P0734 Gear 4 Incorrect Ratio and black carbon/soot on tail pipe. Dash check engine on after a few drives. O2 sensors are clean. MPG about 10 - 11. O2 sensors, injectors, MAF, spark plugs and wires replaced by previous owner. 2003 4.6L. Ideas of what to look at next?

 

P0734 is almost always related to a MAF sensor problem. If you replace it with a cheap EBay sensor instead of a genuine Bosch, you're asking for trouble.

 

Bosch MAF part number 0280217532.

 

That's actually for the 4.0L. The 4.6 is 0280218010.

That said, my understanding is that all Bosch MAF sensors that start with 0280218 or 0280217 have the same sensor in a different housing. When my MAF went bad, I bought the sensor for a VW Jetta and swapped housings. Same sensor, about $30 cheaper.

 

The actual MAF sensor between the 4.0/4.6L = exact same part. The only difference between the 4.0/4.6L = the MAF housing.

 

I replaced the MAF with a Bosch and it cleared the lean left/right codes but this one remained. You're suggesting changing it out again?

 

I would clear your codes again and see if it comes back.

The soot on your exhaust pipe was because the bad MAF sensor fooled the ECU into thinking the system was lean when it wasn't, so it just kept dumping more and more fuel into the system until the long-term fuel trim reached its upper limit and it threw those codes. If your fuel trim is fine now, I wouldn't worry about that.

 

I have cleared numerous times. Comes back immediately as pending and then after a couple drives it goes to fault and trips the warning lamp

 

It's probably your MAF sensor, but I would check your transmission fluid level and condition before you throw another sensor at it.

 

Check every single hose clamp, vacuum line and connection to the primary air intake hose. I have had that code, P0734 Gear 4 Incorrect Ratio twice. Both times it was a loose clamp on the primary intake hose that allowed extra air after the MAF. Fixing that issue corrected that problem and it has never returned.