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Chappy 1/9/2021

2007 Chevrolet Silverado 1500 Classic WT 8 Cyl 4.80L


Poor acceleration with pre ignition 4.8 Silverado

The truck in question was purchased 10 months ago with 110,000 miles.
The engine has a soft loss of power under any load with the exception of WOT with detonation beginning around 2100rpm. Traveling uphill makes the surging more apparent. The power loss is spotty all the way up to 3500rpm when the engine note changes and suddenly comes "alive". It also has an intermittent harsh 1-2 shift. All of this occurs regardless of engine temperature.
I don't have a scan tool that provides real time data but I thought since the sluggish acceleration stopped at 3500 the vacuum was somehow pulling the throttle blade open - I replaced the throttle body with no change in performance.
Then 3 weeks ago the engine light came on during acceleration up an incline - P0330 knock sensor. I was relieved thinking this was the problem. I replaced both sensors and harness with delco parts - no fix
The truck idles smooth has solid fuel pressure - 56psi is the lowest it goes during WOT. New plugs wires and coi

2 Answers

Teddy B

Teddy B 1/9/2021

As I'm reading your question:

Your going to need a Professional Scan Tool or it will be a long journey & guessing it not a
diagnostic strategy.

The Throttle Body as you now know has a motor,gears & springs --- so it's not going to be
pulled open.

I don't want to be Critical or Condescending as I proceed so learn as we go.

You Never Replace Parts until some sort testing results in finding a problem
in a vehicle system.

Then you component test within that system until you find a failed part.

Now I will kindly as I can, give you the best vehicle advice you will ever get
when it comes to vehicles with OBD2 Systems (1996 thru 2021)

This is the number one mistake people that are not professional make.

There are No Vehicle Codes that tell you what is wrong.

There are No Vehicle Codes that tell you to replace any parts.

Replace no parts until you learn a vehicle system & how to test for a problem

Next,then Component Test the parts starting with those that sound like the
name related to the codes & only after you Google or do some research on how
sensors or parts should work & how they should be tested.

You can have many codes & they have nothing to do with anything.
They show up on restart,on scrolling data on a scanner & who knows why.

If a Sensor has Not Failed then consider mechanical or electrical components that caused
the working sensor to set codes,then that sensor is doing what it is there for.

Sensors set codes only after several conditions are met & time frames met,then
even an active code may just be an out of range report that clears upon starting
the motor,just turning the key on or driving.

When you have a code that is there all the time,you need to find out why,I don't personally
clear them to see if they come back,I want to look at data & wonder what's going one.

Did you Replace the Spark Plugs when you first got it ?
Have you had it checked for codes such as cylinder misfires (300 Codes)

It pretty much is telling you a lot.

You get misfires under load,hills are a perfect place to test for that
EGR if you have that system can be an issue under high vacuum conditions (hills)
How about Fuel Pressure at idle or up hills
Blocked Exhaust Converter from years of cylinder misifires
TPS Sensor -- sadly you have to look at data & know your vehicle & not panic
when something goes wrong.
If you keep records of testing,scan tool data you have a comparison to go back to.
If you threw coils & wires at it your still back to a knock from misfires

Data will show you Fuel Trim & O2
That is a good place to start,you can look at both sides of the motor in your case
& narrow down your next step also you need to see cylinder misfire data or what ever
is provided on whatever tools you use.

Check for vacuum leaks --- intake & exhaust
Check for EVAP System issues
Smoke test all those

Check,Test Smoke,Look at Data,Google Stuff

Check the Battery,Power & Grounds

Don't ever assume things are not related

Hopefully the Trans is just confused until the engine runs properly

May just be a Blocked Exhaust or Gas Filter -- never overlook those two
on loss of power issues.


Chappy 1/10/2021

I don't take it as condescending at all - yours is a textbook answer and I appreciate the time you took to answer.
I replaced the plugs wires and coils right after I bought the truck - they were original and needed to go.
The throttle body was really my only stab in the dark - a faulty tps sensor could give inconsistent throttle response and a harsh 1-2. The whole vacuum helping a weak motor was just hope.
After getting the knock sensor code I pulled the intake to find the rear cavity full of water with the harness severely corroded. To me this was definitely the source of the problem with false information coming from a compromised and resistant harness. This is the only time I've had a code stored - no misfires.
At idle I have consistent fuel pressure of 58psi. I drove for 10 miles of hills, flats and on/off ramps with my daughter monitoring the gauge and it only dipped to 56psi during WOT.
At 3500rpm it has all the power it should - it's almost like flipping a switch. So while the fuel filter and cats are good suggestions from you I don't think it applies here but I very well could be wrong.
The EVAP also crossed my mind - I did the easiest thing and jumped the purge solenoid and it gave me a click. Not a definite box to check off but it showed some function. Without a real scan tool I have no way of sealing the system to check for leaks.
The battery is good and cleaning the grounds was my first course of action.
I guess what I'm hoping for is that someone comes across this post and says "I had that same problem and did this" or "there's a service bulletin for that". I know the quickest way out would be to just take it to someone but I just can't do that - foolish pride I suppose.
Thanks again for your time and if there's anything else to say my feelings aren't easily hurt - just don't say anything bad about my dog


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