Hey guys i was driving home the other night it was somewhat cold out (high 40s) and when i started to accelerate i noticed a strange rumbling noise, so i immediately let up on the gas. When i did my RPMs went way down and if felt like it downshifted. The next day it was gone and ran perfect. Any ideas what it could have been? I was thinking maybe the cold weather but im not sure. The only mod i have right now is an intake. thanks!

 

strange rumbling noise

Could it be described as sounding like "marbles in a can"? If so, you were hearing the dreaded winter-detonation sound. It can break the engine apart so I'm glad you backed out of it.

Dan

 

not so much sounding like marbles ina can but a rumbling like the whole truck was vibrating from the rumble. but just in case....what is winter detonation?

 

You probably got some bad gas if it is running fine now.

 

now that i think about it, i did get gas at an unusual place. i normally get gas at at one or two place but i was running low so i got it at the closest place; and it was a cheap gas station. if that was the problem, i still dont know what "winter detonation" is. if anyone can answer please do! thanks. ~Matt

 

my guess on winter detonation would be: colder air has more oxygen, so in a way it would lean out the a/f mix. i'd figure the computer could adjust though. also the lower temp of the air should help the a/f mix resist detonation as well. but who knows..

 

Yes you MAF will compinsate for a certain amount of temp and volume difference. Cold air does not help a heathly A/F. It will ALLWAYS lean out the mixture. If your Border line lean on your A/F a nice cold night will push it over the edge.
Cheap gas ( lower quality, Lower octane, Oxygenated gas ) can and will cause knock on a L and any other vehicle made to run on 91+.
The lower the octane gas = Faster burn rate.
The Higher octane = Slower burn rate.
Lower oct gas will "pre ignite" before spark is set to it and create a flame front. Then when the spark ignites it creates a second flame front on the remaining fuel. When the 2 flame fronts meet it creates a VIOlENT crash between the two of them causing piston and rings to shake and unnatural stress on the rods and bearings. That is what knock is and I hope it help you out.

I left out why high timing causes knock. Same thing as above except you know firing the cylinder off sooner trying to take advantage of a complete burn. Low octane gas will burn to fast once lite. Higher octane gas will allow a slower more controlled full burn with out causing an "crash" in the cylinder.

I hope most of this make sense and it been a while since I studied up on this stuff. I should be 99% correct on this topic.

 

i agree with most of that, except where it comes to the burn rate part. the notion that a higher octane fuel has a slower burn rate, is true in some instances but not in others. from the research i've done, there are quite a few ways for chemical engineers to raise the octane of a fuel, some result in a slower burn rate, but some do not.

also if you could some how have colder air w/ the same # of oxygen atoms in it enter the engine, that would help fend of detonation, but that'd be impossible on the street.

 

Thank you for putting icing on my cake