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2004 gmc envoy_slt_xl
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Discussion Starter · #1 ·
I have read and read a lot of threads about the rough idle issues. I am not sure if I would categorize as a rough idle or just a vibration. I am about ready to pull my hair out trying to figure out this issue :mad:. It has developed this annoying vibration or "rough" idle at around 650-700 rpm when in gear. It seems a bit worse when cold. It seems to run just fine but the vibration rumbling tone is annoying. fyi - It also has never set a code.
Here is what I have done so far.
  • New Air Filter
  • New Fuel Filter
  • Clean Throttle Body (removed)
  • ran Sea Foam into intake using vacuum line
  • New AC Delco Spark Plugs (OEM)

I also contacted PCMforless and ordered an update - went with the core program to ensure I have all of the updates. Still have yet to receive it though.

What I was wondering what else can I check? Or clean?

Any help appreciated.

Thanks,
Rob
 

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2004 gmc
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How many miles? Some members have reported bad heads that showed up with with rough idle, and then they checked compression and got the bad news.
 

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2002 chevy trailblazer_ltz
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25 Posts
It's just an idea, I had a few other cars that started to vibrate in gear at a stop and they had broken a mount. (I drove them hard).

I would have someone stand to the side of the engine compartment looking at the engine while some one else torques the engine, small amount of gas while foot is firmly on the brake and see if a side of the engine jumps or moves a lot. I'm sure it's not the best way, but may give you an idea.

How can you tell if a broken motor mount? Of course they look ok from the top.
 

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2004 gmc envoy_slt_xl
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Discussion Starter · #6 ·
This only has 51000 miles. All of the plugs looked really good - no discolored plugs like any of them misfiring.

I will check the motor mounts and maybe trans mount.

Thanks for all of your help.

Rob
 

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2004 gmc envoy_slt_xl
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Discussion Starter · #7 ·
Well I checked the motor mounts and they seem to be ok. Any other ideas or things to check?


???

Rob
 

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2007 gmc
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159 Posts
Have you cleaned your MAF sensor? checked for any vacuum leaks? check your cat? May be clogging up?
 

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2004 gmc envoy_slt_xl
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Discussion Starter · #9 ·
Have you cleaned your MAF sensor? checked for any vacuum leaks? check your cat? May be clogging up?

Yes cleaned MAF sensor. Yes checked for vacuum leaks and never found any. Never checked the CAT but it seems to breath ok through the exhaust. (stock exhaust)
 

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2005 chevy trailblazer_ls_ext
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Did you do a compression check?

Also on my O5 there is a vibration or rattle at idle. If I go to the back of the truck and put my big foot on the exhaust pipe and push down a little, the vibration will go away. I looked around the muffler area and there is a hanger that seems to be bad.
I haven't gotten around to replacing it yet - partly because I'm lazy and partly because the vibration hasn't gotten any worse and I've just kind of learned to live with it! :D
 

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2004 gmc envoy_slt_xl
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Discussion Starter · #12 ·
Sorry I don't have any fresh ideas, but I should point out there is no MAF sensor in 2004. If it was between the air filter and the throttle body that's the IAT sensor.

Oops! Sorry. I guess I cleaned the IAT sensor.

Also what is the sensor called on the top of the intake? just curious.
 

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Discussion Starter · #13 ·
Did you do a compression check?

No I didn't do a compression check - no compression guage to check. I'll have to see if the auto parts stores have any to rent or check out.

What is the compression suppose to be on these I6?

Rob
 

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2005 chevy trailblazer_ls_ext
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I don't have the exact number in front of me - maybe someone with a service manual can tell you. More than anything else you are looking for a variation between cylinders.
In other words if most of the cylinders measure 170 - 175 pounds, and one measure around or even under 100, then you know you have a problem. I think normally about 20 percent difference is acceptable.


Edit: Just looked in my Haynes (FWIW). According to them, minimum compression is 150 psi, typical is 215. Lowest allowable between cylinders is 70 percent of the highest reading.
 

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2004 gmc envoy_slt_xl
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Discussion Starter · #15 ·
compression checked out ok.
#1 - 190
#2 - 190
#3 - 185
#4 - 190
#5 - 190
#6 - 180

I am thinking of having the dealer do a injector cleaning? Worth it?

I wish there was a way to check the coils too.

Rob
 

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2003 chevy trailblazer_ls
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Well I checked the motor mounts and they seem to be ok. Any other ideas or things to check?


???

Rob
I am not quite sure which thread to answer first....
How did you check the motor mounts???
Have someone stand next to (in front if you trust the person behind the wheel) the TB with the hood open. Some one else is behind the wheel and put the TB in Drive with the brake really pushed down and hit the loud pedal. Does the engine rotate a lot? Same thing in Reverse and what does the engine do? That will most of the time show a broken engine or trans mount.
 

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2004 gmc envoy_slt_xl
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Discussion Starter · #17 ·
I am not quite sure which thread to answer first....
How did you check the motor mounts???
Have someone stand next to (in front if you trust the person behind the wheel) the TB with the hood open. Some one else is behind the wheel and put the TB in Drive with the brake really pushed down and hit the loud pedal. Does the engine rotate a lot? Same thing in Reverse and what does the engine do? That will most of the time show a broken engine or trans mount.
That is exactly what I did to check the motor mounts and the motor really doesn't move or rotate like the mount was broke.

Update: I did buy a new coil and then use it to exchange each coil out one by one to see if the vibration goes away. But no luck I exchanged everyone with the new coil and still is the same. I suppose I could have more than one coil bad but I think the odds are small even though I read a member having 3 coils go bad at once.

I also bought a new Oxygen sensor because the mileage was really bad (10-12mpg in town). I think that did improve the mileage but I will know more tomorrow - going on 100 mile drive.

Also checked to see if the converter is plugged - it's ok.

If it doesn't get better I am thinking an injector cleaning maybe at the dealer. I do have some techron in the tank now but I feel it probably doesn't do as well or does it?

I am running out of ideas..
 

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2003 chevy trailblazer_ls
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Update: I did buy a new coil and then use it to exchange each coil out one by one to see if the vibration goes away. But no luck I exchanged everyone with the new coil and still is the same. I suppose I could have more than one coil bad but I think the odds are small even though I read a member having 3 coils go bad at once.
If it doesn't get better I am thinking an injector cleaning maybe at the dealer. I do have some techron in the tank now but I feel it probably doesn't do as well or does it?

I am running out of ideas..
What will drive the coils nuts and burn them out is when you get a ground problem with the crank sensor and causes the coils to fire multiple times per revolution and get overheated. I would guess you do not have this problem. (backfires, stumbling, etc.)
Techron helps but a prof. injector cleaning is much better. Someone on this site did a write up on doing a really good injector cleaning that will work very well. I would not pay the dealer to clean my injectors, or anything else. The members here have a much better data and general knowledge of the 4.2. Can you get your hands on a scanner that records while you drive the TB?
 

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2004 gmc envoy_slt_xl
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Discussion Starter · #19 ·
No I do not have any connections where I can get a scanner - bummer.

I will be receiving my PCMforless tune today and will install it tonight. I am curious if that makes any difference.

I will look for the injector cleaning link to see what it takes to clean them yourself.

Thanks for the reply.

Rob
 

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2004 gmc envoy_slt_xl
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Discussion Starter · #20 ·
Got the PCMforless core with tune installed last night. Throttle response is much better and it shifts A LOT better - (shift level 1). I still have yet to have the CASE relearn done. This new PCM did not fix my low rpm vibration though like I was hoping. I think I can explain more of the issue now too for someone to help.

It has a "vibration" ONLY at a very specific rpm (~680rpm on tach) whether in gear or not. It seems to be a little worse when in gear but I feel the reason is it is under load is why. It smooths out below and above ~680rpm in gear or not. It still has not set a code.

Here is what I have done so far.
  • New Air Filter
  • New Fuel Filter
  • Clean Throttle Body (removed) - sprayed with B12
  • Ran Sea Foam (1/3 of can) into intake using vacuum line - remaining into gas tank.
  • Techron in Tank (still running on tank - almost empty now)
  • New AC Delco Spark Plugs (OEM)
  • Compression Test - 180-190 all cylinders
  • Oxygen Sensor replaced (new) - This made the mileage go up by 2 mpg.
  • Removed Camshaft Actuator - all 3 screens in place - assuming ok.
  • Checked for Plugged CAT using the intake vaccum method and was ok. (http://autos.yahoo.com/maintain/repairqa/battery_electrical/ques083_0.html)
  • Bought one new Coil and used it to replace every coil (replace, run, test, replace, run, test,...) - no change noticed.
Anyone have an idea why the vibration would only happen at a very specific rpm and no where else in the rpm band?

Any help much appreciated,
Rob
 
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