I have a 1999 Honda civic LX with a D16Y7 and a D16Y8 intake swap. I currently have a check engine light code of p1509 for Idle air control circuit malfunction. I've followed the service manual and tested the wiring. I believe it was the B2 connector at the ECU. Everything checks out. The IACV has been replaced and so has the ECU. I removed the throttle body and cleaned it to remove and build up. I finally ran out of ideas and took it to a honda dealership and they were unable to figure it out also. I've cleaned ever ground I could find, tested the TPS sensor. The car runs but idles at 1100 RPM and the P1509 Code is always there. I've driven it and somewhere between 1200 and 1500 RPMs the car jerks a little bit. Mostly when letting off the gas for a second or when you're speeding up. The code comes right back when I erase it without ever starting the car. I put the key in the on position and erase, comes right back. I'm looking for any help I can get. So far I can't figure it out and neither can the mechanics I've even tested ohms of the new IACV and it seems to be correct. The check engine light started as a p1505 for IACV I believe and once replaced it returned a few days later.

 

How many wires on the IACV plug? Are you going from a 3-wire IACV to a 2-wire IACV?

Are you running the stock P2E ECU?

 

The IACV is a three pin. The D16Y8 swap was done years ago by the previous owner. The ECU is the stock one as far as I know. I replaced it with the OEM one trying to tackle the p1509 problem and it looks just like the old one. The car sat in my yard for a few months after he had a minor fender bender and I bought it cheap. The car was running fine until it sat up. There was no CEL after the wreck so I thought maybe a bad ground but I've cleaned every one of them I could find.

 

P1509 is caused by an electrical problem.

Start by checking for battery voltage on the Yel/Blk wire pin in the unplugged IACV connector.

 

I'm getting 12v with the key in the On position.

 

Describe the Ohm test you did on the IACV and the results.

 

I put my meter on ohms, probed the center pin of the IACV with it unplugged, then I probed each of the other two pins with the opposite lead. My readings were around 22.8 with both.

Thanks for taking the time to help by the way. I appreciate it!

 

Remove the passenger kick panel. Remove the bolts securing the ECU to chassis.

Unplug gray ECU connector B.

Make sure IACV is plugged in, and then turn key to ON(II).

Do you measure battery voltage to body ground individually at pins B6 and B15 in free ECU connector B?

ECU connector B shown in the diagram is viewed from the wire side. Probe the pins from the face side. Make sure the pins match with the indicated wire colors.

 

I tested both and I'm getting 12v on the black/blue and orange with the key in the ON position. There's three connectors on the ECU with B on them but only one with the Black/Blue and Orange wires. It says it's the 25P on the connector.

 

Correction.

Leave key in ON(II) and plug ECU connector B back in.

Individually measure voltage to body ground from pins B2 and B10 by backprobing the wire-side of ECU connector B.

What are the voltage readings?

 

i'm reading 0.03 on both of those.

 

Your results are clearly pointing to a bad ECU. I think you've been here before, but please recount your ECU story.

 

Yeah I replaced the ECU hoping to fix this problem. Never had any problems with it before but it was what the test brought me to when I completed every test I could do from the service manual. I bought a "Tested" ECU and installed it with no luck just a few weeks back. I replaced the IACV twice because the one I had, I found cheap online. So I thought maybe it was a bummed part and I bit the bullet and bought another one at a part store with a warranty. The dealership came to no real conclusion other than the IACV was possibly bad and they wanted $400 to do the job so I tried it myself this week. I've been searching high and low online for something but haven't found anything much so I came to the forums when I found them.

 

My guess is that the problem is related to the Y8 IM swap.

When an IM or engine swap is done, many sensor connectors must be removed. It's obviously critically important to properly re-install the sensor connectors. Unfortunately, there are a few sensor connectors that can be swapped. When this happens, it's not uncommon to blow the ECU. If this is your problem, you may have blown the good replacement ECU.

I highly recommend that you carefully look for swapped sensor connectors using the connector wire colors as a guide. In the 96-00 Civic electrical troubleshooting manual, see pages 24 to 24-11 for guidance on sensor connector wire colors.

 

I appreciate all of your help. From what my buddy has told me it was a direct fit, he didn't change anything electrical. He may have used the throttle body from the Y7 with the Y8. I'm thinking about to spending the $100 and converting it back to the Y7 intake and seeing where that gets me. It's just incredibly frustrating that he did the swap years ago and it's just now causing a problem.

 

Your buddy had to disconnect the sensor connectors. Swapping two connectors is both possible and common in these tech forums. Many people don't think it can happen but they are wrong.

 

My bad I misunderstood you. I thought you were refering to something like changing a 3 pin IACV connector to a 2 or something of that nature. He definitely unplugged the TPS, IACV, and another sensor. Two of them are the same connector for sure. I considered that to be a possibility for a bit but the routing for the wiring seemed to be correct. It was hard to tell from photos online. I'll get the book out and start tracing stuff down and making sure it's all plugged up correctly. Then when I swap it back a Y7 intake I'll at least be certain about it everything that's getting plugged up.

 

The 2P IAT sensor connector can also be swapped with another 2P connector, possibly the power steering pressure switch. Best to check them all.

 

That's good to know! You've been very resourceful. I'm grateful for the help. Honestly you've been more help than the dealership was. I'm about 60 hours into this, with an additional 8 or so reading online. If I weren't so invested personally, I'd probably move on and sell the car. Thus far I haven't found a mechanic to work on it that can fix the car or tell me what's going wrong. Therefore I'm pretty much giving it hell just trying to learn about it all myself to solve the issue. It's got to be something crazy going on. You may be onto something with the connectors being swapped. I'll start on that bright and early.

 

Best of luck with the fix!

 

Thanks! I'll post an update when it's done.

 

There is a sticky thread at the top of this forum with all the wire colors for each sensor. You have to identify where the connector goes by wire color.

Have you confirmed that the IACV is 3-wire? The auto EX intake has a 3 wire IACV which should plug and play on an LX (manual or automatic) keeping the stock ECU and non-VTEC engine. The manual EX is 2 wire-- to make that work properly would require rewiring, swapping the ECU, and swapping to a VTEC engine.

 

The IACV is three pin and all the wiring checks out. I opened the computer and inspected the board and it looks to be in great shape.

 

Be careful about drawing flawed conclusions. Visual inspection would commonly miss many types of failed components on printed circuit boards.

 

We've tried the newest ECU in another 1999 lx and it worked. Is there any possibilities of the TPS playing a roll in all of this? A possibility that the code is being triggered by something that isn't directly related to the IACV.