Hello all ,i have a 1997 f-350 with the 351 it has a dead spot ,or a surge in the throttle about a 1/4 of the way down it runs great up to that spot and after its been doing this for over a year and now the wife drives it so im trying to find the problem.So far i changed plugs wires, cap ,and rotor throttle postion sensor,crank sensor coolant sensor,i even tried another throttlebody from a truck that runs just fine.It does it on both fuel tanks so im guessing its not the fuel pump and i did change the fuel pressure regulator.Does anyone have any ideas ?Thanks

 

Did you ever check for codes before or after replacing parts?

 

yea i did and the only thing that came up was the egr valve that i replaced when the check engine light came on but it is out now ,i did find out one thing and that is it has california emmisions on it and it took a different egr valve that autozone couldnt get so igot that from rock auto.

 

,i did find out one thing and that is it has California emissions

As in OBDII / MAF? did you try cleaning the MAF sensor, clean it with MAF sensor cleaner?

Check it over real good for any unmetered air causing the problem?

Did you actually test any of those parts? Determine if any were out of range, old and new parts both?

 

yes i did clean it ,and all the parts was new,but not ford parts, autozone stuff

 

One of the quite rare OBD-II OBS HD trucks. Try unplugging the MAF and see if the dead spot goes away. Sometimes MAFs just can't be cleaned.

 

Thanks i will try that tomorrow and see what happens

 

Exactly sometime they cause surging and must be replaced to correct it, simply cleaning it doesn't help. However its a good place to start.

Not saying run out buy a new one, one from a bone yard would be fine if decide its time to try another one. Try the meter off that other truck if you haven't, sounds like it's same setup? Always handy have one to do test swaps with, rule out parts in question when testing otherwise fails provide answers.

 

The statement in bold is what concerns me. Many of the aftermarket parts have very poor quality control. I hope you kept the old parts. Since your truck is a California emission OBD-II version the EGR systems uses a DPFE sensor to measure the actual EGR gas flow. The older trucks use a EGR Valve Position (EVP) sensor.

Along with what the others have recommended I would also go back and measure the signal output of the Throttle Position Sensor (TPS). Many folks have had bad sensors right out of the box when substituting aftermarket parts. Invest in a cheap DVM to measure the output value and linearity. Much cheaper than investing in a new MAF only to find the issue is not resolved.

 

Well i did unplug it and the surge is gone so i guess i know what it was now,would it hurt to leave it unpluged till i get a new one or should i just plug it back in fo now ?

 

It'll be fine for the short time, fuel economy will probably go down however. However before you condemn the sensor for sure, do a good check for vacuum leaks.

 

Will do on that but atleast im in the right area for a change lol and thanks for the help

 

Yea before replacing it check for vac leaks, especially the lower intake. It be suspect given your symptoms over say a simple vac line type of leak.

Myself I don't own a smoke machine and never bothered to make one up. I just use a garden hose/water to check for suspected intake vac leaks.
With the engine and exhaust system up to full running temp I Flow water over all matting surfaces working low to high and back to front, doing so while someone watches tail pipe. When/if hit a spot where there is a vac leak, you'll immediately see steam at the tail pipe from the water being sucked in.

When done right it shows if there is a leak and if so exactly where that leak is.

Or you could wipe up a smoke machine if you'd rather not use water.

DIY Smoke Machine for Vacuum Leaks - YouTube

One of many ways to accomplish the task.