Problems fixing rough idle - code P0300 - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum



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Go Back   Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum > 2002 - 2009 TrailBlazer/Envoy Tech > 02-09 Vortec 4.2L I-6 Engine and Drivetrain > OEM Issues

OEM Issues Original Equipment Repairs and/or Problems

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  #1  
Old 02-08-2008, 03:29 PM
grjoyner grjoyner is offline
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Problems fixing rough idle - code P0300

Hi everyone,
I have been reading this forum since I bought my 2002 TB I6 in 2004 and have been helped numerous times by the advice posted. This time, I haven't had much luck fixing my problem after reading through the forums multiple times. Here's my issue:

I have about 66,000 miles on my TB. About two weeks ago, I stopped at a light and all the sudden the engine started running really rough. By the time the light changed the SES light was on solid. As I accelerated from the light, the engine continued to run rough until I was up to around 2.5K RPMs and it seemed to smooth out. At that point the SES light went from solid to blinking. When I slowed down again, the engine started running rougher the slower I got and the SES light went back to solid. I also noticed my gas mileage dropped from around 14 to 10.

I jumped on the forum and started reading when I got home. Based on the information I found, I decided to try a bottle of fuel injector cleaner first. I bought a bottle of Gummout and it didn't make a difference. The next step for me was to check for the spark plug wells filling up with water. I took a look at all the coils and found coil #5 had a bunch of white chalky material on it and carbon inside the boot. I went ahead and bought a coil pack from Autozone and replaced the #5 coil/boot (disconnected the negative battery terminal to clear the SES light). This did not make a difference. The rough idle is still there and the SES is back.

On to plan B - I know a guy at work who owns code reader software (Autotap) and the interface plug. I loaded it up on my laptop, plugged it in and it had a P0301 - Cylinder 1 missfire code. I then started up the vehicle and after running a minute it threw out a P0300 - multiple missfire detected code. I then cleared the codes and the P0300 came back. I then used the software to set up specific monitors for each cylinder and found the cylinder two was continually missfiring but not any others. The only exception to that is that if I rev the engine to over 3K RPM, cylinder 5 starts to missfire a little as well (but only when over 3K). The funny thing is, I never get a P0302 code, always P0300 (and the P0301 hasn't come back either)... So with this info, I decided to try the spark plugs and to move the coils around to see if the missfires would move to a different cylinder. I bought AC Delco plugs, put them in and no difference. I then swapped the #2 and #5 (new one) coils and saw no difference either.

I'm not sure what my next step should be... I don't know if the Catylitic converter could be the culprit, or the fuel filter, compression, etc... Any help would be greatly appreciated!

Glen
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  #2  
Old 02-08-2008, 03:53 PM
mack mack is offline
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Well, If you are only getting misfire on cyl #2 I would focus on it.


You already swapped the coil, so next step is to check compression. If that is good I would check the injector.
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Old 02-08-2008, 04:50 PM
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zamar zamar is offline
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Thinking out loud here but what about a Crank Position Sensor? or Cam Sensor? Something linked to the timing of when the coils are told to fire.

What do you guys think?

Gook luck.
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Old 02-08-2008, 04:57 PM
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ylab ylab is offline
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Also check the wiring, its not only the coil that can result in a mis-fire on that #2 cylinder. Maybe the connector has corroded or the wiring has become damaged.....squirrels love to climb under the hood and chew off pieces of wiring for their nests. If the SES was flashing, that usually means a dead miss was detected and it flashes to warn you of possible damage, mainly the converter, if you continue to drive.
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Old 02-08-2008, 05:12 PM
grjoyner grjoyner is offline
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Compression check?

Quote:
Originally Posted by mack View Post
Well, If you are only getting misfire on cyl #2 I would focus on it.


You already swapped the coil, so next step is to check compression. If that is good I would check the injector.
Sorry to ask, but how do I check the compression and the injector? Is there a "how to" somewhere?
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Old 02-08-2008, 05:22 PM
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RayVoy RayVoy is offline
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One of the guys had a similar problem. Look at his thread, there is some P0300 causes around post #24 and he reported back last night with the cure. It's an easy fix if you have the same problem.

http://forums.trailvoy.com/showthrea...ighlight=p0300
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Old 02-08-2008, 05:34 PM
grjoyner grjoyner is offline
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I'll get some carbon cleaner and give it a shot tonight. Should I use Seafoam, or is there something better?
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Old 02-08-2008, 09:17 PM
grjoyner grjoyner is offline
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ok - here's the latest... I checked the connections and wiring and everything looked fine. I bought a can of seafoam and followed instructions on the web on how to use it. Basically, I poured about 1/2 the can in through the manifold vacuum hose, slowly at first, then enough to stall the vehicle. I turned off the ignition, poured the rest into the crackshaft (through the oil fill hole) and waited 15 mins. When I cranked it up, it smoked like crazy for about 30 seconds or so (as expected), but still was running rough. I hooked the laptop back up to it and cleared the SES light. I noticed that there are no missfires showing on the individual gauges anymore (remember I was getting a constant missfire on cylinder 2). The bad news is it's still throwing a P0300 code, however the SES light did not come back on once I cleared it.

So, bottom line is - I'm still running just as rough as before and still have the random missfire code, but for now the SES light is not on...

By the way - I didn't mention before, but I did clean the throttle body last summer and it still looks very clean.

Do any of you know if I can tell from the Autotap software if the fuel filter or CAT is clogged? It reports values for Fuel trim and O2 sensors (in mV), but I don't what to look for or if these would have anything to do with my problem...

Thank you for all the advice so far!
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  #9  
Old 02-09-2008, 09:41 AM
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PROSPHOTO PROSPHOTO is offline
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Have you ever changed the fuel filter or air filter?

Does the laptop offer real-time misfire information (graph) so you can monitor how random the misfire really is?

With clean filters (including oil), run a bottle of techron through 1/4 to 1/2 of a tank of gas (high concentration); it's a bit better than the gummount

Also, & only b/c this has been done before, make sure all of your coils are on securely and the boots are lined up with the spark plugs (double check)
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  #10  
Old 02-09-2008, 12:01 PM
grjoyner grjoyner is offline
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I have not ever changed the fuel filter. I changed the air filter last summer.

The laptop can be configured to show missfires on each cylinder. I have it set up that way and that's how I was able to tell it was missfiring on cylinder two. As of last night after the seafoam, it isn't showing any missfires on the individual cylinders, but it's still giving a code P0300.

I misspoke in my earlier post about the injector cleaner I used. I actually did use the Techron plus. I used the small bottle (treats up to 12 gals). I ran the gas to almost gone, put in the bottle and about 10 gallons of gas.

By the way, where is the fuel filter? I think I read that it's somewhere around the rear driver's side wheel well?

Thanks,
Glen
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