3/4 throttle stumble

my family van is fuel injected. van used to stumble at full throttle going up hills. now it stumbles at 3/4 throttle accelerating from light. i bought the recommended villager repair book. while trying to ohm the throttle position switch their was no reading except upon opening the throttle from idle and when full throttle position harness disconnected. but with harness removed from tps key on engine not running, a hum varies with throttle opening. and the book called for a 5 volt reference but my van has 11.8vdc.
bought a fuel pressure gauge. fuel pressure is in range and fuel pressure valve works with and without vac. at proper pressure readings. once during pressure test the gauge dropped from 43 to 33 lbs. but never duplicated again.

another problem fuel pump doesn't run when key is on until engine is first cranked. then the pump runs for 5 seconds.

Is your air filter clean? Engine in tune? Might be the TPS. How about the throttle body? Some Fords/Mercs get bad when the throttle body carbons up.

What year/engine/miles?

Is your air filter clean? Engine in tune? Might be the TPS. How about the throttle body? Some Fords/Mercs get bad when the throttle body carbons up.

What year/engine/miles?

i bought the van new and have not abused it. the van is a 1993, 3.0 l, 95,000 miles. at 60k miles put bosh 60k mile pluggs in it. the spark plug wires are in good shape. while driving there isn't the missing that bad sparkplugs produce. it runs smooth until 3/4 throttle and it just starts to stumble and regains itself with more throttle. (this might be why the tps doesn't ohm-there's a wear spot in the resistor)?
i'll check the air cleaner/or replace it.
another thing i remember is the egr would open above part throttle and close suddenly around about 3,000 rpm in park. when the egr closes does it provide much better power? sometimes when driving i'll turn off the a/c switch and at about 3,000 the van gets a burst of power.

It does sound like the TPS, from what I've read.

Not sure about the egr, but the engine may run cleaner above 3K, or the vacuum state of the engine changes, so it closes.

thanks fredjacksonsan. i'll order the tps, and ignition key harness. i read in maxima topics the key switch wears down a electric contact that richens the start mixture.

i ordered a throttle position sensor and installed it. no change in stumble. our weather on the west coast just got colder. now the engine will only run very poorly maybe 40 mph.

i then removed the rubber hose intake to the throttle plate. the throttle plate was covered with globs of carbon around the throttle plate. i used a catalitic converter safe salvent to clean the throttle. at this time i removed the mass air flow sensor. it had carbon on the heater elements. since the weather changed to cold i just replaced the MAF sensor. the engine runs like a dream. good to have a fill for a dependable van again.

thanks to all threads before me talking about cleaning throttle plate, and how replacing different sensors help them.

Sweet. Sorry that I was wrong....but I guess now you have a shiny new TPS. :) Glad it's fixed. Mass Air Flow.....