lsdaddy0

Hi everyone, thanks for the support you give to all Civic people.

Working with: 1997 Honda Civic EX, automatic.
I'm at my wits end. I've replaced the plugs, wires, distributor, IACV, PCV, map sensor, checked for vacuum leaks, and have a "voltage stabilizer" installed. Still, the engine doesn't want to keep a proper idle WHEN THE SUN IS OUT. I know that sounds insane but at night it's fine. During the day, when I stop, the engine drops 100 or so rpm right away, and while I sit there, it drops another 200ish rpms. If I throw it in neutral, the rpms still drop the first 100+/- but not lower. On a day that's stormy or whatever that blocks the sun, it's nearly as good as night time.
Can barometric pressure affect idle? Solar radiation affect electrical? All the Hondas I've had, NONE have been this weird. Any suggestions please.

mcnoople

Do you have the headlamps on in the day? Higher idle with higher electrical load? You are stating how much the rpm is dropping but not the numbers it is at. What is the idle rpm in daytime/nighttime? What is the operating temp? Does it stall?

ezone

Got fault codes?
Do you know what the idle RPM spec is?
Got actual RPM numbers to go with your other descriptions?

lsdaddy0

I only use my lights in the dark or rain. Turning on the headlights in the daytime lowers the rpms more, but not by much. When I switch them off the rpms pop up for a moment. Daytime idle in neutral at normal operating temp is close to 700 rpm. (I don't have scan tool so going on the gauge and cooling fan cycling.) Drops to 300 or less when I stop, in any gear. Night idle is close to 800 rpm and drops at most 100 rpm when I stop. The engine has never stalled. Battery is less than 2yrs old. Alternator was tested good last year. Valves were adjusted last year. No fault codes. Service manual says idle speed for auto is 750 +/- 50 rpm.

lsdaddy0

Also: New fuel filter, o2 sensor, ECT sensor, and thermostat

ezone

If you have CEL on and fault codes, by all means figure those out FIRST.
(I've seen O2 sensor issues that can cause really bad running ---but only briefly, not run bad long enough to set fault codes)

Check if timing belt has jumped a tooth, and check that the ignition timing is set correctly.



If that doesn't solve your idle problems, and assuming everything else is proper you might try.....

Clean throttle body/blade/IAB screw passages (assuming it's equipped with this) thoroughly, and clean IAC valve,

reset minimum air rate using the IAB screw (if equipped, follow a good service manual), disconnect battery to clear ECM memory and purge learned adaptive values, then do a full idle relearn as per service manual direction.

lsdaddy0

No CEL. No fault codes. Timing belt was installed professionally a few weeks ago, I really doubt it jumped a tooth. Ignition timing is set to 14 BTDC. A brand new, genuine honda IACV was installed yesterday. This is the 3rd IACV I've tried in 2 weeks. I want to be clear-the car has done this from day one, before a single thing was changed. Can it be transmission related?

ezone

Pros can make mistakes too.

Anything is possible, but I'm not guessing at a trans yet. How does its fluid look and smell? Any other probs with the trans that would make you suspect so?




If all the basics are done and right, the throttle body cleaning and idle relearn are the first things I'd do.

lsdaddy0

Intake air temp sensor was also replaced. Temperature doesn't matter. Behavior is same no matter the outside ambient temp.

lsdaddy0

I just got back from a drive 7:38pm EST. Current local conditions - 49 degrees, humidity 43%, barometer 29.88 inHg. Idle stable at stop light with headlights on, brakes on, in drive. When headlights are switched off, idle drops 200+ rpm. Turning them back on, the rpms come back up.

ezone

So how did it do after cleaning out the throttle body and doing an idle relearn?

lsdaddy0

No change. Lights on during the day helps keep rpms above 600. I guess I will just keep them on if I can't tolerate a low idle. Thanks for the help.

ezone

ASSuming you did clean the throttle bore and blade thoroughly, and cleaned out the IAB screw air passages so this next idea can actually work....

If I were trying to adjust it by ear,
I'd remove intake snorkel from throttle body, run the engine, get it all warmed up, the radiator fan ran twice, all electrical loads OFF


In the throttle body there are probably two open ports visible.
One leads to the IAC valve.
The other leads to the IAB screw (this whole idea depends on the throttle body having this IAB screw, because I can't see for myself if yours has it).

I'd use my fingers to completely block off the IAC port in the throttle body (so ECM cannot compensate idle speed)



adjust IAB screw until RPM is almost at the desired level (if spec is 750, set this to about 725ish), so at full operating temp the lowest idle speed in neutral/park would be 725.

NOTE: Keep track of how many turns you move this screw so it can be put back to the exact start point later if necessary


Reassemble all, disconnect battery for a few minutes, reconnect, run the engine, let it do an idle relearn again

see what happens.

If it is set too high and surges during warmup then crank the IAB screw in maybe a quarter turn at a time and do an idle relearn/recheck after each try.

Wankenstein

My 98 EX is the same way .. I keep the lights on during the day for smoother idle around 650. Otherwise low idle without electrical load. I've never attempted to adjust idle screw.

tommycarpenter

If your car was built in Canada.. mine was same problem... the relay next to the drivers left foot has faulty solder on it in heat it expands... the relay in question needed to be replaced.. it's the one with the little screw
U changed everthing else trust me... I found this out years ago on honda forum

lsdaddy0

LOL. I figured it'd be something goofy like that. Thanks for the info!