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Hello to All: :)
Ok I have a good one for you mechanic's out their. I have an 2003 Trailblazer with a 4.2L, it is the XLT body.
The problem that I an having is that their is a ruff idle:
1) When I put the vehicle into gear, driver or reverse their will be a slight ruffness to the idle. It is smooth when it is in Park and Nutural.
2) Very ruff when the A/C is turn on and idling. When you drive their is not problems at all runs like a champ.
What I have already done so far to the vehicle:
1) Replace the Spark Plugs with AC Delco Iridium #41103 and gap them at .044
2) Clean the throttle body, no change so I replaced it with a new ACDelco Throttle Body Assy.
3) Replaced the Exhaust Cam Position Actuator Solenoid
4) Reset the cam sensor and crank sensors with my scanner, "Modus"
5) Replaced the MAP Sensor, just because.....
Now I am still at step one, I still have a ruff idle with my vehicle and I have read that it is the Throttle Body and that is 99.9999999% of the problem.
I have read that the coolant sensor could be bad, the O2 sensor is not working correctlly and that I could have a weak coil pak.
Would anyone have a idea of the issue that I am having. I am at the point that it could be a motor or trans mount maybe the A/C Compressor it self.
The vehicle at this time has 94,456 miles, own it from new and I do keep up on my maintenance.
So is their someone that might have a good idea of what the issue maybe.

Thank you for reading this and for any suggestion that you will have.


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Let me start by thanking you for a well written post! It's great to get all the information from someone about their problem. That makes our job much easier!

Anyway, from what you describe, my first thought is always throttle body related issues. A big tell-tale sign is an idle issue when turning the air conditioning on. That is probably the number one common problem on all of the Trailblazer/Envoy models. But, seeing as how you've cleaned, and then actually replaced the throttle body, that is probably not the concern (unless it's just some bad coincidence). Usually after a thorough cleaning and/or replacement, the problem is fixed after a short drive and the idle re-learns a normal position.

Now, for what can probably be the worst-case scenario, which unfortunately is a common issue as well, is that you have a misfire condition at idle only caused by a valve problem in the cylinder head. With a tune up done, and probably NOT a weak coil, that only leaves fuel injectors or a mechanical problem as the culprit. I've rarely, if ever, replaced fuel injectors on any of those models, so that's not a likely cause. That leave a mechanical problem, such as a sticking valve in the cylinder head.

The symptom of a valve/head issue is a misfire or rough idle usually only at idle, or while sitting in park or drive. Normal driving the engine will act fine and run quite good. This seems to be what you are experiencing. The bad news is....this would require cylinder head replacement which can run upwards of $2,000. Trailblazer Misfire Code P0300 At Idle

Now, I'd sure want to positively know if that was the problem before spending big bucks on a repair. This might require you to visit a dealership to get a good diagnosis. I'm not sure you want to spend money 'trying' parts to see if it fixes it, like replacing a coil or whatever else you might suspect. It would be good dollars spent to have someone experienced with this issue take a look at it. You might get lucky and have it be something easy, but I just wanted you to know of all the things it could be.
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In the automotive industry for 20+


   
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ProTech wrote:
Let me start by thanking you for a well written post! It's great to get all the information from someone about their problem. That makes our job much easier!

Anyway, from what you describe, my first thought is always throttle body related issues. A big tell-tale sign is an idle issue when turning the air conditioning on. That is probably the number one common problem on all of the Trailblazer/Envoy models. But, seeing as how you've cleaned, and then actually replaced the throttle body, that is probably not the concern (unless it's just some bad coincidence). Usually after a thorough cleaning and/or replacement, the problem is fixed after a short drive and the idle re-learns a normal position.

Now, for what can probably be the worst-case scenario, which unfortunately is a common issue as well, is that you have a misfire condition at idle only caused by a valve problem in the cylinder head. With a tune up done, and probably NOT a weak coil, that only leaves fuel injectors or a mechanical problem as the culprit. I've rarely, if ever, replaced fuel injectors on any of those models, so that's not a likely cause. That leave a mechanical problem, such as a sticking valve in the cylinder head.

The symptom of a valve/head issue is a misfire or rough idle usually only at idle, or while sitting in park or drive. Normal driving the engine will act fine and run quite good. This seems to be what you are experiencing. The bad news is....this would require cylinder head replacement which can run upwards of $2,000. Trailblazer Misfire Code P0300 At Idle

Now, I'd sure want to positively know if that was the problem before spending big bucks on a repair. This might require you to visit a dealership to get a good diagnosis. I'm not sure you want to spend money 'trying' parts to see if it fixes it, like replacing a coil or whatever else you might suspect. It would be good dollars spent to have someone experienced with this issue take a look at it. You might get lucky and have it be something easy, but I just wanted you to know of all the things it could be.



Hello Again; butchkaz

Thank you for your reply on this matter and your many years of experance in the mechanical field.
Just to touch on a statement you made of a engine code. I do not have any codes at all and never did get any codes. This issue just came up out of no were and by suprise after tune up the vehicle.
I did check on the TSB for bent valve issues on the 4.2L and sticking valve for the head with my local Chevy Dealer just to see if it would be under warranty before I went down that road. BUT? I was told by the Chevy Dealer, "Delillo Chevy in Huntington Beach, Ca." that thay have been having ruff idle issues with the 4.2L and found that the Fan Clutch would be bad or the Motor Mounts "Collapsing or Getting Hard". Now have you heard of anything in this area. I am looking up information on both issues and do see reports of replaced motor mounts "only OEM" and stones in the transmission mount area.
At this time I am pricing my parts and line that I am going to follow next.
Please feel free to leave updates as I will with you on what I fine.

Fire Equipment Mechanic 18yrs.
Motor Sgt. U.S. Army 22 yrs.
Heavy Equipment Mechanic U.S. Army 30yrs.

P.S.
Yes you are very right on the sad news of a bad head or valve was hoping that it might be something else.
As I always say: Plan for the worse and hope for the best, this way when the worse hits it is not so hard and when the best comes then their is relef... :D


   
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It wasn't actually 'butchkaz' that replied to your post, it was me 'ProTech'...so I'll accept your thanks instead of him.

As for your problem, it's not that it couldn't be motor mounts or a bad fan clutch, but that is very rare in my opinion. I've seen way more mechanical problems with the engine than anything else. But, who knows, you might get lucky!

I've actually been to Delillo Chevy during a visit to Huntington Beach and they are a top notch facility. I'm sure your vehicle would be in good hands there.

Best of luck,
ProTech
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In the automotive industry for 20+


   
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Well Thank you Pro Tech for all your help and service I will say I am very sorry that I did not catch that error. Your knowledge was very awarding to let you know.
I took my wife's vehicle to work to take a look at the valve issue that might have been and luck is on my side it is not a stuck or bent valve so I can move on from this point.
I am going to look at the engine mounts this weekend, replacing the struts and shock on the vehicle that I ordered a few weeks ago so while I am doing that I will check out the mounts in the process.
Question for you Pro Tech? and this is just for my knowlegde mostly:
If their was a stuck valve or bent valve issue would have their been a "P" code thrown as you stated earler? I had no codes and no miss fire history so the event of a valve issue just came and gone.
I did do some research on the 4.2L engine and found that the valve timing has a close tolerance and was concerned of pison damage so I look into your idea of the stuck valve and thanks again for the info. and no issue found. I just want to make sure that I am on track too.

Thank you again and I will let you know of these engine mounts


   
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Yes, it will usually set a misfire code....but I have seen circumstances where the miss is so faint that it will not set the light on or set a trouble code, but can be felt in the vehicle. Hopefully it is just a motor mount or other issue.
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My Envoy needs the cylinder head replaced/remanufactured. I was wondering, if I only experience minor problems (i.e. slightly rough idle and flashing check engine light [which disappears when I give it gas] on) is it okay to just drive the vehicle like this? I feel like I've been driving it about a year at least now with these issues and I am rarely if ever at idling speeds. I drive mostly highway, but I do let it idle and warm up in the winter. I plan on getting it fixed at some point, but I am wondering how bad it is for the engine to operate like this?


   
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You can keep driving the truck this way. It will not cause any more damage to the engine. But, you will not be able to pass the emissions / smog test if you need to have one done.
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