A friend called me to describe that his 1999 Civic was surging when the engine was at operating temperature. In addition, the car would crank and not start if the engine was hot.
This appears to be a faulty IAC. Slam dunk! Right? Maybe. Please read on.....
Before beginning, let's start with a little repair history. About, three and half weeks before, we changed his timing belt, water pump, and valve cover gasket. I used a tool named Air-Lift to refill the cooling system. The reason I bring this up is about a week after installation, his heater stopped working, and he called me and implied that maybe the Air-Lift tool damaged his heater core. I suggested he disconnect the heater core and flush it. He didn't completely do exactly as I suggested. He didn't completely disconnect the heater core from the rest of the cooling system. He disconnected one end of the heater core and had the IAC coolant passage open because, I think, he was cleaning his IAC as well. He blew into the open end of the heater core and pushed out pieces of something out the IAC coolant passage. Afterwards, he closed up his cooling system and ran his car. The heater worked, but I'm guessing this is when the Check Engine Light turned on. So, let's begin our study.
My friend calls to tell me his Check Engine Light is on and then continues to tell me how the car is behaving. He says that when the car is warmed up, the car surges at idle and cranks strong but will not start.
Doesn't this sound like a classic bad IAC? That is what he thought.....
Now the back story.... Before calling me, my friend was so confident that the IAC was bad that he watched several YouTube videos, removed it, cleaned it, and reinstalled it. Again, I think he did this at the same time he was flushing out his heater core. Unfortunately, the surging and no start conditions did not clear up. So, he ordered a new IAC which was going to be available the next day. But, for some reason, he decided to call me, described the problem, described his IAC cleaning procedure, and requested I give my input to what the root cause could be, which I found interesting since he already ordered a replacement part.
Looking back, I feel the real reason he called me is that after cleaning his IAC, the Check Eengine Light turned on.
I started out by pulling the codes, which was P0113. But, when I first read it, I thought it said IAC not IAT. But, since I followed the first step in troubleshooting - research, I leveraged the scan tool's Trouble Shooter capability. So, when I entered P0113, I clearly was woken up that the error had to do with the IAT. The scan tool indicated that P0113 meant that the IAT, intake air temperature, sensor's circuit had too high of a voltage.
Knowing that the IAT is a thermistor, I knew that a high sensed voltage could be caused by:
I then proceeded onto the next step, visual inspection. Using the shop manual, we found where the IAT was on the engine. I noticed it looked really good. No physical damage to sensor connector, no damage to sensor tip, and no damage to sensor wiring.
I then configured my scan tool to view manufacture specific PIDs, allowing me to view the Intake Air Temperature PID. It was perfect. It displayed the ambient air temperature within 5 degrees of my handheld infrared temperature scanner aimed at the sensor tip. I was puzzled. So, I started to wiggle the connector and wire as far as I could access. No issues found. So, I figured that the sensor would start to act up as it warms up, so we ran the engine until it reached operating temperature. Still no issues found. I then lightly smacked the sensor and also performed a wiggle test. Still no issues found.
Then it dawned on me. It was located in a plastic housing above the IAC. So, I asked him if he recently removed this housing. He said he did while he was cleaning his IAC. I then asked him if he turned the key on when this plastic housing was off of the car. After a few seconds of uncomfortable silence, he said that is what happened. This is where I made a mistake. I should have reviewed the Freeze Frame data to realize that this error was caused with engine off. That would have clued me in much earlier. So, my mistake cost us about 20-30 minutes.
So, this Trouble Code had nothing to do with the issues he called me more to diagnose. So, back to square one....
Given there was no Trouble Code and the engine was not exhibiting the behavior he asked me to diagnose, I asked him to reproduce the condition. So, we drove for about fifteen minutes in stop and go and highway traffic with the scan tool hooked up. Bingo, while driving, he was able to reproduce the issue.
I noticed that under normal operation and driving conditions, the IAC was at 15% during idle, which to me seemed really low. But, when the engine was in the surging condition, the IAC hovered at 40% during idle. So, this could be caused by PCM trying to open a stuck IAC or the PCM is receiving inputs causing it to open up the IAC.
To determine if the IAC was stuck, I lightly smacked the IAC housing with a screw driver handle while it was trying to open or close. Unfortunately, no change was observed. The engine continued to surge.
To determine if the PCM was receiving weird inputs, I then started looking at input PIDs. I looked for sensors that were out of range, changing frequently, or at some default value. Bingo, as they say, there it was. The ECT was sweeping up and down as quickly as an O2 sensor would. Thermistor sensors don't change that rapidly; they should change in a stair-step like fashion. What was weird is that the ECT sensor was changing in parallel with the IAC. So, I disconnected the ECT, and, voila, the surging stopped. I then reconnected the ECT and the surging started. Just to be thorough, I disconnected it again and the surging stopped. So, I found a 1 to 1 correlation to the surging. But, the next step was to determine why. Was it a bad sensor, a bad connector, or bad wiring?
I turned off the engine and went to KOEO. I then used my infrared heat gun to determine engine coolant temperature and then compared that value to the ECT PID. Way off, at this point in time, the ECT was fluctuating around 40 degrees and the infrared fun reported 180 degrees. I was pretty sure the sensor was bad, but I continued by perform signal integrity testing. I executed the following steps:
All of these tests gave the expected results, indicating that the ECT sensor was faulty.
Once my friend was able to get into the hard fault, I was able to perform all of the above in about ten minutes. He was blown away at how this all went down. He couldn't believe that a little old ECT sensor would cause such a horrible drivability issue.
So, my friend cancelled the IAC order. He instead purchased an ECT and installed it. Car fixed!
GeekDIYMechanic - 1999 Honda Civic - Surging idle & Hot Crank No Start - P0113