One of the most difficult situations in an automotive store is a customer’s concern not duplicated. “We can’t get it to act up.”
A recent example for us, a customer brought in a 2007 Chevrolet Silverado 1500 with an intermittent temperature gauge spike to hot, air conditioning is disabled, cooling fans running continuously, and luckily a check engine light. The code was a P0116 ECT circuit high code. (This code indicates a concern with the coolant temperature sensor circuit, with possible causes of a temperature sensor, wiring harness, the computer, or anything concerning this circuit, and of course silly off the wall problems. (Yes it does happen)
So, of couse we commit to find the problem. The customer had the concern checked by two other shops before giving us the opportunity. It had a new temperature sensor replaced (two times replaced), the connector for the temperature sensor replaced, and various other cooling system components replaced, to address this issue.
The signal reference was inaccurate on a cold startup. We noticed a very inaccurate reading when testing the ECT circuit. It went from an inaccurate reading to a spike and “boom” it was reading correctly. This indicated to us that we could only test, during a cold startup. This is an extremely tight window of testing. It required us to hook up several testing equipment for the duration of this vehicle’s diagnostics.
Our first thought was the temperature sensor is giving the computer a false reading, then, after testing, our suspect was the computer – reading the signal wire incorrectly. We suspected this because of an inaccurate voltage reading on the scanner, compared to the voltage reading on the circuit. The reference signal being accurate at the computer had us suspecting the computer was interpreting the signal internally inaccurate. During all of this routine testing, we still have not duplicated the customer’s concern and the vehicle has not presented the problem. So, we decided we can’t be sure of the computer being the fix, until we get the vehicle to act up. After talking to the customer about the possibilities, and the history of addressing this issue, we decided it would be best for us to wait on any educated guess repairs, and wait until we can get the vehicle to present the concern onsistent enough to test.
We started the vehicle everyday for almost a month, we had to put fuel in the vehicle a few times due to it running and us driving it, in hopes of the vehicle presenting the concern. After several weeks of perseverance, it finally presented. Luckily we had our diagnostic equipment on the vehicle and ready to diagnose.
We determined something was shorting the computer out and shutting it down. After tracing the shorted communication line, we were able to trace down the shorted ECT wire on the valve cover. It wasn’t shorted out enough to present a dead short because of the 5v reference signal intermittently being shorted. Even the scope did not pick it up. The wire was barely rubbed, just enough to cause an intermittent spike in voltage. When we tested for resistance on the circuit, it would pass due to the intermittent short.
A simple wiring repair and a minor engine mount replacement fixed this concern.
The root cause was a broken left engine mount causing the engine to shift slightly, and pulling on the engine harness, on the left side of the engine during acceleration. The ECT wire was slightly rubbed on a small bolt/stud, and slightly touching the coil bracket. This sudden spike of voltage caused the computer to shut down and the computer to indicate the engine was running hot.
A simple wiring repair, and a lot of patience repaired this vehicle correctly and saved this customer a lot of money.
Sometimes being patient and persistent is the key. This is a good reminder of why we are here.