Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum banner

1 - 16 of 16 Posts

·
Registered
Joined
·
7 Posts
Discussion Starter · #1 ·
Quick background story time first - I have an 05 Trailblazer with 115,000 miles that I originally purchased for my wife to drive with our young child. Truck developed a misfire on #4 and after new plugs and coils, there was no change. After a compression test, we found #4 had barely any compression, while the rest were strong. For us, was more cost effective to replace the engine.

After replacement, (what a "fun" job that was!) the engine runs great with no misses or trouble codes, but after a few minutes it seems to stumble and want to die. Wondering where to turn to find a solution, of if anyone has experienced what seems like a random stumbling and stalling at idle or in gear.

Notes: Throttle body has been cleaned. New alternator, battery, coils, and plugs (replaced during the swap.) A/C is discharged, so not in use. Fluids are at their proper levels.

Thanks for the help and guidance!
 

·
Registered
2004 chevy trailblazer_lt
Joined
·
1,117 Posts
Welcome to the forum!

The first thing I would check is your fuel pressure. Normal fuel pressure for your 2005 TB is 50 - 57 psi.

Please measure that and report back for more information.

Good Luck!
 

·
Registered
Joined
·
7 Posts
Discussion Starter · #3 ·
Thank you! I rather like the Trailblazer, but this engine is giving me fits!

Just measured my fuel pressure, and it is sitting right at 60 PSI. Even when the engine starts to run rough and stall, the pressure stays dead on 60.

I also noticed that when the engine starts to run rough, the smell out of the exhaust gets very rich. Where is the next logical step?

Thanks!
 

·
Registered
2004 chevy trailblazer_lt
Joined
·
1,117 Posts
Well, it sounds as if one or more of your fuel injectors might be leaky. First, with the fuel pressure gauge still attached to the test port, shut the engine off and see how long pressure is maintained. If it maintains pressure for several minutes, that is good and shows the fuel injectors are likely not leaky.

Next, using a decent scanner/code reader, see what your short term fuel trim (STFT) and long term fuel trim (LTFT) values are.

Also, while the scanner/code reader is hooked up, scan for codes and let us know what they are.

Good Luck!
 

·
Registered
Joined
·
7 Posts
Discussion Starter · #5 · (Edited)
This last run, the fuel pressure was at 58 PSI, and with the engine shut off, the fuel pressure did not drop in the 5-10 minutes that I observed. I am leaving the gauge on overnight to see if there is any change at all.

STFT is -19.5% at idle. When I press the gas pedal, it rises to 0%. LTFT is 0%.

There are no codes present. I am using an Autel MaxiCheck scanner.

20210529_211255.jpg 20210529_211314.jpg
 

·
Registered
2004 chevy trailblazer_lt
Joined
·
1,117 Posts
Well, the STFT definitely indicates that the engine is running rich. Ideally, when you add the STFT value to the LTFT value the sum should be in the range of -10% to +10%.

Have you inspected the EVAP system to make sure that everything is intact?

Do you have the habit I once had of trying to get every last drop you can into the gas tank at each fill-up? I do know that a fuel saturated charcoal emissions canister can and will cause a rich condition. About 6 months ago I had to replace the charcoal emissions canister on our 99 Buick Century because my Fuel Trim data was in the -16% to -25%+ range. I spent a few weeks tracking down various causes of a rich condition, and everything was fine, so I unplugged the EVAP canister purge valve, drove around and low and behold the fuel trim values went into the normal range.

Check your O2 sensors. If they are not working properly, that can cause a rich condition. I would also make sure your MAP sensor is working correctly as well.

About the only other thing it could be is there is a problem with a valve assembly located on top of the gas tank where an EVAP line connects to. The valve might have failed and is letting raw gasoline get into the charcoal canister. (This can also happen if gas comes out of the EVAP line that goes into the fuel pump assembly.) I'm attaching a couple of files so you can see what I mean.

Hope this helps.
 

Attachments

·
Registered
Joined
·
7 Posts
Discussion Starter · #7 ·
Thank you for your support.

I have not inspected the EVAP system, so that is a good place to go next.

The truck was only on the road for a week before I made the call to replace the engine. I only have filled it up one time since I have owned it (mid February). I have treated the fuel since, of course. I cannot say for sure what happened before I got it, but maybe that charcoal canister became super saturated. It is worth investigating for sure. Hopefully I don't have to drop the tank to fix anything, but it can't be worse than that engine swap.

If memory serves, O2 sensors should bounce between .2 and .9 mv, so those are easy to check. How to I go about checking the MAP sensor?

This is just such an odd problem. The truck ran fine before the swap, albeit with a bent exhaust valve on #4. Oh well, I suppose I'll keep going deeper down the rabbit hole.

Thanks again for the help! I will report back with more information tomorrow if I get the time to work on the truck again.
 

·
Registered
2004 chevy trailblazer_lt
Joined
·
1,117 Posts
Here is how to test the MAP sensor using a digital multimeter (DMM)

Regarding the O2 sensors, the upstream O2 sensor changes it output frequently and you are correct about the voltage range for it. The downstream O2 sensor does not change it's output are frequently as the upstream one does, and it will likely be in the range of 0.5 - 0.9 V DC

Hang in there and I am sure we'll get this figured out!
 

·
Registered
Joined
·
7 Posts
Discussion Starter · #9 ·
Possibly interesting results from this morning:

I noticed B1S1 O2 sensor not moving around much during a test run this morning, so I decided to unplug it. When unplugged, short term fuel trim drops to zero and there is no more rough idle, stalling, or rich fuel smell. I assume the computer substitutes a generic value for the missing sensor. The only other thing is that the truck operates in OL operation, which makes sense, given the missing sensor.

The truck runs great in this situation, but I know this is not permanent because of the eventual CEL. Is there a way that I can bench test my O2 sensor?
 

·
Registered
2004 chevy trailblazer_lt
Joined
·
1,117 Posts
Only way I know of to test an O2 sensor is by back probing the connector and measuring the voltage it produces using a digital multimeter (DMM). If the heater circuit on the O2 sensor is not working, it will set a code. Of course, your Autel MxiCheck should be able to read and show you what the voltages are for the O2 sensor in question.

If you decide to replace the O2 sensor, several on here (me included) recommend staying away from the Bosch O2 sensors and to use one from ACDelco, Delphi, or Denso.

O2 sensors are known to age with both time and mileage so you might consider replacing yours if you never have before. I know you can get both O2 sensors, Denso brand, from rockauto for under $60 plus shipping. That's where I get mine from.
 
  • Like
Reactions: Thetonymartin

·
Registered
2008 chevy trailblazer_lt
Joined
·
87 Posts
I wonder if it's possible to have air getting into the exhaust, upstream of the O2 sensor and that extra air causing the ECM to add fuel? I've heard about leaky intake manifolds here, too, but I'd sure wonder if cyl. 4 injector is dumping too much fuel and wearing out piston rings, fouling plugs, etc. I had a PCV valve fail back in the day and the car ran bad...except at WOT, where it ran normally. I've heard of EGR valves failing, too, but probably doesn't apply here.

Rob in AZ
 

·
Registered
2004 chevy trailblazer_lt
Joined
·
1,117 Posts
RPatton76 - I am sure it is possible for air to get into the exhaust stream ahead of the upstream O2 sensor, but I would think that since the exhaust stream can be viewed as being "pressurized", I am unsure of how much air could actually bee pulled in. Of course I could be dead wrong.
 

·
Registered
Joined
·
7 Posts
Discussion Starter · #13 ·
RPatton76 - For what it's worth, when I changed the plugs while the engine was on the stand, they were not fouled at all, and were still in good shape if I had elected to reuse them. I can check when I get my O2 sensor in since I'll be right there close.
 

·
Registered
Joined
·
7 Posts
Discussion Starter · #15 ·
Final update:

I received and installed the new Denso O2 sensor. Fuel trim is now within the correct range and the sensor voltage is moving like it should. Thank you all for your help in figuring out this issue!

I have never seen or heard of a bad O2 sensor causing these issues. Definitely a new one for me.

20210612_135315.jpg
 

·
Registered
2004 chevy trailblazer_lt
Joined
·
1,117 Posts
Thanks for letting us know that the new O2 sensor worked!
 
1 - 16 of 16 Posts
Top