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2004 chevy trailblazer_lt
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55 Posts
Discussion Starter #1
I had noticed the issue last winter as it came and went. It only seems to be getting worse. After a cold start in the morning and hitting the first red light the engine shakes terribly when in drive and stopped completely (not as bad as a misfire though, been there). I tend to put it into neutral to not draw attention and the shaking stops completely.

I've looked into this a few times but haven't bothered to do anything about it until now since it seems to only progress.

I've done a decent amount of work under the hood as this is my learning car. I think I should reset the PCM - I haven't after I had cleaned the throttle body, replaced the water pump, fan clutch and couple dozen other things I wont go into right now. I read that disconnecting the battery for 10 minutes should reset the PCM. I'll try that and see what happens but I know it'll fall onto a mechanical issue.

Any ideas what it could be? Engine mounts? Transmission? Thermostat?

-KML
 

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2004 chevy trailblazer_ls_ext
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287 Posts
The PCM in the 2004's has a history of causing trouble. Here is a bulletin regarding the subject:
If you have an EXT it was built in Oklahoma City. I'm not sure whether the standard wheelbase vehicles were assembled there as well or not. I had a P1680 code set twice, once in June and again just a couple of weeks ago. My TB falls below the VIN breakpoint so I had the dealer re-flash the PCM. They charged me $125 and it took abut 1-1/2 hours.
 

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2004 chevy trailblazer_lt
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55 Posts
Discussion Starter #3
No it's not an EXT.

I wonder if the throttle is stuck intermittently? In a way it behaves as if I'm pressing the brakes and gas at once, but I'm not.

I've read that some cars need to do a "relearn" after cleaning the throttle body. I wonder if not doing that a couple years ago is contributing to this.

-KML
 

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2007 chevy trailblazer_ls
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62 Posts
Very common. Motor mounts. Use only GM. Further proof of this would be if it does not shake nearly as bad in reverse.
 

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2003 chevy trailblazer_ltz
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187 Posts
Connect a scan tool, look at the data stream. Find out if the IAC and TPS are working properly. While you're looking at that, verify all the sensors and outputs.

The answer is almost certainly in the data stream.

For the record...how old are the spark plugs? What is the cranking compression of each cylinder? Fuel pressure adequate?

Are you buying "Top Tier" fuel, the stuff that has an appropriate amount of detergent additive to keep the injectors and intake ports clean?
 

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2004 chevy trailblazer_lt
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55 Posts
Discussion Starter #6
Just wanted to update this one. Still not fixed, but I have more experience with the issue.
It ONLY happens when in drive & stopped while the engine is still getting to operating temp. No sign of it when in neutral, reverse or park.

I'll have to inspect the motor mounts. I knew it was a common issue since I bought the car, just didn't know if that's what was going on in my situation.

As for that last reply here, I do not have the experience to run those diagnostics but certainly worth knowing!
I'll be sure to check the sensors as well as compression & pressure later on when I can. I had replaced the spark plugs with the OEM AC Delco ones when I replaced a blown ignition coil a couple years back.
 

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2004 chevy trailblazer_ls
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366 Posts
Check that the coolant sensor is ranging correctly, and the thermostat too. Seems it's that grey area where it's not quite warmed up (open loop). There's a few sensors that work in open loop, O2, tps, etc. If you can, check them with a scanner.
I've seen these problems before and they can be a bear to find.
 
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