QuestionI recently swapped my 3.2 for a 3.5 with very low miles BUT the motor had not run in 3 or 4 years.
I swapped the motor, and took it for a test drive during the test drive it skipped a few times. I assumed that it was a sticky valve. No engine codes were on at this time. The skip went away and the motor runs strong and smooth.
But later that day it put on the check engine light, code 0174 lean on bank 2. What are some of the likley causes of this?
AnswerThe 0174 says the left bank is running lean. The powertrain manual lists the possibilities:
fuel line restricted, fuel pump inlet strainer plugged, fuel pump itself; oxygen sensor, sensor circuit shorted to ground, heater operation faulty; TPS volatage higher than 0.92V with throttle closed, TPS sensor sweep, MAP sensor operation, ECT sensor operation, engine mechanical problem, fuel pressure/regulator (high), PCM.
This code says the left bank of cylinders is operating with too lean a fuel/air mixture for meeting pollution standards.
The first suspect is that the device that measures the mixture, the upstream oxygen sensor mounted on the exhaust manifold is no longer operating accurately. But before changing that out:
Did you run low or out of fuel recently as that can cause this code temporarily? If so try disconnecting the battery for a minute which will erase the code and see if it returns.
Do you notice any leaks in the exhaust system (smell or noise)? Repair those first, then erase code similarly.
There are a number of tests of the various parts of the engine that can be done, but I would believe that investing in a new oxygen sensor and installing it yourself would be more economical as a starting strategy. It changes out just like a spark plug but you need to handle the wires at its electrical connection carefully. Use the anti-locking thread compound when installing, and to remove the old one put some penetrating oil on the threads where it screwn into the manifold and let it sit overnight. It may also help to warm the engine before trying to remove it, use a proper sized box-end wrench, and rock it back and forth to loosen up the rust on the thread gradually.
If that doesn't permanently kill the light, then the following checks are appropriate:
cam or valve timing (in case the timing belt has slipped), ignition coil, fuel filter, spark wires/spark plugs, injector stuck closed, manifold pressure sensor, fuel pressure regulator, fuel pump. Sorry to report such a non-specific list, but most of those can be evaluated before throwing money at parts non-specifically. However the cost of such testing will exceed by far the replacement of the oxygen sensor. A competent mechanic with a diagnostic test tool can probably do some measurements to identify the specific parts that is causing the fault.
Interestingly, if you then look at the decision tree it says to take these steps in order:
Check fuel pressure, make sure fuel level is at least 1/4 full (did the car run out of fuel when the code was logged), check out other fault codes first, with engine warmed check for leaks between engine and O2 sensor, check if the 02 sensor is switching between lean and center and rich at least twice in 20 seconds (that is normal), if you have a DRB see what is the long term adaptive memory % value.
Then check secondary ignition with a scope, spray wires and observe scope, disconnect plugs one at a time observe scope, is secondary voltage at least 25kv.
Then check the coolant temp sensor (is it 7 to 14K ohms at 70F and 700 to 1k at 200F).
Then check the MAP sensor and see if it changes
signal and is accurately measuring the vacuum (tee in a vacuum gauge).
Then do typical check for engine mechanical:
read vacuum at idle, valve timing, engine compression, camshaft lobes, check crankshaft sensor slots for debris/deterioration, check exhaust system for restrictions, check PCV system, check drive sprocket timing, check torque converter stall speed, check for power brake vacuum leaks, no contamination of fuel, check for plugged or restricted injector or crossed wires thereto, check the resistance vs temp of intake air temp sensor (spec similar to that of coolant sensor).
There is a 16-step procedure which involves the use of a higher level diagnostic readout box, in the manual. It covers 5 pages which I can copy and postal mail to you.
Roland
