The car is a 92 RS automatic with a 305 TBI, vin E.

Like the title says, I’m having a problem with my idle being way too high. Before I get into the problem, I want to address some previous issues just to be abundantly clear.

Around March or so, I had a problem where the car would suddenly stop running properly (if at all) while the car is wet. Whether it’s because of rain or just a car wash. I made a thread back in June about the problem and chased several things that didn’t even up fixing it. The thread can be found here if you want more information: https://www.thirdgen.org/forums/tech...t-perfect.html

I haven’t updated that post in a while, but the problem went away enough that I didn’t have to worry about it, after replacing the alternator, distributor cap and rotor (twice), and the spark plug wires. All of those are ACDelco branded since the cars hate using other brands. I also installed the wires a little close to the manifold and the insulation on one was burned through. The actual wire inside the insulation was still good, so I moved the wire and wrapped the hole in several layers of insulation tape. Didn’t have any issues with either of those things after that. It drove well in the rain, occasionally the engine light would come up while wet (only while wet), but didn’t affect how the car ran and would not flash a code no matter how I tried.

After that, I had some issues with my idle being inconsistent. It’s always run a little high, with around 900 RPM at the lowest that it would run. I ended up replacing the IAC a month or two ago, and followed the IAC reset instructions on the Iroc Zone website. That brought the idle down to 750-800 like it should be. My IAT and TPS were also replaced during this time, with the TPS due to surging between 17-29 MPH. That fixed a little bit of it, but it still happens. It is more noticeable now that I live in a city and deal with traffic for the first time in my life.

On Monday morning, the 26th, it was raining pretty heavily. The car ran fine in the rain, until I hit a puddle that covered my entire lane and splashed up enough to pour water over my hood. Now, I didn’t have working headlights at the time to be able to see the puddle—but I do know what that problem is and I know exactly how to fix that. After hitting the puddle, the car ran horribly like before I replaced the distributor parts. I was able to get to work without much of an issue, checked things out when the rain stopped and nothing was wet. Clearly I had gotten something wet. After work that day, I stopped by an auto store to pick up parts for the headlights, and the car wouldn’t stay running. The rain had stopped later on and the car had dried by the time I left, and I drove it over to my parents house without any issue, and have been borrowing their spare truck since then.


Okay, now for the current problem. My dad knows much more than I do about engines, and was looking over mine since I had completely given up at this point. He found that my IAC was staying open. I brought over some of the old parts that I had replaced, and went over to help him.

I’m going to try to be as clear as I can with this. While looking over the IAC, we discovered that the wiring for the connector was pretty messed up. Exposed wire, brittle insulation, and the wires were so faded that they all looked identical. We pulled out more of those wires and found that they were routed almost entirely wrong. Pin A was a Light Blue/White, Pin B was a Light Blue/Black, Pin C was a Light Green/White, and Pin D was a Light Green/Black. I had found a wiring diagram that called for Pin A being Light Green/Black, Pin B being Light Green/White, Pin C being Light Blue/Black, and Pin D being Light Blue/White. We repaired the wires and routed them correctly. The IAC is functioning and moving like it should be.

We followed the Iroc Zone IAC reset procedure again, and started the car. It immediately idled (in park) up to 2500 RPM and stayed there. We adjusted the stop screw (I forget what it’s called) for the butterflies as low as we can without damage, and the car idle went down to about 2250 RPM (in park). We tried regrounding the TPS, resetting the ECM, and driving for 40+ miles to gather live data for the ECM to adjust off of. We even swapped the new TPS out for the old one, and both acted the same and had identical Ohm readings in all positions. I currently have the old TPS installed. In gear, the idle is around 1250 RPM.

We searched all over the engine bay and the harness and didn’t find any other broken wires. The only other idea that we had was to check the CTS, and it was reading way off for how hot the coolant was. We swapped it out and had much more accurate readings, but the idle didn’t change.

What we did notice, though, is that the idle (in park) ranges from around 1800-2250. It’ll go up to 2250, slowly move down as the engine adjusts, then it shoots up like the car thinks it’s stalling, even though it is not. The car sounds perfect, and doesn’t have any engine light, and even responds immediately when the TPS is unplugged and plugged back in respectively. The engine also takes in an unreasonable amount of air at idle. Like, 4000 RPMs worth of air at 2000 RPM. It does not pull in nearly as much air when gas is applied.

We’re both at a complete loss as to what is going on. The car does not have any vacuum leaks, even behind the throttle body. The only idea that we have is maybe adjusting the pin that the TPS moves, inside of the throttle body? When we went to install it, it doesn’t sit flush with the holes, and we have to push the pin forward a bit and open it to get the TPS in place enough to screw it back in.

Any help or just a general idea of what might cause this would be much appreciated.

 

If it was working before you changed the wiring - you should put it back how you found it. Regardless of the diagram you found - the IAC is a 4 wire stepper motor and if it's not wired correctly will not step in the direction the ECU wants it to go. Perhaps the diagram you found doesn't match your car, or someone re-pinned the ECU side of the equation - regardless you re-wired it without any evidence that this was an actual problem. Put it back and see if that helps your situation.

GD

 

If you haven't replaced the gasket between the throttle body and intake manifold, you should. They are notorious for allowing major vacuum leak when old.