Conflicted about whether to start preparing for grad school I'm new to this forum and am mainly asking for advice. -- From a conflicted rising junior in undergrad (at an Ivy).

I'm an engineer majoring in ECE with interests in CS (definitely planning on a minor, and if sanity is sacrificed a bit, a double). I've been doing research with a professor in my major and his lab since last summer, after freshman year. So far, I've coauthored one paper that has already been presented at a conference by the first author grad student but hasn't officially been published yet. I know a lot of the advice I've read is to experience industry at least once, which I'm planning on doing the summer before senior year. 

However, I'm conflicted as to whether I'm actually fit for grad school. A lot of the questions I ask myself are, "Do I like research enough? Am I motivated enough?" I definitely enjoy doing research and the topics I can explore with it, as well as the flexibility of hours and such. Often during the semesters I find myself wanting to put more time into research and less on my equally important classes.

Part of me tells myself I should just find a job and settle, perhaps go for a masters. However, another ambitious part of me tells myself to go for a Ph.D. I'm quite conflicted about my path since my past semester was a wreck. My grades were unsatisfactory (1 C and 2 C+'s in major-related classes and a math class) due to my taking of 22 credits (includes 2 research credits -- an A) which tanked my GPA to a 3.1, which isn't horrible, but not ideal.

How do I decide whether I should start prepping for grad school? (Consider that there is an M.Eng program at my school that does not require a GRE, but does require a higher GPA ~3.5.)
A 3.1 undergraduate GPA is indeed low (even from a top school) among successful applicants to a PhD program.  But there are over 130 R1 universities in the US, so it seems likely someone will take you.

I can't advise whether you should get an MS and work for a while before thinking more seriously about a PhD.  What I can tell you is that very few people who start with that intention actually do it.  It's possible to get a master's part-time while you're working but realistically, a PhD requires a full-time commitment and therein lies the problem:  Once they begin working full-time, pulling in the typical $100K+ engineering salary, most people begin spending it.  Worse, they get committed to continuing to spend it.  They take on loans for cars and condos or houses and so on, and then they're handcuffed.  They can't easily leave.

In my own 40+ years in industry, I've known lots of engineers with PhDs.  But I can only recall one person who quit to go get one.  And he had only been working for a year and hadn't yet started spending.  Sure, there may have been others, but I can't remember any others.

So, if you are thinking about a PhD, the best time to do it is probably immediately after you finish your bachelor's.  When I advise engineering students here at Michigan, I point out that I wish I had done that.  I never got a PhD and I regret it.  (And at 68, I think that ship has sailed.)  YMMV.