toyota 4.7 1999 OBD code P1121

[BigDz]

I have run into an intermittent problem with the 1999 landcruiser. After I tow a 5000 lb trailer a fair distance; turn off the motor and try to restart it. I get a P1121 code, the throttle will not respond (it is a electronic throttle). Sometimes it starts, other times it will fire then shut down. Not good when you are miles from home in winter.

This code is "Accelerator Pedal Position Sensor Range/Performance Problem". Can anyone help with the diagnosis? I suspect it could be the TPS. However, why will it only happen after I tow? Obviously, I am at a much higher throttle position(this engine just doesn't have the power of american iron). I am concerned that the whole electronic thottle controller may have to be replaced. Some good diagnostic suggestions would be helpful.

 

[The_Jeff]

This sounds like a job for the factory service manual (ouch) as it should provide a logic tree for how the computer thinks on this topic.

Vague generalities to check:

There are most likely two potentiometers attached to the floor pedal. These would be described as "pedal position sensors." The two pots provide redundancy. One might be sending a different reading than the other. You would expect the redundancy for safety reasons. I'd expect it to drop the motor to idle and let it run at that in the event of a perceived malfunction. It'd be nice to have power steering and brakes on a vehicle that large, not stalling on the highway.

In the electronic throttle widget on the engine is a motor and a "throttle position sensor". If the TPS reading doesn't follow the APS the computer gets mad. One thing to check would be carbon build up on the throttle plate. A $3 can of Gumout and a toothbrush can take care of that for you. Could be some other stuff to check, like Mass air flow. Almost time for a scan tool.

When it's cold, there is less room for the throttle plate to move in, so if there's carbon build up it would bind first in cold weather. I'd check that first because it's cheap and easy.

 

[Gary Allan]

I dunno this system, but since it only occurs after you tow, I think you're a victim of one of two things. As you say you use more throttle during the towing event ..perhaps some "learning" event has occured if the towing is over long enough duration. This may be interpretted as a degradation of the APPS. One would think that their would be a internal subroutine for calibration ..much as there is for fuel trims in open loop operations.

or...

that the APPS is infact defective to some degree using that range of use over that durationand the PCM goes into backup mode.

How does this situation resolve itself? I mean you've obviously not been stranded yet. Does it clear itself after a few attempts? Do you disconnect the battery when this occurs?

I imagine that you're not the first to encounter this. There is probably a TSB regarding just this event somewhere.

Why do I fear a future of "fly by wire"??

 

[JohnBrowning]

Service Manual sounds like a good idea!!! If this is an all electronic throttle it has to have a potieometer(sp) on the fire wall that takes the place of the cable that would normaly run to the pedal. The description you gave sound more like that then a TPS sensor. I am just guessing though. Last time I turned a wrench for a liveing throttle by wire was just a fantasy!

 

[Stooge]

Clean the throttle body first. If you have not ever done it, you'll need a can of Throttle Body cleaner, an old tooth brush, and some time (and beer). There will be a "lip" of deposits just beyond the closed throttle plate (top and bottom). You'll have to use your fingers to keep the plate open while cleaning and the deposits will be HARD to remove. You'll be agast on how much black stuff comes off. I also use an old sock (after initial toothbrushing, wiping the area with cleaner works), put it over my hand and wipe inside the bore alot. You're done when you no longer feel the ridge with your finger nail. The engine, when started, will puff smoke and buck for a minute or two. It will also idle faster until the throttle and computer "readjusts" itself. It will idle smoother now.

Also know that the "throttle by wire" system uses an electric motor to operate the throttle plate (the "i" in iForce - it's intelligent?!). This motor does wear-out, work erractically (like yours) and costs a sh%t-load of money to replace (usually at the Toyota Stealership).

Good luck, and happy scrubbing!!

 

[BigDz]

I agre with you guys on cleaning it first. I never cleaned it and it has 105K mi on it. I also have a Blazer without "fly by wire". I clean that one every 30K or so. You can feel it binding. With an electronic set up, you won't feel anything and I can see how it could screw up the ECU and send an error.

To clarify what I did to resolve it:
1- Pray
2- Pray more
3- get out and kick it.
4- I actually found this last time, that it did not want to stay running. ie. it shut down after firing. I tried small applications of throttle and there was no response or increase in RPM's at startup. It would just shut down. (the throttle was probably binding) Then I gave it about 1/2 to 2/3 throttle and it fired, had high rpm, I released throttle, it shut down. Next I gave it throttle again, and basically featherd it at reliatively high RPM and it seemed to clear up. The Check engine light was on but it cleared after the next three starts which means that it resolved itself.

It's too cold to mess with it today, but I am going to clean the throttle body. I really think that it the most likely solution. It makes more sense than replacing the TPS. If the TPS was bad, it would be doing it more often. I will keep you guys posted.