2004 TB Surges at Idle

Turbospeed

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Member
Jul 21, 2018
8
New England
Yup, another idle surge thread. This TB is my kid's truck. We got it with about 130k on the clock. 144k on there now. Earlier this year I put in a set of AC-Delco plugs and cleaned the throttle body it was dirty and needed a good degunking. The kid is now reporting that her car sometimes surges from idle. I cleaned the throttle body again today, it was still pretty clean. I sprayed down the IAT in the inlet air plenum prior to the resonator with some MAF sensor cleaner. It has not set any codes lately. A few weeks ago when she first reported the problem I noted that it had set P1133, heated O2 sensor insufficient sensing; P0136, O2 sensor bank 1 sensor 2 low voltage; and P0128, coolant temp below t-stat regulated temp. These codes were all flagged as intermittent. The codes have not returned and there is no CEL. I drove it around for awhile yesterday and today and it pretty well behaved itself. We took the truck out when we went for dinner. When we got back home it started to do a slow surge in the driveway. The engine would step up in speed and do a slow decline back to idle. I have new AC-Delco O2 sensors waiting in the wings if needed but I wanted something more solid to go on before I started throwing parts at it. My scan tool doesn't give any indications on whats going on with the throttle body. No TB command nor feedback. I would think that would be important data to see if the throttle body was moving while at idle. I've watched the trace on the throttle position sensor and it looks unremarkable. I can cycle it, the TPS, within a range of 18% to about 85%. I also searched the forums for previous threads on these symptoms. I found one thread where it was suggested to replace the t-stat regulator and coolant temp sensor due to low coolant temp. When the truck is driving around the coolant ranges from 185F. to about 195F. I'm going to look into the possibility of a vacuum leak and check the intake manifold bolts. Any help out there?
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
When you cleaned the throttle body, did you disconnect the battery for 30 minutes?

How's the battery? I've seen weird stuff happen with a weak battery. You should get it tested if it's older.

If you were getting those codes, I'd be looking at replacing those components. Temp seems a bit low. Maybe it's just higher now because it's hotter out. If it tripped that code, the thermostat is likely bad. I'd also look at replacing the O2 sensors although the downstream sensor 2 does nothing for regulating the engine. Upstream sensor 1 throwing that code is getting lazy. Hopefully your cat didn't get plugged up from the bad t-stat and O2 sensor.
 
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Turbospeed

Original poster
Member
Jul 21, 2018
8
New England
When you cleaned the throttle body, did you disconnect the battery for 30 minutes?

How's the battery? I've seen weird stuff happen with a weak battery. You should get it tested if it's older.

If you were getting those codes, I'd be looking at replacing those components. Temp seems a bit low. Maybe it's just higher now because it's hotter out. If it tripped that code, the thermostat is likely bad. I'd also look at replacing the O2 sensors although the downstream sensor 2 does nothing for regulating the engine. Upstream sensor 1 throwing that code is getting lazy. Hopefully your cat didn't get plugged up from the bad t-stat and O2 sensor.

I pulled the fuses for PCM1 and PCMB for about ten minutes or so.
 

mrrsm

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+1 Post #2...^...The Bleed Down works better if you pull power via Negative (-) Battery Cable and disconnect the PCM from any energy source for a minimum of 30 Minutes. When time permits... Pull the MAP Sensor (There is No MAF on your Engine) at the upper, back area of the Intake Manifold and carefully clean out any built- up Oily Residue in the interior hollow of the sensor. It sounds like your OEM ACDelco O2 Sensors are going to come in very handy if these previous tips do not work. This video from @MAY03LT will help with the O2 Sensor R&R. As a Legendary Alumnus at GMT Nation... He always makes these difficult jobs seem like fun:

 
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JerryIrons

Member
Dec 20, 2011
434
I would also disconnect battery as well for at least 1/2 hour. When you cleaned the throttle body, did you pull it completely off, and wipe that out both sides with a rag sprayed with throttle body cleaner?
When you first fire up the vehicle after that, it should run rough for a few cycles until the engine remembers/learns that it has over 100,000 miles haha.
 
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Turbospeed

Original poster
Member
Jul 21, 2018
8
New England
Today I changed out the upstream o2 sensor. Took the little truck around the block to put some heat in the exh. manifold and soaked the sensor with Kroil. Disconnected the battery ground terminal. The upstream o2 sensor popped out easy-peasy. Cleaned the MAP sensor with MAF cleaner. Tried and tried to pull the downstream o2 sensor. Soaked it with Kroil. Ran the engine at idle to heat up the exhaust pipe. She no budge. No matter what I tried I couldn't get that second o2 sensor to break free. If I could put it on a lift I'm sure it would be easier to break it free without breaking it. I'll have to find my MAPP torch and put some heat on the threaded boss the sensor is stuck in. As to the previous throttle body cleaning we removed it from the engine and cleaned both sides of the plate. So we put it back together and it ran very well. Scanned the controls to verify no codes. The kid has her truck back now so we'll see what she reports back with it.
 
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Mooseman

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Dec 4, 2011
25,257
Ottawa, ON
I wouldn't worry too much about the downstream O2 sensor since all it does is report back to the PCM if the cat is working properly. If it is defective, you'll just have the CEL.

Do check for codes after a while. At that mileage, I still think you should replace the thermostat because if it is under-temp, it will kill your cat.
 
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Turbospeed

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Jul 21, 2018
8
New England
The Girl reported that her truck still surged this morning. I told her to give it a little time as the PCM had been reset and was in the process of relearning some of its operating parameters. I will plan on changing that thermostat soon. When I was driving it around yesterday the coolant temp was 185F. I'm still surprised that GM would not provide any scan data on the command and position feedback for the TB. Such an important component in a drive by wire system should have that data available.
 

mrrsm

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Looks like you are back to the suggestions in Post #2 and Post #7 by @Mooseman... And performing an Exhaust Back Pressure Test ...by either temporarily re-removing your Brand New ACDelco Upstream O2 Sensor to relieve some of the exhaust stream pressure while checking for surges idling while In Drive with the Brakes applied... or by going Whole Hog on an Exhaust Back Pressure Test Kit to see what goes on there that might finally reveal The Face of this PITA "Surging Beast"... as a Clogged CAT.
 
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Turbospeed

Original poster
Member
Jul 21, 2018
8
New England
The Girl brought her little truck back home today so I could have another look at it. Of course it behaved itself perfectly until it didn't. We came back to the house and with the AC on the truck would spike up from idle and slowly decay. I hooked up my scanner and I could graph the TPS position spike up, from 12% to 18%, and then slowly come back down. The rise and fall of the TPS position and the engine rpm would repeat over and over. If I turned off the AC the spikes went away. Turn the AC back on and the spikes come back. So the PCM must be telling the throttle body to move. Why?
 

christo829

Member
Dec 7, 2011
497
Fairfax, Virginia
The spikes may coincide with when the AC compressor clutch engages. The PCM tries to compensate for the added load with an increase in rpms rather than have the engine
falter.
 
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mrrsm

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... and if either the Wrong Length Serpentine Belt is installed and/or a less "Bouncy" and Badly Worn Belt Tensioner is presently likewise installed... the ability to "Take Up The Slack" so to speak ...when the Compressor's Magnetic Clutch kicks in will be seriously diminished. Also... if the running Compressor is putting a sufficient Drag on the Engine... it could be on its way out as well. Does this vehicle have any Aftermarket Alternator Hardware/Serpentine Pulley presently installed to handle other power demands?
 
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Turbospeed

Original poster
Member
Jul 21, 2018
8
New England
Thanks to christo829 and MRRSM for your replies. My Actron scanner doesn't give me any data on the AC system, but the compressor clutch cycling makes sense to me. I was graphing the idle speed during the AC operation. I don't recall seeing the idle drop below 600rpm, just the upward spikes and then decay back to idle. This Trailblazer appears to be factory stock with no mods that I can see. There is a short little funky antennae on the upper inside driver's corner of the windshield. I have often wondered if that was an alarm system antennae. As an aside here, I had ordered a new Stant thermostat for installation and it arrived the other day. I looked at the temp rating on it and it was 86C., or about 186F. That's the temp that I currently see with the thermostat that's in there now. So I wont be going through all the aggravation of putting that t-stat in for the same results. I thought the t-stat should be at about 195F. I looked through the factory manual and couldn't find a temp spec for the t-stat. Anyone know of a different source for the t-stat spec?
 

mrrsm

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+1 --^-- ... I wholeheartedly agree with @NJTB... (Hey.., now THERE is a rhyme...) If you visit page three (3) of my "Flickr-Bucket" New Parts for GM 4.2L LL8 Engine... you can see from the side by side comparisons of the Stant vs. the OEM ACDelco flavors of the Thermostat have some very significant physical differences in their spring wire thickness and the dimensions of the Center Brass Button pieces that would make for obvious differences in how they respond when dealing with the flow of the Hot Coolant through the TS Manifold. Selecting the ACDelco is the best choice... no matter what Stant says about its OEM Quality:

https://www.flickr.com/photos/126111508@N07/albums/72157671179967478/page3
 

Turbospeed

Original poster
Member
Jul 21, 2018
8
New England
So I keep hammering away at the surging idle problem on The Girl's little truck as time allows. I replaced the fuel filter, front brakes/rotors, and the front sway bar bushing as part of general maintenance. Winters in New England make working under a truck so much fun. All the parts are in varying degrees of dissolving away. We got a backprobe kit off of eBay, S&G Toolaid 23500, to measure the voltages at the plug on the throttle body. There are two wiper circuits in the TB and the reference voltage to each circuit measured at 5.02vdc. We measured continuity through each wiper and surprisingly came up with different values. TPS1 ohmed out at 3.72kohms across pins C&G. TPS2 measured 2440 ohms across pins H&B. I haven’t so far found any references that say what those values should be. I would think that both wipers should be scaled to have the same range. Unfortunately the wipers and the little stepper motor are sealed and I don’t know how to pull the cover without destroying the TB. Does anyone have a spare TB that can check those wiper values?
1535929408321.png
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Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
The throttle body system is fairly robust with lots of safeguards. If ANYTHING is out of spec, it goes into REP and throws a code.

If you are not getting any codes, the PCM is reacting to something to make it command the throttle like that. Please re-read and confirm what has been suggested thus far has been done.

Another thing you can do is get yourself a Bluetooth OBD adapter and the Torque app on Android to be able to read live data. Both can be had for under $20.
 

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