A couple problems.

[Unofficial]

I know there are plenty of threads on MPG, but I have some other problems too. I figured out that my gas mileage is about 10MPG. I have a completely rebuilt and bored out 350 with headers, and I even have a 700R4 transmission in my truck. The motor has factory heads and I'm running 305 injectors. I didn't change the ECM for the 350 either. I think a possibility is that I've been driving a little crazy, as it is the first week of having my new 350. Would heavy-foot driving habits make that big of a difference?

I don't want to make such a long post, but I've noticed something else. When I'm driving about 15-30MPH, my RPM will change slightly. It pulsates. Like rrRRRRrrrrRRRRrrrrRRRR If you know what I mean... It's not a major jerk or anything, but I don't know what it is. I need to change my transmission fluid soon, would bad fluid cause this?

Thanks for any help or suggestions

[VileZambonie]

You need the prom for the 350 and should replace the injectors. You're most likely too lean and the ECM is adding fuel to compensate. Sounds like you have a lean misfire occuring. Change the prom.

[eventhorizon66]

Vile, I think he did say he went with a 350 PROM in another post.  Is this right, Unofficial?

Maybe an adjustable pressure regulator could help those lower lb/hr rated injectors move a little more fuel.

Oh, and yes driving lead-footed can pay you back with 10 mpg on a 350 truck, even in perfect tune.  Just about anything with a decent V8 under the hood tends to have a jeckle/hyde personality when it comes to fuel efficiency.  Try driving it like grandpa and it might touch 18-20 mpg, when you get the fuel mixure right of course.

[Unofficial]

EventHorizon, I did say I was going to use the 350 PROM, but I never did. Just one thing, this problem occurred even before I put the 350 in. It did the exact same thing with the original 305.

[eventhorizon66]

Maybe a fuel pressure check is in order.  But I'd go with Vile's suggestion on running a 350 prom for your current engine.