did you guys miss the fact he's running bosch o2 sensors and possibly exchanged bad sensors for bad sensors? If the engine thinks he's running lean it's going to keep dumping fuel, right? That's not so good for economy.. I'm just sayin...

 

I work at a parts store, and we sell Bosch O2 sensors (among others, such as NGK).

We've sold hundreds of them. I have seen 1, maybe 2 bad reports on them. They are not that bad of a sensor, and that is not his problem.

OP, I wouldn't aim much towards ignition/spark, but rather towards the fuel side of things as a few people have said. If the pressure is ok, get the injectors tested (or test them yourselves). That's what I would do next.

 

i didn't pay any attention to the bosch brand. i assumed that if he originally had a false lean O2 signal then the O2 swap would have created some positive improvement, even if it didn't fix it. i read that his O2 swap had no effect at all on his 7 mpg.

also, i would expect that a severe lean signal would set a CEL. since he's getting no CEL, then I think? the pcm is not aware of the problem, and thinks the fuel delivery is normal. on the other hand, he's got no CEL for rich, so ..... I don't know....

a scanner would prove that his O2 signal is normal or bad lean.

 

I have a set of spark plugs, I will be putting them in either Today or Sunday, and after driving some distance ill pull them out and see which ones got the worst.

I thought it would have been the PCM as well, because it was throwing an internal PCM error code.

My friend was driving behind me the other day and he called me and told me that he can just see black smoke pour out of the tail pipe when I lay into the throttle. He said "If I didn't know any better I would say its a diesel its smoking so much!"
After hearing that, I'm almost positive its going to be the injectors, because they are really the only thing I have left to change.

 

How about a refresher on what you have changed out.... I don't wanna go paging thru the thread... sorry, feelin' lazy today.

Have you changed to coolant temp sensor for the PCM? (I do believe you have two, one for the PCM, one for the gauge in the dash)

Reading the data isn't going to tell us if the O2 sensor is giving false readings. You will be able to see what it IS reading...... but, without something to compare it to, the data isn't exactly helpful. I would point out however, that no codes, and running rich enough to be mistaken for a diesel does not bode well. These trucks are notoriously sensitive to the brand of sensors that are installed...... I can't count the number of threads I have seen where a member changes some sensor or other from the parts store, only to continue to have troubles. Changing the off-brand sensor for a known good brand, or, one from the dealer, solves the problem.....

The CTS, and O2 sensors are the ones most responsible for fuel economy. If a PCM swap didn't change anything, I would be looking REALLY hard at those two.

 

I know some napa's test and clean injectors. Just another thought.

 

I have changed the Coolant temp sensor that is next to the thermostat in the intake, I didnt see any other sensors for coolant, and when i went to the parts store they only had one type of sensor.

I went to the junk yard and bought a set of "new" (to me anyway) injectors for $50. I am putting them in right now, I'll post back the results, most likely later tonight.

(edit): Got the new injectors in, the idle seems to be a little more smooth, hopefully after driving for a little while I will know if I am getting any better mileage.

 

Got the injectors changed, still averaging around 6-7 mpg, after about 120miles.

I can't even think of what to change next.

 

How is the compression?

Is the truck running on all cylinders?

Is the air filter clean?

Are the brakes sticking?

Are the wheels free to turn?

How many metal tabs were in the connector for the coolant temp sensor? The engine one has one and the gauge has two. Both my '94 motors had 2 coolant temp sensors.

 

The compression was right around 130 when I last checked it, I will try to do another compression test this weekend.
I would assume its running on all cylinders, it doesn't sound like it has a misfire
I replaced the air filter
I was able to spin all four tires when i had it on jacks.
I believe there were two metal connectors in the coolant temp sensor, where is the second temp sensor? I changed the one that is up and to the right of the thermostat when looking at the engine from the front of the truck. I wasnt able to find a second one. I could change that this weekend as well.