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Code P0505

Tiny
LUIS MEDINA
  • MEMBER
  • 1998 HONDA CIVIC
  • 1.6L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 150,000 MILES
So, I recently swapped motors on my 1998 Civic Coupe with another d16y7 from a 1999 lx model everything was mostly the same except the engine harness had like 2 or 3 more connectors for the ECU and different distributor connector, but just reused my original harness and distributor from old motor and reused intake manifold because the one that I swapped in had cruise control on manifold. Now my question is, that I keep getting the code p0505 for the idle and I don't know what to do anymore. I replaced all gaskets when I put engine in and cleaned the throttle body, also has new air filter, I also checked for vacuum leaks and found none so what other thing should I check for? I also bought new IACV and used one, but I still get the same code. It drives smooth and everything but when it's idling or sometimes when I come to a red light my Check engine light goes off and my idle goes up and when I put scanner and clear code my idle drops back down like if nothing happened. Any help please I do appreciate it and thanks in advance.
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Thursday, September 30th, 2021 AT 5:28 PM

24 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,772 POSTS
Hi,

The code indicates an issue with the idle control system. This could be the result of a faulty IAC (which you replaced), a throttle position sensor, or even an engine coolant temperature sensor.

If you look at the three pics below, I attached the diagnostics. It will walk you through the different things to check.

Let me know I this helps or if you have other questions.

Take care,

Joe

See pics below.
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Thursday, September 30th, 2021 AT 9:16 PM
Tiny
LUIS MEDINA
  • MEMBER
  • 27 POSTS
Nope I tried all those things probably a bad ECU?
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Saturday, October 2nd, 2021 AT 8:12 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

Anything is possible, but it's odd that the ECU wasn't an issue prior to the engine replacement. When you checked the wiring, was it done at the ECU or only at the components? We are missing something simple.

Let me know.

Joe
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Sunday, October 3rd, 2021 AT 4:10 PM
Tiny
LUIS MEDINA
  • MEMBER
  • 27 POSTS
Okay on cold starts the engine goes up to 2,000 RPMs and automatic goes down to 1,500 RPMs and from there it drops down like normal while getting to operating temperature while on operating temperature it idles around 900-800 RPMs, but if I just let it keep idling the check engine light comes on code p0505 and my idle goes up. So, like I said I changed the IACV from old motor, same thing then I bought new IACV same thing again, I put new throttle body gasket and intake gasket still get code p0505 and I forgot that my model is a DX model. I did what you said also and changed the ECT and TPS from old motor and nothing. I checked for vacuum leaks with brake cleaner but no change on idle, think I am going to check again and spray as the engine is warming up in the morning.
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Sunday, October 3rd, 2021 AT 9:04 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

It has to be something electrical. When the idle is high, check the throttle position sensor angle. It should be approximately 10%. This should be done with the engine at operating temperature. However, I would like you to do it when cold as well, when the idle speed is too high.

Let me know.

Joe
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Monday, October 4th, 2021 AT 6:47 PM
Tiny
LUIS MEDINA
  • MEMBER
  • 27 POSTS
Okay hear me out on this the motor I pulled out was bad according to mechanic it would shake, bog, hesitates/misfire and I would get random misfire even though I changed everything from spark plugs, spark plug wires, whole distributor and even changed the head gasket prior from taking it to a mechanic. It ended up getting worse because when it got up to operating temperature and I would turn off the car the car would just crank and not turn on but when the motor was cold again it would turn on just fine. So, I replaced motor with a good maintained motor according to previous owner but now I'm getting the same exact symptoms as the other motor, and again mostly new everything so it must be the ECU.
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Tuesday, October 5th, 2021 AT 10:37 AM
Tiny
LUIS MEDINA
  • MEMBER
  • 27 POSTS
The only thing with this motor that I put in is that even though the engine gets to operating temperature and I shut it off and turn it on when it has been up to temperature it starts right up not like old motor, still though how is it that this motor has the exact same symptoms as other and yet I was told by the mechanic that my old motor was no good because it was overheating for a long period of time because of a blown head gasket. The motor I put in has I good head gasket because I tested it. It was one of the first things I did when I got it. I'm seriously confused, lol.
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Tuesday, October 5th, 2021 AT 11:06 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

I didn't realize these were the same type of symptoms you had with the old motor. With that in mind, certainly, it could be what you're thinking. Or a problem with the wiring.

However, this is where the manuals are conflicting. If the code is present, according to the manual, it's related to a faulty IAC valve, vacuum leak, coolant temperature sensor, or clogged port in the throttle body.

The only way I find anything on a possible PCM/ECM issue, is if P0505 is not present. When the code is present, the manual identifies specific possible causes.

Take a look at the two pics I attached and let me know your thoughts. Also, let me know if you cleaned the ports on the TB, confirmed the ECT signal is within spec, and if you are using the original throttle body.

See pics 1 and 2 below.

Joe
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Tuesday, October 5th, 2021 AT 6:39 PM
Tiny
LUIS MEDINA
  • MEMBER
  • 27 POSTS
Yes, I cleaned the ports on the throttle body and it still has original throttle body and I also swapped throttle body from old motor that was also its original throttle body and has for the ect output was around 190-200 on the freeze frame data on scanner when code p0505 came on and just this I erased the code to see if it would come back on and it did put this time with random misfire on cylinders and p0505 again. While the engine is idling it's like shaking/hesitating and while it's doing that, I could hear like little sounding pops out of the muffler. I don't know how to really describe it lol. I'm assuming that's when it's misfire but like I said this is what the old motor would do minus the high idle. The old motor would shake/hesitate and pop sounds out of muffler and even the misfire on random cylinders code came on that motor as well, Same symptoms. It shouldn't have misfire though if it's getting spark and fuel, I double checked everything.
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Tuesday, October 5th, 2021 AT 7:21 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,772 POSTS
Hi,

The idea that it is the same is likely indicating the problem part was never changed. I don't like to say it's the PCM, but that is what is left. The manual is saying no, but if everything other is good, it is pointing in that direction.

The noise you hear from the exhaust, could it be a backfire? Also, by any chance can you get a substitute PCM to try?

Let me know.

Joe
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Tuesday, October 5th, 2021 AT 7:49 PM
Tiny
LUIS MEDINA
  • MEMBER
  • 27 POSTS
It sounds like backfire and yes, I can get an ECU from the junkyard of the same car hopefully that's the problem.
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Tuesday, October 5th, 2021 AT 8:19 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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I don't like spending money. Make sure you can return it if that isn't the issue. Hopefully, that will take care of the problem.

Let me know.

Joe
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Tuesday, October 5th, 2021 AT 8:31 PM
Tiny
LUIS MEDINA
  • MEMBER
  • 27 POSTS
Not ECU I swapped ECU from junkyard and still the same thing.
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Tuesday, October 12th, 2021 AT 11:10 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

I was hoping that would take care of the problem even though the manual said differently. Does it act exactly the same with the different ECU?

If it does, I'm starting to question if there is something different about the engine. However, the engine itself really should be the same.

Are you certain the throttle plate is fully closing and there are no vacuum leaks? I know you indicated checking, but I want to confirm.

Also, if you perform the portion of the test I highlighted below, what happens?

Let me know.

Joe
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Tuesday, October 12th, 2021 AT 5:22 PM
Tiny
LUIS MEDINA
  • MEMBER
  • 27 POSTS
Okay, I erased the p0505 code yesterday and today I plugged/covered the lower port hole on throttle body and the idle went down but it did not stall/shut off when I covered it. After like 10ish minutes the code p0505 popped up again and this was the freeze frame
DTCFRZE = P0505
FUELSYS1 = CL
FUELSYS2 = N/A
LOAD_PCT(%) = 25.9
ETC (*F) = 190
SHRTFT1(%) = 3.1
LONGFT1 (%) = 3.1
MAP(inHg) = 7.7
RPM (/MIN) = 993
VSS(MPH) = 0
IAT(*F) = 82
TP(%) = 11.8

Hope this helps.
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Wednesday, October 13th, 2021 AT 2:38 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

If the idle dropped, the next thing is in the procedure is to set the idle speed. Have you tried this? See pic below.

Joe
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Wednesday, October 13th, 2021 AT 5:46 PM
Tiny
LUIS MEDINA
  • MEMBER
  • 27 POSTS
I moved that screw before but don't know how to adjust accordingly to manufacture specification.
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Wednesday, October 13th, 2021 AT 6:01 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,772 POSTS
Hi,

Take a look at the pics below and let me know if that helps.

Joe

See pics below.
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Wednesday, October 13th, 2021 AT 6:21 PM
Tiny
LUIS MEDINA
  • MEMBER
  • 27 POSTS
Went to the junkyard again and got a OEM distributor because I currently had a aftermarket one which I replaced two times in the past and as soon as I installed the OEM one it didn't misfire on all cylinders like before. I'm hoping that was the problem as for the idle it stays at 1,500 RPMs and drops down slowly as the engine is warming up and idles 1,000 or slightly below 1,000 the screw didn't really do anything. Also, when I shift to drive or reverse it drops to like around 750-800ish.
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Thursday, October 14th, 2021 AT 4:25 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

Has the code (P0505) returned? This is good news! The idle can be lowered with the screw I mentioned. That determines how much air is allowed past the IAC valve. See if you can adjust it more now that it's running properly.

However, what you indicated the idle speed to be really doesn't sound too bad. A little high, but not bad at all.

Let me know. Also, thanks for the update.

Joe
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Thursday, October 14th, 2021 AT 6:22 PM

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