'99 Vortec 305 runs rich/performance improvements

[Old Blue]

My '99 K1500 has a 305 crate engine with approx. 25,000 miles. It seems to be running really rich. The exhaust smells really strong (stronger than my grandfather's 1995 C2500 with a TBI 350). It still has the catalytic converter and both upstream and downstream 02 sensors (no check engine light). I'm sure it's not supposed to be this rich. Plus, I've noticed that my fuel economy is dismal at best. I don't know what I need to do to improve this condition.

Also, a question for those of you still running a 305: What kinds of things have you done to improve power? My truck has the 4l60e and 3.73 gears. It accelerates ok on flat roads, but I have to give it a fair amount of throttle to keep it at 55 or above when going up hills. I didn't know if there were common upgrades that could boost power that also wouldn't break the bank. Thanks in advance.

 

[pro17]

Can you check for codes? (you didnt mention there were any, but my truck came with the check engine and ABS light in the dash cluster physically busted, and we found plenty of codes, unaware of, several years later).
BTW how long as it been doing it

 

[Old Blue]

The check engine light comes on when I start the truck (as it's supposed to), and then promptly goes off. It never once has come on during my ownership. The only warning light that comes on (and stays on) while driving is the ABS light, which I was told by the previous owner was bad vehicle speed sensors. I looked inside the tailpipe today and it was coated in soot (such as one would find in a chipped or tuned diesel). It doesn't smoke or backfire, which are two of the symptoms of running rich that I found while researching this problem.
I'm wondering about a temp sensor that might be telling the computer that it's still cold, thus dumping more fuel in than it needs. Again, I'm looking for advice because my knowledge and training is centered around vehicles equipped with carburetors.

 

[Old Blue]

BTW how long as it been doing it

The exhaust has smelled really strong since I bought it back in June. I only started thinking something might be wrong now because I'm getting ready to go on a trip. I hadn't really taken it out on any long distance on the highway, so I didn't realize how sluggish it was, or the poor fuel economy. Recently, I've started taking out on longer stretches of highway and I'm only getting around 14 mpg. A friend of mine has a truck almost identical to mine but with a 5.7 and oversize tires, and he averages 17-19 mpg. Granted, he has put a cold air intake and true dual exhausts on his, so that may account for some of the difference.

 

[DerekTheGreat]

I highly doubt your friend is getting 17-19 with oversize tires. Even with the mods, I put duals on my Town Car and noticed nothing.. I've got a 5.7 in my '89 K series and it gets 14-16 depending how I stab it and such. Your buddy's speedometer is most likely off and so is his gas hand when it's time to refuel. Anyway, when's the last time your truck saw fresh plugs? Have you had a look at them to see if it's running rich or using oil? If they're sooty black that's a good indicator of an engine that's running rich. I'm not a vortec guy but I'm left wondering why the o2 sensors aren't picking it up. Seeing as it has an upstream and down stream sensor wouldn't that be able to tell too if the cat isn't operating correctly?

 

[inscarguy]

my suggestion is the basics first, do a complete tune up if it has not had one, clean the mass airflow sensor(could be the culprit), throttle body in and out. get a scanner or go to a parts store and have it scanned just to cover all the bases. I have seen codes w/o throwing the CEL

aside from that it could be anything, bad wires, weak coil, coolant temp sensor, anything.

 

[pro17]

The exhaust has smelled really strong since I bought it back in June. I only started thinking something might be wrong now because I'm getting ready to go on a trip. I hadn't really taken it out on any long distance on the highway, so I didn't realize how sluggish it was, or the poor fuel economy. Recently, I've started taking out on longer stretches of highway and I'm only getting around 14 mpg. A friend of mine has a truck almost identical to mine but with a 5.7 and oversize tires, and he averages 17-19 mpg. Granted, he has put a cold air intake and true dual exhausts on his, so that may account for some of the difference.

At the very least, I would get one of these:
https://www.ebay.com/itm/New-BAFX-E...-Tool-Android-OBD-Reader-Scanner/251488093341
And then on your smart phone, purchase an app like torque pro, and that will show you fuel trims, data from the sensors, etc. Before you start throwing parts at the vehicle, at least look at what the computer is seeing.