QUESTION: I have a 2006 Chevy Cobalt LS, 2.2-liter engine with a manual transmission. It has 186,000 miles and is a very good commuting vehicle. Recently it was idling rough and I read that cleaning the throttle body might solve the problem. All went well with the cleaning; I removed the throttle body and moved the "butterfly" to clean it more thoroughly. But after I was done, the vehicle fell into a high-idle syndrome of some sort. It now idles at 1,500 rpm rather than 800, which is normal.
Someone suggested that I drive it in first gear and "force" the rpm down to 800 or so. I tried this and it worked for a while, but as soon as I hit the gas pedal, it's back up to 1,500 rpm. I have been told I will need to take it to the dealer to have the computer "re-flashed." Is that true?
ANSWER: I suspect the high idle speed after removing and cleaning the throttle body is due to a vacuum leak. Are you sure any and all vacuum lines that you may have removed, disconnected or disturbed during your service were properly reconnected? Did you install a new gasket between the throttle body and intake manifold when you reinstalled the throttle body? An air or vacuum leak would allow unmetered air to enter the induction system downstream of the mass airflow sensor, causing a lean air-fuel mixture that creates a high idle speed.
GM suggests cleaning the throttle body and throttle plate with a clean shop towel and its Top Engine Cleaner or equivalent. GM does not recommend using strong solvents because of potential damage to electrical components, sensors, seals and O-rings.
The throttle body on this engine incorporates two throttle position sensors and an electronic throttle actuator motor. I'm hoping none of these were damaged during the cleaning.
You might try disconnecting the battery for 60 seconds. In the next few driving cycles, the electronic control module may relearn the correct idle speed. I'm betting on an air or vacuum leak.
Q: I am banking on you solving this problem. We purchased our 2010 Ford Escape XLT/AWD 4-cylinder new in March 2010. Four months later, a rattle developed when traveling at 1,500 rpm. The rattle is heard underneath the vehicle but doesn't occur all the time. The rattle can be heard over the radio and lasts 10 to 20 seconds and then it stops. The dealership finally replaced the serpentine belt tensioner and the rattle ceased for several months. Today, the third tensioner replacement was put on our vehicle. The mechanics seem to be at a loss as to what is causing this to continue to occur. Our dilemma is that the three-year warranty is up in March and we need a solution to this ongoing and extremely frustrating problem.
A: Ask the dealer to check Ford service bulletin 11-10-26, dated October 2011. According to my Alldata database, this bulletin covers a rattle or drone between 1,300 and 1,400 rpm generated by a resonance in the serpentine accessory drive belt. Ford has issued a new alternator/generator pulley kit to address this issue.
As I first read your question, your description of the rattle reminded me of the typical sound caused by a cracked heat shield mount on a catalytic converter.