Ok to start off with i dont know hardly anything about the 2.5 or much ford stuff. My stepdad and i are trying to figure out what is wrong with a friends 98 ranger with a 2.5. The owner said it was fine then on his way home from work it just wouldnt go over 45mph and on idle the engine shakes. he took it to two or three different places and they replaced the injectors, the fuel sending unit, checked the cat, we checked the MAF and nothing. when we put a computer on it there arent any codes but when you go to live reading the o2 sensor has zero voltage. we put a voltage tester on the plug and had voltage. we even put a different sensor on and still nothing. Timing looks to be right. Only thing i can think of is maybe the pcm? can anyone give us some ideas?

 

Yes, could be a failing PCM but the fact you can "talk" to it, i.e. get data, would mean it is at least partially working.
I would start engine then pull off the MAF sensor connector, see if CEL(check engine light) comes on, it should, and then shut off engine and read the codes, if there are no codes this time then PCM does have an issue.

This is just general not, ford specific, info:
Computer controlled engines do not(can not) use the O2 sensors until engine is warmed up, so if the trouble also occurs when engine is cold then it won't be an O2 sensor issue.
O2 sensors must be heated above 600degF to work(read oxygen levels), this takes about 5 minutes on cold engine and 1 or 2 minutes on warm engine.
During this warm up time the computer is in Open Loop operation, it is running preset air/fuel mixes from tables in memory, once warmed up the computer switches to Closed Loop operation and uses feedback from the O2 sensors to fine tune air/fuel mix.

The lima 4cyl engines, 2.3l and 2.5l, that used dual coil set ups had a common issue with the ignition module(ICM) causing problems, these can be tested at most auto parts stores or with a volt/ohm meter.

Another issue that could cause the symptoms is a stuck open EGR valve, or bad EGR modulator.
The EGR valve should only be open when engine is under load, i.e. foot on the gas, rich fuel/air mix, if it is allowing exhaust gases into the intake at other times engine would stumble and lack power.
With engine at idle remove the vacuum line from the EGR Valve, there should be no vacuum present on the hose, if there is then the EGR modulator could be stuck open, follow that hose back to EGR modulator.

If there is no vacuum on EGR valve hose then put your vacuum hose on the EGR valve and suck on it, i.e. open EGR valve manually, engine should start stumbling more and then die as EGR valve opens.
If there is no change or very little change then replace EGR valve.

I assume the other "shops" checked fuel pressure.

And used a vacuum gauge to check that a vacuum leak wasn't the cause.
PCV valve hose is another common issue with Rangers, they tend to crack and leak badly as they get older