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Discussion Starter #1
Yeah, sorry that my first post is to ask for help.

I have a 2004 ls. Sometimes, and I think it may be related to the AC, my idle will jump to 900 rpm. I hear a click before this happens. I can spend between 15 minutes and all day with the rpm that high. I don't hear anything when it drops down and I usually don't know it's happened until I realize that I don't have to stand on the brake as hard when slowing/ stopping. I've changed the air charge temp sensor on the intake tube, the map sensor on the intake plenum and the throttle body. I've done multiple idle relearns (3min on-1min off-3min on)

The reason I think it might be related to the AC is because yesterday, when it was 30 deg., I had the defrost on for about 15 minutes and when I changed the heat to the floor, the rpm was back up to 900 and stayed that way for about an hour and a half. That's when a light bulb went off in my head. This started about 3 months ago, and I run the AC regularly.

I have almost 200K mileage and I drive the truck daily, averaging about 55 miles a day. The truck is my service/work truck. I'm not sure if anyone needs any more info, please feel free to ask and I'll answer.
 

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2004 chevy trailblazer_lt
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I was going to suggest cleaning your throttle body, but then I saw that you already changed it. Assuming it was a new throttle body, it should be clean.

My suggestion is to disconnect your battery for 60 to 90 minutes so the PCM unlearns your driving habits, and then reconnecting the battery and start start driving as you normally do.

I believe that when the AC compressor engages, the PCM tells the throttle body assembly to up the idle in order to handle the additional load. When the AC compressor disengages, the PCM then tells the throttle body assembly to go back to "normal".

You might want to reseat the electrical connectors to the throttle body assembly, the MAP sensor and the air temp sensor as well to make sure they are making good electrical connection. Also check the various ground connections to the engine block, the firewall, and to the body to make sure they are electrically good as well. A poor ground can cause all sorts of gremlins to occur.

Have you plugged in a code reader/scanner to see if there are any codes present?

Good Luck!
 

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Discussion Starter #3
OK, I'll try the 'disconnect battery for 60-90 minutes" and do another relearn. The longest I had the battery disconnected was 15 minutes when I swapped the throttle body out.

I don't remember the p-code, but the only thing my scanner picks up is the pcm almost setting a 'too lean' error. I don't know what the criteria is for actually setting the code. I've gone over every mating surface and vacuum hose connection on the intake system I could find, looking for vacuum leak. Also, before I swapped the throttle body's out, I tried cleaning the old one (multiple times). If I checked the tb when the idle was at 900, there was about an eighth to a quarter inch gap between the throat and plate, and when at 600-650 idle the plate was fully closed. And I hear a click from the engine bay when it decides to jump to 900 rpm.

I can't stand the idea of swapping out the pcm and paying the stealership a ton of money to do a relearn (cam/crank, I believe).

One good thing that came out of the tb swap was my trans doesn't kick out of torque converter lock-up as badly as it used to. That used to drive me bat-**** crazy. I swore I would get whip lash at 45-55 mph. Seems to stay locked up at 50+ mph. 45 mph still acts up, switches in and out of lock up so fast I have to get up to 50 just to keep from making myself sick, but that's a different issue altogether.
 

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2002 chevy trailblazer_ltz
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Two things.
Cheap "new " throttle bodies are sometimes faulty right out of the box.

Secondly it sound like you need to do a relearn on this with a quality scan tool. There you can see in real time the throttle percentage with the pedal. Makimg sure its proper.

That click is in fact your AC compressor. It kicks on and the PCM will adjust accordingly to make up the load on the engine.

Good luck
 

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2004 chevy trailblazer_lt
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Two more questions. How long have you owned your TrailBlazer/Envoy, and did you ever change the crankshaft sensor?
 

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Discussion Starter #6
I've owned it about three years, and no, I've never changed the camshaft sensor.

@xcursion88- I also assumed that the click was the ac. Unless something odd is happening with the compressor, it's only heard when the idle jumps up to 900 rpm. And it will set the 900 rpm idle while cruising. At that point, I know that the rpm will be high when I stop.
 

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2008 chevy trailblazer_lt
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If it never slows down, I'd wonder if it's the throttle body issue, or perhaps some small vacuum leak or something, that the computer ignores and just makes the idle faster.

Rob in AZ
 

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2004 chevy trailblazer_ls
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Mine does the same thing, will idle at 900 for anywhere from 15 minutes to a day, but always goes back to 600. I chased it for about a year (tightened intake manifold bolts, checked wires, cleaned the throttle body, etc.). Nothing helped. I once replaced the SAIS valve, that seemed to stop it for about 3 months. Occasionally a code P0171 will come up, but it clears itself. Learned to live with it.
PS: I also chased this with a Tech 2, but it would never do it when the scanner was hooked up.
 

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Discussion Starter #9
OK... I think I got it, and it should be credited to a stealership mechanic. As someone who troubleshoots for a living (appliance tech), this one caught me off guard. I took the truck in to have the ECU software updated for the fan clutch. The mechanic told me that the ECU was already updated (previous owner is a mechanic for a different stealership). I asked about how I can stop the fan with my thumb when the engine was hot. The mechanic said (with a wink) that I was throwing a code for the fan clutch relay (no, I wasn't, but I also wasn't about to argue it).

On my way home, I stopped off at my neighborhood (ie-locally owned) auto parts store and purchased the somewhat expensive solid state cooling fan relay, installed the relay and almost had my thumb removed by the fan when I restarted the truck. That was 3 days ago and my idle hasn't jumped to 900 rpm since.

So, I'm going to mark this as solved.
 

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2004 chevy trailblazer_lt
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I'll keep my fingers crossed for you! In the meantime, can you provide a part number for this "upgraded" relay?

Thanks!
 

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Discussion Starter #11
It's not an upgrade. The cooling fan relay is a solid state relay, ACDELCO 15016745. It's one of two solid state relays. The other, if I remember correctly, is for the PCM.
 
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