Ghost in the machine engine problem.

Hi everyone.



I have a real mystery on my hands. My 2000 Tundra (189K miles, 4.7l, 4wd) started to run rough at idle and when you step on the gas, nothing happens. It starts fine, then when the idle drops down to normal it starts acting like it's not firing on all cylinders. I took it to my neighborhood shop that I've been using for years, and they had it for a few days and are now scratching their heads. Here's what was found:

1. No check engine light. The only fault code was "Slow O2 sensor feedback" or something to that effect.
2. They checked the rear O2 sensors. Working perfectly.
3. They checked the front O2 sensors. Here's where the mystery is:

A: If you unplug both front sensors, the engine runs fine and the truck drives like a champ.

B: Plug in the right sensor, engine runs bad, doesn't register that the left is unplugged, no fault codes.

C: Plug in the left sensor, engine runs bad, doesn't register that the right is unplugged, no fault codes.

D: They installed new O2 sensors and the same scenario as A-C.

E: they checked the continuity/resistance on all pins/plugs, everything normal.

They looked at the mass air flow sensor. No issues. Put in a new one just see what would happen, still no issues.



The only thing we can think of is there is something wrong with the ECU as far as internal circuitry goes. So right now I'm driving around with the front O2 sensors unplugged. This is a real ghost in the machine thing and I'd love to hear if anyone has heard of this before. Please let me know what you think.



Thank you!

 

It could possible the I. A. T. Sensor. Basically a low idle sensor.or something wrong with throttle body. Hard for me to diagnose on the internet.

 

If you step on the gas and nothing happens, sounds like your throttle position sensor, or something on the throttle motor itself. Have someone press the gas while you watch it. Pull the throttle tube if need be.

 

Welcome....

It's important to know the actual code number it reported and if there are any pending codes which means the Check Eng light will not light for pending. What type of code reader are you using ?

Until the A/F sensors warm up and other criteria is met, you will be running in open loop which means the ECU will use base fuel maps and e/G will run like a champ. By unplugging front sensors you'll continue running good but never reach closed loop.

Without heading too far out into left field, A/F and O2 sensors are two trip logic before setting a permanent code. So without knowing exactly how this was tested I'll defer.

What would be important to know is what type code reader is being used and is it capable of showing pending codes.

I'm not the one under the hood, so it's difficult without knowing answers to questions posed.
First reaction is a lot of jumping around the problem that is right in front of the mechanic.

 

My first thought was something related to throttle body, throttle position sensor, which I see others echoed.

But everything Bubba says paints a much less "could be that" or "sounds like that" picture, maybe would help the mechanics.

You know you can take your truck to a Toyota dealership and have them do diagnostics on it, right? With weird stuff like this, if I can't figure it out on my own, that's what I'm usually doing. Let the certified OEM mechs investigate.

Hopefully your mechanic used OEM parts... or you may be adding all kinds of other unknowns into the mix.

 

What brand of 02 sensors were used? Our trucks are picky about o2 sensors, bosch specifically. Use denso sensors.