Detonation (or pre-detonation) can be caused by a number of things. It happens when the air/fuel mixture ignites before it's supposed to.

It can be caused by an engine that's running too hot, increased compression caused by carbon build-up in the combustion chamber, hot spots also caused by carbon in the chamber, excessive engine load, lean fuel mixture, or a few other things.

One thing you can do is check the condition of your spark plugs. Pre-detonation will erode the plugs very quickly. For a visual guide, look here...

http://www.geistware.com/rcmodeling/...lug_face_2.jpg

Hope this helps.
John

 

Anything at this point helps!! Thanks John.

 

You just gotta love this forum!!

 

Would the PCV valve cause this noise? Noticed it was past the time to replace it, I have 67,000 miles on the truck now. Damn, I feel like I'm grasping at straws now.

 

[QUOTE=JohnG6]Detonation (or pre-detonation) can be caused by a number of things. It happens when the air/fuel mixture ignites before it's supposed to.

It can be caused by an engine that's running too hot, increased compression caused by carbon build-up in the combustion chamber, hot spots also caused by carbon in the chamber, excessive engine load, lean fuel mixture, or a few other things.

One thing you can do is check the condition of your spark plugs. Pre-detonation will erode the plugs very quickly. For a visual guide, look here...

http://www.geistware.com/rcmodeling/...lug_face_2.jpg

Hope this helps. Pulled a plug to inspect and found nothing as described in the visual guide, so what now? Will a detonation problem cause the kind of noise I describe at the beginning of this thread? On a side note, when going under overpasses, the noise echos like it's coming from an external source on the truck

 

It's VERY easy to find out if it's detonation or not..............premium gas........regardless of what you may hear, you are not going to screw up an engine by running a couple tanks through it.

Wouldn't hurt to replace the PCV valve and make sure the elbow isn't cracked - and look for other vacuum leaks.

 

That's cool. I'm trying to runout the rest of the 87 octane so I can fill it w/ 93. Already looked for vacumm leaks and couldn't find any.

 

Also mitch put some good fuel injector cleaner through it.I like the Lucas myself

 

Sounds like a good idea. How's that Xcal workin' for ya?

 

Well, took the truck to a dealership yesterday (I hate doing that) to have a lariat cluster reprogrammed and told the mech. of my issues with the rattle so he did a once over with his computer diagnostics and found nothing wrong. Trucks pcm isn't throwing any codes out there either, so he doesn't think I have a lean condition with the engine. He thinks it is just something loose and the noise is a resonece issue as I'm driving along the road. All I know is, this thing is driving me absolutely insane trying to figure out what it is. Still going to try the premium fuel as well as an injector cleaner. Little side note, he told me not to wait for 100,000 miles to change the plugs because they get a carbon build up on them that causes them to get stuck in the chamber and then they break off when trying to remove them. So I guess I'm changing my plugs this weekend! Will keep posting as far as noise issue is concerned.

 

If you are going to put cleaner in the gas, I recommend Techron. This also helps get rid of valve deposits.

 

Thans for the tip
glc!

 

I would HIGHLY recommend NOT running any fuel additives. It is most likely a loose muffler/cat heat shield, or loose muffler/cat baffling. See it all the time, and it WILL DRIVE YOU CRAZY! My exhaust system makes a stupid rattling sound at 1800rpm under light load ONLY if I have gone WOT in the recent past. It's something loose in the muffler, and it has done it since the day I bought it. 7 years and 160,000 miles ago!!! On a stock truck, forget detonation, but if you suspect it, (because of K&N or whatever) just fill up with premium. If you put 1/4 a tank or more it will raise the octane considerably, so you don't have to run it down all the way. BUT BE AWARE, high octane fuel WILL change the engine's exhaust note. It WILL "quiet" it down, because the PCM will not raise the spark advance enough to compensate for the lower octane. (or at least the 2valve will not) So it may "fix" the sound.

Crawl under the truck and beat the muffler, shields, cats with your hand or a rubber mallet, and see if they rattle.

Good luck and keep us updated.

 

Funny you mention all that, crawled under the truck a few days ago and checked entire exhaust system and found nothing loose. Just filled up truck with premium with w/ a 1/4 tank of 87 still in it today!

 

Thing is - you can't tell if it's loose until it heats up . Theres a TSB on the issue for the 97-03 model years.