It is possible you have a blown plenum gasket

 

Ugh. I've heard it said that every 2nd gen dodge on the road has a leaky plenum gasket, its just a matter of how much.
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Anyway, I reved the engine and after holding it at ~ 3000 rpms for a few moments going up a hill the check engine light started its blinking, and I got these codes: p0306 p0300 p0302 p0304 all engine misfire codes. What would do that?

 

im new here and im not the smartest mechanic around either when it comes to automobiles. i do know a bit though. my expertise are motorcycles and ATVs. my opinion on losing gas mileage with a free flowing cat-honestly, i dont see how it would cause your mileage to go down. if anything it would get better. less restriction=more power, better flow. less restriction=better mileage. it will not cause your truck to run rich either. if anything, lean but thats where the computer and fuel injection come in play. it will adjust automatically. as for this so-called "back pressure". i wish some one could explain this to me. what is back pressure supposed to do and be? thats got me confused. from my understanding and experience, the freer flowing the exhaust, the better. just my $0.02.

 

It is understood that too little backpressure will harm low end torque. This is true in some respects. The key to an efficient exhaust system is exhaust velocity. A free flowing exhast pipe that is too large creates a slower gas velocity, hurting power.
That's it in a nutshell.

 

A variety of things can cause multiple cylinder misfires.
Vacuum leak, clogged fuel injectors &/or worn rotor/cap/wires some of the more likely.

 

just put a glasspipe on there to have some back pressure

 

I gutted my cat after it plugged up on me. I was an hour away and had the g/f bring down to me the sawzall. I cut the cat off and drove home with open headers. lol It sounded pretty aggresive. Oh yeah this was last year after 12 inches of snow. Why can't this stuff go bad in the nice weather. I hollowed out the cat with a pry bar and had the local mechanic weld it back up for me. So far the c.e.l. only goes on after I leave it idling for long periods of time. I do have a Flowmaster muffler that makes it sound nice too. As for the emissions testing for inspection if you have the type where you have some put a probe in your tail.....pipe you more than likely will fail. How ever if they only plug into the dash OBD2 port your good as long as there are no check engine lights. I did some digging if you start having the c.e.l. come on you can purchase something called a cat delete box. You wire it into the harness where you would put the O2 Sensor and viola, no light. Ahem for off road use only Good luck

 

thats what i did, sounds nice as fu*k too

 

Oh yeah if your not sure if its plugged use a temperature gun to get a reading from before the cat and after . If the cat is plugged the temps will be different.

 

Actually, the temperature is supposed to be higher on the downstream side if the cat is functioning properly.