Jake brake problem?

I have a 2001 d12c that has suddenly started missing and having tons of diesel smoke.

First it was the crank shaft positioning sensor, it wasn't smoking then and that cured the miss.

Very shortly after, it started missing and tons of diesel smoke, the code came up saying it was "mechanical fault, injector #4" so I bought and injector and that didn't fix it, same code. So we changed #3 with #4 and got the same thing, injector #4, same miss and tons of smoke.

Re adjusted the valves, injectors and jake and it stopped smoking and missing.

The driver then drove it about 250 miles and when going down a hill using the jake brake it suddenly started missing and smoking even worse than before. With the jake on there was flames coming out of the stack. The engine still idled, a little rough, and still smoke, a lot.

Oil psi good, coolant temp good, no leaks, but fault is now "mechanical injector #5".

So here is my thought, it is not an injector at all, the jake is not closing off, reducing compression, not burning all the fuel. It isn't running rough enough for one cylinder to be the only thing.

Checked all electrical connections and everything is fine.

Jake solenoid?