O.k. so this past weekend i replaced the spider assy with the upgraded version,oh 2000 jimmy 4.3 auto. blah blah blah. i was having a slight ruff idle, but ran very good, getting 16 mpg around town, and 19 hwy, not to bad right, so did the replacement and the moter run great, i mean wow what a difference, night and day difference, but the mpg is lower, now i baby this money pit better than anything, super easy on the pedal and all that, any ideas????? currently mpg dropped to 13 around town, dont have a hwy mpg yet....
thanks in advance,
Robert

 

SES light on?
Any codes in memory?
Did you check fuel pressure?
Does the engine reach operating temperature 195F?
4X4 front axle might be engaged due to a stuck vacuum switch.
I see you have messed with the torsion bars... have you checked front chassis ride height and wheel alignment?
Non stock wheel & tire size, did you recalibrate/flash the PCM?
Each one of these things can and will affect fuel mileage.

 

Captain, no SES lights or DTS codes, not checked fuel pressure, the 31" tires been on for over a year,and the aligment is correct,everything is as it was. the origianal mpg i was getting im not complaning about, i was just expecting the same or better, but the vehichle runs great.
i did do a gear change ot 3.73 to compensate for the tires, and did not flash the ECM. ????

 

Seems hard figure that the spider would change mileage. Just changed my spider but truck is not on the roAD yet so dont know the mileage

 

If the SES light is off, (provided it works) the OBD system is doing its job and no problems have been detected. Fuel pressure is critical on these engines, so whenever there's a driveability issue, it should be the first thing to check. Fuel pressure is not monitored by the PCM so if there's a problem, the SES light will not necessarily illuminate. Key ON, engine OFF, fuel pump running, pressure must be 60psi to 66psi and must remain above 55psi for 10 minutes after the pump shuts off.

The transfer case vacuum switch has a habit of leaking vacuum or sticking on these vehicles. If the switch leaks vacuum while in 2WD, it can engage the front axle. The transfer case does not engage the front driveshaft, so you're still in 2WD. Sometimes the only symptoms are a drop in fuel mileage, or "crow hopping" in the front end.

Adjusting the torsion bars changes front chassis ride height, which directly affects wheel alignment substantially, which in turn affects fuel mileage.

Injector pulse width, transmission shift points, ignition timing, speedometer accuracy, ABS operation, (and other things) are all affected when changing wheel & tire size, and axle ratios. Whenever a change is made, the PCM needs to be flashed to calibrate it correctly to improve fuel mileage and performance.