04 TB EXT 4.2 4x4 205,000 miles. Idles high at 3000 rpm and will accelerate by itself as well. It will idle normally if I turn on the AC but when the AC cycles it idles up again. Just replaced the fuel pump and cleaned and tested the CPAS. It ran fine last year until the fuel pump went out and I couldn't replace it till now. I'm planning on doing an engine flush and oil change with synthetic oil and 2 quarts of MMO. PVC service with B12 and throttle body cleaning. What else could it be cause I've done all this before and put maybe 15,000 miles since? Any suggestions welcome thanks!
SOLVED! 2004 TB 4.2 high idle 3000 rpm
- Thread starter Machineman6981
- Start date
Hey Machineman are you reading any codes? Is this high idle something that has been slowly getting worse or did it come on by itself more quickly? In days of old if an engine began to idle high often it was an intake leak. Does the engine idle high both when cold as well as when the engine is fully warmed up? I would start by putting a code reader on it and see what codes are showing.
The CEL isn't on but it has a P0016 and P0014 stored in the PCM. It started this after I replaced the fuel pump. I don't think it's a leak because of how it will idle fine with the AC on. It also will slowly idle down with the AC off but as soon as I shift from park it idles up again. It is also not my brake booster cause nothing happens if I press the brakes in park and not shift. It idles up as soon as I shift out of park.
Mooseman
Moderator
Those codes would point to a bad CPAS, cam sensor or problem with the camshaft phaser. They could affect the idle but not to that extent.
Clean the throttle body and reset the PCM by disconnecting the battery for 30 minutes. That usually fixes idle issues.
Clean the throttle body and reset the PCM by disconnecting the battery for 30 minutes. That usually fixes idle issues.
It doesn't just happen upon start-up it stays this way the whole time I drive. If I don't have the AC on I have to brake very hard to get it to stop due to the 3000 rpm idle. I'm going to try to clean the throttle body and PCV then reset the system and see what happens. Thanks for the suggestions
gmcman
Member
- Dec 12, 2011
- 4,656
I would start with the EVAP hose at the throttle body. Remove that and plug the port at the TB, you can cover it with your finger..see if it idles back down.
Remove the plastic intake resonator and lightly touch the throttle plate....see if it moves very easily. You are looking for a broken tooth in the gears that controls the throttle plate. Should be tight and only move under force....should not have any loose freeplay.
Check the vacuum hose between intake runners 3&4.
Also, check your intake manifold bolts, a 1/4" ratchet works good. Don't overtighten, they are just past snug at 89 INCH pounds.
You are looking for an unmetered air leak.
Did this happen all of a sudden, or after a dead battery or a jump start? Any prior maintenance just before this happened?
Remove the plastic intake resonator and lightly touch the throttle plate....see if it moves very easily. You are looking for a broken tooth in the gears that controls the throttle plate. Should be tight and only move under force....should not have any loose freeplay.
Check the vacuum hose between intake runners 3&4.
Also, check your intake manifold bolts, a 1/4" ratchet works good. Don't overtighten, they are just past snug at 89 INCH pounds.
You are looking for an unmetered air leak.
Did this happen all of a sudden, or after a dead battery or a jump start? Any prior maintenance just before this happened?
This is important to Know:
The GM 4.2L LL8 Engine controls POWER by Controlling and Regulating AIR INTAKE. As long as the PCM reads Excessive Air in the System from the MAF, IAT, MAP, VSS and O2 Sensors, from the Intake Side through to the Exhaust, it will try to make corrections by adding fuel.
If allowed , the PCM will attempt to achieve a Stoichiometric Balance of a 14.7:1 Air to Fuel Ratio. Naturally, this type of situation creates the real possibility of having a “Runaway Engine” if not controlled and constrained.
The PCM programming is supposed to have such Safe Guards by using its various sensor inputs and it also maintains a Memory of the Throttle Plate Position, adjusting for Wear and Poor Performance from being Dirty over time in order to prevent any “Over or Under Performance” NOT intended by the deliberate actions of the Driver.
But if there are any ‘unsealed” areas around or cracks in the Intake Manifold or any Loose Vacuum Hoses, then ‘un-managed air’ will get in and by-pass some of the Sensing Mechanisms the PCM relies upon for Total Air Control.
The relationship between the ACC Pedal Positions and the Opening and Closing of the Throttle Body Butterfly Plate is the Primary Way that the PCM will be able to SHUNT the whole operation into LIMP MODE if anything goes sideways and prevent any possibility of having a loss of control over the Motor.
If your Vehicle is “Accelerating on its Own”, these are the Main Components to be concerned with:
How to Test the Trailblazer ACC Pedal:
https://easyautodiagnostics.com/gm/4.8L-5.3L-6.0L/app-sensor-tests-1
https://easyautodiagnostics.com/gm/4.8L-5.3L-6.0L/app-sensor-tests-2
https://easyautodiagnostics.com/gm/4.8L-5.3L-6.0L/app-sensor-tests-3
Video General Procedures For How to Test the APP Sensor Pedal:
How to Test the Trailblazer Electronic Throttle Body:
https://troubleshootmyvehicle.com/gm/4.2L/electronic-throttle-body-tests-1
https://troubleshootmyvehicle.com/gm/4.2L/electronic-throttle-body-tests-2
https://troubleshootmyvehicle.com/gm/4.2L/electronic-throttle-body-tests-3
Updating the PCM using a Tech 2, GM MDI or a VXDIAG-NANO Scan Tool and the TIS2000 SPS Pass-Thru Technique with the Latest Calibrations would also help as well.
The GM 4.2L LL8 Engine controls POWER by Controlling and Regulating AIR INTAKE. As long as the PCM reads Excessive Air in the System from the MAF, IAT, MAP, VSS and O2 Sensors, from the Intake Side through to the Exhaust, it will try to make corrections by adding fuel.
If allowed , the PCM will attempt to achieve a Stoichiometric Balance of a 14.7:1 Air to Fuel Ratio. Naturally, this type of situation creates the real possibility of having a “Runaway Engine” if not controlled and constrained.
The PCM programming is supposed to have such Safe Guards by using its various sensor inputs and it also maintains a Memory of the Throttle Plate Position, adjusting for Wear and Poor Performance from being Dirty over time in order to prevent any “Over or Under Performance” NOT intended by the deliberate actions of the Driver.
But if there are any ‘unsealed” areas around or cracks in the Intake Manifold or any Loose Vacuum Hoses, then ‘un-managed air’ will get in and by-pass some of the Sensing Mechanisms the PCM relies upon for Total Air Control.
The relationship between the ACC Pedal Positions and the Opening and Closing of the Throttle Body Butterfly Plate is the Primary Way that the PCM will be able to SHUNT the whole operation into LIMP MODE if anything goes sideways and prevent any possibility of having a loss of control over the Motor.
If your Vehicle is “Accelerating on its Own”, these are the Main Components to be concerned with:
How to Test the Trailblazer ACC Pedal:
https://easyautodiagnostics.com/gm/4.8L-5.3L-6.0L/app-sensor-tests-1
https://easyautodiagnostics.com/gm/4.8L-5.3L-6.0L/app-sensor-tests-2
https://easyautodiagnostics.com/gm/4.8L-5.3L-6.0L/app-sensor-tests-3
Video General Procedures For How to Test the APP Sensor Pedal:
How to Test the Trailblazer Electronic Throttle Body:
https://troubleshootmyvehicle.com/gm/4.2L/electronic-throttle-body-tests-1
https://troubleshootmyvehicle.com/gm/4.2L/electronic-throttle-body-tests-2
https://troubleshootmyvehicle.com/gm/4.2L/electronic-throttle-body-tests-3
Updating the PCM using a Tech 2, GM MDI or a VXDIAG-NANO Scan Tool and the TIS2000 SPS Pass-Thru Technique with the Latest Calibrations would also help as well.
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First I want to thank everyone who has taken time to help me on my issue. I changed the oil using 5 quarts of synthetic 5w30 and 2 quarts of MMO. I cleaned the throttle body, IAT, and PCV system with B12 and reset everything by disconnecting the battery and now it idles fine!! I'm not sure what did it but I'm happy!! Thank you all again!
Mooseman
Moderator
That's great however I wouldn't run it with that much MMO for too long as the oil is thinned out too much.
I think it's likely the throttle cleaning and reset that fixed it. Nothing in the oil would have fixed your idle issue except the codes if the VVT system was gunked up. Keep monitoring for codes to make sure they don't return.
I think it's likely the throttle cleaning and reset that fixed it. Nothing in the oil would have fixed your idle issue except the codes if the VVT system was gunked up. Keep monitoring for codes to make sure they don't return.
When you transition back to the (7) Quarts of Mobil1 & either a Mobil1 or K&N Oil Filter... If Time Permits... Could you "Autopsy" the present Oil Filter and let us know what you find on the heels of using the MMO -Motor Oil Mixture?
The thing I'm particularly interested in knowing is whether or not any "High Mileage Black Carbon Mung or Build Up" has broken loose and then gotten lodged inside the Paper Fan-Folds of the Oil Filter.
This stuff to be concerned about often looks like "Large Coffee Grounds" and if sufficiently Large In Size ..it can get ingested and trapped inside of the Under Screen side of the Oil Pick Up Tube.
The Unintended Consequence of using "Loosening" Oil Treatments can be the paradoxical Blocking Off of the Oil Ingestion, suddenly starving the Engine of Motor Oil... so beware any signs of sudden changes or drops in your Oil Pressure for these reasons.
One last issue regarding this is the problem of the GMT360 LL8 4.2L Motors is that the Oil Pressure Gauge on the IPC...is a FAKE Device and the only way to know what your actual Oil Pressure is would be to install a special Accel Oil Port Adapter (Metric to NPT) adjacent the Front Oil Filter Gallery.
Note that the Oil Pressure Sensor Threaded Port in the Passenger Side of the Engine Block is NOT the Same Size as the Illustrated Front Oil Gallery Plug (M16 X1.5MM) where the Accel Adapter Threads In after removing this Aluminum Hex Plug:
That carefully installed item gets used in combination with installing an Analog Oil Pressure Hose and Gauge Combo so you can directly observe what the Gerotor Oil Pump actually manages to produce for adequate Oil Pressure while pumping just over 11 Gallons Per Minute when working properly. Nominal Oil Pressure for this Motor is 12 PSI at 600 RPM at Idle and around 65 PSI at 3,000 RPM.
I know this seems a bit excessive... but you've introduced a change in the *lubrication environment* deep inside of that Engine now that will deserve your undivided attention over the next few Oil Change Cycles until you know there won't be any "Oily-Under-Pressure Surprises"...
The thing I'm particularly interested in knowing is whether or not any "High Mileage Black Carbon Mung or Build Up" has broken loose and then gotten lodged inside the Paper Fan-Folds of the Oil Filter.
This stuff to be concerned about often looks like "Large Coffee Grounds" and if sufficiently Large In Size ..it can get ingested and trapped inside of the Under Screen side of the Oil Pick Up Tube.
The Unintended Consequence of using "Loosening" Oil Treatments can be the paradoxical Blocking Off of the Oil Ingestion, suddenly starving the Engine of Motor Oil... so beware any signs of sudden changes or drops in your Oil Pressure for these reasons.
One last issue regarding this is the problem of the GMT360 LL8 4.2L Motors is that the Oil Pressure Gauge on the IPC...is a FAKE Device and the only way to know what your actual Oil Pressure is would be to install a special Accel Oil Port Adapter (Metric to NPT) adjacent the Front Oil Filter Gallery.
Note that the Oil Pressure Sensor Threaded Port in the Passenger Side of the Engine Block is NOT the Same Size as the Illustrated Front Oil Gallery Plug (M16 X1.5MM) where the Accel Adapter Threads In after removing this Aluminum Hex Plug:
That carefully installed item gets used in combination with installing an Analog Oil Pressure Hose and Gauge Combo so you can directly observe what the Gerotor Oil Pump actually manages to produce for adequate Oil Pressure while pumping just over 11 Gallons Per Minute when working properly. Nominal Oil Pressure for this Motor is 12 PSI at 600 RPM at Idle and around 65 PSI at 3,000 RPM.
I know this seems a bit excessive... but you've introduced a change in the *lubrication environment* deep inside of that Engine now that will deserve your undivided attention over the next few Oil Change Cycles until you know there won't be any "Oily-Under-Pressure Surprises"...
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Sure I can open the filter when I change the oil again. I'll post pics of what it looks like. I've done an engine flush on it 3 times and this will be the second time I've done the MMO. This TB had a very sludged up engine when I got it and I was trying to get it cleaned up as best as possible. This forum actually helped me with most of the engine work I've had to do. I lost my login info for my other account so I opened this account. My screen name used to be Dieseldog and all my adventures with this machine are here!! I'll have the pics of the filter in a few days.
Outstanding, "Dog"... and as you recall from your 'past life'... we have a very decent group of Mods here that I would 'Bet a Brace of Gutenberg Bibles' could look into blending your two accounts ...if you PM them about this issue.
And if you decide that having the 'Peace Of Mind" that only an Analog Oil Gauge will provide to you... Amazon sells THIS Device for installing into that Engine Oil Gallery:
Oil Gallery Metric-to-NPT Adapter Kit for Under $10.00:
https://www.amazon.com/dp/B0894LDF76/?tag=gmtnation-20
...and here is a fine assortment of Oil Pressure Tester Kits to choose from for attaching the 1/4" NPT Oil Pressure Gauge-Tester Hose and/or the Kits Adapters to it as needed:
https://www.amazon.com/s?k=oil+pressure+tester+kit&crid=1UN9J7WVVXPF7&sprefix=oil+pressure+tester+,aps,120&ref=nb_sb_ss_retrain-deeppltr_1_20
And if you decide that having the 'Peace Of Mind" that only an Analog Oil Gauge will provide to you... Amazon sells THIS Device for installing into that Engine Oil Gallery:
Oil Gallery Metric-to-NPT Adapter Kit for Under $10.00:
https://www.amazon.com/dp/B0894LDF76/?tag=gmtnation-20
...and here is a fine assortment of Oil Pressure Tester Kits to choose from for attaching the 1/4" NPT Oil Pressure Gauge-Tester Hose and/or the Kits Adapters to it as needed:
https://www.amazon.com/s?k=oil+pressure+tester+kit&crid=1UN9J7WVVXPF7&sprefix=oil+pressure+tester+,aps,120&ref=nb_sb_ss_retrain-deeppltr_1_20