Stumbling at random?

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CharlieB

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Okay I swapped a new 5.7 into my 97 and I finally got all the bugs worked out for the most part except for no tcc lockup. But anyway, when the truck is idling and I give it just a touch of gas it wants to stumble sometimes then it'll pick like its suppose to a second later. Does the same thing in gear as well. Doesn't do it every time tho. Any ideas? Thanks
 

Big D

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I had that happen a couple times, it did it mostly when it was cold, eventually it wouldnt start, it was my Ignition Control Module.... but like posted above, it could be a lot of things.
 

CharlieB

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The truck hasn't had cats for years now so that's not it. I re did all the gaskets so I know it's not a vacuum deal. All my grounds are clean, made sure I did that putting the motor back in. The tcc won't even lock up when I ground the solenoid out either
 

SAATR

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Get a DMM and check the output of the TPS. It has 3 wires:. 5V reference, ground, and signal. Check voltage at each wire with the key on until you can determine which is which. Connect between the signal wire and ground, and make sure the voltage is around .5 to .7 volts at 0 throttle and around 4.5 to 4.7 volts at full throttle. Have a helper run it through its full range, slowly, to check the sweep and look for drop outs. If you have a code reader that reads live data, you can do the same by looking at the TPS % with the key on. Should go between 0 and 96 or 98%. Also, if you have said code reader, look at the ECM's coolant temp and, with the engine cold, make sure it is at or very close to ambient temp. I don't remember what units the MAP sensor displays in, but it should be around 14 psi, 750 to 760mmHg, or 1 Bar. If it is a lot different, there is a problem.

Pull the MAF and inspect the resistors. If they are anything but shiny silver, hit them with some MAF cleaner. ONLY clean the MAF while it is cold and disconnected from power.

If all else fails, it is could possibly be the EGR. I had the same symptoms and an EGR valve code stored. Replaced the EGR, no more code, no more stumble.

Guessing and throwing parts at the problem gets expensive, quickly. You may be money ahead to pay a GOOD diagnostic tech for a couple hours of his/her time to nail the problem down.


Edit:. What skylark said is also a fair possibility. I didn't rehash. Also, make sure the throttle blades are clean and the throttle body is free of buildup.
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