Idles at 900

NJTB

Original poster
Member
Aug 27, 2012
612
Flemington, NJ
I have a 2004 I6 Trailblazer, 80xxx miles. About 6 months ago it decided it wanted to idle at 900 rpm.
When this first happened, I disconnected the battery for 1/2 hour, the idle came down to 600, I thought it was fixed. Well, since then it's happened several times, disconnected the battery, worked great for about 150-200 miles, then started all over again. Once in a while it will return to 600 rpm on it's own.
It doesn't matter if the A/C is on or off.
I checked for codes with a scanner, there are none. Took the throttle body off and cleaned it twice. The temp gauge is straight up, maybe a tick to the left. According to the scanner, this is ~190-210.
There are no driving issues, still gets about 19-20 miles a gallon, and idles smooth. It's just one of those annoying little things that drive me crazy. I do over maintain the thing, oil every 3,000 miles, trans, coolant, PS fluid, transfer case fluid more often than the manual suggests. Always used OEM parts.
The only repair, other than maintenance, was a O2 sensor about 15-20000 miles ago.
My initial thoughts are drive it until the Check Engine light comes on (I know it will) or to ask the learned people on this forum if maybe they know.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
I found on a couple of local offroad buddy's cars, that if the FPR vacuum hose was disconnected, the idle went to 900. Can you take a look at your fuel pressure regulator hose? It's been discussed before with pictures so you should be able to locate a picture with Google image search in case you don't know where it is.
 

BRomanJr

Member
Dec 9, 2011
371
NJTB said:
I have a 2004 I6 Trailblazer, 80xxx miles. About 6 months ago it decided it wanted to idle at 900 rpm.
When this first happened, I disconnected the battery for 1/2 hour, the idle came down to 600, I thought it was fixed. Well, since then it's happened several times, disconnected the battery, worked great for about 150-200 miles, then started all over again. Once in a while it will return to 600 rpm on it's own.
It doesn't matter if the A/C is on or off.
I checked for codes with a scanner, there are none. Took the throttle body off and cleaned it twice. The temp gauge is straight up, maybe a tick to the left. According to the scanner, this is ~190-210.
There are no driving issues, still gets about 19-20 miles a gallon, and idles smooth. It's just one of those annoying little things that drive me crazy. I do over maintain the thing, oil every 3,000 miles, trans, coolant, PS fluid, transfer case fluid more often than the manual suggests. Always used OEM parts.
The only repair, other than maintenance, was a O2 sensor about 15-20000 miles ago.
My initial thoughts are drive it until the Check Engine light comes on (I know it will) or to ask the learned people on this forum if maybe they know.

Just a thought, what brand O2 sensor did you use?
 

gmcman

Member
Dec 12, 2011
4,656
Are the edges of the TB butterfly clean? The entire edge? I would check the intake manifold bolts also possibly allowing an air leak. They could seal up when the engine warms up.
 

NJTB

Original poster
Member
Aug 27, 2012
612
Flemington, NJ
the roadie said:
I found on a couple of local offroad buddy's cars, that if the FPR vacuum hose was disconnected, the idle went to 900. Can you take a look at your fuel pressure regulator hose? It's been discussed before with pictures so you should be able to locate a picture with Google image search in case you don't know where it is.

I checked that Roadie, and yes it's hooked up (air plenum to FPR). Since you mentioned it, I DIDN'T check to be sure it had vacuum, there was no fuel in the line. I'm gonna have to do that. Thanks for the reply.
 

NJTB

Original poster
Member
Aug 27, 2012
612
Flemington, NJ
BRomanJr said:
Just a thought, what brand O2 sensor did you use?

AC Delco. When the O2 sensor first went, the TB ran poorly, but no CEL. When the CEL finally came on, it was almost not drivable. Pulled into Autozone, bought the tool, walked to the Chevy dealer, got the sensor ($100 !!!) and installed it. Ran good till about 6 months ago when the idle thing started.
Thanks for the reply.
 

gmcman

Member
Dec 12, 2011
4,656
NJTB said:
Since you mentioned it, I DIDN'T check to be sure it had vacuum

I'm almost certain that line to the resonator to the FPR does not see vacuum, unless the air filter gets real clogged. Unless something happens at WOT but I don't know.
 

AV8ER

Member
Apr 19, 2012
260
Mine did this a few times. It was during the two years i drove around with the CEL lit up like a christmas tree (fan clutch long story) so i never knew which of the 5 or so codes it was. Anyways, If I reset the codes it would idle back down to 650 like usual. If I recall it was linked with the CPAS code in my case. Usually if it idled at 900 and i stomped on the gas it gave a nice growl because the valve timing was off. After i cleaned the CPAS ive never had that problem again.
 

NJTB

Original poster
Member
Aug 27, 2012
612
Flemington, NJ
Thanks for all the replies.
Yesterday I drove ~200 miles and the TB idled fine, no problems. Go figure.
The next chance I get, the intake manifold bolts, etc, etc., are going to get checked. I was hoping the fix was going to be stupid easy, but it looks like it's digging time.
 
Jan 21, 2012
58
NJTB said:
Thanks for all the replies.
Yesterday I drove ~200 miles and the TB idled fine, no problems. Go figure.
The next chance I get, the intake manifold bolts, etc, etc., are going to get checked. I was hoping the fix was going to be stupid easy, but it looks like it's digging time.

just a question ? does this happen just after you fill the gas tank? that's what happens to mine and lasts for about 50-60 miles then goes back to normal 650 RPM and I have never been able to track down the cause. never throws a code. I just live with it. If anybody comes up with a fix then I will fix it.
 

NJTB

Original poster
Member
Aug 27, 2012
612
Flemington, NJ
Enderbygrandpa said:
just a question ? does this happen just after you fill the gas tank? that's what happens to mine and lasts for about 50-60 miles then goes back to normal 650 RPM and I have never been able to track down the cause. never throws a code. I just live with it. If anybody comes up with a fix then I will fix it.

The vast majority of the driving I do with this car is hiway. It sits for a week, I fill it up and drive it to PA to work on a house I own there, it's about 100 miles each way. Most of the time the TB starts to idle fast about 5 miles from home (about 195 miles into the trip), and stays that way until I pull into the driveway and shut it off. It will then idle at 900 until I disconnect the battery for 1/2 hour.
Once in a while it will idle fast about 130-150 miles into the trip, and then the rest of the way home.
Point is it's getting tiresome disconnecting the battery weekly.
I followed the suggestions on this list (thank you all) with no improvement (intake manifold bolts are 11 in. lbs, not ft. lbs!!!) and am now leaning towards a flakey sensor, one that will read within the parameters MOST of the time. I'm also figuring whichever sensor it is is on in open loop (O2, TPS, coolant, MAP) as I'm thinking the raised idle is because the PCM thinks the engine is colder than it is, like one minute into a dead cold start (maybe).
So now I'm driving around with the scanner hooked up, watching.

PS: to the poster who suggested the gas cap-I failed state inspection TWICE for a bad gas cap. It's got a new one.
 

gmcman

Member
Dec 12, 2011
4,656
When you disconnect the battery, do you just fire it up and drive away? Perhaps try the re-learn for the throttle body, have you done this yet? Are you absolutely certain the edges of the butterfly are clean. The reason I stress this, if you have a bit of crud on the edge this can keep the butterfly from fully closing to where it needs to be. The area of the butterfly that's clean creates an air gap and the PCM cannot close the butterfly more to reduce the RPM. This is not really an issue but just something to ensure that is clean to eliminate any possibilities.
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
NJTB said:
The vast majority of the driving I do with this car is hiway. It sits for a week, I fill it up and drive it to PA to work on a house I own there, it's about 100 miles each way. Most of the time the TB starts to idle fast about 5 miles from home (about 195 miles into the trip), and stays that way until I pull into the driveway and shut it off. It will then idle at 900 until I disconnect the battery for 1/2 hour.
Once in a while it will idle fast about 130-150 miles into the trip, and then the rest of the way home.
Point is it's getting tiresome disconnecting the battery weekly.
I followed the suggestions on this list (thank you all) with no improvement (intake manifold bolts are 11 in. lbs, not ft. lbs!!!) and am now leaning towards a flakey sensor, one that will read within the parameters MOST of the time. I'm also figuring whichever sensor it is is on in open loop (O2, TPS, coolant, MAP) as I'm thinking the raised idle is because the PCM thinks the engine is colder than it is, like one minute into a dead cold start (maybe).
So now I'm driving around with the scanner hooked up, watching.

PS: to the poster who suggested the gas cap-I failed state inspection TWICE for a bad gas cap. It's got a new one.

It might be easier to pull fuses #10 & 28 if it's the PCM that's the problem. That way there's nothing else that needs to be reset.
 

NJTB

Original poster
Member
Aug 27, 2012
612
Flemington, NJ
gmcman said:
When you disconnect the battery, do you just fire it up and drive away? Perhaps try the re-learn for the throttle body, have you done this yet? Are you absolutely certain the edges of the butterfly are clean. The reason I stress this, if you have a bit of crud on the edge this can keep the butterfly from fully closing to where it needs to be. The area of the butterfly that's clean creates an air gap and the PCM cannot close the butterfly more to reduce the RPM. This is not really an issue but just something to ensure that is clean to eliminate any possibilities.


I completely removed it twice to clean it. Both times the most crud the throttle body has in it was some oily sludge, but no carbon. The thing was CLEAN when I put it back on. Used Brakleen and a rag, and even made sure the mounting surface was cleaned and even, and the orange silicone seal was in good shape.

When I disconnect the battery, I leave it for 1/2 hour.
 

Mypetropig

Member
Dec 29, 2011
226
It could be a cam position sensor. When mine was acting up it would stumble then jump to 900 rpm. Disconnecting the battery would help for a short while. Finally after what felt like months it turned the cel on. The part was cheap and easy to put on. No problems since.
 

gmcman

Member
Dec 12, 2011
4,656
Did you by chance spray the silicone seal with the brake cleaner? If i'm not mistaken the trichloroethelyne in brakecleaner eats up silicone.
 

NJTB

Original poster
Member
Aug 27, 2012
612
Flemington, NJ
gmcman said:
Did you by chance spray the silicone seal with the brake cleaner? If i'm not mistaken the trichloroethelyne in brakecleaner eats up silicone.

When I cleaned the throttle body, I removed the seal to check it for dents, cuts, or anything else that may cause a vacuum leak. Installed it with a couple of drops of 'dragon snot' (weatherstrip adhesive) to hold it in place.
The other day I hooked up the scanner and took it for a ride, and paid close attention to the coolant temp. When the dash gauge reads 210, it's really anywhere between 190 and 210, and I've read that on this forum before so I guess that's normal. At any rate, the coolant sensor is good.
Also watched the TPS, no glitches or hang ups, and the O2 sensor seems to be varying voltage properly.
The whole trip, everything was within specs.
The next thing I'm going to try is to clean the IAT (intake air temp) sensor in the tube to the plenum. I'm kind of reduced to pokin and hoping.
 

christo829

Member
Dec 7, 2011
497
Fairfax, Virginia
gmcman said:
Did you by chance spray the silicone seal with the brake cleaner? If i'm not mistaken the trichloroethelyne in brakecleaner eats up silicone.

Not trying to be pedantic, but Brakleen (tm) changed their formula, so it no longer contains TCE. It does still have
Acetone, Toluene, Methanol and Heptane. If it's labelled as "non-chlorinated", it's the "no TCE" formula. The Acetone
may still damage plastics, but it's a bit more sensor friendly than the old stuff. I still have a screwdriver with a "custom"
handle thanks to grabbing it after using the old formula.

To the OP, when my '04 did this a few months ago, it turned out that my cam sensor wire harness had one wire that was
slightly damaged. I think I may have done it the last time I took the belt off. Repaired that, and it's behaved so far.

Good Luck!

Chris
 

NJTB

Original poster
Member
Aug 27, 2012
612
Flemington, NJ
I posted some time ago (can't find the original thread) about the TB idling at 900 intermittently after getting off the hi-way. There were a bunch of suggestions that I thank you all for, but after checking what I could without fixing the problem, I decided to just wait until the CEL came on.
About two months ago a P0410 popped up, did a search here and found it was the SAIS valve. Researching the posts here, I removed the valve and found the plunger was stuck, freed it and lubricated. Bingo, no more code and no more fast idle. It's been fine for about 2000 miles now, so I'm calling it fixed.
On a side note, two weeks ago the thermostat died and I replaced it. Fun job. Walked out this morning and saw anti freeze on the ground, lifted the hood, and it's leaking from the water pump. Oh, joy.
 
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triz

Member
Apr 22, 2013
746
You must be just passed the 100k mark. That is when I started to replace a lot of things.
 

Blckshdw

Moderator
Nov 20, 2011
10,665
Tampa Bay Area, FL
I merged your threads to keep everything together.
 

NJTB

Original poster
Member
Aug 27, 2012
612
Flemington, NJ
triz said:
You must be just passed the 100k mark. That is when I started to replace a lot of things.
92xxx. I noticed at 60xxx things started to go south, repaired them and yes, I guess 100,000 is the next plateau.
Thanks for finding the original post.
 

PProph

Member
Dec 7, 2011
220
thanks for reporting your fix! that helps users in the future!
 

6716

Member
Jul 24, 2012
821
may as well get the fan clutch while you're after the water pump
 

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