Intermittant 900 rpm Idle, P0171 Code & Reduced Fuel Mileage

scarecro

Original poster
Member
Apr 16, 2012
2
2004 Trailblazer LS 4x4; 82K miles; owned from 10K. After reading many posts both here and at TrailVoy regarding my above referenced "problems", I purchased an Actron CP9580A scantool. Only discovery found was low coolant temp, so replaced the thermostat, CTS, radiator hoses and anti-freeze. Mileage has improved as I would expect with higher coolant temps, 195-205 on the Actron. However, I still have experienced the 900 rpm idle and P0171 code. Have replaced the spark plugs, cleaned the TB, replaced the TB gasket, sprayed all around the intake manifold with carb cleaner, all vacuum hoses are sound and properly connected, new air filter, new fuel filter and replaced the upstream O2 sensor awhile back. The other two common denominators with the high idle/P0171 code are use of the cruise control for extended periods and altitude change, Bay Area to Tahoe (100 to 7,000 feet) and return. The '04 TB does not appear to have a barometric pressure PID as I have seen on some other vehicles. And I cannot imagine how using the cruise control could affect these issues. I only mention it because someone else over at TV had shared the same problem. Also, I had my tranny serviced at the dealer recently and asked for any PCM updates. They said mine was current.

So, any ideas or suggestions for further diagnosis?
 

tblazerdude

Member
Dec 4, 2011
321
You are not the only one with high (unusual) idle. 900 RPM is what my truck sat at yesterday, and as soon as I turned the heat on it dropped to 600 RPM. I have no idea what the relation is, but it went away. It has happened before and it always goes away. No cause consistent and no remedy consistent. The only thing I could think of was a sticky thermostat (replaced mine with CTS about 2 years ago, and switched to prestone "every make every model inc. aluminum" yellow stuff). No codes. It does it much more in winter... I wonder if there is another temperature sensor that might need to be replaced, but I can't find one. Can you get it to idle normally like me? Or is it just constant high idle? Another cause I have noticed, of the high idle, is right after fueling up. The engine never fully warmed up, (my local gas station is less than a mile away) and I filled the tank up. After starting the engine I got 900 RPM, it was cold out, but only maybe 40 degrees fahrenheit. Not below zero.
 

Matt

Member
Dec 2, 2011
4,030
Check all your vacuum hoses for leaks and good connections. Also, check to make sure that your airbox and "tube" going to the TB have all the hoses connected to it.

Also could be a cracked exhaust manifold (hope not). You may want to check the tightness of the bolts.

These do affect the idle and cause P0171 codes. HTH
 

scarecro

Original poster
Member
Apr 16, 2012
2
tblazerdude said:
Here are the pictures of my intake hoses and FPR hoses. Anything look amiss?

View attachment 10179

View attachment 10180

Yes. Your vacuum line from the fuel pressure regulator should be connected to a source on the other side of the throttle body. At least that is where my stock set-up is connected.

Thanks for your other suggestions. Doesn't make any sense that using the heater would affect the high idle, but willing to try anything if it helps.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Here's where the FPR hose is supposed to go in a stock installation:

View attachment 23605

Where it should have gone.

View attachment 23606

That nipple on the intake resonator is on the wrong side of the throttle body, as noted. It's TRUE purpose on the vehicle is for EXT/XL long wheelbase trucks, to provide a vacuum source to a hot water valve on the passenger's side of the firewall used only for the rear HVAC heater core. For all SWB trucks, it should remain capped off.
 

Attachments

  • 100_4647.jpg
    100_4647.jpg
    80.6 KB · Views: 21
  • 1.jpg
    1.jpg
    69.9 KB · Views: 89

forrestnummy

Member
Oct 6, 2013
15
the roadie said:
Here's where the FPR hose is supposed to go in a stock installation:

View attachment 10215

Where it should have gone.

View attachment 10216

That nipple on the intake resonator is on the wrong side of the throttle body, as noted. It's TRUE purpose on the vehicle is for EXT/XL long wheelbase trucks, to provide a vacuum source to a hot water valve on the passenger's side of the firewall used only for the rear HVAC heater core. For all SWB trucks, it should remain capped off.

So I have messed with this high idle for a little bit now. I think I have it solved. The high idle happens either right after I fill up or within the first 50 miles of the fill up. The issue looks to be part of the EVAP system. There is a purge valve which is up front on the lower, driver side of the engine and it pretty easy to change out. I went through the wheel well with long extentions. The other two are in the back. One is on the side of the gas tank and is a little valve not much bigger than a pack of cigs. The other is the charcoal canister that is under the spare tire on the drivers side. It is fairly big and bulky. The problem is one of two things. Your over filling your gas tank and it is overflowing into the charcoal canister and saturating the charcoal and the high idle is there to burn the extra fuel and fumes off. Another issue is the hose/fill tube has a small crack in it and will cause random high idle. The crack is hard to see most of the time. Usually very small crack. But the problem 90% of the time is the charcoal canister gets over filled. You can test it by not filling your car up all the way. Stop it before it gets to the full and for sure stop it before the pump auto shuts off. This will eliminate the problem with the gas overflow into the canister. Drive around through a couple of tanks of gas, not filling up all the way on either fill up and see if the high idle returns. IF it does return then you need to look toward you exhaust manifold either being cracked or not tight. The trailblazer exhaust manifold bolts were actually too long which caused them to come loose. Should of been a recall but you guess it, no one has died so GM doesn't care. The exhaust is a pain in the ars to take off and most of the time where they crack is a place that you cant see until you take it all the way off. Which again GM knows about the design issue but did nothing to make it right for those that purchased the trailblazers. You can see the new exhaust manifold design on the replacement part. There are two exhaust pipes that used to be connected, now they are separate cause the vibrations would crack the two in half. So they just made them into two parts so it wont happen again. If you purchase the new exhaust make sure that you get the new bolts, so your not back in there in 6 months getting them tight again.
 

IllogicTC

Member
Dec 30, 2013
3,452
For anyone trying to run a forum search on their p0171 code, here's a helpful video. In this video's case, it turned out the SAIS was sticking open.

[video=youtube;ViL7RX_22A4]http://www.youtube.com/watch?v=ViL7RX_22A4[/video]
 

Forum Statistics

Threads
23,442
Posts
639,384
Members
18,664
Latest member
SooMenn22

Members Online