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Welcome, I'm Chris (aka Moose), my normal schedule online is around lunch and after dinner past midnight.
Can you go into more detail?
The best way to find a vacuum leak is with a can of flammable brake cleaner. Spray shot burst around all seals, hoses, gaskets on , and to and from the intake manifold. If you spray a leak the rpms will change indicating you have sprayed the source of the leak. can you try that.
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When you apply the throttle the vacuum goes down. You may have heard the air flow being sucked into the now open throttle body.
I have the steps to remove and install the intake if you want to replace those gaskets and see if that helps. Is that what you want?
My next step would be a power balance test to see if one cylinder is weaker than the others. Confirming those cylinders are getting good spark, checking those cylinders compression, running a short term fuel trim test to see how lean it is running, checking the fuel pressure.
The MAF sensor tells the engine how much oxygen is coming into the engine. The O2 sensors tell the computer how much oxygen is left over after combustion. Your running lean, that means to much oxygen is detected in the exhaust. Which means oxygen is being sucked into the engine form a source not metered or not enough oxygen is being burnt up inside the cylinder from a partial misfire.
Hello again Chris here,
See the links below.