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Discussion Starter · #1 ·
The vehicle had a long term P0017 code while people tried in vain to fix it (replaced camshaft & crankshaft sensors, VVT actuator solenoid, PCM, camshaft phaser etc). Finally went ahead and replaced the timing gear (turns out the intake side timing chain guide plastic insert had broken away so crankshaft was advanced compared to the camshaft).

Put it all back together, started it (did not touch gas pedal) and it ran fine without a code, idling at around 650-670. I let it get to running temperature while I was bleeding the cooling system and everything was looking great.

Then I gave the gas pedal a little touch to see how it sounded, up to about 1500 rpm and it came back down it started idling at 900+. Short term fuel trim is now 19.5, long term is 27.5 and we're getting the P0171 code. I have driven the car around the block to see how it works otherwise, and we have had one occasion where it idled around 670 for a few moments, then kicked back up.

I can't see or hear any vacuum leaks, but without running the car my 02#1 sensor is .45 volts, but once the vehicle is running it gets down to 0.005 volts.

Since it didn't start the high idle until I hit the gas pedal, I'm struggling to think this is a vacuum leak (although I could be wrong).

I'm wondering whether this is a fuel delivery issue, a vacuum leak or a long term residual of the vehicle having being driven for a long time with the P0017 code - or something else entirely.

Any thoughts would be gratefully received.
 

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2004 chevy trailblazer_lt
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Welcome to the Forum!

First question, which engine? We really need to know. Second question - how many miles.

Now, assuming it is the 4.2L I6 engine, double check and make sure the Engine Coolant Temperature (ECT) Sensor is connected and working properly. You should be able to watch the temperature on a halfway decent OBD II code reader/live data scanner. If the ECT is not reporting properly, the ECM can make the engine do some strange things.

It appears that the O2 sensors are working properly, BUT, when the engine is in closed loop mode (normal engine operation), the upstream O2 sensor's (#1 sensor) voltage should be bouncing around all over the place from just above 0.000 V to 0.700 volts. The downstream O2 sensor should be relatively stable and not changing much or as rapidly as the upstream O2 sensor.

If your upstream sensor's output voltage stays around any given voltage and does not bounce around, I would be suspicious of a bad sensor.

Good Luck!
 

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2004 chevy trailblazer_ls
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I chased the same hi idle, P0171 code for years. Tightend the intake manifold, cleaned the throttle body, checked for vacuum leaks, etc., etc. All the sensors were reading within range, I simply couldn't find it. FWIW, the fuel tims were in the range of -3 +3, LT and ST, on a flat road steady speed, so correct.
Eventually, a P0410 popped up (SAIS valve), tried cleaning it, didn't work, so I replaced it. The hi idle went away for about 6 months. Go figure.
Over time, it happend less and less, and rarely happens now. When it does, I clear the code, the idle comes down, and it's good for many months.
I've read on one of the TB forums on person noticed on a customers car the air cleaner cover wasn't quite on correctly, he reset it, and a P0171 he wasn't even lookin for went away.
 

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Discussion Starter · #4 ·
Welcome to the Forum!

First question, which engine? We really need to know. Second question - how many miles.

Now, assuming it is the 4.2L I6 engine, double check and make sure the Engine Coolant Temperature (ECT) Sensor is connected and working properly. You should be able to watch the temperature on a halfway decent OBD II code reader/live data scanner. If the ECT is not reporting properly, the ECM can make the engine do some strange things.

It appears that the O2 sensors are working properly, BUT, when the engine is in closed loop mode (normal engine operation), the upstream O2 sensor's (#1 sensor) voltage should be bouncing around all over the place from just above 0.000 V to 0.700 volts. The downstream O2 sensor should be relatively stable and not changing much or as rapidly as the upstream O2 sensor.

If your upstream sensor's output voltage stays around any given voltage and does not bounce around, I would be suspicious of a bad sensor.

Good Luck!
Thank you so much for your reply, and I'm sorry for the lack of info, I'm new to the forum.

Yes, it's the 4.2 I6 engine. Vehicle has 210,000, but the engine itself only has about 110,000 because we got it replaced under warranty after the original spun a bearing.

The upstream 02 sensor just sits at 0.005 volts as soon as the vehicle starts. Before that, it just bobbles around in the 0.45 volts region, but as soon as get it started...it does not move at all, just stays stuck at the 0.0005 level, so I will go ahead and get that replaced ASAP.

Again, thank you so much, I really appreciate the help.
 

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Discussion Starter · #5 ·
I chased the same hi idle, P0171 code for years. Tightend the intake manifold, cleaned the throttle body, checked for vacuum leaks, etc., etc. All the sensors were reading within range, I simply couldn't find it. FWIW, the fuel tims were in the range of -3 +3, LT and ST, on a flat road steady speed, so correct.
Eventually, a P0410 popped up (SAIS valve), tried cleaning it, didn't work, so I replaced it. The hi idle went away for about 6 months. Go figure.
Over time, it happend less and less, and rarely happens now. When it does, I clear the code, the idle comes down, and it's good for many months.
I've read on one of the TB forums on person noticed on a customers car the air cleaner cover wasn't quite on correctly, he reset it, and a P0171 he wasn't even lookin for went away.
Thanks for the reply, I really appreciate it.

I had a weird 410 code before I stripped it down to do the timing chain, but once I put it all back together that code had gone away. I did read that someone else had had issues with the SAIS, so maybe I need to get that check valve replaced anyway, since I have read that it can fail and cause issues.

My short term fuel trim is coming up at 19.5, while the long term one is 27.5, which might make sense if the Oxygen sensor mentioned in the post above is messed up, and the idle comes up high, so the valve might be an issue. I think I'll replace it and see how it handles.

Thanks again for the help, I really appreciate it.
 

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'05 Chevy TB EXT
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When all gets said and done, I got money on you contaminated the B1S1 Oxygen Sensor with some glue or sealer or anti-seize compound ... or coolant ... or prelube oil ... or sumpthin'. .
 

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Discussion Starter · #7 ·
When all gets said and done, I got money on you contaminated the B1S1 Oxygen Sensor with some glue or sealer or anti-seize compound ... or coolant ... or prelube oil ... or sumpthin'. .
Thanks for the reply, I really appreciate your help, and I think you are probably right :). I'm also wondering if the car was just running in a crappy state while it was doing the P0017 code, and the sensor got contaminated from just horrible emissions.

I am wondering though, because since the initial perfect idling when I first started it up, now it idles high (950 ish) as soon as you start it up, even if I clear codes and pull the battery terminal for a few minutes. I thought the system didn't take input from the upstream O2 sensor until it went closed loop (although I am totally willing to be wrong about that). Would the messed up sensor affect idling immediately?

Again, I really appreciate your wisdom on this. My wife loves this trailblazer, and really wants me to keep it running :)
 

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2004 chevy trailblazer_lt
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DO NOT DISCONNECT THE BATTERY - REMOVE THE APPROPRIATE FUSE(S). Why? Because when you reconnect the battery, the HVAC actuators inside of the dash are commanded to run a recalibration procedure which stresses the old brittle plastic gears inside the actuators and the brittle plastic gears break and leaves you unable to control where the air comes out, or control the temperature of the air, etc. Replacing at least one of them literally requires the removal of the entire dash! So, if you ever need to actually disconnect the battery, such as in the case of needing to install a new battery, be sure and use some kind of Keep Memory Alive device to avoid the HVAC actuator recalibration routine.
 

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2004 chevy trailblazer_lt
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The reason why the engine idles "perfectly" at start up is because the engine is in open loop mode which means that ECM is ignoring input from various sensors, such as the Oxygen sensors, until the engine warms up to a specific minimum temperature that is programmed into the ECM. So, the ECM is using various factory pre-programmed data for the engine to run until the engine comes up to a specific temperature.

Now, when that minimum temperature is obtained, the ECM then switches to closed loop mode and uses the data reported by all the engine sensors - Oxygen, Engine Coolant Temp, Manifold Absolute Pressure, Intake Air Temp, Throttle Position Sensor, Camshaft Position Sensor, Crankshaft Position Sensor, etc. - to determine how the engine should be running. This is the point where your idle speed increases and the engine runs rough and runs rich.
 

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Discussion Starter · #10 ·
DO NOT DISCONNECT THE BATTERY - REMOVE THE APPROPRIATE FUSE(S). Why? Because when you reconnect the battery, the HVAC actuators inside of the dash are commanded to run a recalibration procedure which stresses the old brittle plastic gears inside the actuators and the brittle plastic gears break and leaves you unable to control where the air comes out, or control the temperature of the air, etc. Replacing at least one of them literally requires the removal of the entire dash! So, if you ever need to actually disconnect the battery, such as in the case of needing to install a new battery, be sure and use some kind of Keep Memory Alive device to avoid the HVAC actuator recalibration routine.
Thank you, I didn't know that. I will get one of those asap.
 

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Discussion Starter · #11 ·
The reason why the engine idles "perfectly" at start up is because the engine is in open loop mode which means that ECM is ignoring input from various sensors, such as the Oxygen sensors, until the engine warms up to a specific minimum temperature that is programmed into the ECM. So, the ECM is using various factory pre-programmed data for the engine to run until the engine comes up to a specific temperature.

Now, when that minimum temperature is obtained, the ECM then switches to closed loop mode and uses the data reported by all the engine sensors - Oxygen, Engine Coolant Temp, Manifold Absolute Pressure, Intake Air Temp, Throttle Position Sensor, Camshaft Position Sensor, Crankshaft Position Sensor, etc. - to determine how the engine should be running. This is the point where your idle speed increases and the engine runs rough and runs rich.
[/
Problem is it only ran perfectly in open loop on the very first start up. The second I kissed the gas pedal it has run high every time since. Now it runs at high idle immediately after starting, no matter what I do (cleared codes/battery disconnected (which I will obviously not do anymore)).

I'm still going to go ahead and replace the O2 sensor, and I'm also replacing the check valve for the SAIS valve, and I'll update here when I find out more.

Thanks again for the knowledge and wisdom, I really do appreciate it.
 

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2004 chevy trailblazer_lt
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Our pleasure and good luck!
 

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Discussion Starter · #14 ·
Look @ your fuel trims w/ engine @ 2500 rpm. If they drop (move toward zero) you have a vacuum leak. While you are there also monitor your O2's @ 2500 rpm.
Thanks for the tip. I took it upto 2500 rpm, short term fuel trim was still 19.5, so according to what you taught me, this is NOT a vacuum leak.

Interesting enough, the upstream 02 sensor is now just staying at 0.005v when running or not, so I think its my O2 sensor that needs to be replaced, which I'm doing next.|

Thanks for the input, I really appreciate it.
 

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Thanks for the tip. I took it upto 2500 rpm, short term fuel trim was still 19.5, so according to what you taught me, this is NOT a vacuum leak.

Interesting enough, the upstream 02 sensor is now just staying at 0.005v when running or not, so I think its my O2 sensor that needs to be replaced, which I'm doing next.|

Thanks for the input, I really appreciate it.
I had an O2 sensor sudden death once after doing a bunch of other work. Yeah, probably ended up contaminated. I did spend some time trying to find out if it was just the sensor or a true lean condition.

But that was silly. It was a really old sensor with I-had-no-idea how many miles were on it. Yours is at least 110K ish old (if new with the engine replacement). That's long enough. They do go downhill. My guess is that if you toss in a new one (stick with AC Delco) you'll be good to go.
 

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Discussion Starter · #16 ·
Just wanted to update everyone on the outcome. You guys were right about the O2 sensor. I took it for another run before taking it into a mechanic to get the O2 sensor replaced (because I decided that if someone was gonna strip the threads getting it out, it wasn't going to be me) and got the P1139 heated oxygen sensor code. The mechanic replaced the O2 sensor and got everything re-learned and balanced out, and the trailblazer is running great.

Thanks for all your wisdom and help, I really appreciate it.
 

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2004 chevy trailblazer_lt
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Thanks for letting us know!
 
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