P0300/P0302 and Flashing CEL - Chevy Colorado & GMC Canyon
 
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post #1 of 4 (permalink) Old 03-02-2017, 08:43 PM Thread Starter
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P0300/P0302 and Flashing CEL

Hello all,

I've been getting the P0300/P0302 issue on my '05 I5 Chevy Colorado in the last few weeks. It was happening in conjunction with a really rough idle. One year ago I also had the rough idle but cleaned the throttle body and replaced the plugs and things evened back out.

In the last month or two, the rough idle returned with a vengeance and I began to get the P0300 and P0302 DTCs.

So far I have:

+ Cleaned the MAF Sensor
+ Removed and thoroughly cleaned the throttle body
+ Replaced #2 spark plug ("old" plug had no abnormal signs of wear)
+ Swapped #2 coil with #3 coil. Error code remained on P0302.
+ Seafoamed through the vacuum line at the throttle body (this caused an immediate reduction in rough idle)

After the Seafoam last week, things seemed better, but now I'm getting an intermittent Check Engine Light which is flashing and has me slightly panicking. When flashing, the truck is idling rough again. It only flashes on a cold start and after decelerating, and only every now and then. Not reliably.

My next planned steps are to replace the fuel filter and to do a Techron Fuel System Cleaner on my next empty tank. (Also an oil change).

But beyond all of that, I'm at a bit of a loss for what else I should be checking and trying for. I am aware of the P0300 service bulletin from several years ago, but I just started having this problem and these codes in the last few months, which is a little late for the service period in that bulletin.

Does anybody have any other suggestions for troubleshooting and repairing this issue? I would really prefer not to have to go get a leakdown test, if possible. I'd also like to avoid replacing the entire head since the truck is 12 years old and just doesn't need that level of investment any longer.

Thanks for your help everybody.
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post #2 of 4 (permalink) Old 03-03-2017, 06:50 AM
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The "check engine" light flashes when the condition is causing potential damage to the catalytic converter. it's possible the rate of misfire is getting higher, causing the converter to overheat. Are you getting a different code when that happens?

You could check the spark plug firing by removing the coil and spark plug, connect them, ground both the spark plug and coil, and operate the engine.

Do a careful inspection of the coil connector and wiring. Also disconnect the black connector from the PCM and check the terminal for the #2 coil. Also check the condition of the wire. The wire for the #2 coil should be located near the center of the plug and the color code of the wire is Red w/White stripe.

Although a bit remote, how old is the battery? Voltage is supplied to the coils directly and the PCM controls them by the ground side of the circuit.
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post #3 of 4 (permalink) Old 03-03-2017, 11:13 AM
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Originally Posted by chrisrodkey View Post
Hello all,

I've been getting the P0300/P0302 issue on my '05 I5 Chevy Colorado in the last few weeks. It was happening in conjunction with a really rough idle. One year ago I also had the rough idle but cleaned the throttle body and replaced the plugs and things evened back out.

In the last month or two, the rough idle returned with a vengeance and I began to get the P0300 and P0302 DTCs.

So far I have:

+ Cleaned the MAF Sensor
+ Removed and thoroughly cleaned the throttle body
+ Replaced #2 spark plug ("old" plug had no abnormal signs of wear)
+ Swapped #2 coil with #3 coil. Error code remained on P0302.
+ Seafoamed through the vacuum line at the throttle body (this caused an immediate reduction in rough idle)

After the Seafoam last week, things seemed better, but now I'm getting an intermittent Check Engine Light which is flashing and has me slightly panicking. When flashing, the truck is idling rough again. It only flashes on a cold start and after decelerating, and only every now and then. Not reliably.

My next planned steps are to replace the fuel filter and to do a Techron Fuel System Cleaner on my next empty tank. (Also an oil change).

But beyond all of that, I'm at a bit of a loss for what else I should be checking and trying for. I am aware of the P0300 service bulletin from several years ago, but I just started having this problem and these codes in the last few months, which is a little late for the service period in that bulletin.

Does anybody have any other suggestions for troubleshooting and repairing this issue? I would really prefer not to have to go get a leakdown test, if possible. I'd also like to avoid replacing the entire head since the truck is 12 years old and just doesn't need that level of investment any longer.

Thanks for your help everybody.
Well looks like you eliminated a coil/plug problem. I'm guessing the seafoam making it better is a fluke or psychological ;) The first step in diagnosing is a good visual inspection. Have you checked very closely for a vacuum leak? Seems likely because its only at idle? Right? You could aso try swaping fuel injectors like you did with the coils to see what happens. I hate to say it but checking compression on #2 (at least) would be a good idea just so you know you aren't chasing your tail. (only at idle, points to low compression too) Good news is, I think you said its intermittent, so if thats the case, not likely compression. Oh, also, does the idle go way down when it happens? If so maybe a throttle position sensor or throttle body gone bad (sensor in it) I think auto parts places stock throttle bodies for these trucks. There must be a reason why ;)

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post #4 of 4 (permalink) Old 04-25-2017, 08:38 AM
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If your rough idle is anything like the video's linked in post #3 below, I'd say just take it in for a leak down test at a dealership. Lots of people with the same symptoms as you go through the same steps only to find out that its bad valves.

Hesitation at Start Up - Chevrolet Colorado & GMC Canyon Forum

Looks like you are doing all the right things with troubleshooting. Good spark, clean intake, clean fuel system, and you identified that the misfire stays in one place. That, to me, means injector #2 or valve problem.

If the rough idle is intermittent, it very well could be the valve problem. I just got my truck back yesterday, got a valve job and fixed my intermittent misfires (P0300, mostly on cyl's 1 and 5). Went through a huge list of basic repairs before the valve job and nothing fixed it. The valve problem is far too common to overlook on these motors.

The leak down test at my GM dealer provided compression results and the % leakage from the valves (about 27% leakage on cyl's 1 and 5). Took about 2-3 hours, $140 iirc
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