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2003 gmc envoy_sle
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Discussion Starter #1
I have an '02 Envoy, 4.2L with 99,000 miles. I now have a Random misfire problem. I have performed the TB cleaning, and fuel injector replacment.

What is the most common problem for this? The misfire occurs randomly during idle, and quite significantly under a part throtle load. Once it downshifts, the misfire disappears.

Could this be an EGR valve?
 

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2002 olds bravada
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2,369 Posts
We don't have EGR valves on our engines. My advice to you is to change your spark plugs with ac delco iridiums (41-103). The manual states that spark plugs should be changed at 100k miles, and you're close to that mark. That should fix your problem, but you should also inspect air/fuel filters, fluid changes, etc. Good luck!
 

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2003 gmc envoy_sle
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Discussion Starter #3
Thanks for the reply.

I changed the plugs at about 77k. I did used AC Delco Iridium plugs.

You're right on the EGR. I was checking the manual, and it does not show an EGR valve.

Would the O2 sensor (pre Cat) be at fault? I believe the voltage is supposed to be between 0.1 and 0.9 volts.
 

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2002 olds bravada
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Yes, an oxygen sensor can cause misfires. I don't know the specific voltage range values, but I think you are correct.
 

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2004 chevy trailblazer_lt
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997 Posts
The front O2 sensor output should vary rapidly between 200 mV and 900 mV. It is read by a scan tool. (Don't try measuring output with a multimeter; it won't work.)

Are you getting a trouble code for a random misfire? If so, you should pull all your plugs again, make sure they are 41-103 AC Delcos and that all the coil packs are attached properly. (They are very often not attached properly after a plug change.)

You can also look at the cam position sensor actuator as it may be missing a screen or need cleaning.

If this doesn't help, you should get a spark plug tester and test each coil pack for good spark.


If you are NOT getting a trouble code, I would have your fuel pressure tested for possible fuel pump failure.
 

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2003 gmc envoy_sle
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Discussion Starter #6
My DTC is P0300 (Random Misfire).

The MIL flashes on when the engine starts to misfire under a load.

I changed the plugs to AC Delco 41-103 several months ago. I had a temperature problem, but this has been resolved with a new thermostat.

I will check the cam position sensor (CMP) to see if it is dirty. But the car runs fine except for an occasionaly "hiccup" at idle and when it's under a load.

My scanner shows the O2 sensor output changing a lot and quite rapidly. Is this normal?

Any other ideas??
 

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2003 gmc envoy_slt
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357 Posts
A code P0300 may mean that one or more of the following has happened:

* Faulty spark plugs or wires
* Faulty coil (pack)
* Faulty oxygen sensor(s)
* Faulty fuel injector(s)
* Burned exhaust valve
* Faulty catalytic converter(s)

* Faulty camshaft position sensor
* Defective computer

C.B.
 

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2003 gmc envoy_sle
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Discussion Starter #9
Thanks, CBWILSHA, for the information.

Wouldn't the code be more specific if the problem were a specific spark plug wire, plug or coil. In this GM computer code, it should show a specific cylinder where the misfire is taking place. I would think the "generic" code (P0300) would be something that is "common" with all cylinders.

Just trying to understand why the Random Misfire code is appearing.
 

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2002 olds bravada
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the code stands for Random/Multiple Misfire, so it may be more than one cylinder thats misfiring. Test each coil pack for a good sharp spark, and check the conditions of the spark plug while you're at it.

Have you checked the oxygen sensor values yet?
 

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2003 gmc envoy_sle
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Discussion Starter #11
Haven't checked the O2 sensor yet.

Unfortunately, my daughter had to take the Envoy back to Dallas with her.
It may be a week or so before I have it back.

I'll check the O2 sensor. What is the best method for checking the coil packs?
 

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2003 gmc envoy_slt
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Thanks, CBWILSHA, for the information.

Wouldn't the code be more specific if the problem were a specific spark plug wire, plug or coil.
Yes, if the problem was always a specific cylinder, for example, #3, the code would be P0303--the last digit of the code indicating which cylinder has the problem.

C.B.
 

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2002 chevy trailblazer_ltz
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You know going through the same problem, If I had to start over again, I would start with compression test. Last thing you want to do is spend a bunch of money to find low compression and be hit with a huge bill- Takes 15 mins to do and alleviates allot of stress. Just my :m2:
 

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2003 gmc envoy_sle
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Discussion Starter #14
Can anyone tell me what the Pre-catalyst O2 sensor reading should be at idle? Mine is changing quite rapidly (~every 1-2 seconds).
 

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2004 chevy trailblazer_lt
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Generally, the O2 sensor pre-cat should be reading between about .1 - .9 volts, and
fluctuating rapidly. The 1-2 second change rate that you describe sounds close,
and could even show faster, but sometimes scan tools don't refresh fast enough
to catch that rapid a change.

Just because it shows the P0300 random code doesn't mean it's really happening
on all cylinders. Sometimes the computer can't differentiate properly, but knows
it's having misfire issues, so it generates the "general" code. A friend of mine had
a bad bearing in his water pump, and it was knocking badly enough to trigger
a misfire code. It doesn't sound like that's anything you have to worry about,
but just an example of how the PCM can be confused. The problem might be
on two cylinders, and still generate the "random" code.

Definitely check all your coil packs for dirt or carbon tracking that might provide
a bleed path for voltage. Make sure there's no water or oil in any of the plug
wells, and make sure the wiring that goes to the coil packs is on correctly and not frayed
anywhere.

If you can, checking the compression is a good idea as well, just to make sure.

Check the plugs, in case one is damaged or faulty. Doesn't happen that often
that I can see, but is still a possibility.

If the problem fades when you shift to neutral, that cam position actuator or sensor
could be the culprit, but let's rule out the coils and plugs first.

Oh....have you run any fuel system cleaner through it? Maybe one of those
injectors isn't running as efficiently as it should be.

Cheers-

Chris
 

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2003 gmc envoy_sle
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Discussion Starter #16
Envoy Random Misfire - P0300

Christo829:

Thanks for the details. It sounds like you have some experience with this.

I will run some cleaner through it when I get it back. My daughter has it for the next week. I should have it by the weekend. I replaced the injectors and plugs last year (~20k miles ago).

The problem goes away when it's not under a load. For instance, when you are accelerating on the highway, or going up a hill. If the transmission downshifts, or the load is reduced, the misfire goes away.

I will let you know what I find out.

Thanks,
 

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2002 chevy trailblazer_ltz
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Just a thought but, if the purge canister gets saturated in gas and plugged could his throw out a misfire code even if it is evap related.
 

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chevy trailblazer_lt
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P0330 Trouble Code

All-

I thought I would post to let you know my experience. I have a 2002 Trailblazer, with the 4.2, about 125,000 miles. The engine started running rough, service engine soon light came on. I pulled the code and it was P0330, random misfire. Upon diagnosing the issue by checking all of the coils for spark, I found cylinder 1 coil to be shot. It was strange how the computer diagnosed this as a random misfire, but it was isolated to cylinder 1. I hope this helps some of you!
 

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2004 chevy trailblazer_lt
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It would be nice if the computer can detect the strength of every spark and give us feedback on exactly what was wrong. It would also be nice if the computer could replace all the plugs for us, clean the throttle body on a regular basis and parallel-park our cars.

They don't.

(Oh wait ... they do have computers that parallel-park cars. This is to replace "driving" and "experience" and "training" and "practice," I suppose ...)

The computer actually uses what is called "fuzzy logic" when it determines a loss of power or a slightly rich mixture. It can predict the misfire condition quite readily (it needs to - this is mandated for emissions reasons) but it has a harder time detecting why the misfire occurs. This is why the same code is thrown when a plug is bad, a coil is bad or even a valve is bad.

It also has a much harder time determining which cylinder misfires. If it is very clear which cylinder it is, and occurs on every power stroke, then it throws a cylinder code. If it is not quite clear enough or it doesn't occur on every stroke, it throws the random misfire code.

This is why we always advise proper diagnostics before changing parts. It would be costly to swap out all six coils when it is likely only one is the problem, so a $10 spark tester can go a long way to helping diagnose a bad coil.
 

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2005 chevy trailblazer_ls
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22 Posts
I have similar issues some days its fine has plenty of power other days its misfiring and idling rough at stop lights I cant hook up the OBD-2 to read it I guess I have a fuse out somewhere. anyone know where that fuse is??? any help would be greatly appreciated
 
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