P0303 - Chevy Cobalt Forum / Cobalt Reviews / Cobalt SS / Cobalt Parts
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post #1 of 17 (permalink) Old 09-11-2017, 08:24 PM Thread Starter
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P0303

Just had a P0303 And check engine light today. This was after a fun drive in combat traffic going to DFW airport.
So far, this is the only engine error the Blue Cobalt (BC) has had issued since I became its new owner. After about 10-20 miles, and several starts, the check engine light went out.
This evening I connected a cheep ODB scanner and checked for errors. P0303 confirmed, @ 900 rpm 78F outside air temp 192F coolant temp. All other parameters seemed nominal.

As a reminder, this is an 08 LS now with 120k miles, 7k miles on new AC plugs, 100 miles on fresh air filters & oil & oil filter and the original OEM coils. I guess the BC needs the the memory cleared and watch it for a bit to see if ithe error returns.
BTW, still getting 34-35 mpg @ 65-75mph over 60mile driving legs.....I do like this fun little car.
And, it only used 1-2 cups of Mobile 1 oil during the past 7k miles

BC

Last edited by 1966tbird; 09-11-2017 at 08:28 PM.
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post #2 of 17 (permalink) Old 09-12-2017, 04:22 AM
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Don't get fooled by P0303. GM never figured out why there ECM puts out this code when it doesn't which code to put out. Most common cause is first HOČ starting to go bad, sometimes a dirty MAF but it's never a cylinder 3 misfire. If your OBDII reader can read misfire you can double check this.

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post #3 of 17 (permalink) Old 09-12-2017, 08:57 AM Thread Starter
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I do not think the $30 Actron can but I know my Tech2 can display missfires since memory was cleared. ~3k miles ago (while messing with BCMs)
Also, I need to check but I believe I replaced both O2 sensors 7k miles ago as simple 100k mile pm. I have not touched the throttle body yet. But the MAF was cleaned during the 100k mile PM using multiple semiconductor grade solvents. Again this was at 113k miles when ebay delivered the BC.
The BC has 250mi since oil & filters were changed, when the error was set, so a piece of FOD could have lodged on the filiments on the MAF sensor. Fuel filter not changed, no fuel pressure reading still getting very good mileage. And the mil light turned off on its own.
This time, i will verify which cam chain tensioner is installed.

I could use some help, before i start though, tracking down the correct page in the Tech2 to check the errors & check for missfires.
Or
How to use the Tech2 to check P0303.

BC
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post #4 of 17 (permalink) Old 09-12-2017, 06:12 PM
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I check misfires with the 5$ ($4USd)Torque Pro APP. Too friggin complicated with the TechII drop down menu system, unless you know where to go. One wrong turn and your off to a wild goose chase.

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post #5 of 17 (permalink) Old 09-13-2017, 08:55 AM Thread Starter
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Today, I Connected the Tech2 and pulled up the missfire data on the BC.
Misfire History had - total of 4 for cylinder 2, and ~6500 for cylinder 3. Cylinders 1 & 4 had no misfire history logged.
In misfire graph with a total X value of 32, only cylinder 3 displayed anyhing and only around idle.

I swapped coils between cylinder 1 & 3 and tested again. Cylinder 3 still shows an occational missfire during idle. No noticeable missfire at higher rpm (~2000 rpm). Now the engine is toasty enough I wish not to juggle hot spark plugs. Once the engine cools some, I will swap spark plugs 1 & 3 and test again.

While messing around the powertrain in the Tech2. I found the fuel section and toggled the fuel injectors off & on. One at a time in order.
Each cylinder displaying missfire as the fuel injector was dissabled (1-2 sec) and cleared up as soon as it was enabled. In all, this took about 20 min . Now typing the message out allowing the engine to chill.

BTW, the Permatex copper antisize was most welcome on the coil pack threads.

BC
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post #6 of 17 (permalink) Old 09-13-2017, 04:05 PM Thread Starter
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Pulled spark plugs 1 & 3.
#1 looked like the slightly used ACDelco with clean white insulator surrounded by soft black soot on steel.
#3 looked similar to #1 except ....there was a 30 degreesd arc of amber stain on the center insulator about 2mm down from the exposed iridium electrode.

Swapped spark plug locations and re-ran tests..........Same results......slight misfires on cylinder 3 at idle......no misfires at higher RPMs.
Idle speed at 784 rpm.
Nominal temps of 192-212F for coolant IAT of 90F with outside air near 80F.

DTC record has misfire passed this ignition cycle, passed last ignition cycle, history of pass & fail with MIL light enable set....MIL light is On.
History also has 2275 miles since DTC cleared ( when swapping & testing BCMs a month or so ago)
Also verifyed both O2 sensors were replaced ~7k miles ago.

BC
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post #7 of 17 (permalink) Old 09-13-2017, 07:59 PM Thread Starter
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It suggests a possible dirty injector...... ???

BC
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post #8 of 17 (permalink) Old 09-14-2017, 04:34 AM
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GM has a great fuel treatment in a bottle.



I'm sure other brands work good too but I've had great results with this stuff.

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post #9 of 17 (permalink) Old 09-14-2017, 08:47 AM Thread Starter
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I Pulled the fuel rail #4 looked like it has the most carbon on it.....swapped fuel injector #3 with #1 and ran again.
This time on startup, I kept rpm up a bit untill I could get the Tech2 booted. By this time I noticed the MIL light off.
I went to DTCs and checked. History had P0300 passed this cycle, passed last cycle with history of pass & fail.

I then went to missfire data and saw only the history of 7k missfires on #3. And 2 or 4 missfires on cylinder 4

Now dropping rpm down to idle ~800 rpm. Cylinder 3 stats showing misfires.
Also checked, I still have the OEM O2 sensors in the replacement Denso boxes from RockAuto.

So, since MIL is off and the Tech2 said no errors,, I cleared DTC history.
Still showing cylinder 3 logging misfires at idle.

Coby7 the injector cleaner would not be bad and throwing $5.00 for general purpose is not out of line.
BTW, throttle bore looks nominal and going out now to check cam tensioner. And its flat or type A...bummer on that.

---------- Post added at 03:07 PM ---------- Previous post was at 02:06 PM ----------

So what I know is
Not the spark plug.
Not the fuel injecetor.
And the wiring loom covers just disintegrated and now most of the top engine wiring harness is bare wires. This will not last long.

Speculating: Could a slightly dirty Mass Air Flow cause decrease in fuel injector pulse width causing a lean misfire?
Also a slow front O2 sensor could indicate too rich causing fuel pulse to decrease to the point of lean misfire?

---------- Post added at 03:47 PM ---------- Previous post was at 03:07 PM ----------

Happy Camper here!

Pulled the air filter box & removed the MAF sensor.
Doused it in clean maf cleaning fluid and reinstalled both.

Misfires gone.
So that what I found on the BC. YMMV

BC

Last edited by 1966tbird; 09-14-2017 at 07:25 AM.
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post #10 of 17 (permalink) Old 09-14-2017, 10:37 AM
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Told you a P0303 was tricky.

Every minute spent angry is a happy minute lost for ever and ever!!!
Drives 2016 Cocoa Buick Encore 1.4L Turbo auto KL4CJDSB2GB652675
Drives 2016 Sunset Orange Trax LT1 1.4L Turbo auto 3GNCJPSB8GL142716

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