ughmas

Hello everyone, long time civic owner first post, would love some help with this issue, driving me nuts!

1998 Civic DX coupe all stock 5 sp

The story/problem:

Was driving fine one day, sitting at a red light in neutral, Car throws a CEL, idle starts going crazy (bouncing like its hitting a limiter (baw baw baw).

Pulled over, turned car off for 10 seconds, started back up, CEL still on but car idling fine now.
Same thing happens again if i let it idle for 20+ seconds starts going crazy again. Have to turn car off to fix it.

other symptoms-
if idling and press the accelerator breifly, when the rpms drop back down the engine skips when it hits bottom
slightly rough shifting due to weird idle

Went and got CEL read, P0505, suspected dirty/faulty IACV, and was told this is most likely the issue.

Took apart IACV, it was pretty dirty, cleaned with carb cleaner and toothbrush, got it looking like new, let air dry, re-installed and cleared codes.

On first startup after doing this car was running good, but soon the same thing happened. figured it went bad.

SO I ordered a new IACV, installed it, cleared codes.
first startup car idling high, it slowly settled down to where it was supposed to be, idling perfectly, let sit for a few minutes still idling good. Drove around for 20 minutes or so car running PERFECTLY, idling good, shifting smooth, smile on my face i think i have it fixed.

Get back to driveway, sit idle for a minute or so, CEL comes on idle goes crazy again baw baw baw s***.


-Checked for vaccum leaks, none whatsoever, checked for air in coolant, none.

-When the IACV Is UNPLUGGED (before startup or during idle) it does the problem IMMEDIATELY

Im thinking it might be the TPS, am I missing something? I've searched the internet high and low.

Thanks

motegicivic

Did the that cel come back?

Also check the wiring to the IACV.

When was the last tune up?

ughmas

Yes p0505 comes back whenever i let it idle for like 20 seconds when its warmed up

last tune up as in...? its got 94k on it so I think i need to do the timing belt soon, but everything else looks ok.

Ill take a look at the wiring thanks

Freshdot

does this car have a FITV? if so clean that out as well. sometimes they also need to be adjusted.

Prboy

Fitv?

Freshdot

fast idle thermo valve located underneath the throttle body, has coolant lines going to and from.

Former User

A stock 98 Civic does not have an FITV.

Former User

Yes, voltage test the TPS and resistance test the ECT sensor.

If those are fine, adjust the idle speed (see FAQs sticky).

ughmas

Yes the symptoms sound like a FITV problem but the 98 does not have one unfortunately!

I will get a multimeter today and check out the TPS and ECT thanks

motegicivic

You said the code comes back, then check the actual wiring to the IACV, pull back the harness tape or covering inspect to see if frayed burnt etc.

Check inside the connector for corrosion, if needed get some electronics spray and clean it out.

grndslm

I would clean the IAC valve one more time. I cleaned mine the other day and thought I had it clean and was about to install it, but as I was walking to the car, there was still one grain of sand or something that totally obstructed the valve's ability to move freely. You obviously know that piece is very delicate, and it is the main way to control the idle. If cleaning it once solved the problem for 20 minutes, then I think you have a good idea of what the problem was.

While you're re-cleaning it.... clean the throttle body as well. Not sure if you did that or not, but it's worth a shot.

Definitely make sure to unplug the battery for a few minutes and then when you hook it back up, make sure to let the car find its idle again for a few minutes. I've seen some people suggest 5 to 8 minutes, but I guess as soon as it hits its lowest point, it's probly ok to drive.

ughmas

grndslm,

Appreciate the advice but as i said i bought a brand new IACV after the problem returned and it still is doing the same thing. (I also cleaned the throttle body when i took it apart the first time)

I checked the Wiring it looks fine and Volt tested the TPS yesterday, it checked out fine .55V at closed throttle increases smoothly. which was frustrating I thought for sure it would be the TPS.

I haven't tested the ECT yet, is there anything else that it could be??

Former User

Why?

Also look in the FAQs sticky about how to adjust the idle speed.

Only post again when both have been correctly done.