Hello and thank you. Baffled by my Trailblazer

[doctor al]

Old retired mechanic here with a perplexing issue. I have a 2006 trailblazer ext 2 wheel drive with I6. Vehicle has always run fine and been trouble free. However, 3 times it has acted weird as to where it won't run over 55mph. The first time this happened, it turned out to be a bad tire. I have since replaced all tires with new. Now, when pulling my enclosed trailer, (16') it started the same 55 mph max issue after running through a rough stretch of road. (potholes-construction). I have to figure there is some sort of sensor causing this action, but, I can't seem to figure it out. (tow package maybe?) I'm in the middle of moving my stuff from Arkansas to Florida and that means somewhere, my code reader is safely tucked away. Both times this happened, I was able to unhook the battery for a bit and let the computer re-set, but I'd really like to find what sensor is causing this.

 

[Blckshdw]

When you say it "won't run" do you mean the engine stalls out? Or behaves like there's a speed limiter, and won't accelerate past 55 mph?

Do you have any dash lights lit up? Any other strange behavior like vibrations at speed or noises?

 

[TollKeeper]

Sounds like to me that the transmission is overheating and putting the TCM/ECM in limp mode.

 

[mrrsm]

Welcome to GMT Nation...

This problem generally has Two Issues to investigate:

(1) A Clogged CAT.

(2) A Failing (CKP) Sensor [Crankshaft Position Sensor] ...or its Wire Harness Connections.

If it comes down to cases and the (CKP) Sensor must be R&R'd... then a CASE Re-Learn via using a Tech 2 or a comparable High End Scan Tool will be required.

 

[doctor al]

When you say it "won't run" do you mean the engine stalls out? Or behaves like there's a speed limiter, and won't accelerate past 55 mph?

Do you have any dash lights lit up? Any other strange behavior like vibrations at speed or noises?

Acts like a speed limiter. No dash lights and no vibrations or noises. First noticed it going down hill into Birmingham and had to keep pushing the gas to run 55. Downhill!!! most of the time, it'll coast at 65-70 down a hill.

 

[doctor al]

Sounds like to me that the transmission is overheating and putting the TCM/ECM in limp mode.

I have a trans temp gauge and it runs between 180-200. never any higher.

 

[doctor al]

Welcome to GMT Nation...

This problem generally has Two Issues to investigate:

(1) A Clogged CAT.

(2) A Failing (CKP) Sensor [Crankshaft Position Sensor] ...or its Wire Harness Connections.

If it comes down to cases and the (CKP) Sensor must be R&R'd... then a CASE Re-Learn via using a Tech 2 or a comparable High End Scan Tool will be required.

Wouldn't you think either one of these issues would act up at other times? Running back and forth to Arkansas fully loaded but with no trailer, no problem at all. Only when I'm pulling the trailer and then, only when it hits a really rough surface. Scratching my head.
Here's one... borrowed my brothers 6x10 enclosed. Got all the way to Memphis and then bam, 55 only until I pulled over and unhooked the battery for a few about 10 minutes and then no more problems for the next 150 miles

 

[doctor al]

One more thing, I doubt it has to do with the weight I'm pulling. I have no problems towing our boat, although I haven't hit any rough roads with it as there are not potholes or construction between home and the water.

 

[doctor al]

Is it possible there is some sensor somehow connected to the tow package that acts up when the weight suddenly jumps or bounces around? I haven't found any such sensor... but that doesn't mean there ain't one. Us old coots just don't know where to look.

 

[mrrsm]

The clue that draws attention to the (CKP) Sensor has to do with the "Bumpy, Rough Road" phenomena occurring close aboard to these power diminishing events. That strikes me as possibly relating to the Reluctor Wheel that has 60 discreet "Notches"...Minus 2 of them ...cut into its outer orbit to demarcate the position of the #1 Cylinder as it approaches Top Dead Center. This means that each "Point" on the OD of the Reluctor Wheel represents 6 Degrees out of a 360 Degree Circle.

That Reluctor Wheel is made of Mild Steel and in some other engine Platforms has been known to Break Loose and Spin Free of its Weld On Points at the front of the Crankshaft. Since "Timing is EVERYTHING" as we all realize for efficient combustion in each cylinder and the expected power it should produce, anything that might alter either the (CKP) Sensor itself in the Lower Engine Block or even the Reluctor Wheel itself rolling around directly underneath it can affect Engine Power if they become changed in any way.

There have even been some instances where the Reluctor Wheel was simply CRACKED all the way through from its center around the Crankshaft to its Outer Edge. And so the Engine only manifested these Power Loss and Stammering Events with Timing Problems at a High Enough RPM Level to allow the Reluctor Ring to EXPAND away due to the Centrifugal Force acting well enough to change its Outer Dimensional Shape and likewise... its Relationships with the (CKP), an obviously Subtle and Very Frustrating Problem to have to Diagnose.

The PCM relies upon the MAF, the IAT, the MAP, the CPS, the CPAS, the ACC Pedal, the Throttle Plate Position, the Upstream O2 Sensor ...and the (CKP) Sensor ... to allow the PCM to decide on when to Fire the 'Coil Over Plug' Ignition System once inside each Cylinder per each 4 Stroke Event BTDC and provide the Spark Plugs with the necessary Secondary Voltage while coordinating How Much Fuel to Deliver and When it gets SPRAYED by the EFIs ... ALL based upon All of these relationships All behaving as they should every 720 Degrees of Rotation on every single Four Stroke Engine Cycle.

So running over a "Bumpy Road" just might allow a Loose Reluctor Wheel to "Chatter" in position where it rides around on the Crankshaft Base Circle just enough to skew the Reluctor Induced A/C Voltage developing inside of a normally operating (CKP) Sensor while it inductively tries to create 60 Alternating Current Events (minus two) using a Permanent Magnet and a Fine Coil of Wire for signalling the PCM to "TIME" these events in succession... by creating an A/C Sine Wave and alerting the PCM with Variations in Signal Frequency and Amplitude.

One Last Thing...that really is The First Thing the AIR entering the Engine encounters... Replacing a Dirty Air Filter as a possible obstruction during demands for Engine Power and Torque would not hurt...either. (Sez... The Oldest Coot of them ALL...)

 

[doctor al]

The clue that draws attention to the (CKP) Sensor has to do with the "Bumpy, Rough Road" phenomena occurring close aboard to this power diminishing. That strikes me as possibly relating to the Reluctor Wheel that is cuut into 60 discreet "Notches"...Minus 2 of them to demarcate the position of the #1 Cylinder as it approaches Top Dead Center. That Reluctor is made of Mild Steel and in some other engine Platform has been know to break free of its weld on points at the front of the Crankshaft. Since "Timing is EVERYTHING" as we all realize for efficient combustion in each cylinder and the expected power it should produce.

Anything that might alter either the (CKP) Sensor itself in the Lower Engine Block or even the Reluctor Wheel itself rolling around directly underneath it can affect Engine Power if they become changed in any way. The PCM relies upon the MAF, the IAT, the MAP, the ACC Pedal, the Throttle Plate Position, the O2 Sensor ...and the (CKP) Sensor to allow the PCM to decide on when to Fire the Ignition System on each 4 Stroke Exsvent BTDC andprovide the Spark Plugs with secondary voltage while and coordinating How Much Fuel to Deliver and When it gets SPRAYED ...ALL based upon all of these relationships all behaving as they should.

So running over a "Bumpy Road" just might allow a Loose Reluctor Wheel to "Chatter" in position where it rides around of the Crankshaft Base Circle just enough to skew the Reluctor Induced A/C Voltage developing inside of a normally operating (CKP) Sensor while its inductively tries to create 60 Alternating Current Events (minus two) using a Permanent Magnet and a Fine Coil of Wire for signalling the PCM to "TIME" these events in succession.

Looks like the CKP will be the thing to check. Thank you. 👍

 

[Mooseman]

I'm thinking a clogged cat. You could remove the O2 sensor and try it. If power and speed picks up, bingo. And since you say it also does this while going downhill, especially if engine braking which requires more air flow, it could point towards the cat or clogged exhaust.

The only speed limiting thing that I know of on this platform is the REP (reduced engine power) which would turn on the engine down warning light and would certainly store a code or two when it happens and a CEL. This is in relation to the drive by wire system if something goes sideways to prevent runaway engine situations.

If there was a problem with the crank sensor, it would throw a code for it. There is certainly the crank to cam correlation that would at least throw a code. And if the reluctor wheel was loose or broken, it wouldn't run at all, or at the very least, run like hot garbage.

 

[mrrsm]

...and MORE Good News... The 2002-2007 flavors of the 4.2L all sported a "7 Notch" Cast-In Cast Iron Reluctor Wheel triggering 6 variations +1 for finding TDC on the #1 Cylinder and providing Signals for the (CKP) Sensor that is fitted through a Hole in the Lower Driver's Side of Engine Blocks... which WOULD include YOURS.

The 2008-2009 GMT360 4.2L Versions bumped up to the LS Style of the "58 Tooth Reluctor Wheel" (60 -2) as previously described with its particular weakness for occasional failure. So there is no possibility of losing the Correct Timing in your Engine caused by a problems previously explained above for your Engine's "Bottom End": Your Motor's Crankshaft Looks like THIS:

(THREE OF SEVEN CAST-IN CAST IRON NOTCHES ARE VISIBLE HERE):



Problems can develop after a Long Engine Life where the Timing Chain S-t-r-e-t-c-h-e-s, the Timing Chain Guides and Chain Tensioner wears out and the relationship between the Exhaust Camshaft and the Crankshaft positions of the #1 Cylinder at TDC become "very sketchy" with the result being that the PCM is unable to reconcile Engine Ignition Timing with EFI Spray, Compression, etc. ...and so it goes sideways.

Source... Calvin's YouTube Video

PS...

In cases where the Inner Ceramic CAT "Precious Metal Coated Ceramic Honey-Combs" are damaged from excessive "Rough Riding", The Honeycombs can FRACTURE and collapse inside of the CAT container and obstruct the Flow of the Exhaust Stream.

You can investigate THIS by sliding a Piece of Cardboard under the SUV and the lay on it under the Passenger side, right by the Exhaust CAT location. Then with an Ice Cold Engine...VERY GENTLY use a Dead Blow Hammer to 'tap, tap tap' upon the bottom areas of the CAT Can... listening for what sounds like "Pocket Change being Dropped into a Ceramic Bowl". If you can hear THAT sound... it would be definitive for proving this to be a 'Cracked (Baffled) Ceramics' issue.

 

[doctor al]

Well, update time. I removed the ABS fuse and I have had no issues while towing my trailer this last trip. I'm guessing somewhere in the brake system, a sensor was not liking the jostling the trailer was making. I can live with the light on the dash.