Hi everyone. New member, first post.

I have a '93 W250 5.9 Magnum, with about 66k miles (actual, not overflow), and it's not running very well recently.

What I've done:
I had to cut my EGR pipe off to gain access to the crank sensor a while back, because the fasteners at both ends were impossibly stuck, but I have disconnected and plugged the vacuum port that feeds the EGR valve, so it should not be opening, although I have not verified that it is closed all the way. I intend to install a block-off plate but have not yet completed that upgrade.
I have replaced the coil, distributor, cap, and rotor. I have new plugs and wires to put on today.

Symptoms:
No check engine light.
Idles normally, most of the time. A few minutes after a cold start, it will idle very poorly for a few seconds, and then recover.
Cruises on highway normally, on roads with little to no incline.
Acceleration from a stop is very poor. Acceleration and hill climbing while moving are very poor.
Sometimes, when I lift off the gas, it will pick up a bit of acceleration.
Sometimes, I'll step on the gas, get poor acceleration, then after a few seconds, it will begin to accelerate normally.
Best performance seems to be at idle and cruise with some light acceleration in the 55mph to 70mph range.
Fuel economy doesn't seem to be suffering. It's in the 12mpg range that I've come to expect from a truck like this.

Theories:
Intermittent, but major vacuum leak.
Worn out spark plugs or bad wires. Replacing them all today, to see if it makes a difference.
MAP sensor may be slow to respond to changes in manifold pressure.
Throttle position sensor may be failing to read correctly in part or all of its range.
Timing chain may have jumped, although I tend to doubt this, due to the relatively low mileage of the engine.

 

UPDATE!

It runs much better at part throttle when cold than it does when fully warmed up.
I found that it performs as expected at WOT, even when warm, but occasionally backfires hard through the intake.

 

Disconnect the O2 sensor, and see if it runs any better.

 

It's interesting that you suggest that. A year ago, the truck quit running, because the sensor ground reference became disconnected (I suspect rodent activity) for the crank sensor and speed sensor. Given the direction that the wires from the O2 sensor go, I wouldn't be the least bit surprised if it's the same problem. It's not easy to get at the O2 sensor connector, so it'll probably be the weekend before I can try it, but I think your suggestion has substantial merit.

 

I was able to get in there and unplug the O2 sensor. It made little if any difference. I'm going to troubleshoot the wiring this weekend, and see how that goes.

 

Test the throttle position sensor. Use an analog meter. Digital simply won't get it. You should see a SMOOTH increase from less than 1 volt at closed throttle, to greater than 3.5 volts at WOT. If it bounces around AT all in the range, replace the sensor.

 

Throttle position sensor tests ok. The ground wire to the O2 sensor is showing no continuity to ground, so I think it's safe to say that's a large part, if not all of my problem. I suspect that the crank position, speed, and O2 sensor grounds all go to a common point, and that common point failed at some time, about a year ago.

 

Ran a new ground wire to the O2 sensor, and it runs much better now. It's still not perfect. There's still some hesitation when launching, and when accelerating, but the improvement with just reconnecting the O2 sensor ground is dramatic. I didn't get around to putting the new plugs and wires in yet, so maybe I'll do that, and see what happens.

 

Be sure and reset the PCM as well. It may have learned some bad habits when things weren't working as they should. (just disconnect the battery for 15 minutes or so.)

 

Try removing the oxygen sensor. It should be on the left (drivers) manifold. Be sure to check timing.