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I have a 2009 Buick Enclave CX with ~150k miles. I begin having issues accelerating/retaining speed going uphill. I had assumed it would be transmission related, but I had it scanned and the codes all pointed to piston misfires.

Seeing as the spark plugs hadn’t been changed, we started by changing all six. We also addressed a code related to the air filter on the front driver’s side. Since we had to remove the intake manifold, we also replaced that gasket.

With everything replaced we tried to start it. At idle, the rpm meter kept oscillating between 700 and 1300. After clearing codes and rescanning we got a code relating to an ignition coil. We disassembled again, changed the ignition coil, and reassembled. At this point the engine wouldn’t start. It would turn over when turning the key but it would not actually start. We then did a capacitive discharge thinking the computer may have needed a reset. This did not change anything. When we scanned again, we got no engine codes at all.

We are at a loss and would appreciate any input.
 

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Since it ran before you did the ignition coil, start there, take it apart just like you did to replace it and make sure every connection is rock solid. Check the plug wires make sure they are all seated correctly.
 

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From looking at what you started with, what you did and what you have now, I would say you may have a sizable air leak at the intake manifold. One coil or faulty spark plug will not prevent the vehicle from starting. The fact that you said the engine rpm's were oscillating leads me to believe you have an intake leak. They are oscillating due to the engine control module (ECM) is unable to correct the air mixture to stabilize the idle. If that leak (Assuming) is large enough the fuel mixture will be too lean for the engine to start/run. Try blocking off the air cleaner completely and see if the vehicle will start. If it does that's your problem. Also did you use AC Delco plugs? "Some" vehicles do not run well on aftermarket plugs. Keep us posted and we will try and solve your issue. Good luck.
 

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Also what spark plugs did you use as replacements? These engines do act up if the OEM or superior plugs aren't used.
 

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So many little steps to do this work. Let’s assume you got the manifold and gasket back on as it should be.
There should be a vacuum hose on the front of the manifold, about 1/2” and also one on top of the manifold for the PCV system. Did you connect the throttle body connector properly. You may have also disconnected the mass flow meter for the filter change, again is the electrical connector properly seated.
When you connected the large hose between the air filter and throttle body, are you sure the hose properly went over the end connections. Are the gear clamps tight.
Did you look in the throttle body before connecting the hose to verify the ‘butterfly’ was closed?
Lots of little items and again I’m assuming the plugs are properly installed with the proper gaps and the ignition coils are on the plugs correctly and the wiring connections are fully seated.
Good luck and let us know what you find and if you still need assistance.
 
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