I have a '94 Mercury Tracer. Startup and cold idle are OK, but after it's warmed, up it stalls. If I turn on the air conditioning, it will idle OK. Is it the valve seats or a sensor? The process of elimination can be expensive.
A When dealing with drivability issues, always remember the "KISS" principle -- start with the simple stuff first. In this case, the reason the engine continues to idle with the air conditioning engaged is the step-up in idle speed commanded by the powertrain control module (PCM). The first things to try would be to clean the injectors and decarbonize the induction system with a fuel system/fuel injection cleaner like Sea Foam or GM's Top Engine Cleaner. Then check for any type of vacuum leak that could lean out the idle. And check for splits or tears in the rubber boots and hoses downstream from the throttle body and MAF (mass airflow sensor). The idle speed should be 780 rpm (plus or minus 50 rpm) with the air conditioning off.
Q I have a '94 Geo Tracker with the 1.6- liter engine. After the car sits overnight, it never seems to start on the first turn of the key. On the second turn of the key, it seems to start right away with no problems. I'm concerned about this because with winter coming I'm afraid it might put excess wear on the battery and perhaps not start at all on the coldest mornings. Where should I start the diagnosis?
A Have a shop connect a fuel pressure gauge to the end of the fuel rail to check the fuel pressure. The fuel pressure should be 36-43 psi with key on/engine off when the fuel pump first runs for three seconds. With the engine running, the fuel pressure should be 30-37 psi. Ten minutes after shutdown, the fuel pressure should still be 26 psi or higher.
The typical cause for a delayed but normal cold start is a bleed-down of fuel pressure after shutdown so that there's inadequate fuel pressure at the injectors at startup. Try repeatedly turning the key to the "on" position for several seconds then "off" before cranking the engine. Doing so may rebuild the fuel pressure so that the engine starts the first time you crank it. If so, remember this simple technique for those cold winter mornings. Where does the residual fuel pressure escape? A leaky diaphragm in the fuel pressure regulator or a faulty fuel pulsation damper.
Another remote possibility is a weak battery or battery connections that pull voltage down below about 10 volts when the starter is first engaged.
Q My wife's car is a 2002 Saturn Vue AWD V-6 with only 29,000 miles on it. The problem started in 2008 at approximately 19,000 miles. While driving, the throttle will intermittently cut out. The engine keeps running but there is no throttle response when the gas pedal is depressed. To recover from this situation, the car needs to be restarted. I have had this happen at least 50 times. The dealer found P2135 and P2176 codes indicating an issue with the TPS (throttle position sensor). They cleared the codes but the Service Engine Soon light comes on again the next day. The throttle plate and connector were replaced and 2,000 miles later the problem was back.
A According to my Alldata automotive database, DTC code P2135 is set when the two TPS sensors are out of correlation by more than 6.5 percent. P2176 is set when the engine control module cannot determine the minimum setting for the TPS sensors when the key is first turned on.
The primary suspect, which was addressed with the throttle plate and connector replacement, was a poor connection at the engine harness-to-throttle body connection. GM later issued another bulletin addressing this scenario (P2135 code), TSB #03-08-45-003D dated February 2006 that identifies a poor connection in the 40-pin instrument panel-to-body inline connector located behind the left hinge pillar molding. Inspect this connector for bent or corroded pins.