cowgirl terri

Hi all!

I have just joined and I need some advice from the brain trust on this forum

My 2008 Silverado has started idling rough and is showing a flashing CEL. I took it to my local mechanic and he pulled codes p0121, p0300, p0420, p0449.

The truck runs ok, idles a little rough , no loss of power and does not make any engine noises, it hesitates for a second upon acceleration but pulls fine after that.
My mechanic, who is a good guy but I just want check says I need a new engine because of a low compression issue, this sounds a little extreme so I thought I would ask before I go see a dealer for a new truck.

I do not know if he did a compression check or not

Thanks in advance!

Terri

oilcanhenry

If he did a compression test, that might help but he would have to pull all your spark plugs to do that..

Code P0300 means that you probably have a bad coil pack, most of the time. What motor do you have in your Chevy?

cowgirl terri

I have a 1500, 4.8L

oilcanhenry

Well, if your mechanic does a wet oil cylinder pressure test, the 8 cylinders should be within 15 percent of each others pressure to be considered normal, but if all less then 130 PSI or so the motor not up to specifications and likely needs repair. You might also have a throttle position sensor issue, as well as a catalytic converter decline, going by the OBD II codes:

P0420:- Catalytic Converter- "EFFICIENCY BELOW NORMAL".

P0121:. Throttle/Pedal Position Sensor/Switch issue.

P0449:evaporative emission system vent control circuit intermittent issue.

P0300: multiple cylinder misfire code when your engine starts running rough and the check engine light comes on,If your 2008 Silverado 4.8 GM V8 pickup is burning oil between oil changes, then it's likely worn out pistons/piston rings and/or cylinder walls.

Likely have some valves that need tending to as well, if that's the case. It will really come down to what the compression test shows, since a worn cylinder or two will cause misfire issues, but that can also be due to many other factors as well.

Back in the day, we'd use a can of STP to help the rings seal up the chambers on older, high-mileage engines, but with today's high-tech motors, it's probably not a good idea, particularly with a turbocharged motor (which yours isn't, BTW).

cowgirl terri

Well thanks Henry, I asked and he did not do a compression check and some of my friends are saying what you are.
This truck runs pretty well and has been cared for oil change wise and doesn't use any oil, I"m starting to think maybe this is plugs or wires.
I will update and thanks for your help

oilcanhenry

You're welcome,Terri. I'd would be looking into the ignition system. If its been awhile since the plugs were changed, that _might_ help.

Last time I pulled a code P0300 it was a bad coil pack, BTW. Not on my vehicles but someone else's.

tech2

the only code to worry about right now is the misfire dtc.
the tech hasn't done enough testing to confirm anything yet.
see if the fuel trim is running lean at idle but ok at 2500rpm...if it is, you have a vacuum leak...check the intake gaskets. you want the fuel trim to be +/- 10%
-do you have a hard start after refueling?
-have the tech watch the misfire counters to see which cylinder is accumulating the misfires...if its associated with one bank or one cylinder, it a good clue

Rednucleus

What's your mileage?

cowgirl terri

My thanks to all for the advice here, this is my first non equine forum and I didn't know what to expect.
My husband picked up a set of plugs and ,after a little grease and sweat, got them changed out last evening.
Instantly feels back to normal!
I am feeling I should kick up a little fuss about the "you need an engine guy" but I really don't like leaving any bad reviews on a business, anyone can have a bad day I expect, but still feel I ought to let them know
so they can correct some of the in house issues they may have with service staff.
Bedside manner needs improvement as well, I love my truck and felt a little sad about potentially having to say goodbye to it.

Oilcan and Tech2, Thank you!

cowgirl terri

Well I am back again, My truck felt great for a few days and now it has the cel light on and it is flashing on acceleration. It goes back to steady when slowing down and foot off the gas. The truck seems smooth but it is down on power and shifts a little hard from 1 to 2

We noticed when changing the plugs they all appeared very lean , real white. I now am thinking intake leak based on some of the symptoms I am describing.

My husband did the plugs, the wires and bought an HEI tester, all cylinders snapping strong and loud.

any thoughts appreciated