If you didn't buy from a dealer (who should be on the hook for this issue if you did), you'll need to do some troubleshooting to determine why the engine is running rich/reading high HC emissions yourself or with the assistance of a qualified repair shop/dealer.
Generally speaking, some possible causes - in no particular order - include but may not be limited to:
- Bad/fouled mass airflow sensor (MAF) and/or manifold absolute pressure sensor (MAP)
- One or more bad fuel injectors not metering fuel properly
- Worn spark plugs, plug wires, distributor cap, and/or rotor
- Failing oxygen sensor(s)
- Damaged/worn catalytic converter(s)
- Clogged air filter (less of a candidate with fuel injection, but still)
Again, this is not an exhaustive list of possible causes. If the Check Engine lamp is on, reading the stored codes will likely point you in the general direction to start with. If not, you've got some detective work to do unfortunately.
Locating a service manual for your chassis will help in this regard. Helm Inc.
has most factory service manuals available for purchase. For online diagnostic & repair information, I find Alldata DIY
to be extremely valuable. Also, even though your coach is based on the Ford F-53 motorhome stripped chassis, a lot of F-550 truck information is relevant as well.