I mentioned before that my '87 EFI 302 had failed emissions, but only in the "Low Speed" hydrocarbons. High speed was just fine.

The codes say "45". My booklet says "Thermactor air flow is always upstream during Engine Run Self Test. The guide usually linked from this site says:
<table table="" style="font-size: 100%;" border="1" cellpadding="2" cellspacing="0"> <tbody><tr><td><center></center>
</td> <td>Thermactor air not bypassed during self test.</td></tr></tbody> </table>
OK, I could see how the thermactor system might mess with low speed HC. What should I look for here? The Haynes/Chilton describes an air cleaner, air pump, air bypass valve (this seems to be called backfire supressor elsewhere?), air control valve, check valve, and vacuum differential valve. I'm not sure which of these parts I actually have, I think I have the pump, 2 check valves, and bypass valve.

What's the best place to start? This stuff is extremely difficult to access in the E150 van here. I saw a lot of stuff in the book about pulling off the air pump hose for example and I'm not even sure how that's going to be practical to "feel" the output with the engine running. Vac check I guess... I carefully replaced a lot of the vac hose a few years back and that stuff should be in good shape. There is no backfiring on this vehicle.

In fact, just what sensor did the computer use to make this error? I'd like to see if I can rule out anything that can't cause this code, even if it's still possible that it could break.

I also got a code 13- "RPM out of spec during normal idle operation or DC motor does not follow dashpot". However, my idle sounds just fine really. I don't get it.

 

OK I located the relevant parts. Not as hard as I feared.

The air bypass responds to vacuum. I just pulled a vac with a hand gauge, it held it, I blew into it and it flowed until I released the vac and it closed off. Sounds good. Found the air control valve, took off a hose, watched the valve itself move in response to vac. It did not leak vac. Looks good.
Started the engine with that hose off. Lots of air blowing out of the air bypass to air control valve hose. I surmise that the air pump is working and the air pump's cleaner, wherever that is, must be good enough.

That leaves the check valves, and of course the computer solenoids. I don't really picture those as likely suspects.

I didn't see any sensor that the computer would use to throw that code either.

Any ideas on this code or the low speed HC emission issue??

 

Whoopie! It passed!
What changed? Well, plugs, ignition wires (which was probably unnecessary), rotor, distributor cap, retimed it, and that "3 oz/10 gal acetone" additive trick was flushed out by several tanks. The problematic "low speed HC" went from 458 to 152.

You know, those plugs were old yet weren't in bad shape. No visible damage and they were basically correct on gap. Nor did that rotor or cap look noticibly bad.

That leads me to suspect one thing- I wonder if that acetone trick actually increased the emissions badly? Frankly, that's an obvious suspect here. They say it's supposed to give you better mpg and lower emissions, I've just been experimenting with it for kicks and figured it would be a safe bet to add it for the test. Could it have really done something that bad?