engine bucking, hard to start, bad mpg

My son has a 99 Sierra with a 4.3 (tbi I think) and an automatic. About 83k miles.
In the last few months we have noticed a drop in MPG from about 18 to 15.
From time to time the truck is hard to start. It just turns over and when it does start there is a cloud of black smoke and a rough idle for a min then it smooths out. Soot in the tail pipe too.
And sometimes when you are driving the engine bucks. Like a monentary loss of power. At a very slow speed (idle) today the engine stopped twice in parking lot driving.
Fuel filter is less than 1000 miles old. New coil, and a bottle of gumout has been run through the system. Spark plugs and wires are about a year old.
What should we do next....????

Are you getting a check engine light? If so check to see what codes you are getting. Could be an o2 sensor or coolant temp sensor. Lots of different things that could cause your problems. Have the truck scanned for trouble codes and then repost them.

Last year for TBI was 95. You have a Vortec. My neighbor had a 97 Vortec that did the same thing and it turned out to be a bad fuel pressure regulator. Basically, the engine was flooding (due to the leaking fuel pressure regulator). Hard starts when warm and black smoke (rich condition) too.

I did a fuel pressure test and the pressure dropped to zero in about 10 seconds after shutting the engine down (it should hold pressure).

//2000CAYukon

No Check engine light. We will try to get a scan done just in case.
Thanks for the info.
We will post as we learn more.
Vortec not TBI, my mistake.

Ok, here we go.
1 no codes found
2 R+R fuel regulator. Had to remove the top plastic cover on the intake to get to the regulator.
3. Started right up and no big puff of black smoke.
4. Took it out for a drive- within a block of autozone the check engine light came on. Pulled in and they came out and read the codes. 171 and 174. lean on both sides. I had a loss of power too.
They suggested a new mass airflow sensor.... none in stock.
I drove to another close by store, not in stock. but I did see a can of cleaner for MAS. Purchased it and pulled the sensor in the lot and cleaned it. Reinstalled and drove to the next store. Asked them to reread the codes. same codes 171 and 174. This person said to try to just replace the fuel filter and add some Lucus fuel system cleaner.
Hmmmmmm. Sounds cheaper than a mass to ok, I purchase the filter and fuel cleaner.
Pour it in and start for my brother in laws garage to change the filter. within a block of the second autozone the check engine light goes off. Go figure.
Truck is driving great now....?
Well while at my BIL's shop we reread the codes (no change) and then clear them. I also pulled the O2 sensors and cleaned and soaked them.
We put everything back together and drove it home. Truck is starting and driving fine now.
No codes! Yea....
Thanks 2000CAYukon and everybody.

We also pulled all of the ground wires we could find and cleaned the connection points and reinstalled.

We never did get the fuel pressure tested.
They had a tester at Autozone but said it would not go high enough. (only to 100psi)
What psi do I need to test this system?

We never did get the fuel pressure tested.
They had a tester at Autozone but said it would not go high enough. (only to 100psi)
What psi do I need to test this system?

Fuel pressure at idle should be 60 - 65 PSI. A 100 MAX PSI tester is fine for this truck.

//2000CAYukon

Thank you for the info.

I think I will pick up a tester and check the pressures. It is always good to have an excuse to buy a new tool.