Intermittent idle issue: '88 Civic

Have you replaced the map sensor?
No Raven I haven't. My understanding is the Map sensor will throw a code if bad. Also am not experiencing any fuel economy issues or power loss or ANY other issue. I believe my mechanic checked the voltages out on the Map sensor when he looked at it last. I see this part can cost upward of $300...though I do see others a lot cheaper and am not sure what the differences are. The cold weather definitely tamped down the frequency of the "incidents"...even after the car is warmed up good. It has to be some cryptic electrical issue with one of the sensors that gets more aggravated based on certain temperature/humidity conditions (none of which I can reliably recreate in the winter!!). I think all that's left that could cause this that hasn't been replaced is the MAP...TPS...IAC Valve. Hoping the warm weather in the spring will finally expose the culprit.


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Do this simple diagnostic test --> Unplug the O2 sensor to see whether the problem is eliminated.
OK boys here's the 3/22/17 update on the mystery idle issue. I was getting antsy for spring and we finally had a 50 degree day so I tried an experiment. I got the car to go into it's bobbling mode after warming up for 10 minutes. As usual it would bobble for a little bit and then correct. I unplugged the idle air control valve and the car then goes into base idle of around 500 rpm. Left it there for 15 minutes without incident....purrs like a kitten. So I figured it's either the idle air control valve or what my mechanic figures....some sensor sending some info to the computer and then the computer telling the valve to close off. I got a refurb of the exact same idle air control valve and decided to install not a mechanic but it wasn't too bad. Had another warm day yesterday and decided to idle it for a while. No went into it's bobble mode so it is NOT the idle air control valve. And also there is no screen on the 1988 version of that part. It is probably 25 degrees out today and had the car out for 4 or 5 issues.....If it was 50 degrees out the car DEFINITELY would have done its' bobble thing.

So to recap here's what's been changed:
Distributor (which was bad originally)
Timing was checked
Spark plugs AND wires
O2 sensor
Coolant Sensor
Intake air Sensor
Idle Air Control Valve

Not sure if the Map or TPS sensor would cause my problem. Of course no codes ever thrown! The car did have a burnt valve before all of this started for which the head was pulled and serviced. I'm wondering if something was put back wrong or maybe a bad gasket somewhere. Other than this issue the car runs perfectly! Definitely seems that ambient air temperature lessens the chance of this condition as again I had the car out 4 or 5 times today (very cold out) and the coolant temp was definitely warm and of course no issues whatsoever!
4/6/17 update....I noticed a post on a Honda Technical Forum that talked about a service bulletin 91-031 that was issued back in 1991. It pertains to my vehicle and talks about erratic idle performace after coming to a stop with the throttle fully closed. It states the probable cause is "Inadequate grounding of the throttle angle sensor circuit." The "fix" involves replacing the existing ground wire with a new ground harness. It's supposed to get warmer finally next week and am thinking of taking into a Honda Dealer to see if they still have the parts involved in the fix. The flat rate time to fix is 20 minutes.
I'm wondering if a mystery ground would be "less" mysterious when it's cold out and more apt to go bad when it's warmer or if it's rainy/humid. Next question for the forum would be what happens when the ground acts up on the TPS? Does that give bad info to the computer which then may tell the IAC valve to close? Would appreciate anyone with electrical knowledge to give me their thoughts.


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A 26-year old TSB problem likely already reared its ugly head and was addressed long ago.
I know I never had this problem before since I am the original owner. I'm wondering if a wiring harness was jostled when I had a burnt out valve serviced...thus possibly triggering the above issue. With no codes after this long a period I'm down to the TPS or Map sensors and they are not exhibiting any of the symptoms normally associated with their malfunction...thus the thought of some wiring/short issue.


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A bad TPS wire would throw a TPS CEL code. However, testing the TPS wires with a multimeter is pretty easy, as is calibrating the TPS.

It's best to rule out the simple stuff first.

Have you reset the base idle speed and bled the cooling system as detailed in the service manual?

Does the ECU LED light work?

Stock D15B2 engine?
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Haven't touched the idle speed....the car idles perfectly 99% of the time. When I disconnected the IAC plug during a rare time it was bobbling the idle drops down to 450 and holds bobbling. Cooling system was bled/checked by mechanic...ECU light works, It's the original stock engine.


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When I disconnected the IAC plug during a rare time it was bobbling the idle drops down to 450 and holds bobbling.
Done recently? How did you monitor engine rpms?

If you truly measured 450 rpm, this is the problem because the base idle speed is clearly set way too low. You must increase it to 625 rpm.


Other items to consider:
Has the PCV valve been replaced or checked?
Has the alternator been tested?

Cooling system was bled/checked by mechanic.
Recently? How?

This is the correct bleed procedure:

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Thanks for all your help boofoo. Let me answer a couple of your points.

IAC VALVE.....When I disconnected the Idle Air control valve it was one of the rare times I could get the car to go into it's bobble mode ...not hunting....but bobbling as if the car is going to stall. I disconnected it while it was bobbling and the idle leveled off perfectly. I let it run for 20 minutes longer (disconnected) and it held with no issues...I just looked at the tach at that point and I thought it was around 450 but it could have been higher. I've never touched the idle adjustment and 99.9% of the time the idle speed seems fine. Bottom line in my non mechanic mind was the IAC valve was closing and "choking" the engine when it shouldn't be. I bought a used, cleaned and tested IAC valve and put it in.....same it's not the IAC valve.

AIR IN COOLANT....Mechanic assured me system was bled properly when I had valve work done. I don't lose coolant and have no heating/overheating issues.

ALTERNATOR....Have not had checked but this has been going on for a year and I have no charging issues...Alt lights going on or dimming of lights.

PCV VALVE....Mechanic checked this.

The only 100% sure things I can say is the problem is DEFINITELY affected by Ambient air. If it's cold out the odds of the idle doing it's bobble dance are slim to none. When it's warm out the car also has to warm up before it may do it's thing. Another possible critical clue is the problem is DEFINITELY affected by humidity. If it's a rainy humid 50 degree day or more you will see it go crazy at every stop....or if the car sits out in the rain it may kick in after the first time you stop....i.e. the car does not have to be warmed up. Almost sounds like something gets shorted out under the right instances...that's why when I saw that old Tech Bulletin 91-031 about a grounding issue with TPS I thought I stumbled on something? Again I'm not a mechanic and don't know if heat or humidity would cause a grounding issue to get worse OR are grounding issues either a YES or a NO...there's not sometimes it's grounded and works and sometimes it doesn't. The bulletin talked about "occasional" erratic idle issues when the throttle plate is closed (which I assume it is at idle)


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The tach in your cluster is highly inaccurate below 1K rpm. The only way to set the base idle speed properly is to monitor the engine rpm using an accurate external tach. Look into an inexpensive non-contact tachometer, and properly set the idle speed.

Correct me if I'm wrong, but since the mechanic's work, you replaced the ECT sensor. Any work that may introduce air into the cooling system necessitates a cooling system bleed. In addition, nothing you've mentioned verifies that the mechanic properly bled the system. I am always highly skeptical about mechanics' assurances. It's so easy to properly bleed the cooling system. Just do it yourself.


Checking whether a PCV valve is stuck closed is very simple. Also verify that the breather system is not clogged.

If you have a multimeter, testing the alternator is also simple.
OK boys I'm back for an update here in October of 2017. Had the car at a Honda dealer who didn't find jack. They suggested changing the throttle body which I did.....didn't change a thing. Also tried a thermostat and another O2 sensor that was OEM. No luck. My last hail Mary was cleaning all of the ground connections....No luck. Finally after doing the grounds my mechanic took a flyer and changed the plugs...AGAIN....No luck but he did notice an oil smell after the car did its' bobble thing. He pulled the plugs immediately and noticed oil on the electrode end of one of the plugs....the AHA moment. He then did a leakdown test and it indicated that the rings were no good on one of the pistons. He said the "bobble" was being caused by oil occasionally dripping on the plug end that is causing a misfire which makes the tachometer jump around. Won't notice it until car warms up...i.e. at a lower idle speed and sitting for a bit.
Now my options are sell the car for next to nothing.......replace the motor with a rebuilt......pulling the engine and sending it to a machine shop for a tear down (EXPENSIVE)....OR a fourth option which I'm leaning towards. The machine shop guru (who did the head two years ago) said to tell my mechanic to change the rings with the engine in place...i.e. pull the head (which only has 7k miles on it)..pull the oil pan etc and remove the pistons...replace the rings and reinstall. Has anyone done this before and if so how long did it take and what was your result?

P.S. The car runs like an absolute monster...other than this issue


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Do you think you made progress? And at what cost?
If the car didn't look and run well I would have pulled the plug when I burnt a valve two years back. Certainly the process was a headache but it was educational too. I have a mint shape 1991 Accord too so it wasn't a lack of vehicle situation. Bottom line is I now know the culprit to a very tricky problem that nobody (including the dealer) could figure out. Two very knowledgeable people (mechanically) told me if I don't want to get rid of the car, the path I was pursuing was the most cost effective option of the ones I mentioned and has a good shot of success. Current plan is to get the work done around the holidays when the mechanic has time. In the interim the car is still driveable.


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Post the cylinder compression and leakdown results.

I'm skeptical. Why would the dripping oil only affect the engine at idle?
I didn't do the tests so I don't have the results. I stopped in after the mechanic pulled the spark plugs after he smelled oil. One of the plugs had oil covering the first couple of threads above the electrode end. The mechanic did the leak down test the next day (as he works on many cars each day) The results indicated a ring issue. I've known this mechanic for years and he is honest as the day is long and is also considered by those in the know as one of the best mechanics around.
I'm guessing the problem exposes itself more at lower RPM's at idle AFTER warm up. I have noticed the "bobble" on occasion when moving where my foot is off the gas. Car has a standard tranny and sometimes when you're taking a turn and your foot if off the gas the idle will do a quick bobble. Maybe at higher RPMs the oil is just being burnt off and when you're sitting at a light it pools up and drips on the plug tip.


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Let us know how it turns out.

Would be more convincing to see the actual cylinder compression and leak down test results.

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