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2004 chevy trailblazer_ls
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Discussion Starter · #1 ·
Its nothing serious but i was drivin my TB today back from school and went to drop my friend off. While i was waiting outside in the car i was playing with the shifter, just changing gears and stuff.

Then the RPM jumped up from the normal 600 to 1K
then engine was warm and the car was still on.
It stayed at 1K for most of the day and then went back to 600

Anyone know whats up?
Fan Clutch maybe ?

My Throttle body is clean (just incase)

Thx
 

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Western Canada Chapter
2003
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6,411 Posts
It makes me think that while you were sitting it got a little warm so the PCM commanded an increased idle speed and fan clutch engagement to allow better AC and cooling of the engine.:undecided
 

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2002 gmc envoy_slt
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I dont know about that... When I sit in my Envoy or leave it idling for a while, the idle never leaves around 600rpm but what does change is the sound of the clutch fan. It will go from being not noticeable, to after idling a while and kicking in, and you can hear the roar of the clutch fan, working- cooling down the engine.

Ive never had the idle of my Envoy change when hot. Ive actually overheated it once too after I replaced my t-stat. And when I say overheat - Im talking temp gauge PEGGED to the right. :x The only thing I noticed was that day, the clutch fan was really loud...lol.
 

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2004 gmc
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When I was experimenting with the efans, and stressing the engine in hill climbs to the point of overheating, the RPM didn't budge. I'm very sure the PCM algorithms will command the fan clutch to be 100% engaged, but not bump up the engine RPM at idle to assist cooling. And I was watching commanded fan RPM versus actual engine RPM using EFILive at the same time.

Did your temp gauge stray off 210? All I can think of for an RPM bump is a temp sensor problem where it thinks the engine's cold. Here's the stock PCM chart of desired RPM versus ECT:

 

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Western Canada Chapter
2003
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When I was experimenting with the efans, and stressing the engine in hill climbs to the point of overheating, the RPM didn't budge. I'm very sure the PCM algorithms will command the fan clutch to be 100% engaged, but not bump up the engine RPM at idle to assist cooling. And I was watching commanded fan RPM versus actual engine RPM using EFILive at the same time.

Did your temp gauge stray off 210? All I can think of for an RPM bump is a temp sensor problem where it thinks the engine's cold. Here's the stock PCM chart of desired RPM versus ECT:

Hey Bill

Good point. I wonder if in fact the thermostat opens too much or is stuck open and overcooling and his temp sensor is in fact a little off throwing it into the high idle parameters? would be interesting to see Dudes actual engine temps and his gauge :undecided
 

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2004 chevy trailblazer_lt
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Was searching and ran across this thread... My idle stays at 1k after long road trips w/ highway driving. Very annoying and I have no explanation. I don't think it's A/C related because it's done it during the winter months too.

Anyone have suggestions on where to start diagnosing this? I have a clean throttle body and I just did the seafoam thing... Normal idle is very smooth now.
 

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Western Canada Chapter
2003
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Well the OP never returned to validate any findings or re-occurrences so it is tough to make a conclusion.

What kind of miles on the truck? Have you changed your thermostat yet or any of the other things like fan clutch or water pump? (Just trying to get a little history)

Like the Roadie was saying, the PCM commands all of the idle parameters based on inputs. If it is getting bad info or if something is not reacting in the planned way to those inputs then the idle will be unreliable. When I changed my thermostat I cured a bunch of issues that included idle, performance, and mileage concerns. :m2:
 

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International Chapter
2004 chevy trailblazer_ls
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3,900 Posts
Its nothing serious but i was drivin my TB today back from school and went to drop my friend off. While i was waiting outside in the car i was playing with the shifter, just changing gears and stuff.
Then the RPM jumped up from the normal 600 to 1K
then engine was warm and the car was still on.
It stayed at 1K for most of the day and then went back to 600
Anyone know whats up?
Fan Clutch maybe ?
My Throttle body is clean (just incase)
Thx

Was searching and ran across this thread... My idle stays at 1k after long road trips w/ highway driving. Very annoying and I have no explanation. I don't think it's A/C related because it's done it during the winter months too.

Anyone have suggestions on where to start diagnosing this? I have a clean throttle body and I just did the seafoam thing... Normal idle is very smooth now.
Welcome to the 600-900 RPM with no reason club you guys :D been a member since last year, tried every thing, nothing worked :no: been to the dealer 100 times, nothing changed except my bank account :duh: no particular reason, does not have a patron either, cold, hot, highway, city hell, sometimes it happens to me while i am in the drive way warming it up, sometimes after a long 3 hour drive it just jumps to 900 :rolleyes: just got use to it, only thing i can think of Africandude is we have sister trucks, may be they were infected from day one and it was hidden there till now :crazy::laugh:
 

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Western Canada Chapter
2003
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6,411 Posts
Welcome to the 600-900 RPM with no reason club you guys :D been a member since last year, tried every thing, nothing worked :no: been to the dealer 100 times, nothing changed except my bank account :duh: no particular reason, does not have a patron either, cold, hot, highway, city hell, sometimes it happens to me while i am in the drive way warming it up, sometimes after a long 3 hour drive it just jumps to 900 :rolleyes: just got use to it, only thing i can think of Africandude is we have sister trucks, may be they were infected from day one and it was hidden there till now :crazy::laugh:
Ah Dude .... your problem is you don't drive fast enough!:excited::bonk::crackup::duh::dielaugh:lets see that speedometer pic again!
 

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2004 chevy trailblazer_lt
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156 Posts
I will do a search on how to check my fan clutch... I haven't noticed any temp variances though... Wouldn't I see the water temp rise/drop if it were any of these three issues?

It's annoying that when I stop after highway driving it's like mini-whiplash when I let off the brake... Not to mention it can't be good for gas mileage and I assume it indicates a problem. :hopeless
 

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2003 chevy trailblazer_ltz
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Well guys the same thing happens to mine. Very rare but I've seen it. Needle is just under 210 where mine usually runs anyway and it usually happens if I was being hard on it. RPM will run at about 900rpm til I get it shut off or sometimes just as I am about to get to where I am going. Don't know what the acutal cause is because my temp gage doesn't move either way. Sometimes makes it more of a pain to stop too. I used to have a 92 Nissan Stanza that would do that, but it would change rpms with the ac compressor running or not when it was to temp. No other side affects with that one I couldn't fix by the use of the clutch. Sometimes I miss my manual transmissions.:D
 

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2004 chevy trailblazer_ls
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Well, I may be committing a cardinal sin in my first post by bringing up a thread that is 2 months old, but I have something to add that may be useful.

I have an '04 4.2L with 105k miles. I have the high idle problem too. It runs right at 1k rpm after highway driving, but only after I use the cruise control. It doesn't seem to matter how long I've been using the cruise control (1 minute or 3 hours) and it doesn't always happen after using the cruise, but it has never happened under any other circumstances. Also, sometimes when I shut it down under high idle and then start it back up while the engine is still hot, I have experienced it start up and run with high idle (not every time, just occassionally). If I shut it down under high idle and the engine completely cools it doesn't start back up under high idle, ever.

I am the original owner and have had this problem as long as I can remember. I took it in 3 times under warranty for the dealer to look at it and, of course, they couldn't replicate the problem.

It is annoying to me because it makes it harder to stop at a red light and there seems to be a rough shift down to first gear while stopping with the hig idle. I'm sure the torque converter can take the strain, but it reminds me of the old cars that would shift funny when the carburetor butterfly would stick open.

Don't have any solutions, just another input that may help us solve this problem.

Scott
 

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2004 chevy trailblazer_ls
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Discussion Starter · #16 ·
Hey guys.
Thanks for the Input.

Its been a few months and the idle hasnt returned.....Until.........
about a week ago the 1K idle came back,Again, it happened when i was playing whith the shifter changing gears while sitting still.??

I do think my fan clutch might be bad but not going to replace it anytime soon as there r no other problems with mt TB.

I guess we r just gona have to deal with it untill we can figure it out ??
Which is ok for me cus it makes my TB feel faster on takeoffs,
hehe :thumbsup:
 

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2004 gmc
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In a demonstration of the power of the larger community, I was trying to fix NickNorie's identical problem a couple of weeks ago. Intermittent 900 RPM idle, accompanied with a P0171 code (sometimes didn't light up the SES lamp) that implied lean running. Usually started the 900 RPM operation after a hard launch, but not always.

Twice when I had EFILive hooked up to him, and it was idling at 900, I cleared the code, and the idle went right down to 600! I never found out the root cause in my driveway, but Nick did later.

He had his intake resonator replaced with a CAI tube, and connected the vacuum line to the fuel pressure regulator to the ENGINE side of the intake manifold (where there's a little capped off test port) instead of the air filter side of the throttle body butterfly valve. That vacuum line has to go before the throttle body to regulate the injector fuel pressure properly.

This might solve the mystery for some members.
 
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