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Why was the tank replaced and did you also replace the fuel pump?
How often do you use your 4wd?
If you are positive there are no oil leaks, try replacing the PCV valve. Seems like GM had a recall on the PCV years before that but it's a cheap part and if it sticks open, it will burn off oil vapor by drawing it into the intake and leave no evidence. A good valve will rattle freely when removed from the engine and shaken. Another test is to remove the vacuum hose from the valve while the engine is running and plug it with your finger. The rpm should drop by about 50 so just a slight drop in rpm with the PCV valve removed from the system. If there is no change in rpm, the valve is stuck closed. If the change in rpm is greater than 50, the valve is stuck open.
Are you still having problems with your Malibu?
How's the oil pressure when cranking? Does the oil light go out? Do you have NO compression or LOW compression?
I believe you said you had the tires balanced and the wheels aligned. Did you also have the tires rotated? Assuming the alignment was done correctly, this will be due to a bad tire. A tire can develop an internal chord separation that cannot necessarily be seen from the outside. This can cause what is called a tire pull. A tire with this problem cannot be repaired but must be replaced, however, sometimes if placed in a different position on the car, the pull will not be as noticeable. This will be one of your front tires. Try swapping the tires on one side (front to back and back to front) and see if it helps.
If the fuel filter was old, what else might be old? This concern could be caused by a lot of things; worn spark plugs, spark leak or high resistance in the plug wires, dirty mass airflow sensor, plugged air filter, damaged catalytic converter.... the list goes on. Do you have and trouble codes stored? Are there any other symptoms? How does it start? How does it idle? Is it worse when the engine is warm or cold? There are many possibilities so please provide some more information and we will try to help you out. Go ahead and check the transmission fluid. Should be pink and clear. Put some on your finger and rub fingers together. Feel any grit? I only say this because you just purchased the car. I do not believe your problem is in the trans.
You will likely need more than a cold start. There is a certain specific drive cycle required to run the monitors. For some reason, most techs don't do this but it will actually verify the repair. Here are the steps in case you want to try it yourself. 1. Cold Start. In order to be classified as a cold start the engine coolant temperature must be below 122°F and within 11°F of the ambient air temperature at startup. Do not leave the key on prior to the cold start or the heated oxygen sensor diagnostic may not run. 2. Idle. The engine must be run for two and a half minutes with the air conditioner on and rear defroster on. The more electrical load you can apply the better. This will test the O2 heater, Passive Air, Purge "No Flow", Misfire and if closed loop is achieved, Fuel Trim. 3. Accelerate. Turn off the air conditioner and all the other loads and apply half throttle until 55mph is reached. During this time the Misfire, Fuel Trim, and Purge Flow diagnostics will be performed. 4. Hold Steady Speed. Hold a steady speed of 55mph for 3 minutes. During this time the O2 response, air Intrusive, EGR, Purge, Misfire, and Fuel Trim diagnostics will be performed. 5. Decelerate. Let off the accelerator pedal. Do not shift, touch the brake or clutch. It is important to let the vehicle coast along gradually slowing down to 20 mph. During this time the EGR, Purge and Fuel Trim diagnostics will be performed. 6. Accelerate. Accelerate at 3/4 throttle until 55- 60mph. This will perform the same diagnostics as in step 3. 7. Hold Steady Speed. Hold a steady speed of 55mph for five minutes. During this time, in addition to the diagnostics performed in step 4, the catalyst monitor diagnostics will be performed. If the catalyst is marginal or the battery has been disconnected, it may take 5 complete drive cycles to determine the state of the catalyst. 8. Decelerate. This will perform the same diagnostics as in step 5. Again, don't press the clutch or brakes or shift gears. If this sounds like something you don't want to try, just drive the car normally for a couple days and the monitors should run on their own. Just make sure you drive it in town as well as on the highway. Good Luck!
Try this site. Just click on "Free TSB's and Wiring Diagrams" and agree to the terms. Then select "Wiring Diagrams" and enter your vehicle information. Let me know if you need additional help. Good luck! http://www.wilsonautoelectric.com/index.html
By "heat actuator" are you referring to the electronic actuator mounted to the air box inside the cab?