Our stored codes are P0171 and P0174. These are fuel system lean codes for each bank of cylinders on the engine. The fuel mixture was too little fuel or too much air to set the codes and the problem affected both the right and left banks (banks 1 and 2).
Now we are in the OEM diagnostic system but let's exit that and go into OBDII briefly to take a look at some data related to the problem. OBDII is the government mandated onboard diagnostic system and while it does have more limited diagnostic abilities in some aspects, it also has a lot of useful data if you know where to look. Mode 2 would be "freeze-frame" data related to the time the code set.
The freeze frame for P0171 indicates then engine was at idle and cold. The engine has to be warm enough for the fuel control system to go into closed loop or the sensor readings will be ignored. Closed loop puts the computer in control of responses to sensor inputs. So, while 161 F isn't completely cold it is below normal operating temperature for the engine. Fuel trim shows the computer adding fuel. The freeze frame shows us a car that is sitting still, at idle, and warming from a cold start. Lean fuel under those conditions would usually be a vacuum leak. Remember, lean is either not enough fuel or too much air. Engine vacuum is strongest at low rpm and little fuel is needed to run the engine. If the freeze frame indicated lean fuel under a heavy load we would suspect a fuel delivery problem. If we consider that the vacuum leak is under cold idle conditions then it is likely to be a leak under the upper intake or plenum. Very common on these engines.
The next step is to verify the leak. We want to set the scan data to monitor short term fuel trim. Then we'll spray my good friend Berryman's spray carb cleaner at the base of the upper intake. If the fuel trims respond to the cleaner then we have verified the leak is at the upper intake gasket.
We need to let the cold engine warm just enough that we can see fuel trim start to be controlled by the computer. That would be the shark fin waves on the left of the scan waveforms. When the carb cleaner is sprayed about half way into the event you can see it is picked up by the engine vacuum leak. The computer sees it as fuel and starts commanding lean. That's our solid test result. A leaking upper intake gasket.
As always, thanks for reading!
Kenneth Hayes
G&G Auto Repair
