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FIXITMAN, Mater Technician
Category: Pontiac
Satisfied Customers: 17368
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Still trying to figure out why my 2005 Bonneville Gxp (LD8

Customer Question

Still trying to figure out why my 2005 Bonneville Gxp (LD8 4.6 Northstar) is reading lean fault codes. Po171& Po174 banks 1&2 lean, at the same time for a few months now. I've done endless forum searching and Google searching and haven't tried this site yet. I recently got it back from the GM dealership and they did some work to it but didn't fix it. They did a full inspection and there are no vacuum leaks, the fuel filter was replaced, and the entire fuel system including the throttle body, fuel rails and injectors were all cleaned by their machine, and i replaced the rubber intake boot. The ses light went away until the next day. It had the codes po171, po507 for iac rpm high, and po300 misfire. I cleared the codes and the light came back on the next day and still have the same old 171 and 174 but the other codes weren't there. I want to get this fixed asap and it's driving me crazy. My only ideas left are coils, faulty o2 sensors, or the intake manifold gaskets. I read on a forum that a guy fixed his by retourqing the intake gaskets as it was causing a vacuum leak. If the gaskets were bad wouldn't that mean milky oil? The oil is fine but I want to try the retourqing. I could really use some ideas or advice from a GM technician that knows about Northstar motors as well. Thanks
Submitted: 1 year ago.
Category: Pontiac
Expert:  FIXITMAN replied 1 year ago.

Hello, my name is***** you for the question. On this engine this is caused by one of three things most of the time. It is either low fuel pressure fro a failing fuel pump, a leaking intake gasket, or a bad air flow meter. I always check the fuel pressure first. If that is fine then a smoke test of the intake is next. Re-torquing the bolts may help, and no, a leaking intake gasket will not always allow coolant to leak into the crankcase. If all of these are fine then the air flow meter may need to be replaced. I hope this helps. Please let me know if you need further assistance.

Customer: replied 1 year ago.
I think tomorrow I am going to try spraying carb cleaner around the intake gaskets and then retorquing if necessary. I know the bolts torque to 89 in/lbs spec but should I loosen the bolts all at once and torque in a certain pattern, or loosen one at a time and torque in a certain pattern? Or do they not need loosened at all to retorque?
Customer: replied 1 year ago.
Also any input on the po507 Iac rpm higher than expected and the po300 misfire? I know that if po171/174 is set than the po171 po507 & po300 will not set at the same time vise verca.
Customer: replied 1 year ago.
Anyone want to reply? So far your answer states everything I already know so not really satisfied
Expert:  FIXITMAN replied 1 year ago.

Sorry for the delay in getting back with you. A vacuum leak will cause all of these codes because it makes it impossible for the engine to properly calculate mixture and hold a good idle.

Customer: replied 1 year ago.
I know that a vacuum leak will cause lean codes, but I have no idle issues and it idles and runs just fine. The dealership shop checked everywhere for vac leaks and didn't find any, I also sprayed carb cleaner around the intake manifold gaskets and no change in idle, so no leak. I hear no hissing or sucking or any signs of a vac leak. I've cleaned my maf sesnor, the shop cleaned the throttle body, fuel rails, injectors, and put in a new fuel filter. I'm not sure what I want to try next. Do you have any more ideas?
Expert:  FIXITMAN replied 1 year ago.

Pull the oil filler cap off at idle and see if there is a lot of suction. Most of the time if it is the intake it leaks on the bottom. What about the mass air flow sensor. Has it been tested?

Customer: replied 1 year ago.
I've done some reading and have found that a failing mass air flow sensor can throw off the pcm and cause these codes even if it is cleaned or cleaned improperly. I thought I cleaned it fine just sprayed it with maf cleaner and let it dry but who knows. I'm thinking of getting the maf sensor tested tomorrow. How does a maf sensor just go faulty all of a sudden? The car does have 173xxx miles and I'm sure it's the original one.
Expert:  FIXITMAN replied 1 year ago.

These can fail all of a sudden due to their fine wire design. The wires that pick up air flow and temperature change can fail easily.