Okay guys, I've gone through a few tanks since my M1 install that fixed the plenum leak, but I had yet to truly check my fuel mileage. I didn't get the most accurate check, but approximately 10 miles after my low fuel light came on, I had only gone 228 miles on my odometer. Being at just a hair under 1/16th before I put some gas in it, I'm guessing I could have gone another 10-15 miles safely. However, this still gives me under 10MPG, which is horrible for a 5.2.

Also, my idle has been ridiculous (900ish cold, 750-800 warm), ever since I got the truck. While doing the manifold install, I cleaned the throttle body, including the IAC and its port thoroughly with cleaner and brushes, but I'm still getting a bad idle and an obnoxious sucking noise when cold and when coming down from a high RPM. I'm pretty sure the IAC is ready to be replaced, but I'd like some verification. Also related, it runs very rich cold, as I can smell gas pretty well from the exhaust, but that may just be that it hasn't started reading the sensors, although the tip of my exhaust is coated black like it has been running rich for a while. Warm, it seems to have a slight miss at idle.

The plugs were replaced last summer, when I got the truck and started working on it with my dad. Unfortunately, I hadn't read up on the plugs, and I ended up with Bosch Split-Fire coppers. I checked them the other day, and there was no wear and just the normal light brownish color on the plug.

Other than what is previously mentioned, it had a good tune-up last summer with MSD 8.5mm wires, an Accel cap, MP 14x3 air cleaner with K&N filter, etc. The cat has NOT been replaced, but my dad doesn't believe it to be clogged.

What I want to know is what may be causing these things. I'm pretty sure I'm gonna give a pre-cat o2 sensor a try, since I dont think it has been replaced, and possibly a MAP sensor. The truck has 237k, so I'm sure it's time. But under 10mpg is ridiculous, even if 70-80% of my driving is in town.

What do you guys think?

P.S. Quick summary for those who don't want to read all of that:
Bad idle
slight miss at idle
horrible mpgs
replaced old manifold with an M1
runs rich
sucking noise when cold and letting off from a high rpm (3-4k)

 

I get 11MPGs with that much city driving, so you're not too far off.
Although, my plenum might be blown.

As for running rich when cold, that's just the computer default.

The idle is probably the IAC, but I don't know that much about it.

 

Never judge MPG by using the gauge. Fill up!!!
And BTW, in my experience, Dodge low fuel lights come on with 5 gallons left in the tank.

 

Well, mine doesnt smell rich at cold startup. (dont smell like gas or anything) And i dont have black soot on the tail pipe either. So i wouldnt say its normal..Unless mine is running different from others. I have an all stock truck tho. Id def check the o2 sensors. How about the codes? Checked for any yet?

OP Are you sure that the MPG isnt from a heavy foot tho?

Oh yea, you say you had a plenum leak? Them o2 sensors. They could be screwed up from the plenum leak.. You know...from oil...

 

hows the cat?

 

Try cleaning the TB, replace IAC and Map sensor. Get a Haynes manual and test the TPS.

 

for the mpg problem - replace front 02 sensor.

for the idle problem - replace IAC, don't forget the o-ring. save the old IAC temporarily. if that doesn't fix it, there was nothing wrong with the old one, congrats, you now have a spare, now spend another $45 and replace TPS.

 

TPS has been replaced a month ago (forgot to mention).

My dad is dead set that my cat is not clogged, but I convinced him that after I get it inspected next month, its getting gutted.

I replaced the IAC today, now my truck finally idles down to around 650 warm. Much better. The plunger was froze up pretty bad keeping the port in the TB shut at all times.

As stated in my original post, I cleaned the TB thoroughly while installing the M1. It was horrible, but when I replaced the IAC it was still squeaky clean. I'm probably going to invest in a pre-cat O2 sensor tomorrow, see if that helps.

I found out today that the MAP sensor is 2x as much as I though it would be, at around $100. Yuck.

But it's definitely running rich according to the tailpipe. It should not be that sooty.

I wasn't sure how much was left and I wasn't carrying much cash on me to the homecoming game last night, so instead of chancing it, I threw $10 in it, which made me so mad that I messed up my mileage check, but better safe than sorry.

Going to test drive in the morning to verify the sucking noise is gone. It sounded so crappy, I was embarrassed to drive by all the kids waiting for the bus down the route I take to the high school. And hell, it would almost feel as if it was about die at some points.

 

wow thats worse than my 360, i run the a/c and get 14.5 city and i got 191K on it (runs like a champ)

 

Yea Im pretty sure there's something amiss.