check engine light and odd noises

i have no idea whats going on with my truck. Its making a wierd rumbling noise when i rev it up. Sounds like it doesnt have a muffler. it will rumble real deep and pop. The check engine light comes on if i drive it for over an hour. when i turn the truck off and back on the light turns off. Would the cat make the check engine light come on? i want to get it checked out but everytime i turn off the truck the light turns off so i cant check it... any suggestions besides driving the shit out of it til it comes on and then checking it? its a 91 chevy full size with a 350

it could be the cat or something in the muffler. test your oxygen sensors to see if they need replacing.

how can i test the o2 sensor?

i dont think there's a way to test the o2 sensor. And yes, the cat can cause your check engine light to come on. It'll probably pull a code like "auxilary control emissions" or "manufacture control" Wat you can do is pull into wherever you're getting your light checked like Autozone or something like that, but dont shut the car off, the key needs to be on anyways for the code reader to actually read the code. Hope this helps.

EGR valve

try to look for a haynes repair manual and it shows how to check the sensors. or look online for an article.

check engine light=emissions shit. The light on my expedition came on and my truck stumbled hard when I pulled away from an intersection, or on any acceleration. Turns out it was one of my ignition coils, no spark means crappy air fuel mixture, making the check engine lights come on. Do yourself a favor, go somewhere that has a scan tool and get that done cause you could be chasin problems from here till next year.good luck

i just pulled the o2 sensor out and cleaned it. see if that helps. this is the second time that the check engine light has come on. now i know not to turn it off...

i replaced the o2 sensor and its still being a bad bad truck... I smelt the exhaust and its not super gassy smelling like it may be running lean.... any other suggestions?

the check engine light hasnt been coming on... but its still making the noise. i have changed out the 02 sensor and put new plugs in it. If i hook up the code reader while the light is not on will it still say anything? And i noticed it only does this after i have been driving a while like after it gets hot.