I have a 1990 Bronco II just for bad weather days in TN. I have tried to get it to pass emissions 3 times with no luck. It is producing too many hydrocarbons and they make me idle it between 2000-3000 rpm's every time. Some articles I have read said to adjust the Ltrim. I have no idea what that is. What is the timing supposed to be? Will changing the oil help? Please before I get disgusted and sell it for too much less than what I have in it. Dewmel.

 

Most of the time you get a hi HC reading it is due to the ign system.
New plugs , wires and the correct timing setting should take care of most of it.
Also it should have the correct thermostat(195) in it to better complete the burning of the fuel in the cylinders.
If the engine has been running rich the oil is probably loaded with extra HC also.
Change the oil.
Last , but not least , is the converter(s) still on the vehicle?
I forgot to mention , check the fuel pressure regulator for not controlling the fuel pressure to the injectors or leaking extra fuel into the intake by way of the vacuum line.
O2 sensor may not be doing a good job.