The_Honda_Guy

Working on a 97 DX D16Y7 5 speed. Car is currently throwing a code for the idle air control motor and idling around 1000 RPM. I first tried to remove the IACV and clean it, then installing a new gasket. Let the car run and bled the cooling system properly and that seemed to fix it for a short time. After a couple weeks the car started idling high again and threw the code, so we replaced the IACV with a new one from Honda, bled the system again, and was fine again for a short while. Car has again started idling high and throwing the code. My first guess is that it could be a TPS problem. Sometimes when you're driving slow in first gear and getting on the throttle lightly the car jerks. I had this same problem with my RX-7 when the TPS was bad and out of adjustment. Also, I thought I remember searching for this and found someone that had the same problem and thought it was related to the TPS also. Anyone experienced this same problem?

The_Honda_Guy

Nobody has anything to input on this? Guess I'm just the only one to run into this problem...

The_Honda_Guy

Well, I finally started seriously digging into this this morning. I grabbed the Hayne's manual and went out to the car. I first decided to check the TPS to see if that was in fact my problem. Here's what I got:

Reference voltage from PCV was approximately 4.9 volts - yellow/blue wire to green/black wire. This checks out correctly as the voltage should be approximately 5.0 volts.

Signal voltage checked with red/black wire to battery ground. Throttle fully closed was approximately .4 volts and 4.6 volts fully open. The voltage slowly increased from .4 to 4.6 and was a clean sweep with no voltage spikes or jumps. This also checks out correctly as the readings should be .5 volts to 4.5 volts.

Nextly, I decided to check out the IACV. Here's what I came up with:

Battery voltage is supposed to be at the center terminal. I got about 12.5 volts with the engine off/key on and measuring from center terminal to battery ground. This checks out correctly.

Nextly, I needed to check for correct resistance between the center terminal and each of the outer terminals. I found 0 ohms between the center terminal and each of the outer terminals. The resistance is supposed to be between 16-28 ohms. Obviously this seemed to be a problem. I looked up the wiring diagram for the IACV and found that the center terminal (yellow/black wire) came from the PGM-FI relay and was measuring correctly. I dug out the PCM from the kick panel and found the wires for the two outer terminals (blue/black and orange). I measured between the center terminal from the connector since I knew that it was getting the correct voltage and back-probed the two wires directly off the PCM connectors. I still found 0 ohms of resistance between them. The only thing I can think of at this time is that the PCM has a fault in it at these two wires.

What do you guys think?

Shaggy143Music

Now when you checked resistance from the center terminal to the outer terminals did you do this on the IAC or on the harness? I am not sure but I think you are supposed to do that measurment on the IAC. It sounds like you were measuring resistance of the wires from the pcm to the IAC and if you were then it should be 0 ohms. I have misunderstood what you did though.

Also did you use just a DMM for your TP test or a scope?

The_Honda_Guy

Finally, a good reply! I realized what I typed may have been hard to read. lol

Like I said, I first tested the resistance between the center terminal and the two outer terminals at the IACV connector with it disconnected. That's when I found out that I had 0 ohms of resistance between them. After finding that out I decided to go all the way to the PCM to see if it was that the terminals on the connector were bad or if it was the wiring or what it was for sure.

I pulled the PCM out so I could get at the wires and left the connector unplugged at the IACV. I then took one of the leads from the multimeter and hooked it to the center terminal on the connector and then used the other lead to back-probe both of the other two terminal wires directly off the PCM. I still found 0 ohms of resistance between the center terminals and the two outside terminals. This is why I was thinking it was a PCM issue.

For the TPS test I used my Snap-On digital multimeter. I realize that there COULD be dead spots in the sensor since you can't see every position exactly, but when I say I opened it slow I opened it SLOW. I've done quite a few of these tests with other TPS's, so it's not the first one I've done. Like I said though, I highly doubt it's a TPS problem now since I can watch the throttle position on my scan-tool also and it makes no jumps in it's open percentage. Also, it's showing the correct open and close percentages and is measuring the correct numbers.

I'm thinking of taking the PCM from my friend's 97 DX manual D16Y7 and trying it in my girlfriend's car to see if it does the same thing. IIRC, it should be the same PCM since it's got the same motor and transmission.

Shaggy143Music

from what im getting it sounds like you tested the wires from the PCM to the IAC and if that is what you did then it should be 0 ohms, you wants no resistance in wires, but if i am still misunderstanding you then replacing it with a known good PCM should give you a good idea of it too.

The_Honda_Guy

I guess this would explain it even better.

The test simply states to test the resistance between the center terminal (yellow/black wire) and then two outer terminals (orange and blue/black). When you test between them, you should see 16-28 ohms of resistance. When I tested the wires straight off the connector terminals I had 0 ohms of resistance between the center terminals and each of the outside terminals.

I decided to make sure that the wiring was good and that the connector wasn't bad. What I did then is trace the orange and blue/black wires back to the PCM since the yellow/black wire comes from the PGM-FI main relay. I knew the yellow/black wire was testing correctly since it had battery voltage like the Hayne's manual says it should. Since I knew that wire was good, I hooked one of the leads of the multimeter into the IACV conector center terminal (yellow/black wire). Then I hooked the other lead into the orange and blue/black wires by back-probing them at the PCM. I still found 0 ohms of resistance, meaning that the wiring wasn't the problem and that it wasn't shorting out somewhere between the PCM and IACV connector. Basically I did the connector test but instead of testing the two outside terminal wires at the connector I tested them directly at the PCM while still using the center terminal wire at the connector.

I also checked Alldata's diagnosis steps. Alldata says to check to warm up the vehicle until the radiator fan turns on, then let the vehicle idle with all the loads off. Then you check to ensure that the ECT sensor is reading bewteen 194-200 degrees F and the TPS is reading approximately 10 degrees. The ECT sensor is reading 194 degrees F and the TPS is reading 11 degrees, so these check out correctly. Then it says to check the idle, which obviously mine is around 950-1000 RPM, meaning it's off.

After that it says to take off the intake and put your finger over the lower port on the TB (not the one leading into the IACV). I put my finger over it and the idle didn't change. It then says to check for vacuum leaks and to ensure that the throttle plate is completely closed. Just to make sure, I unhooked the throttle cable to make sure the plate was closed and it still read 11 degrees, meaning it was closed with the cable attached.

I started hunting for a vacuum leak then. I wiggled the different vacuum hoses and the idle didn't change. I knew that the TB gasket and IACV gasket were good and torqued properly since I had replaced them. The only other place I could think of a vacuum leak was the intake manifold gasket where it bolts to the head. I noticed what seemed like a piece missing out of the gasket between the 2nd and 3rd cylinder runners. I sprayed some Seafoam Deep Creep on the area and noticed what looked like bubbles and then seeing what seemed like the spray being sucked into the intake ports. I'm possibly wondering if it's got a slightly leaking intake manifold gasket.

Shaggy143Music

Spray some brake or carb cleaner or some propane where you suspect the vacuum leak. you will notice a change in idle speed if you do that and it is a vacuum leak

IhateworkingoncaRs

I thought that the ohm test was suppose to be performed on the IAC not the wires?

The_Honda_Guy

Well, I dug into it a bit more and figured out what the problem was. I would have never imagined what this could have been and basically stumbled on it.

One of the things I was wondering is if possibly the throttle cable was binding up and holding the throttle slightly open. I decided to try and turn the throttle plate closed by hand to check this. As I put some pressure on it I noticed that the idle went down quite a bit. I then started wondering what the hell was keeping it open.

I then took the throttle body off and started inspecting it. I decided to take the plate itself out and lightly sand it with some emery cloth on the edges and then lightly sanding the throttle body where the plate sat closed. While doing that I noticed there was a screw next to the throttle wheel where it turned open and closed. There was a nut on this screw that was for some reason loose. Somehow the nut had broken the yellow loctite-type stuff loose and had worked itself slightly down the screw shaft, thus holding the throttle plate very very slightly open. I backed the nut off a bit, thew a small bit of Loctite on it to keep it from doing the same thing again, and re-assembled the throttle body. Started the car up and checked the RPM with the scan tool. Low and behold the idle had gone down quite a bit.

I then let the car warm up all the way and kick on the radiator fan, then return to idle. Idle was sitting at a steady 680ish RPM, no surging, and no CEL. Went and drove the car and brought it back, still at 680ish RPM at idle. I guess that very very slight bit that the throttle plate was getting held open was enough to make it idle 300 RPM high. The hood says the idle should be 670 RPM + or - 50 RPM. Also, I checked the TPS to make sure it was correct and it was sitting at 9%, which is damn close to the 10% the Hayne's manual says. Also, spark timing was at 12 degrees BTDC, which is dead on with it being + or - 2 degrees.

I couldn't believe how stupid this problem turned out to be. My guess is that since the throttle was getting held open slightly the IACV was getting pissed that the car was idling too high and it couldn't really control it, thus the reason it was throwing the CEL.

Also, I went back through and sprayed brake cleaner around all the vacuum sealing sources and couldn't find anything leaking. I wiggled every single vacuum line and sprayed them all down too. I couldn't find a damn thing causing a vacuum leak. I must have spent two hours spraying everything down and looking and listening. Sucks that it turned out to be what it was since I have spent nearly a year hunting this down.