Rocks hitting inner fender sound

[magnumb]

When I give my truck more than 1/2 throttle and it's under load i get a sounds just like rocks hitting inner fender, but it's not. Sound comes from mid to rear area of the truck. Any clue what that could be. Very odd.

 

[someotherguy]

Probably pinging (spark knock/detonation) - year, model, engine, miles, any engine work you've done whether maintenance or mods, details would be helpful.

Richard

 

[90halfton]

Definitely sounds like pinging. I would check timing, maybe see how your plugs and wires look while you're at it. Agree with some other guy. Need more info.

 

[magnumb]

Ohh boy....1995, GMC Yukon 5.7 TBI with 230,000 miles.
You think the reason i'm hearing it towards the rear is the pinging noise is traveling through exhaust?

Adjustable fuel pressure regulator running about 14 PSI.
MSD 6A ignition.
87 octane
Timing is at 0 degrees.
NGK gaped to I think .035 and running 1 step colder.
No oil burn...just rear main seal leak.

Runs like a champ...plenty of get up and go and still pulls hard for having 230k.

 

[c1500deathtrap]

Try taking all the rocks out of your tires.

 

[someotherguy]

Ohh boy....1995, GMC Yukon 5.7 TBI with 230,000 miles.
You think the reason i'm hearing it towards the rear is the pinging noise is traveling through exhaust?

Adjustable fuel pressure regulator running about 14 PSI.
MSD 6A ignition.
87 octane
Timing is at 0 degrees.
NGK gaped to I think .035 and running 1 step colder.
No oil burn...just rear main seal leak.

Runs like a champ...plenty of get up and go and still pulls hard for having 230k.

That's enough miles for it to possibly have some carbon buildup in the heads.

Before wondering about that though I would be suspicious of an EGR valve failure. With the engine off, use a Mityvac to see if you can pull vacuum on the valve itself. If it doesn't hold, it's bad. Not to make EGR troubleshooting sound that simple, but in my experience the negative backpressure EGR's used on most TBI small block applications, this is the most common failure mode.

If you do end up replacing the EGR, use AC Delco/Delphi ONLY...and check the part # carefully before choosing. The other brand/one-size-fits-many rarely work properly on TBI's.

Not sure what to say about the sound seeming like it is coming from farther back; it could just be your perception of it...or those of us saying it's pinging could be wrong just because we're doing the long distance diagnosis.

Richard

 

[magnumb]

The intake gasket was changed about 10,000miles ago and the EGR passages were cleaned and EGR checked out. I've had EGR issues in the past and it was replaced with AC Delco unit and remember spending alot of time figuring out the correct part#. That was about 30,000miles back...so i think the EGR is good to go.

I may try retarding the timing and see if the sound goes away and if it does i'll replace plugs with stock gap and move back to zero degrees to see what happens.

Could carbon buildup still cause ping after running say less than 5min in 50 degree weather?

 

[90halfton]

This may sound stupid, but check for loose things banging around, and/or vibrating. Had a Harley years ago that had an exhaust sheild clamp come loose and a certain throttle range/rpm it sounded just like pinging. Tore my hair out tuning it til I figured that out. Not saying that applies here, but worth a quick look.

 

[magnumb]

I did get under their and root around and nothing popped out at me. Was thinking the cat, but i hit that with a rubber mallet and nothing could be heard.
It seems to be related to load placed on engine and pinging does make sense....actually the only reason i think i'm noticing the sound is that i cut a hatch in floor for the fuel pump and recently changed fuel lines and never closed that hatch....maybe i just cover it back up and forget about it

 

[mountie]

I have concerns with MSD..... It is not manufactured like it used to be... I find MSD to be unreliable.