Power loss and fuel milleage

[PIAMG]

1999 GMC Jimmy Envoy. 8 months ago milleage went from 500 Km or 310 miles to 350 km or 210 miles. Since then the tranny was rebuilt. New Magnaflow Cat and Muffler installed. Wheel bearings and breaks. Two fuel pumps (non-OEM) Injector and intake flush and clean, new plugs, throttle body spacer and Edgetech Programer. Cold air intake installed and removed (gas milleage worsened). Fuel pressure is good and engine and tranny scanned. No codes or check engine lights. It continues to run like a carburated vehicle running rich. When off the gas it feels as if there's a jake break (engine retarder). Does not back fire but hesitates under full throttle.
Lots of power available just no go. Any suggestions? Thanks

['98jimmyon22s]

i'm having the exact same problems with my '98 jimmy. i've recently done a tune up with 60 dollars worth of new ac delco plugs, new plug wires, dist. cap and rotor button. i'm also having trouble with the transmission blowing fluid out of the vent tube. i'm assuming some of that is tied in with the 4x4 problems that i've hopefully found an answer to from another thread. could the loss of power and sluggish performance be linked to the recent brake job i did? i read a thread that said the rear brake calipers need to be replaced at the same time. all i did was replace the pads and rotors in the rear and just the pads in the front. any help with our poor power and fuel mileage would be greatly appreiated!

oh and by the way, if anyone has any clues on the tranny fluid coming out of the vent tube, it all seemed to start when i replaced the water pump(had to disconnect the trans cooler lines). i checked them for blockage and they seem to be fine. i installed an external trans cooler just in case there was a blockage inside the radiator with no luck. someone please help!!

[MT-2500]

Originally Posted by '98jimmyon22s

i'm having the exact same problems with my '98 jimmy. i've recently done a tune up with 60 dollars worth of new ac delco plugs, new plug wires, dist. cap and rotor button. i'm also having trouble with the transmission blowing fluid out of the vent tube. i'm assuming some of that is tied in with the 4x4 problems that i've hopefully found an answer to from another thread. could the loss of power and sluggish performance be linked to the recent brake job i did? i read a thread that said the rear brake calipers need to be replaced at the same time. all i did was replace the pads and rotors in the rear and just the pads in the front. any help with our poor power and fuel mileage would be greatly appreiated!

oh and by the way, if anyone has any clues on the tranny fluid coming out of the vent tube, it all seemed to start when i replaced the water pump(had to disconnect the trans cooler lines). i checked them for blockage and they seem to be fine. i installed an external trans cooler just in case there was a blockage inside the radiator with no luck. someone please help!!

Better to start a new thread with problems on your own truck.
MT

[MT-2500]

Originally Posted by PIAMG

1999 GMC Jimmy Envoy. 8 months ago milleage went from 500 Km or 310 miles to 350 km or 210 miles. Since then the tranny was rebuilt. New Magnaflow Cat and Muffler installed. Wheel bearings and breaks. Two fuel pumps (non-OEM) Injector and intake flush and clean, new plugs, throttle body spacer and Edgetech Programer. Cold air intake installed and removed (gas milleage worsened). Fuel pressure is good and engine and tranny scanned. No codes or check engine lights. It continues to run like a carburated vehicle running rich. When off the gas it feels as if there's a jake break (engine retarder). Does not back fire but hesitates under full throttle.
Lots of power available just no go. Any suggestions? Thanks

Several things needs checked out.
A complete fuel pressure check.
All engine tempt and air tempt sensors and readings.
MAF sensor reading.
Is all 02 sensors working good?
Air fuel mix ratio?
Transmission lock up working good?

Fuel pressure test.
Hard to start cold.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak.
Post back fuel pressure readings.
MT

Is the engine reaching operating tempt of 195-200 degrees?