p2138 code supressed by unplugging MAF

[scottmcphee]

The back story:

 

Full disclosure: I've been driving for months with a p0139 rear O2 slow to respond. Even after replacing it, ECU shows stuck at reading a 0.6v output, always. So there's probably a plug or wiring issue there I haven't chased down. Just living with it. And no cruise control, with a check engine on. Fuel economy unchanged.

 

I can live with that ... but p2138 pops up out of nowhere last week. This is unliveable because it cuts throttle response, and car goes into limp mode.

 

p2138 is a drive-by-wire code whereby sensors in the accelerator pedal (and or maybe throttle body plate angle sensors) are in disagreement, and the car's response is to throw this code and go into limp mode. Limp mode cuts power to the throttle body, so the plate falls to an almost shut position - being held open by a spring - to allow a fast idle, around 2500. In limp mode, pressing on the accelerator does nothing - there is absolutely no response at the throttle.. it's a dead pedal. A dangerous place to be on the freeway / merging, turning left, etc.

 

I'm in the process of diagnosing the cause of p2138. Most people say "new pedal / sensor / assembly" is required because the 2 (redundant) sensors for pedal position are in disagreement. Worn carbon traces, dirt, etc. could be the cause. I beg to differ, the pedal is not the problem... read on.

 

Others say clean the throttle body and check for debris in throttle position sensor module (which is riveted to my throttle body by the way). So I clean that TB and I had to drill the rivets holding the cover of the sensor to get inside it, and clean as a whistle. I see the 2 hall effect (magnetic) redundant sensors in there... no debris, no wear items, nothing to clean. Bolt it back together onto the TB. Reconnect, with a bit of dielectric grease on contacts. No sign of dirt in connector or wear of wires going into connector.

 

Working back at the pedal end of things, I can unscrew the sensor cover and clean the sensor well with alcohol. Clean plug contacts at the pedal. Dielectric there too.

 

Go for drives between each step of this, still throwing p2138. Randomly within a drive cycle or two...

 

I go crazy, and rig the pedal sensor so voltage output of primary sensor is jumpered to voltage output of the secondary sensor. In other words, the redundant sensor outputs in the pedal assembly are now identical voltages for any given position of the pedal. With this setup there is no physical way to have "discrepancy of A / B sensor voltages" from the pedal. (How I jumpered the sensors is covered in another thread).

 

So I drive around with this experiment on the pedal sensor, and sure enough p2138 comes within 20 minutes. Yes, it could a wiring / connection issue between pedal and ECU, I get that.. and will check for that too. But for damn sure, it's not because of discrepancy of A / B voltages from the pedal. So people, replacing your pedal because of a p2138 is NOT A SURE THING that will solve your problem.

 

BUT here's where it gets weird, and the purpose of this post.

 

Somewhere along the way I read the suggestion of unplugging MAF sensor gets rid of the propensity of throwing the p2138 code.

 

So I unplug MAF sensor. Car is now driving fine and stops throwing the p2138 code.

 

Yes it reports other codes for MAF, IAT, sensors not reading right... blinking cruise control light, check engine on... MAF related codes now p0102 MAF too low, and p0113 IAT sensor too high

 

BUT the car has great (normal like) drivability... not going into limp mode. Throttle response to pedal is great, no glitches, no high idle, nothing erratic, ... just normal. This is with the pedal sensors still rigged with primary and secondary tied together! Car is not detecting or complaining anything funny about that experiment as I drive around now with a non-redundant pedal. (I recognize the safety concern here and will restore pedal to normal redundancy soon and when I do I'm pretty sure it won't make a difference while MAF remains unplugged).

 

Why did the car suddenly stop caring about p2138 with a MAF unplugged?? Apparently p2138 could be a red herring code for something else going on. I'm calling this out because people are replacing all kinds of parts in the drive-by-wire chain, throttle bodies aren't cheap... And after the big spends.. p2138 comes back.

 

Anybody got a theory for my case of unplugging MAF makes it go away?? Bad MAF sensor?

(I know the rear O2 is bad - constantly reading 0.6v). Maybe with no MAF input, the ECU runs its basic map that skips over several other operating condition checks... like the drive-by-wire system.

 

I conclude: trusting the throttle position is not as important to the ECU as reading incoming volume of air. That's a scary thought! (Bad software design in the ECU?)

 

Whatever it is... unplugging MAF seems to be a way cheaper solution than (semi-random) replacement of a bunch of parts in the car if you just need it drivable. I will check wiring continuity and pin grasp of each pair of connecting pins in the drive by wire system next... At least I'm drivable while this is going on.

[GEE-OTTO]

I wouldnt drive with the MAF unplugged. With that P2138 you basically have the TB and the pedal assembly. Ive read threads where the owner replaced the gas pedal to fix the issue, have the TB calibrated and replaced the entire TB. Have you looked at the TB lately to see it is caked with oil residue?

[scottmcphee]

Throttle body was cleaned thoroughly, while off the car. Brass throttle plate is shiny and new looking. Throttle throat clean and shiny aluminum.

 

Regarding calibration: I use my SSM tool to watch %pedal press to %throttle opening relationship while pressing the accelerator pedal. Both are showing operation basically as you'd expect over their whole operating range. By the way, the relationship goes non-linear and by the time the pedal is pressed 60% to the floor the throttle has opened to 100%. In other words, the last 40% of pedal is all for nothing except to make you feel good about "flooring it", I guess.

 

Can anyone else confirm or compare their findings of the pedal to plate relationship?

 

Pedal Travel and throttle percentage advancing and decel are smooth, not jumpy.

 

The car drives smooth and proportionately powerful over the whole pedal range, which confirm the pedal and throttle are basically doing what they're supposed to be doing ... Just fine. Idle is fine. Starts are easy. Pedal and real throttle response are great. So why the p2138 code... Half of the redundant sensing is not reaching the ECU so even though primary operation is great, the car jams out and codes on loss of some sensing.

 

But why does it stop complaining about all that when MAF is unplugged? Easily distracted ECU I guess.

[scottmcphee]

Another thread and theory points out that unplugging front O2 is another way to distract the car from complaining about p2138. Some have found that replacing front O2 solves the p2138... And this was done with no complains or codes from the car about the O2 sensor being bad or marginal.

[orndog]

Throttle body was cleaned thoroughly, while off the car. Brass throttle plate is shiny and new looking. Throttle throat clean and shiny aluminum.

 

Regarding calibration: I use my SSM tool to watch %pedal press to %throttle opening relationship while pressing the accelerator pedal. Both are showing operation basically as you'd expect over their whole operating range. By the way, the relationship goes non-linear and by the time the pedal is pressed 60% to the floor the throttle has opened to 100%. In other words, the last 40% of pedal is all for nothing except to make you feel good about "flooring it", I guess.

 

Can anyone else confirm or compare their findings of the pedal to plate relationship?

 

Pedal Travel and throttle percentage advancing and decel are smooth, not jumpy.

 

The car drives smooth and proportionately powerful over the whole pedal range, which confirm the pedal and throttle are basically doing what they're supposed to be doing ... Just fine. Idle is fine. Starts are easy. Pedal and real throttle response are great. So why the p2138 code... Half of the redundant sensing is not reaching the ECU so even though primary operation is great, the car jams out and codes on loss of some sensing.

 

But why does it stop complaining about all that when MAF is unplugged? Easily distracted ECU I guess.

 

 

I recall reading somewhere that there is a coating on some of these and cleaning until shiny takes that coating off, causing problems.

[scottmcphee]

I recall reading somewhere that there is a coating on some of these and cleaning until shiny takes that coating off, causing problems.

 

Sounds like blinker fluid to me, snake oil.

[scottmcphee]

UPDATE: Damn it.

 

Unplugging the MAF did seem to clear up the p2138 ... for a day.

 

Running with MAF still unplugged today the car coded with p2138 from a cold start rolling down our driveway.

 

Maybe it was coincidence that MAF unplugging seemed to suppress this code.

 

One step forward, one step back. Square one on that idea.

[SCASEYS]

http://forums.nasioc.com/forums/showthread.php?t=803788

[scottmcphee]

Update: cleaned the grounding strap points on the engine block (manifold for the ECM electronics, and block for the battery (-) post for starter, and (-) strap to chassis from the battery (-).

 

And... this solved a p0139 (slow to respond rear O2 sensor)... and (drum roll please) for a couple days running now... no recurrence of the p2138 code!! Knock on wood. But I think that has got it.

 

 

The rear O2 is now being read by the ECM correctly and BtSSM tools shows fluctuates of voltage levels matching those observed using VOM on the sensor itself. And it's NOT slow to respond.

 

I believe bad grounding was culprit of both. Rear O2 sensor bad reads, and the drive by wire throttle discrepancy error.

[scottmcphee]

Update again... WELL THAT ONLY LASTED A COUPLE DAYS

 

My ECM has gone "deaf" to the rear O2 sensor values. The sensor is outputting its nice range of values from 0.1 to 0.8 ... but ECM is reporting the rear O2 as a constant 0.6v .... WHEN I KNOW IT'S NOT.

 

What is causing my ECM to be "deaf" to the rear O2 sensor values?

 

A few days ago, was reporting fine after cleaning up ground lugs. Today.. deaf again.

[Regacy]

Most sensor signal lines are terminated with resistive loads in either a pull-up (to Vcc) or pull-down (to Rtn) configuration. Those terminations are normally found within the ECU. An intermittency in the ECU circuit board artwork may have occurred within one of those resistive terminations or a connector contact/crimp is pizzled and it sounds to me like you know how to look for that stuff.

[scottmcphee]

I know what you're saying, and I actually had the cover off my ECM looking for any obvious signs of problems, scorching, cold joints, etc.. I didn't start probing it closely, its all conformal coated inside making probing a little more difficult.

 

I did test the rear O2 sensor socket on the harness up in the engine bay looking for a bias voltage on the sensor input lines there, thinking maybe a pullup is in play. Nothing appreciable with my VOM.

 

The sensor itself is a thermopile that generates between 0 and 1 volt, which is a pretty fragile range to be carrying all the way back through the engine bay to the ECM. I think it's shielded, the schematic says so, but even still... you'd think by now they'd be putting a little amplifier near the sensor.. nope.

 

 

Anyway... it's currently looking like the tool I am using to display ECM values is .. a little funny. I'm checking with the maker. It should be showing me a drop to zero volts when sensor is unplugged, I think, but it's not.

 

If the tool is true and ECM really is stuck at 0.6v on the input, I will disconnect the wiring from that input to rule out the wiring journey through the car ... if the ECM alone is showing 0.6 with nothing attached to the pin, then the issue is internal to ECM. Dreading that...

 

Here I am talking about an O2 issue in a p2138 thread I started! Sorry.