Greetings,

Got myself a "great deal" on a '93 D21. 4wd, regular cab, 2.4. 257k hard miles.

I bought it as is knowing it had an exhaust leak and that AT LEAST that would need repairing before getting it to pass smog (required here in Humboldt for transfer of ownership only). After a bunch of basic repair and maintenance work, with it running fairly smoothly, I took it in for it's smog. It passed everything except HC at idle.

HC at idle (971rpm)- 131 ppm (Max 100, Avg Passing 17)
HC at 2500 rpm- 76 ppm (Max 170, Avg Passing 13)

It passed CO at both idle and 2500rpm.

The test was done first thing in the am, cold engine, on a wet day, with CRC's "Guaranteed to pass" still in the tank after using half a tank and then topping it off. Not ideal conditions. The truck is currently reg'd non-op, so driving it around is risky at best.

TO THE POINT- Do I dig into why it failed further? Or do I chalk the un-burned HC up to a cold motor and try again with the truck warmed up after having pushed through all of the fuel currently in the truck?


The following has been changed in the last month:
Oil
02 Sensor
Oil, air, fuel Filters
PCV Valve
Transmission, transfer case, and diff (f&r) oil
Cap/Rotor/Wires
Plugs
Exhaust manifold (source of exhaust leak)
EGR valve
Coolant

I am more familiar with carb'd engines, so have done very little on the fuel management side, but I have done the following:

Cleaned out any accessible wiring harness connections (using CRC electronics cleaner)
Removed and cleaned the MAF sensor CAREFULLY
Cleaned out fuel vapor canister with compressed air

With such a huge difference between idle and 2500rpm, my gut is that the vehicle simply warmed up between the two emissions tests and is more efficiently burning fuel once warm. Or that there's a passive vacuum leak that is self sealing under load. For reference, it blows whitish smoke on warmup which dissipates once sufficiently warm.

Any input is appreciated and welcome.

Lots of possibilities, including valve deposits, leaky EGR valve, or one or more failing injectors.

I'd probably run some injector cleaner through it in higher concentration than specified, possibly even a Seafoam treatment, then take it in for testing while at full operating temperature.