Odd RPM Problem
 

I got a '92 Civic DX back in January and I've been doing my best to fix all of the terrible neglect that she suffered under her PO. She's had an odd idling problem the whole time, where it will surge up about 400-500 rpm randomly at idle, but now that Summer's come around, another problem has shown up. Once she's warmed up, the idle is only around 300-400 rpm (judging by the in-dash tach, at least), and when I let off the gas and it isn't in gear and going somewhere the tach will drop all the way to 0 and then recover to that 300-400. I've already cleaned the EACV like everyone says to try and fix the roaming idle, and I've tested per the manual to see that it's actually still any good. Like I said, it only happens once it's warmed up all the way. When I start her in the morning, she runs perfectly. Does anyone have any ideas/experience with this kind of problem?

It has a D15b in it, btw. Non-V-TECH. I can't say how well it was installed since I bought it like that.

use the search, and look for "idle surge." There are TONS of threads on the subject with all the information you could ever need to diagnose and cure the issue.

also, it's VTEC

Yes. I've done this and all I see is "Clean the IACV, Clean the EACV. Clean it. Clean it." The surge isn't really a problem right now. I want to know why it's idling so low.

well did you clean the iacv? (idle air control valve)

Don't trust the dash tach to show idle rpm exactly. If the engine doesn't shudder or stall, idle is OK. With EACV wire disconnected, should slow down to about 450, where it will start to shudder.

Low coolant level is another common reason for the idle to surge. Check it in the radiator itself (not the tank) with the engine cold.

Well, that sounds right about when I unplugged the EACV, but I still don't like the way it pretty much bottoms out whenever I let off of the gas and it isn't in gear. It's definitely falling too low because I can hear the AC actually decline for a moment with the engine. I've been wondering about the coolant, and when I went to look at it the radiator was full, but the reservoir was dry and a bone, and judging by the black sludge that was in the... straw?... going into it, it had been that way for a long time. I need to drain and refill it and see if anything happens.

Idle issues are idle issues, most of them are caused by the same types of problems...

Did you read through this thread? https://www.hondacivicforum.com/foru...le+SUrge+Issue

Any CEL codes?

Clean and adjust the FITV and replace the PCV valve.

@ Forty04 - Very useful thread. Thank you. My first bet is going to be looking into my coolant system. After looking into the reservoir and seeing it in the condition it was in, I'm assuming that there is some work to be done in that area. Once that's done I'll continue from there as per your "check list". A couple of questions:
1) I don't believe that my D15b has a FITV, and I'm having trouble telling exactly whether or not it does. What should I look for. I know that it's supposed to be right under the TB.
2) Would you remind me what a TPS is, please?
3) Are there any videos or something that could demonstrate what a vac leak sounds like?

@ RonJ - There won't be any CEL codes unless the Check engine light comes on, right? I haven't looked because it hasn't ever given me a light, just finicky annoyance.

The D15b7 does have a FITV. Also check for a clogged PCV valve.

Does the CEL work?