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Serious lack of power under acceleration

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fwc

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Everything was running fine until two weeks ago...

On a "cold" engine it starts fine, idle is a bit rough with some surging. When I first put it into gear the idle drops a couple of hundred RPM and then comes back very rough.

Acceleration from a stop is horrendous, like I have a wheezing four cylinder under the hood. It surges and jumps and then smooths out after 2000 rpm, when the tranny shifts it bogs down again until the RPM builds up.

Running on the highway, it seems pretty good although I can feel a slight miss every once in awhile. Acceleration from speed is fairly smooth although it doesn't always kick down a gear. Once it has all warmed up and run on the highway for a bit, acceleration around town isn't too bad, but still not perfect.

91 Jimmy, stock 350 TBI and auto, only things that haven't been replaced within the last few years are the injectors and fuel pump.

ALDL logs attached. This is from a "cold" start, warmed up for about a minute and then driven a couple of miles. Any thoughts?
 

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Check your fuel pressure. Have you changed the fuel filter recently? It's a good idea to change it at least once a year.
 
Yes, O2 sensor is installed and about a year old. No SES light and no codes.

Picked up a new fuel filter last night, I will put that in today. The one that is in there is about a year old. I will try to check the fuel pressure, not sure if I have the proper guage for that though.
 
Base timing is set to 0. Cap, rotor, and ICM are fairly new.

I replaced the fuel filter, injectors and fuel pressure regulator diaphragm since I already had the parts. That seemed to help a bit. Checked the fuel pressure and it registred 9psi.

I also replaced the coolant temperature sensor and that made a huge difference. Still a slight bit of sluggishness at low rpm but at least it is driveable now.
 
fwc said:
Base timing is set to 0. Cap, rotor, and ICM are fairly new.

I replaced the fuel filter, injectors and fuel pressure regulator diaphragm since I already had the parts. That seemed to help a bit. Checked the fuel pressure and it registred 9psi.

I also replaced the coolant temperature sensor and that made a huge difference. Still a slight bit of sluggishness at low rpm but at least it is driveable now.
Click to expand...

Buy an adjustable fuel pressure regulator and set it so you have 12-13 psi and i'll bet your problem is solved.
 
I had checked the fuel pressure at the filter outlet and it registered 9PSI. There was no way I could figure out to get the adapter on the fuel line at the throttle body. There jsut wasnt enough space. Where would an adjustable regulator go? How would it change what the pump is putting out?
 
It doesn't matter where in the fuel inlet line you check for pressure it will be the same. The adjustable regulator goes in place of the stock regulator.
 
It may not be the regulator, it may be the pump. But 9 PSI is to low, 12 to 13 is good. Your BLM all show lean as well telling us 9 PSI is to low. Your TPS is also 6.5 I think and should be closer to 5.4. IAC looks good so no adjustment on throttle plates but TPS does need to be adjusted.
 
Hey eagle mark, Thanks for taking a look at the winaldl logs. I already have a new pump, just havent put it in yet. I will try to get that done this weekend but will adjust the TPS first. Thanks!
 
The TPS on a TBI aren't adjustable by loosening the screws and adjusting like a TPI set-up. You can however bend the little arm to adjustment the reading.
 
4X4HIGH said:
The TPS on a TBI aren't adjustable by loosening the screws and adjusting like a TPI set-up. You can however bend the little arm to adjustment the reading.
Click to expand...
Your correct, but the TBI TPS has enough slop that it is (slightly) adjustable. Usually if you have to bend the tang it's a wore out throttle shaft, or new rebuilt throttle shaft or ? and not just a changed TPS....
 
The TPS on my TBI unit will not move at all with the bolts really loose.
 
If your base TPS voltage is off on TBI, perhaps resetting base idle (idle set screw plus the correct procedure to get the IAC to "relearn") would fix it, instead of trying to move the TPS?

Now if I could only figure out why the fairly early 6.5L diesel/4L80E applications don't use base TPS voltage to test/set, and require a calibrated gauge to test/set...
 
Well I was able to "adjust" my TPS mounting position by elongating the screw holes along the arc of rotation and get it set to read 0.59V. That helped a bit but same drivability problems exist. I am dropping the tank on Friday to install a new fuel pump. If that doesn't fix the issue, then I am really at a loss.... Beyond that maybe the cat is plugged? It is only about four years old...
 
Ok, replaced the fuel pump and that seems to have solved everything. Also replaced the rear most body mounts while I had the tank down since I wasnt able to do those when I did the rest. Thanks everyone for your help!
 
Awesome, I bet it feels good! (both how it runs, and the sense of accomplishment!)

Congrats. :)
 
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