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Old 04-15-2012, 08:04 PM   #1
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Default P0300 code with no apparent symptoms

I have a 2004 GMC Sierra 4.8L, 2WD that is constantly throwing a p0300 code. The truck runs fine at all vehicle and engine speeds. Other than the CEL being on, and flashing at highway speeds, there appears to be nothing wrong with it. So far I have tested/cleaned each individual fuel injector, checked fuel pressure (~59psi), cleaned the MAF sensor, check each ignition coil, replaced the ignition wires, checked for vacuum leaks, and replaced the u-joints. Next I want to test the exhaust back pressure, but I am not hopeful in finding anything wrong. Other than a shake/shutter between ~52 and ~58 MPH the truck runs/starts/drives great. Any help would be appreciated as I am getting to the end of my rope. My inspection sticker is 2 months out of date and I really want to get this taken care of.

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Old 04-15-2012, 08:13 PM   #2
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You may have already read that the crank position sensor can cause p03XX series of codes - 0300 is a random misfire all cylinders if I remember.

It won't always be noticeable and if the CEL is flashing, driving is a no-no, (you already know that and it'* easy for me to say (smile))

The shudder could be front brake pads or rotor (I think tie rods would get worse the faster you drive)

luck,greg
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Old 04-15-2012, 08:19 PM   #3
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The shake could be your tire balance. Usually the brakes will only cause it when you apply them.
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Old 04-15-2012, 08:22 PM   #4
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Thanks for the reply Greg.

Wont bad crank position sensors cause a no-start problem? Or are you talking about trying to re-learn the computer to the CKP sensor?

I also forgot to mention that I replaced the plugs. The old ones looked excellent when I took them out, but the truck had ~200k on it when I bought it (early January), and I thought it couldn't hurt.

I also noticed that when I was recently carrying a heavy load (~2500 lb Bridgeport milling machine) the CEL was flashing when driving ~30 MPH. That was a new one for me.

I didn't think the shudder was related to this, but I have heard that off balance drive shafts can trick the knock sensors into thinking the engine is misfiring. Its possible that it is coming from the front rotors, but when I was doing the rear brakes, I noticed one of the axle shaft seals was blown pretty badly, so I am going to replace that bearing and seal.
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Old 04-15-2012, 08:39 PM   #5
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I've had (2) seperate vehicles in the last 10yrs. that were bad front pads causing a 50mph shimmy (slight)

In my experience, any unbalanced/warped rotating part will either set a harmonic drone or get worse with faster rotation.

luck,greg
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Old 04-15-2012, 08:47 PM   #6
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I plan on replacing my front brakes soon, so I'll let you know about that. You're probably right. Right now the shudder is only a minor nuisance. I need to turn off that CEL so I can get my inspection sticker. The cops are like bees around this college town!
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Old 04-15-2012, 08:55 PM   #7
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I believe the CKP will not always cause an operator noticeable event (except CEL)

I hate replacing parts (you understand) is it poss to just remove your CKP and "brakeclean" it and the wiring?

My 4.3 was throwing p0300, but I upgraded my spider inj ***'y to resolve it

And finally, I'm not suggesting the rotors may be bad, just the pads.

I'm not saying I'm cheap, but I replace my undrilled rotors when I see daylight thru them.

as rjblazer said, if it'* the rotors, you'd know it when stopping

luck,greg
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Old 04-15-2012, 09:05 PM   #8
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I'll take a look at my service manual tonight, but I don't think you can get to the CKP sensor without removing the water pump.

What are people getting for fuel economy with these 4.8'*? I am getting 14-16mpg, seems a little on the low side, but not low enough to suggest poor running conditions. Anyone agree/disagree?
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Old 04-15-2012, 09:11 PM   #9
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Rear gear? - Tire size? - city? - highway?

I've got a 4.6, 3.43 gear, 235/75/15 (205 is stock) I get 15/city - don't know hwy yet

I've only had it 2mos. - with the 180deg stat and leaking inj, I got 11.5 city

luck,greg
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Old 04-16-2012, 09:07 AM   #10
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i would say thats low, but mileage is mostly based on the driver. you ever look at the throttle body. check the butterfly with it open for carbon build-up
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