Dr.Phil

I've been an unregistered "lurker" in this forum for years and consider it to be a first rate resource for Toyota Tacoma information.
This is my first post and I'm hoping that I can get some insight on what the issue with my Tacoma might be.

Vehicle Info:
2002 Toyota Tacoma SR5, TRD
3.4L V6 Engine
Truck has Factory Alarm & Cruise Control
No modification done to Truck other than the audio system and K&N Replacement Air Filter.

Problem:
When accelerator is depressed, RPM increases to about 2,500 and then it will sputter.
If throttle is opened all the way, engine will backfire through the intake.
If fuel return hose is restricted, the problem goes away.

Relevant History:
Truck has had check engine light on for quite some time now but runs fine and gets same gas mileage. (19 mpg average)
Error Codes present prior to new problem arising = P0171 System too Lean (Bank 1)
Current Error Codes = P1135 Air/Fuel Sensor Heater Circuit Response. (Bank 1 Sensor 1)
Immediately prior to issue presenting itself, I was troubleshooting and eventually located a cross short between the lighting and brake light circuit.
This cross short occurred inside of the dual filament bulb inside the tail light cluster.
(One filament blew and subsequently welded itself to the post of the other filament.)
The cross short caused the brake lights to come on when the head lights were energized.
It also caused the instrument lights and the marker lights to become energized when the brake lights were activated.

Troubleshooting Steps already taken:
In accordance with Toyota Factory Service Manual DI-242 (Problem Symptoms Table), I started troubleshooting the fuel system by following DI-448.
While doing so, I noticed that if I pinched the fuel return hose while manipulating the throttle butterfly, the acceleration symptom would go away.
This led me to believe that the fuel injection system was not getting enough fuel pressure to operate properly.
Starting with the easiest item to eliminate, I replaced the fuel filter. (No Joy)
The next logical culprit in my mind was the fuel pressure regulator.
I needed to replace the valve cover gaskets anyway, so I changed the regulator. (No Joy)
The only items left in the fuel system were the fuel pump and all the plumbing.
I replaced the fuel pump and the hoses between the fuel tank & hard lines. (No Joy)

As most of you know, the service that I have already performed is quite time consuming and has parts that aren’t exactly cheap.
So, I come to you all with my heart in my hand, hoping that one of you is able to help me resolve my problem.
My thoughts at this point in time is that I either have a partial obstruction in the fuel system or an electrical / electronic problem that is preventing the fuel pump from putting out the necessary fuel pressure or the ECM from operating properly.
Since the symptom was not present prior to working on the electrical problem, I’m inclined to lean toward an electrical / electronic issue.
To take the electrical wiring to the fuel pump out of the equation, I disconnected the power to the fuel pump and connected a “Jump Box” directly to it via the BN7 Connector. (No Joy)

Since replacing the ECM is quite expensive and also not a returnable part, I want to exhaust all other possibilities prior to going down that road.
My next steps are to hook up a fuel pressure test set to determine if the issue is Fuel Pressure / Volume or Fuel Injection / ECM.
If the test set indicates a low fuel pressure condition I will focus on the wiring going to the fuel pump.
If the pressure is within limits, I will check for proper fuel volume.
If fuel volume is out of limits than I will look for fuel line / hose obstructions.
If fuel volume is within limits and fuel pressure is within limits then I will conclude that the ECU is causing the problem by telling the fuel injectors to dispense the incorrect amount of fuel into the intake.
If it is determined that the ECU is causing the condition, I will use a multi-meter and O-Scope to test the relevant sensors and replace any sensors that test out of limits. (MAF, FA, and O2 sensors)
If error condition still exists, replace ECU.

Any thoughts or suggestions are much appreciated.
Cheers,
Phil

Jomoka

You should try measuring the fuel pressure directly, before throwing too many parts at it.

Sounds like a lean running situation, based on the fact that pinching the fuel return line and spiking the fuel pressure up temporarily helps.

Given the replaced parts - it's unlikely to be a fuel pressure issue. If the pump wasn't putting out enough fuel, or the filter was clogged - pinching the return line wouldn't help (the FPR whouldn't be letting fuel through until it got up to the regulated pressure). If the FPR was bad - replacing it should have helped.

Kind of gets back to the ECU's determination of how much air is going into the motor. Which is negatively affected by a bad or dirty MAF, and/or any significant air leaks in the intake system after the MAF (allowing unmetered air in that doesn't get appropriate amounts of gasoline).

I will say that when I first bought my car the previous owner had stuck an over-enthusiastically oil K&N oil filter on it - the resulting oil flowing through the intake fouled the MAF, and it had developed a very slow throttle response and popped up O2 sensor codes occasionally - basically the MAF was reacting very slowly and when the throttle was abruptly changed it would result in lean or rich spike, depending on which way. K&N's aren't great ideas IMO - for cars with hot wire MAF sensors.

Dr.Phil

Thanks for the reply!






I had recently "refreshed" the K&N air filter, so I will clean off the MAF and test the system with the filter removed.
That should either solve the issue or eliminate it from the equation.

Dr.Phil

Thanks to the suggestion from Jomoka...

I was able to solve the mystery for under 10 dollars and less than 10 minutes labor.
Mass Airflow Sensor...
Two screws N' a good dousing with MAS Cleaner and life is good.

The truck was down for several weeks when I was troubleshooting the electrical short and I forgot that I had "recharged" the K&N filter at the same time that I first started to work on the truck.
I had started the truck and it ran fine, but I had not driven it while troubleshooting.
Fast forward, $200+ in parts and 4 solid days of labor and I can't figure out whether to jump for joy or cry.
Oh well, I needed to change the valve cover gaskets anyway and after 183K, it can't hurt to replace the fuel pump with a nice upgraded Bosch pump.
Hopefully this post can serve as a reference to some poor soul having a similar problem.


Have a Merry Christmas everyone!


Cheers,
Phil