Captain_Fook

I've been a long-time lurker here and have checked out previous threads thoroughly to see if anyone else has had this problem, but can't find anything relevant.

A few weeks ago, a low speed, rough idle problem started to develop (it struggled to maintain 500rpm at a standstill in drive, a little faster in neutral, but with the a/c on the engine shudders and almost stalls). At the same time, a check engine light came on while I was cruising at around 70mph on the freeway. It was code 0131 - low voltage on primary oxygen sensor (which I presume is the one in the top of the exhaust manifold).

So I assumed that the sensor was done (the car has 149k miles on the clock and I have no record of it ever having been replaced), replaced it, and reset the check engine light. While driving around local roads the light stayed off, but once again, the light came on after around 2-3 minutes at freeway cruising speed. I've now reset the light several times, and the exact same thing happens on every attempt - after 2-3 minutes of freeway cruising, it comes back on with the same code. The rough idle problem remains.

In an attempt to solve the rough idle problem I have replaced the PCV valve (there is no record of it ever having been replaced in the car's service history, and the old one didn't rattle at all when I shook it, so it clearly needed replacing), TPS and MAP sensors. Replacing the TPS sensor achieved a slight improvement: the car now idles almost normally with the a/c off, but the speed drops and I feel vibrations that I'm not happy with when the a/c is on.

I'm guessing that the next step in trying to address the rough idle issue is to remove and clean the idle air control valve. But I can't see how this would be related to an o2 sensor fault. Logically, though, it would seem that these two problems are related to each other, because they both appeared at around the same time.

All the other life-limited components I can think of that could be an issue have been replaced within the recommended intervals: oil, oil filter, air filter, spark plugs, leads, cap and rotor.

The gas mileage this car is getting is still OK (between 30-35mpg depending on the freeway/urban mix of driving), acceleration feels normal, engine temperature is normal ... in fact, apart from the rough idle and the check engine light that comes on at a high cruising speed, the engine is showing no sign of anything being wrong. So this o2 sensor problem clearly isn't causing the ECU to inject an incorrect fuel mixture.

So I have two questions.

1 - Is removing and cleaning the IAC valve a sensible next step? If so, can someone point me to a detailed guide for removing and reassembling the throttle body? I don't feel totally confident about this (especially the removing and replacing/recalibrating the throttle cable part), though I have found a tutorial on this site for removing the IAC valve from the throttle, and am confident that I can handle that. Also, where can I buy replacement gaskets for the throttle body and IAC valve?

2 - If anyone has any insights on the o2 sensor/check engine issue, they would be greatfully appreciated.

Many thanks in advance!

nub

Something to look for would be a crack in the exhaust manifold near where the primary O2 mounts. Remove the heat sheild so you can see what's under it in that area.

An exhaust leak in this area would cause the sensor to output low voltage due to the extra oxygen content. The ecu would see this as a lean condition and adds more fuel causing a rich condition.

You don't need to remove the throttle body to access the IACV. On a Y7 with the air box removed it's right there on the front of the throttle body. 4 7mm bolts that have phillips heads also. It has an o ring seal you should be able to reuse. I beleive the oring is a dealer item only but I've been wrong before.

Captain_Fook

Many thanks. I've just taken the top heat shield off and looked at the manifold, and cannot see any signs of any cracking (looked with a magnifying glass for hairline cracks, too). I also removed the sensor, cleaned it with a compressed air spray, put copper grease in the threads and refitted it to the torque setting in the Haynes manual - it didn't feel loose, though. I haven't had a chance to take the car on a freeway drive to see if that cured the P0131, but tomorrow's commute will establish that.

I did wonder about a manifold crack, but ruled it out because if the ECU was being fooled by an air leak into setting the mixture too rich, then surely the gas mileage would deteriorate significantly? It's now been around 1,000 miles since the rough idle and P0131 fault first appeared, and the mileage I'm getting is the same as it always has been.

The bottom two bolts on the IAC valve assembly look difficult to get at without removing the throttle body, but I'm going to have a try next weekend. A variable angle adaptor on a socket wrench might work. Thanks for the heads up.

Captain_Fook

The same thing happened on my commute this morning: the CEL lit up 2-3 minutes after reaching freeway cruising speed, the fault being P0131.

I've now ordered a complete new idle air control valve. I found a new one on Ebay for $43 (including a new gasket). A replacement gasket alone is $20, and so if I'm going to invest the time in disassembling the IAC valve from the throttle body, an extra $23 to replace it with a whole new one seems worth doing. Assuming that it's here by the end of the week, I'll fit it on Sunday and report back.

94_civic_dx

my car used to do the same thing cruising around and after a few minutes the cel would come on and rough idle always 02 code cleaned iac and fitv and new 02 kept doing it tightend 02 in a few more turns works perfect now maybe try that good luck

Captain_Fook

I removed the throttle body, cleaned the carbon deposits from the the plate and all other surfaces with isopropanol and a shop towel, lubricated the spring with some lithium grease, replaced the idle air control valve with a new one and reassembled.

The IACV was done, alright. There were thick carbon deposits all over the valve cover itself, one of the vanes looked slightly bent and there was a lot of resistance when I tried to rotate it by hand. When everything was reassembled and the car restarted, the rough idle was totally gone.

The one slight gotcha was that after restarting the car, the idle speed was really fast (just under 2,000 revs in neutral and with the a/c off) for the first 20-30 miles. I'd disconnected the battery while doing the repair, and suspect that the ECU was learning the characteristics of the new IACV. Anyway, after around 30-40 minutes of driving around local roads, it settled at around 1,000 revs idling without the a/c, and 800-900 with the a/c on. This is a little faster than the Haynes manual says it should be, but I'm not really worried about that. I'm guessing that with more driving, the idle speed will come down a bit more. So as far as the rough idle is concerned, it looks like mission accomplished.

Replacing the IACV with the throttle body in situ would have been impossible. The injector rail would have prevented me from getting any screwdriver or socket wrench into the two fasteners underneath the coolant pipe attachments, and even if I could have got the fasteners out, there wouldn't have been enough clearance to remove the valve itself. The only tricky part of the job was getting the coolant pipes off. They were really tough to shift and there was very little room to get a pair of pliers in to lever the bottom one out.

No luck on the CEL / P0131, though. It came back on again on the freeway with the new IACV in place, just like it did with the old one. I think I'm now down to two possibilities: either the replacement o2 sensor I bought earlier was a faulty one, or the catalytic converter has failed. The heater voltage to the sensor is fine, and there is continuity on the signal wire. I don't really like driving around with a CEL on, but if I replace the sensor again and the problem is the cat, I'm just going to destroy another sensor. So I'm going to wait until the car's next smog test is due in September. If it fails that I'll replace the cat and sensor at that point, but if it passes, just replace the sensor again (with a Honda OEM one, probably).

Thanks for everyone's suggestions.

gearhead2228

hello honda tech, i am a new user here but have been using this forum for a while now to find various info. since i am a new user i cannot make my own thread but i need help asap so i decided to post here since i am having a similar issue. sorry if this is rude or anything i dont mean any offense.

i recently swapped a zc dohc black top in my 88 civic, about 3 or 4 months ago. since i put it in ive been slowly fixing/upgrading when i get the money. recently however my car has been running really bad and i was hoping someone could help as i have done hours of research and haven't found a solution. heres a list of problems:
- when in neutral idle drops to around 200-300 rpms and acts like it is going to stall (this only happens when the car gets to operating temp and the IACV closes) (also it doesnt happen 100% of the time im in neutral, kind of intermittent, about 80% percent of the time)
- the idle will "always" drop to 200-300rpms when turn on the head lights
- hesitates when accelerating anywhere under 2000rpms
cranks 4-5 times before starting (kinda sounds like a smokers cough)


in order to try and fis these issues i have:
-replaced iacv and pcv valve
-check the alternator, battery, fuel filter, and for any vacuum leaks (all checked out ok)


I willl admit i am kind of a newb so any advice or help is appreciated.

Captain_Fook

Hmm ... the only thing that comes immediately to mind is the air filter.

As for my problem, a new o2 sensor solved it. My local O'Reilly had a Bosch 13007 for $40 which they said I could return if it didn't cure the check engine light problem. It did: I fitted it on Tuesday and did a round trip from Loma Linda to Las Vegas yesterday (480 miles), with no sign of any CEL.

I think what fooled me was that when I replaced the sensor initially, I assumed that the cause of the CEL had to be elsewhere as the result of assuming that the replacement sensor had to be OK. The problem was that both the original sensor and the one I initially replaced it with were bad.

The moral of the story: caveat emptor when it comes to buying cheap, no-name Chinese parts on Ebay.