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I have been working on this 1999 Honda Civic DX now for almost 8 months. It started with a rough idle one night when I was on the way home from work. I was able to make it home and start checking the net, including here for some help. I went to my nearest Advance and got an OBD reading and it said it was my IAC. I went to a local junkyard and pulled the IAC from a 1998 Honda Civic. I installed it… it worked for a couple minutes and the surging idle started again. I went back to the parts house and got my OBD scan again and it registered IAC. This time I ordered a new one from Autozone with a lifetime warranty. I installed it and then I still had the same problem, surging idle after a few minutes. I took it back to Advance and this time it said it was the TPS. I looked it up and most people said it was unlikely, but I went ahead and changed it out, calibrated it, etc. Now I get about 5 good minutes and surging idle again. Took it back to the parts store… got 2 codes, IAC and Map sensor. I changed out the MAP sensor, and it was not fixed. The light stayed off for a while… I went for a drive… about an hour, no problems other than the occasional hiccup in the idle when I was at a light. I made it about 5 minutes from my home and then I started having the surging idle consistently again at the red light I was at. JIC, I pulled the second IAC I had put on, took it back, just in the rare (very rare) instance that I got a bad part… I have now put the IAC back on and the idle problem still exists. I have also replaced most of my smaller vaccuum hoses and the smaller radiator hoses (they were starting to look really worn) going to the IAC, etc. I burped the coolant, as seen on ETCG… I also ran the car for 10 minutes at about 2500 RPMs with the heater on, etc., trying to get it to settle out. I have now taken the panel off to get the the ECU. I took it out and looked inside and I did not see any burns/dark spots on the board. I did smell the familiar smell of burning transistor (my wife blew up one on a PC she had one time), but I did not see any bulging. PLEASE, for the love of God, what can I do? I am on the verge of spending more on this little Honda than the car is worth… Thank you again and thanks Eric for all the advice you have given us.
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Idle problems suck lol, I’ve always had trouble tracking them down.
Check your ignition (plugs, coils, wires) for faults. I had an accord in my bay one time that had just enough spark leak in the #3 wire to screw up the idle, but not enough to register a misfire.
Does it run rich and/or use more fuel? If so, check your ECT sensor, if the computer always thinks the engine is cold, that could cause your rough idle.
I’m sure I’ll think of more possibilities after a couple beers 😉 good luckSo something as simple as spark can cause surging idle? I have a deep seated hatred for all cars post 1985 (last carbs and the beginning of the invasion of the machine). Will check that…
Yessir. New cars suck for that very reason, anything out of spec even just a tiny bit can throw the whole system off.
I once chased a misfire for 2-ish weeks on my old Taurus, only to find out the fuel filter was restricted lol. I fixed a couple vacuum leaks and got a lot of new parts in the process though, so I guess it wasn’t all bad.That I’m not sure of.
On the one hand, anything that affects the idle theoretically could cause an IAC circuit code, but on the other hand an IAC circuit code should be in the IAC circuit.
It occurred to me earlier a bad PCM could cause this, if it’s not sending/receiving proper signal to/from the IAC.
It’s also possibly your p0505 is what I call a “ghost” code. A code that literally has nothing or very little to do with your actual problem, sometimes the on board diagnostic will send you to the “default” or most likely cause of a fault.
The best advice I can offer is to go through your entire idle circuit, obviously the IAC and TPS are good if you replaced them, I’d check for vacuum leaks, check the PCV system, make sure your throttle linkage is properly adjusted, MAF sensor, anything that relates to idle and fuel/air ratio. Wish I could point you in a more specific direction and save you time and energy, but like I said, idle problems aren’t my forte.
Pretty much every time I get a rough idle in my bay, I go through the whole system unless it’s something extremely obvious that I find right away.
Best of luck, sorry I can’t be more help.Ya.. code cleared. I have also checked for vacuum leaks and have been unable to find anything. I guess this weekend I will start going through the plugs and what not…
It’s just weird that it threw a code for idle valve and you replaced it and still happens. Have you checked that the idle openings in the throttle body are clean? Could be gunked up
And also remember, if this is a ghost code, check your EGR valve, and do a fuel pressure test.
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Does the D16y6 Honda Civic in fact have an EGR valve? If so… is someone would post a picture of the valve and tell me where to find it. Mine is a MT, but I looked for the EGR valve and could not find it. I have also seen several posts saying that the 1999 Honda Civic DX doesn’t have one…
The DX doesn’t have one. As far as I know.
Never mind the EGR valve. Did you clean the ports in the throttle body that are used by the IACV when you replaced it?
And also, did you properly bleed the coolant system after replacing it? This is a must as air will get into the system and cause erratic idling
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