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Discussion Starter · #1 ·
This is my first post so let me start off by saying hi to everyone. I've been dealing with the dreaded rough idle & the ocassional cutting off. I,ve done a lot of searching on the site and have found a lot of good info. Let me tell ya what I've done and see if I can get any other possible help. Have taken the throttle body off and cleaned it (several times), changed the spark plugs (delco's), changed the #3 coil pack, changed the fuel filter, all were changed with OEM parts. I have even disconnected the coil's one at a time to see if the idle stayed the same or got worse to see if there was another bad coil that had not set a code. Is there another way to test/read them. Any additonal help/thoughts would be greatly appreciated. Like the title says I'm about over it and ready to get rid of it. It's an 04,I6 2wd w/65K on it.

J
 

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2003 chevy trailblazer_lt
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425 Posts
welcome to the site. i would try a bottle of fuel injector cleaner and see if that helps. and a bottle costs like 5 bucks
 

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2007 chevy trailblazer_ls
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Good start, here are a couple of possibilities....Bad or sticking injector causing the missfire/stall, bad fuel pump cutting off or not producing enough preasure, or a bad preasure regulator and last but not least a vacuum leak. Start off with a preasure test and run it till the symptom is replicated. this will detect most of the fuel problems. To check for a bad injestor, the "long screw driver or medal rod" placed on the end of each injector can diagnose a sticky or failing injector, it just takes patience. A simple vacuum test will indicate a leak and subsequent start to finding it.:m2::thumbsup:
 

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Discussion Starter · #5 ·
Good start, here are a couple of possibilities....Bad or sticking injector causing the missfire/stall, bad fuel pump cutting off or not producing enough preasure, or a bad preasure regulator and last but not least a vacuum leak. Start off with a preasure test and run it till the symptom is replicated. this will detect most of the fuel problems. To check for a bad injestor, the "long screw driver or medal rod" placed on the end of each injector can diagnose a sticky or failing injector, it just takes patience. A simple vacuum test will indicate a leak and subsequent start to finding it.:m2::thumbsup:

I'm not sure how to perform a pressure test on the fuel system while using the truck. Please give me some direction on how to accomplish this. Where would I hook up to to perform the vacuum test?

thanks for your help
J
 

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2007 chevy trailblazer_ls
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184 Posts
For the fuel preasure test look for the shraeder valve located along the fuel rail to the front of the engine. Look for it on the driver side, very front of the engine just behind the timing chain cover. Look for a large "tire valve cap" and unscrew it. This is where you would place the guage. To test it, you simply hook it up, start the engine and observe drops in preasure, eratic needle movement etc. To do a vacuum test, there is a test port in the intake plenum just behind the fuel test port in the intake plenum. It has a plug over the port. Remove it, place the test guage here and observe the needle movement. You should observe steady needle movement, give it throtle and the needle will drop and recover. If its bouncing around, vacuum leak indicated, or if it's low..... leaking intake manifold, ignition problem etc. Best bet is to get yourself a manual (I use Haynes) and you can get more specific numbers of where the reading should be.:thumbsup:
 

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Discussion Starter · #7 ·
For the fuel preasure test look for the shraeder valve located along the fuel rail to the front of the engine. Look for it on the driver side, very front of the engine just behind the timing chain cover. Look for a large "tire valve cap" and unscrew it. This is where you would place the guage. To test it, you simply hook it up, start the engine and observe drops in preasure, eratic needle movement etc. To do a vacuum test, there is a test port in the intake plenum just behind the fuel test port in the intake plenum. It has a plug over the port. Remove it, place the test guage here and observe the needle movement. You should observe steady needle movement, give it throtle and the needle will drop and recover. If its bouncing around, vacuum leak indicated, or if it's low..... leaking intake manifold, ignition problem etc. Best bet is to get yourself a manual (I use Haynes) and you can get more specific numbers of where the reading should be.:thumbsup:
Thanks for all your help. I will do these test and over the next couple of days and see what i get for readings. once again thanks
J
 

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Discussion Starter · #9 ·
Just to clarify, it is not setting codes now?
I've been dealing with this since last July. It has set a code or 2 over the past year, P0303 (replaced #3 coil), had the code for the cam shaft sensor and I took it out cleaned it checked for oil around the connector (there was no oil) and put it back in. I 've had a misfire on another cylinder swaped the coil from that cylinder to another to see if the misfire followed and the code has not returned. It has also had the timing over advance code 1 time when
it had the cam shaft sensor deal but it has not retuned since I cleaned that sensor.

J
 

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2007 chevy trailblazer_ls
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184 Posts
With the codes you have received and the randomness that you are getting them at, it is possible that the crank sensor is bad/going bad. This sensor tells the pcm/ignition module when to sent spark, so if it's intermittent, it can cause faulty coils/missfire codes. For an $80 part, it could be piece of mind to try that. (I'm guessing at the cost, it's what the last cost was for one of these sensors on another vehicle a year ago) Short of have the ODB II system completely scanned and a "freeze frame" take while the vehicle is run through it's paces etc. it sound like a bit of a guessing game. Unfortunately the hand held code reader won't do this, I have a program I run on my laptop called "Autotap" and it will do all this diagnostic stuff.:m2:
 

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2004 non_gmt360 other
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610 Posts
Thanks for all your help. I will do these test and over the next couple of days and see what i get for readings. once again thanks
J
Here's a pic of the schraeder valve. right in front of the fuel tank.

Probably a stupid question but how long has it been since you changed the fuel filter ?
 

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2007 chevy trailblazer_ls
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O.K. this is right out of the manual: "The Crankshaft Position Sensor (CKP) provides the PCM with an crankshaft position signal. The PCM uses the signal to determine the spark sequence (firing order) for each cylinder. The PCM also uses the signal to precisely control ignition timing and caculate engine speed (RPM). The signal is used by the ODB II system for misfire detection." Does any of this seem familiar with your symptoms????:mad:
 

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Discussion Starter · #17 ·
O.K. this is right out of the manual: "The Crankshaft Position Sensor (CKP) provides the PCM with an crankshaft position signal. The PCM uses the signal to determine the spark sequence (firing order) for each cylinder. The PCM also uses the signal to precisely control ignition timing and caculate engine speed (RPM). The signal is used by the ODB II system for misfire detection." Does any of this seem familiar with your symptoms????:mad:
ok I did the fuel pressure test and the vacuum test and it all seemed ok.
From what your saying here it seems like that (CKP) is possiibly the culprit
I can get one from the dealer for $28. I've asked a couple of different dealers what the cost would be for a "case relearn " and they say that they dont't know what that is and keep talking about updates for the PCM or a re-flash. From what I've read on the forums here that those are not what have to be done and that there is actually a procedure called a "case re-learn" in there in their service manual for replacement of the (CKP). Is this correct?
 

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2004 gmc envoy_slt_xl
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ok I did the fuel pressure test and the vacuum test and it all seemed ok.
From what your saying here it seems like that (CKP) is possiibly the culprit
I can get one from the dealer for $28. I've asked a couple of different dealers what the cost would be for a "case relearn " and they say that they dont't know what that is and keep talking about updates for the PCM or a re-flash. From what I've read on the forums here that those are not what have to be done and that there is actually a procedure called a "case re-learn" in there in their service manual for replacement of the (CKP). Is this correct?
I had the same issue when asking the dealer. Ask for Crankshaft Angle Sensor Error Relearn. I got a quote anywhere from $50 to $150 to do the procedure.

Good luck.
Rob
 

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2007 chevy trailblazer_ls
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Right out of the manual:'A crank sensor variation relearn procedure must be performed using a TECH 2 scan tool. Have the PCM reprogrammed by a dealer or service department etc. etc." And of course they don't give any information on what that procedure is or what it's supposed to do. One thing for certain is, that after replacing the sensor, disconnect the negative battery cable for a few minutes to clear any codes, stored freeze frame data and stores sensor variables. This is done because the new sensor will operate a different variable than the old one and the PCM will adjust other parameters as a sensor "ages" to ensure the most effecient operation etc. of the engine. Hope this helps.

Andrew:thumbsup:
 
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