MY 85 240 has failed emissions 3 times. Very high HC(hydrocarbon levels)

Have replaced/swapped the following:

plugs, oil, AMM, idle control valve, flame trap, O2 sensor

Seems to have gotten worse since I did the work!

Any suggestions?

 

I would:

- make sure your timing is 12 degrees at idle, and that it doesn't "jump around"
- measure the o2 sensor voltage at idle, on the green wire, with everything connected. it should cycle quickly between .2VDC and .8VDC
- measure the resistance of the coolant temp sensor at the ECU.

 

On top of the above suggestions, also:

-Pull the vacuum hose from the fuel pressure regulator and ensure it isn't letting fuel into the vacuum line
-Investigate the condition of your catalytic converter (how old is it? does it look particularly rusty and heat damaged?)

 

When you did the AMM job did you adjust the CO content? That kinda calibrates the Lamba Sond (O2 sensor). If you got LH 2.2 FI system that is done by a screw on the AMM (it's covered by a cap next to the plug for the unit). After you've confirmed everything else if is in good shape, that can be adjusted. For a procedure on that... I don't have my manuals with me, but I'll get them at Thanksgiving. Someone else may know too. If you have already checked it, then all of this is old news.

 

As you probably know, high hydrocarbons is unburnt fuel exiting the tailpipe.
Check fuel pressure, basic timing, air intake obstructions(clean filter?), air intake leaks after the AMM.
Also replace spark plugs, check leads rotor cap.
Also check for vacuum leaks at intake manifold.
Spark timing is usually not a big factor on HC, it usually affects NOX more.
O2 sensor is not a big part of the picture idling either, although at speed it is. If teh O2 sensor is more than 50k miles old, I'd replace it, they are a wear item.
I'd also throw an oil change at the engine, it helps the rings seal.

Regards, Andrew.

 

A classic trick is to remove the air filter just before test. Check at home first so that you make sure there is no loose stuff in airfilter housing.
This wont fix your problem but it will take it a long way towards a good test, unless its completely off that is.
How far off was the test?