2005 Tacoma 4.0L Hesitation and No Throttle Response

[swiss]

Truck: 2005 Tacoma DC LB 4.0L with Auto, 70,000 miles, dual Optima Blue Tops using National Luna products, no other major engine/electrical modifications that should relate
Symptoms: Truck has little/no throttle response and hesitation upon acceleration. When symptoms occur the truck feels like it has no power. Biggest symptom though is the little to no throttle response. I can have the throttle at WOT and get no response. Symptoms are sporadic and seem to occur when the truck is cold. Once the truck warms up and/or been cycled on off a couple times the problem goes away but the CEL remains on for a few more cycles.
CEL is on and codes pulled were a P2121 and P2127
Troubleshooting: Took the truck in to the Toyota Dealership and had them run codes and troubleshoot. P2121 is Throttle / Pedal Position Sensor / Switch "D" Circuit Range / Performance. The truck had not had the 90L Pedal Assembly recall completed yet so that was the first step. New pedal assembly and Code 2127 remains. P2127 is Throttle / Pedal Position Sensor / Switch "E" Circuit Low Input. After testing it was found that the voltage was low at throttle position #2. The connectors were removed to check and clean contacts. That did not resolve the issue.

At this point, the dealership recommends replacement of the ECM wiring harness and/or Replacement of the ECM. They say that the only three components to the system are the ECM, Wire Harness, and Throttle Pedal Assembly. I’m still not sold that it’s the ECM or Wiring harness. Both those are pretty big ticket items and I’d hate to replace all that and still have the issue persist. I’m looking for input, ideas, and different approaches from other Toyota owners. Thanks.

 

[SC T100]

If the codes are telling the dealer the TPS (or whatever it's called) is bad, why not just replace that and not the ECM? Just because they "cleaned" them doesn't mean that sensor isn't shot. At the most I would think replacing the throttle pedal assembly would be a better way to go first (if the TPS is integral and can't be replaced seperately). You may want to have another dealer give you a second opinion before dropping a ton of cash into a new ECM/ harness.

 

[keezer37]

The position sensor does not look to be part of the pedal assy. I believe it is behind the instrument cluster and to the right. I would have gone for this were it not for your comment of there being a fault on the input side of Switch "E" and looking at the prints, it is just the sensor - wiring harness - ECM.
What throws me is that it happens when it is cold. I would usually be inclined to go after the sensor for this reason and simply because they tend to have a higher failure rate.
Were it the harness, you'd have a better chance of an intermittent fault. I would normally do a resistance check on the line for switch "E" within the harness but you're dealing with labor cost here.
The loss of power is probably the ECM going into a fail safe mode.

No good suggestions other than to move the harness if possible to see if the fault code goes away.

 

[swiss]

If the codes are telling the dealer the TPS (or whatever it's called) is bad, why not just replace that and not the ECM? Just because they "cleaned" them doesn't mean that sensor isn't shot. At the most I would think replacing the throttle pedal assembly would be a better way to go first (if the TPS is integral and can't be replaced seperately). You may want to have another dealer give you a second opinion before dropping a ton of cash into a new ECM/ harness.

I should clarify, The TPS code went away after the replaced the entire pedal assembly and thus why they didn't proceed with further troubleshooting of the TPS. Currently I only have P2127 which is the low voltage code for the Pedal Position Sensor. The removal and cleaning of the plugs was for the P2127 code to see if it was just a bad contact between the new pedal assembly and the ECM.

The position sensor does not look to be part of the pedal assy. I believe it is behind the instrument cluster and to the right. I would have gone for this were it not for your comment of there being a fault on the input side of Switch "E" and looking at the prints, it is just the sensor - wiring harness - ECM.
What throws me is that it happens when it is cold. I would usually be inclined to go after the sensor for this reason and simply because they tend to have a higher failure rate.
Were it the harness, you'd have a better chance of an intermittent fault. I would normally do a resistance check on the line for switch "E" within the harness but you're dealing with labor cost here.
The loss of power is probably the ECM going into a fail safe mode.

No good suggestions other than to move the harness if possible to see if the fault code goes away.

Which position sensor are you referring to? I am trying to get all the sensors on this vehicle figured out for one. Are you commenting on the Throttle position sensor or the pedal position sensor? The way I understand it the P2127 is for the pedal position sensor which there are two of built within the pedal assembly for redundancy and if the signal from either does not match the system throws a fault.
What sensor are you referring to behind the dash? I don't want to rule anything out just yet. I'm reading the schematics the same way...which is probably why the dealership believes it can only be either the wiring harness or ECM...because those are the only two other components in the system. I'm still not sold on this as I am a skeptic and do not fully trust automobile diagnostics and their computers...especially when it comes to electrical gremlins...I've seen some strange things happen.
As far as I can tell the issue is intermittent and cold related. If I run the vehicle and cycle it a few times it runs normal and reads normal parameters.
Being a digital signal from the sensor I would assume the dealership can test for good signal strength. I have played around with the harness, plugging and unplugging it from either end with no change in results.
I appreciate the input and keep bouncing ideas my way...this brainstorming might lead me to some light bulb theory. Thanks

 

[keezer37]

http://www.ncttora.com/fsm/05+/data/ileaf/06toyewd/06toypdf/ewd/2006/tacoma/h/em01d3.pdf Page 6 to see the schematic of both.

They are calling it "Accelerator Position Sensor (A19)" what I was referring to. There is also the "Throttle Position Sensor" which looks to be on the throttle body (T1).

Both can be viewed here: http://www.ncttora.com/fsm/05+/data/ileaf/06toyewd/06toypdf/ewd/2006/tacoma/g/em01d0up.pdf

TPS - Page 2
APS - Page 5

Check my info, perhaps I'm looking at it wrong.

Perhaps the TPS on the throttle body would be susceptible to damage given it's location.

I would trust the engineers who designed the truck before I trusted the technician. The more self-diagnostics are engineered into vehicles, the less training techs get - opinion.

 

[keezer37]

My bad. I'm relying on pictures instead of the write up on the APS.

 

[keezer37]

Was the APS (APP sensor as the manual calls it) replaced as part of the recall? If so, are they assuming because they replaced it, it's not faulty out of the box?
The manual states to check the APP sensor first. http://www.ncttora.com/fsm/05+/data/ileaf/06toyrm/06toypdf/06rmsrc/rm2006ta/0050051.pdf Page 2

 

[swiss]

Keerez37, Thanks for all your help and and input. It is greatly appreciated.

I am beginning to have a bleak outlook on this.

As far as i can tell it The APS/APP, Wiring Harness, and ECM seem to be a closed circuit of their own. And since the tech is going by the book on this one that's what they are going to trouble shoot. By the sounds of it they feel that since they replaced the pedal assembly(with the APS/APP) that portion of the system is fully functioning. (something I'm not sold on yet)

The schematic of the APS/APP is spot on and confirms that there are two sensors in the pedal assembly and if both sensors are not reading within spec the vehicle goes into limp mode (which I believe is what I am experiencing).

By the looks of it the Throttle Body assembly houses the TPS, which seems to be correct. I had the unit apart in my hand yesterday cleaning the TB. Since I'm not getting any more codes relating to the TB the tech isn't pursuing that. Even though replacing the pedal assembly cleared the P2121 code I'm still not sold that the TB Assembly and TPS are not involved in some way and am considering replacing the unit.

I am not sure how accurate the pictures represent the actual system. I believe you can take those as a general reference of the approximate location of each sensor. For example the TPS is on the Throttle Body and it's hard to reference the exact location of the TB on that picture. And the APS/APP does look like it is located behind the dash when in fact it is part of the pedal assembly.

I dropped the vehicle off at the dealership last night so the could do cold start diagnostics when the issue occurs (even though they have driven and diagnosed the vehicle with the issue present before as well) I told them I want a complete diagnosis and that I want to know without a doubt what the issue is. I am not willing to blindly throw expensive components at my truck in a trial and error method of troubleshooting. Lets see what they come up with.

Thanks again for all the input thus far!

 

[RMP&O]

My 04 gave me some problems like this. It was either the APS or the TPS. I replaced the entire TB assembly (only way you can get the sensors from Toyota) and my problem went away.

I would be looking for a bad ground or maybe a wire that has been grounding somewhere. I had another problem kind of like this all summer. Solved it yesterday, my rear coil had a wire that was grounding out and causing my cam position sensor to get very confused.

Just because it is not throwing codes does not mean everything is A-OK. The entire time my TB was failing the CEL never came on, well maybe it did for about 2 seconds like 2x. The other recent problem, CEL might come on once every 30x the problem reared it's ugly head. The CEL would then go off.

Nearly everytime I have had a problem like this the techs could not solve it, saying you need to change this part or that part but the problem would only go dormant for a few days and then come back after parts were changed. Spending many hours on CT, TTORA and other websites reading and reading and reading is how I have been able to trouble shoot and solve all my problems. By reading a ton I am able to narrow a problem down to just a few things. Go tot he tech, explain and tell them what to look for and BAM, they figure it out and fix it! I find if I just turn them loose and say my truck is not working right, fix it. Well, they 9x out of 10 cost me money but don't solve any problems, sometimes they even create new problems!

 

[swiss]

Solution: IA1; Connector Joining Wire Harness and Wire Harness, Instrument Panel Wire and Engine Room Main Wire (Left Kick Panel)

There is a locking connector at the left kick panel that sits between the PPS/APS and ECM. The connection was not seated properly and locked.

Lesson Learned, when reading schematics pay attention to all the little details and sub notes, don't just assume that one wire goes directly to another location. Simple solution to a problem. Also, don't take the dealers word for problems and actually take the time to troubleshoot potential issues.


Thanks to all those that helped lead to the solution. BIG thanks to keezer37 for all the input! It's good to know that there are competent people out there willing to lend a hand! :beer:

 

[keezer37]

Cool. Glad you found it. Money denied the dealership, victory!

Your experience and RMP&O's comments reaffirms my belief that techs get zero electronics training beyond checking codes and tech manual familiarization.