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Yesterday my 2005 Honda Accord was running at a low idle when cold and stalling out when warm. Today I resolved the issue but I'm not sure what exactly I did that resolved it or what the original cause was. I'm looking for advice on what the cause could have been and what else I should investigate to prevent the issue from occurring again.

Symptoms:

  • At cold start engine was idling very low, around 500.
  • Once vehicle was at temperature the idle would drop further and the vehicle would stall out if I didn't keep the idle up w/ the accelerator.
  • No check engine light.

Troubleshooting steps performed:

  1. Suspected poor air-flow. I removed and inspected the air filter. Air filter was fine. Started car with air intake open (air filter removed) and no change in behavior.
  2. Suspected poor battery or poor battery connection. I disconnected the battery, cleaned and lubricated the terminals, topped off battery with water, reconnected. No change in behavior.
  3. Suspected bad Mass Airflow Sensor. Disconnected MAF electrical, restarted motor. No change in behavior.
  4. Suspected engine controller needed reset. Disconnected battery, reconnected, held motor at 3000RPM for a minute. No change in behavior.
  5. Suspected bad gas / water in gas tank. Added 1 gal fresh fuel. Fuel gauge at 1/4. Added 1/4 bottle methyl alcohol fuel dryer (HEET). Ran motor at 3000 RPM for a minute. No change in behavior.
  6. Let vehicle sit overnight. Tested vehicle the next morning, 3000 RPM, no change in behavior.
  7. Suspected coolant system might have air in it (and this would be impeding operation of the Idle Air Control Valve IACV?). I removed the cap to the coolant reservoir and lifted the tube so I could observe coolant overflowing into the reservoir. I turned off the cabin fan and moved the heat to high, started the engine and held the idle at 2000 until the engine was warm. The coolant reservoir was just under MAX when the engine was warm. No change in behavior; engine still stalled when warm.
  8. Suspected stuck IACV. At the advice of Eric The Car Guy (see below) tapped IACV with back of screw driver while revving engine. No change in behavior.
  9. Suspected bad Throttle Position Sensor (TPS). Disconnected TPS electrical, restarted engine. Idle was fine! Check engine light now on.
  10. Shut engine off, reconnected TPS, restarted engine. Idle was fine. Check engine light still on.
  11. Took car to gas station, filled it with gas, on the way back check engine light went off. Low idle is still rough and acceleration is a little hesitant but it no longer stalls.

Would appreciate suggestions on what the cause could have been and what (if anything) I should look at next. Thanks!

See also http://www.ericthecarguy.com/fuel-and-emissions/952-how-to-solve-honda-idle-problems

Update: Never found root cause of this issue, but now I have a new one. See 2005 Honda Accord 4cyl revving idle when warm

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    You still have a rough idle? Also, the connection to the TPS may have been not so good, then when you pulled the connection and replaced afterward, it gained a good connection. You may still be having an issue with the TPS, but it's decided to clear itself for the mean time. I'd suggest when you get a chance, to consider putting a new TPS in, as this will probably rear its ugly head in the future. Great bit of detective work and write-up, BTW :D – Pᴀᴜʟsᴛᴇʀ2 Nov 3 '15 at 23:16
  • How well does the engine rev? Does it sound forced? Like if the exhaust were blocked in some way – race fever Dec 18 '15 at 16:32
  • I don't remember seeing this question before. +1 for the methodical approach taken in trying to rule out the possible root causes! – Zaid Dec 27 '15 at 5:42
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Given the tests already completed and the stated symptoms and the lack of trouble codes, the most likely failure is an inoperative or stuck closed idle air control valve (IACV). The fact that is runs with the throttle open a bit verifies the engine and fuel control system operates reasonable well.

  • Test 8 shows that the OP believes the IACV is fine – Zaid Dec 27 '15 at 5:39
  • @Zaid Indeed, I suspect otherwise, all the other results point to it as the next part to check. The tap on it test is not enough. I should have explained that in the answer. Tapping on it will not often unstick it. I take them off the throttle body, clean them, test them with power and ground supplied in both the open and close directions to verify free movement in both directions. I clean the throttle body also. I test the command signals with and oscilloscope. The OPs test source is a little short of the mark in my opinion. – Fred Wilson Dec 27 '15 at 6:41
  • Can you elaborate on how I might test the Idle Air Control Valve (IAC) electrically once I have it removed from the throttle body? I'm hesitant to apply a voltage to it for fear damaging it. – moof2k Jan 3 '16 at 18:31
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    @moof2k It is OK to test it by applying 12v. Connect the yel/blk wire to 12v. Connect black to ground. Now connect blk/red to ground just one tap at a time. The shutter blade should flip. The blk/red is the computer control wire, this is where the signal from the PCM is checked. It takes a oscilloscope to properly test it because it switches quickly. – Fred Wilson Jan 3 '16 at 21:48
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I love the methodical approach taken to solve this issue. In the absence of concrete data from a scan tool it will be hard to definitively tell what the root cause is, but there are some prime suspects.

The one thing made conspicuous by its absence is the condition of the throttle plate.

It is possible that the throttle plate was partially stuck due to the accumulation of gunk on the back face. If the throttle plate was open more than what it should be at idle, this could explain the runs-well-when-cold, not-when-hot behavior.

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