Well,I'm Back and I am at a major Impass in the saga of equalizing the forces in the universe. I have had to replace a replacement fuel pump, after plenum and intake gaskets, Hughes plate on bottom of the plenum and rebuilt injectors including new connectors, this after having done a complete tune up,plugs,wires cap,rotor and crank sensor {Mopar}. I now have had to replace a 6 mo old alternator bad regulator,and now I get a code 51/and code 21. The cat I replaced 2 years ago along with the o2 sensor and it still gets fluctuating rpms, poor acceleration ,misses,backfire, stalling . If I understand correctly this should be taken and have fuelsync done @ Dodge or someplace that has DBR III or equivalent scanner to time things together. Code 21 seems to denote o2 circuit voltage to computer not fluctuating.Code 51 lean f/a #1. SCI code $4B which I have no clue what that is . So my fellow warriors I am stuck here in space with no hope of returning without some positive vibrations and the "COLLECTIVE" knowlege of those who have traveled this part of the Galaxy. Commander Blamram your humbled servant............

 

The O2 sensor isn't switching, and the PCM thinks it's running lean. Of course, the code 51 could be a result of the code 21...... Disconnect the O2 sensor, clear codes, go for a drive. See if it runs any better.

Also, year/engine/etc of your truck would be good to know.

 

Code 51 also means the fuel trim is maxed out, so between this code and code 21 which indicates the O2 readings aren't working it's very possible the engine is running way rich which can cause all sorts of drivability issues. Could be the O2 sensor (others can chime in which brands to use and which to avoid) or the sensor wiring. Wouldn't be the first time damaged 02 sensor wiring is throwing the PCM off.
I wouldn't worry about the fuel sync at this time. It's basically a cam position sensor that tells the PCM if 1-8-4-3 or 6-5-7-2 are the next cylinders to get fuel. It doesn't provide timing for individual fuel injectors.
Out of curiosity, what's the story with the alternator ? Even in 94 the regulator function was in the PCM.

 

tried the o2 disconnect no luck,
1994 ram1500 5.9 L gas 187000

 

94 ram 1500 5.9efi

 

Took truck in due to discharge in charging system, regulator was bad in alternator, took to O'Reilly they checked and replaced free,installed and gauge returned to normal operation ,took for a test drive with scanner hooked up and
jumped into it the tachometer read 3000 rpm, scanner read 730 to 1100 rpm is it possible the speed sensor is crap .also I'm not so sure about the computer,it seems to be throwing codes ,I fix the problem ,clear it then code goes away,it has always had a code 37 from day 1 which never goes away,but this truck used to smoke the tires without even trying has me perplexed.

 

Regulator is part of the PCM. If replacing the alternator fixed it, all well and good.

Tach gets its cue from the PCM for what to read, if it doesn't agree with what the PCM thinks engine RPM is, that's a cluster issue.

 

You can check the descriptions of the codes here. You need to sort out the 02 issue first.
What was the engine actually running at? You can tell between 1100 and 3000 rpms.

What brand 02? Do not use Bosch! They will crap out fast, Use NTK only.

 

I believe that the o2 is a bosch,but wont swear to it and the 730 to 1100 were readings while actually driving with scanner hooked up while driving.

 

also is it NTK OR NGK like the spark plugs, oh by the way THANK YOU with your knowledge it helps tremendously......