Jdmfsm.info and charm.li may have some factory manuals you can use for diagnosis. If it's true JDM though there may not be manuals you can read.
this post was submitted on 13 Oct 2025
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Thanks for the links. I'm pretty used to Google translating these things at this point. It's funny every time I find a manual for this Japanese rig it's in Russian.
Replace parts until it works? That shit is black magic, the one in my 90’s prelude required a wax plug to melt that then allowed vacuum to operate a valve, this valve then allowed some exhaust gas into the plenum on top of the block.
What you need is the Factory Service Manual, should be roughly the same size as a 90’s phone book and was a requirement for vehicles sold in the US.
A wax plug is truly insane. Parts for the emission system are pretty slim pickings, mostly just used off eBay. I think my bigger problem is the carb needs a rebuild something fierce. I have the carb kit and the time now so I should be able to tackle that soon. The fuel air mix screw has a little rubber washer in it that is pretty much gone so that can't be helping any.
Pure Rube Goldberg… today it would be solenoids, temp sensors and the ECU powered up 24/7, which I guess is the advantage.
It also made a random robot-farting noise about 8 hours after cooling down which I believe was the evap purge canister venting. Gave me the heebie-jeebies a few times when I was out late at night, smoking a cigarette.
Another tip I've done for pinpointing vac leaks, the bodge job farmer way. If you don't have a smoke pen and don't want to go buy one, use some brake cleaner (non chlorinated) or starter fluid. Give it GENTLE TINY sprays around vacuum connections and components and see if the engine idle changes. An increase in idle means it's drawing the fluid in and burning it, aka a leak. Do it sparingly and wait a minute between sprays so you don't have a buildup of flammable vapors.
Incidentally this is a great way to also find bad spark plug wires...
Car Wizard uses a smoke machine to find vacuum leaks. As in, injecting smoke into the system so you can see where it comes out. I thought that was pretty clever.
I've got one, that's how I found the leaks! Super handy to have around.
For vacuum leak? I usually 1)look for any visible wear on all the hose and vacuum stuff 2)listen to the hissing and find out where, or 3) get a can of throttle body cleaner and start spraying on place i suspect there's a vacuum leak. If the timing drop then there's leak there.
My main leak is coming out the top of this thing, which I think I've ID'd as a vacuum amplifier. It's plumbed from the intake manifold on the bottom and runs between the vacuum advance and the charcoal canister. From what I remember it holds a vacuum on the bottom but doesn't seem to stop the flow of anything between. There's used ones on eBay, but I doubt they're in much better shape.
I'm tempted to try and pry the top off of it and see if I can glue it back into functioning, although part of me wonders if it's supposed to purge air out of the top.
The vacuum advance pod is pretty weird too. It's got two diaphragms stacked on top of one another. One advances the distributor and the other I'm guessing signals back to the system that the advance is on. Not sure if that part is working as that pod doesn't hold vacuum, but maybe it would if the advance was pressurized? I haven't figured out how to test that yet, maybe I'll apply vacuum to the advance and put a 2nd gauge on the signal one and see if it holds anything.