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Username Post: Silverado 1500 4.8 P0300 code at high RPM only??        (Topic#218964)
jawzzy 
Member
Posts: 8

Reg: 04-11-04
06-11-09 02:10 PM - Post#1715255    

I got this code last October when I accidentally drove home in 3rd gear....went all winter and spring till last weekend pulling my boat up north, and it returned for the first time since then...the check engine light flashes, then stays solid until it decides to flash again once in awhile (I presume when it flashes, the misfire is actually occurring). I've read ALOT of the suggestions on fixing this, and can add this:

because of a slight miss in the engine idling at stops, 40,000 miles ago I changed the plugs, wires and all 4 oxygen sensors with Delco original equipment, but still no better, so I lived with it till this day (it's really not that bad, just annoying to know it's there). Now I get this P0300 code when using 3rd gear which causes higher RPM's. I reset the light Sunday, and no problems again so far. Even when I get the light though, I notice NO CHANGE in engine performance whatsoever, just a solid or flashing engine light......so, with the problem being related to strictly high RPM's, I'm hoping perhaps someone can direct me to a more specific area to search and repair thanks in advance for any suggestions or ideas!!!

 
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2plus2 
Member
Posts: 467
2plus2
Loc: Langley,BC
Reg: 02-03-05
06-11-09 04:01 PM - Post#1715326    
    In response to jawzzy

how many miles?
my astro did something like this,miss fire between 60-70 mph cruising down the hwy
ended up needing the injectors cleaned
69 canadian pontiac 2+2
factory L48 350 chev,TH350,PW,PT
99 silverado rclb 4.8 nv4500 14bsf 4.10's locker 6" procomp 35 bfg m/t's 9k winch cutout flares
92 s10,bead locked 38 tsl's,d44,9inch,spools,sm 465/np208/np241,exo cage


 
jawzzy 
Member
Posts: 8

Reg: 04-11-04
06-11-09 08:03 PM - Post#1715479    
    In response to 2plus2

125,000 miles, and I use injector cleaner regularly It's not the speed, but the RPM's that I use when towing or in 3rd gear, usually above 2500rpm to 4000rpm up the hills I also had the injectors cleaned when I tackled this last time 40,000 miles ago.

 
Bad Bowtie 
Valued Contributor
Posts: 4543
Bad Bowtie
Age: 42
Loc: TX
Reg: 07-04-03
06-12-09 06:51 AM - Post#1715670    
    In response to jawzzy

When were the fuel filter & PCV valve last replaced?
GMC. We are Professional Grade.

http://www.cardomain.com/id/2004VHO


 
jawzzy 
Member
Posts: 8

Reg: 04-11-04
06-12-09 09:17 AM - Post#1715744    
    In response to Bad Bowtie

the fuel filter about 3 years ago, but now that you've mentioned a PCV valve, I was going to say I didn't have one cause my Oil Change place has never mentioned it to me with this truck, and they ALWAYS did with my old one...so I went out and looked, and sure enough I do, but it's under the Vortex cover, so I guess the Oil guys never notice it. I'll change it and see what happens, although it couldn't be that simple could it Thanks!!!!!

 
Bad Bowtie 
Valued Contributor
Posts: 4543
Bad Bowtie
Age: 42
Loc: TX
Reg: 07-04-03
06-12-09 11:50 AM - Post#1715866    
    In response to jawzzy

I'd replace the fuel filter too, every 12-15K.
GMC. We are Professional Grade.

http://www.cardomain.com/id/2004VHO


 
timberjack 
Forum Newbie
Posts: 30

Loc: Thunder Bay, Canada
Reg: 01-30-07
06-12-09 06:03 PM - Post#1716064    
    In response to Bad Bowtie

I had the exact same conditions you describe a few years ago.

Above 2200 rpms or so, and the CEL would flash, trigger the p0300 code, but no noticeable miss could be felt. I did everything i could think of, fuel, air filters, pcv valve, plugs, wires, seafoam. I finally conceded and took it to the dealer. They suggested much of the same, but finally were able to track down the miss to a specific cylinder with their scantool. Mine would only show the random misfire and no other details.

Eventually, the tech spent some time on the phone with a tech in Detroit, who suggested they change the valve springs on the suspect cylinder. According to the fellow at GM in Detroit mentioned there have been some issues as described for certain 4.8L engines, and some of the valve springs were weaker than they should have been, thus allowing the valves to open late, and close early, not much, but enough that the computer could pick up the miss, and trigger the CEL.

It worked for my truck, though they only changed those springs on that cylinder. Now 3+ years later, i have the same thing happening again, only at higher rpms, so i'll open up the rocker covers and probably change all the springs myself this time.

One more place to look.
2000 K1500 Silverado Extended cab Short box.
4.8L, 5 speed manual, 3.73's with a G80.


 
Smitty_Chevy 
Contributor
Posts: 834
Smitty_Chevy
Age: 65
Loc: Atlanta & Yazoo, MS
Reg: 10-29-08
06-13-09 03:00 PM - Post#1716443    
    In response to Bad Bowtie

  • Bad Bowtie Said:
I'd replace the fuel filter too, every 12-15K.



I respectfully disagree. That's a waste of time and money in my opinion. If someone is getting that much crud in their fuel it's time to change where they buy their gas not fuel filters.


 
jawzzy 
Member
Posts: 8

Reg: 04-11-04
06-13-09 03:13 PM - Post#1716454    
    In response to Smitty_Chevy

well I spent the day installing new iridium Bosh plugs, FUEL FILTER :(, PVC, and cleaning the air filter and system, and no change....I can now make the misfire happen on demand...just stay at 2400RPM or higher for about 20 seconds, and the check engine light starts flashing for the P0300 code. Any RPM below 2400, and no misfires recorded, so go figure. Guess I'll have to take it in so they can hook up the BIG scanner and see if it can decipher any of this to a more specific problem. Like above, As posted above, I'm concerned it may be my valve springs. Most of the plugs were in normal condition, but a couple were a bit wet. I would think if the Cat's were plugged, it would misfire at any RPM wouldn't it

 
Bad Bowtie 
Valued Contributor
Posts: 4543
Bad Bowtie
Age: 42
Loc: TX
Reg: 07-04-03
06-15-09 11:45 AM - Post#1717521    
    In response to Smitty_Chevy

  • Smitty_Chevy Said:
  • Bad Bowtie Said:
I'd replace the fuel filter too, every 12-15K.



I respectfully disagree. That's a waste of time and money in my opinion. If someone is getting that much crud in their fuel it's time to change where they buy their gas not fuel filters.





Let's.
I'd gladly spend $12 bucks for a fuel filter & every year. I'd get my money back with-in a year too...
GMC. We are Professional Grade.

http://www.cardomain.com/id/2004VHO


 
jawzzy 
Member
Posts: 8

Reg: 04-11-04
06-24-09 02:02 PM - Post#1722637    
    In response to Bad Bowtie

Update: Mechanic is baffled (as I expected ) Spent 5 hours already, and can't find the problem...which has been narrowed down to misfires ONLY between 2400 and 2800 RPM..above or below those RPM's, and NO misfires....so still looking!!!!!!!!!!!!!!!!

 
Billy_Bob 
Contributor
Posts: 290

Reg: 05-25-09
06-24-09 08:52 PM - Post#1722900    
    In response to jawzzy

Well might try here...
http://www.random-misfire.com

 
jawzzy 
Member
Posts: 8

Reg: 04-11-04
06-25-09 03:15 AM - Post#1722946    
    In response to Billy_Bob

Thanks!!! but been all over this page already, and still trying more of his suggestions.

 
DeLong 
Banned Member
Posts: 2868

Loc: Rural Illinois
Reg: 06-06-02
06-25-09 08:04 AM - Post#1723058    
    In response to jawzzy

My 98 lumina did the same thing so changed everything you mentioned.

It ended up being a coil pack.
00 Yukon SLT XL 5.3
98 Lumina Police Package 3.8
88 2500 4X4 5.7
87 Mustang LX 5.0
67 Mustang Fastback 289
65 Mustang Fastback 289


 
Bad Bowtie 
Valued Contributor
Posts: 4543
Bad Bowtie
Age: 42
Loc: TX
Reg: 07-04-03
06-25-09 08:35 AM - Post#1723077    
    In response to DeLong

I would take those Bosch spark plugs back & get new AC Delco iridiums.

Hope you figure out the problem soon...
GMC. We are Professional Grade.

http://www.cardomain.com/id/2004VHO


 
jawzzy 
Member
Posts: 8

Reg: 04-11-04
06-25-09 01:21 PM - Post#1723233    
    In response to DeLong

already changed the coil packs on the 2 troublesome cylinders, and no change, but thanks!!!

 
jawzzy 
Member
Posts: 8

Reg: 04-11-04
06-26-09 08:06 PM - Post#1724151    
    In response to jawzzy

FIXED!!!!!!!!!!! 4 days and 10 hours at least of labor later. It was faulty cam sensors that ONLY acted up between 2400 and 2800 RPM's. Unfortunately it was about the last sensor that they had to check after checking EVERY other one in the truck, but at least I know now they're all good He said he only charged me about an hours labor over the Cam Sensor job itself, plus for a Injector cleaning that they did to try in the beginning, for a total or $475.00, so for over 10 hours of labor, I guess I got off easy...a dealer would've just started the clock and kept billing me! Anyway, I hope this helps anyone else down the road with similar problems. I do remember reading one post somewhere that had the same cause, but no mention of the specific RPM's involved.

 
Bad Bowtie 
Valued Contributor
Posts: 4543
Bad Bowtie
Age: 42
Loc: TX
Reg: 07-04-03
06-27-09 09:23 AM - Post#1724381    
    In response to jawzzy

WOW!!

Well, thanks for posting back what you found out!!
GMC. We are Professional Grade.

http://www.cardomain.com/id/2004VHO


 
Billy_Bob 
Contributor
Posts: 290

Reg: 05-25-09
06-27-09 11:26 AM - Post#1724455    
    In response to jawzzy

Ok, let's see if I have this right as to what was causing this problem...

The engine computer detects engine speed (RPM) via the camshaft position sensor and/or the crankshaft position sensor???

And it detects misfires when it is expecting one of the above to maintain RPM's or slightly increase RPM's, but instead there is a slight drop in engine speed.

(Each time a cylinder fires, there is a slight burst of speed, but if a cylinder does not fire, that slight burst of speed will be absent.)

So now you have the camshaft position sensor which is not always working. This would send "pulses" to the computer. And let's say is misses a few pulses at higher RPM's, then the computer would interpret this as a slight drop in engine speed and a misfire!

Is this correct anyone?


 
Smitty_Chevy 
Contributor
Posts: 834
Smitty_Chevy
Age: 65
Loc: Atlanta & Yazoo, MS
Reg: 10-29-08
06-27-09 08:30 PM - Post#1724773    
    In response to Bad Bowtie

  • Bad Bowtie Said:


Let's.
I'd gladly spend $12 bucks for a fuel filter & every year. I'd get my money back with-in a year too...




I've got one car with over 171,000 miles on it, another car with over 121,000 miles on it, a truck with 49,000 + miles on it and another car with 7,100 miles on it. That's a total of 348,000 miles. If I had changed filters every 12,000 miles I would have replaced 29 filters by now. At $12 bucks a piece that would be $348.00 not counting the time to change 29 filters.

I've never replaced the first filter on any of them yet and all of them run just fine.


 
Bad Bowtie 
Valued Contributor
Posts: 4543
Bad Bowtie
Age: 42
Loc: TX
Reg: 07-04-03
06-29-09 01:11 PM - Post#1725820    
    In response to Smitty_Chevy

"Smitty"
Let's just agree to disagree.....
GMC. We are Professional Grade.

http://www.cardomain.com/id/2004VHO


 
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