5.7 TBI throttle response

[Krosati]

The Suburban I bought a couple of months ago seems to be sluggish from take off. I had some problems with the charging system but I had the alternator converted to SE and it charges fine now. After spending a few weeks driving it, I find that gas milage is only 7.33 MPG!
In order to rectify this, I started checking the manual for possible causes.
Here's are the symptoms:
1. When cold, truck drives great and throttle works smooth from idle to 70 MPH.
2. Once warm (not fully up to operating temp),idles fine but starts from dead stop are sluggish unless at least 1/2 throttle.
3. Once fully warm, idle seems to vary a 100-200 rpms and starting from a stop (sometimes) causes the engine to die.

Here's what I have done so far:
1. Cleared codes (none present but did it anyway) I do not have any new codes stored in the ECM.
2. Confirmed open loop on cold engine
3. Confirmed open loop at idle and closed loop above 500 rpm on warm engine.
4. Replaced temp sending unit (on front of manifold near t-stat).
5. Replaced t-stat with 195 degree unit. Spring side is in manifold.
6. Replaced O2 sensor
7. Replaced TPS. It is a non adjustable TPS.
8. Removed air cleaner to observe the fuel spray in TBI. There seems to be a few droplets every few seconds but mostly just a fine mist. Purchased injector cleaner and added to full tank. Still running last half of this tank so milage may increase a bit.

Any ideas as to what to check next? I am replacing the fuel filter and spark plugs Sunday to see if this helps.

[crossy]

I won't act like i am a good technician here but the TBI is very simple. since you replaced the TPS & O2 sensor the only things that could really be left are, EGR, MAP, IAC. I don't beleive that any of the new style converters clog like the old ones used to. whenever mine had problems i paid to have it scoped and check the A/F ratio . Air/ Fuel . there has to be a problem with a TBI gettting 8 MPG.
I know goodwrench & 75 Chalet are pretty good with the taech parts of the system. good luck, DAVE

[1FaastC10]

8mpg may not be that bad. what are the specifics on the truck?

1/2 or 3/4 ton? 2 or 4 wd? what trans? gear ratio? tire size?

[Krosati]

Gmc Suburban. 1/2 ton 2wd, 5.7 w/700R4 trans.
Unsure of axle ratio as of yet.
Engine replaced at 127,000 with replacement Goodwrench engine. Now has approx 36,000 miles.
Research shows milage should be 12 to 18 mpg. I expect at least 12 from TBI since my older cars have always averaged better than that with 4bbl carbs.
I'm sure some milage will return if I can solve the sluggish response of the TBI

[racedvl]

My milage with my 87 4x4 burb with a TB 5.7L - 700R4 - 3.42 - 31x10.5 R15 coming home from Pigon Forge was ~ 17 MPG.

[apstguy]

Definitly chage the MAP and IAC sensors. They measure stuff to do with air which bases all the specs in the engine on (timing & amount of fuel). These MAP sensors on TBI seem to be little PITA's.