAVE NOT BEEN CHECKED.
  
  
• WE CANNOT FIND A REASON FOR THE ENGINE TO BE HAVING THIS ISSUE

The shop is reputable, with several folks very familiar with Gen 1’s, and at least three mechanics had taken a stab at it by the time they threw in the towel. No one was happy. But at least they only charged for 2hrs of diagnostics…

***DOES ANYONE OUT THERE HAVE AN IDEA WHY THIS CAR IS STRUGGLING??***

All suggestions very much appreciated.

 

Remove your upstream o2 sensor from the exhaust and try it.

Of course you'll have a code for unplugging it and it will be very loud

 

Welcome to TW!

 

Welcome!!

My 2 internet guesses:
clogged cat?
Easy test, unbolt exhaust somewhere b4 cat and test drive with open pipe!
See if bogging stops

remember
Serious overheating with affect both the engine AND transmission
Check/Smell tranny fluid

 

It would be nice to try another ecm.

 

I would hook up a scanner and look at fuel trims and such.....driving/idle

I would smoke test everything........even the brake booster, pinch it off


as far as an ecm....since its auto....those are cheap and plentiful
may not be bad to have a backup......for prosperity
I sure do

 

Basic stuff first. Do you have a scan tool? I would verify that the check engine light illuminates then goes out after the motor is running. Then I would check if there is communiucation with a scan tool, if there is communication are there any codes, present or pending? has the fuel pressure been checked and or the fuel filter been replaced? The fuel filter on the 4 cylinders is kind of difficult so it gets overlooked quite often. Compression looks good.

 

Welcome to TW!

 

Let us know the outcome btw.... another idea I thought of is if possible remove or connect another knock sensor wrapped up and isolated from the world. Still needs to be plugged in, and go for a drive to see what happens

 

Thanks all! I'm going to do a little research and will follow up.

 

Partially Clogged catalytic converter?

 

It’s out of time just enough to kill power.

 

This would be my vote based on the info you've given.