2004 Thunderbird 82 000 miles. The car ran great last summer, stored for the winter. This spring I put in a new battery. Now the car stalls at idle just like you turned the switch off. At highway speeds it sometimes has a loss of power. The car must be slightly warmed up before these problems show up. The check engine light comes on and code p0174 lean bank 2. Here is a list of things I have done. New plugs/coils, new fuel filter, new oxygen sensor sensor 2,1 smoke tested engine, checked the wiring, reseated grounds, cleaned the fuel injectors. Cleaned mass airflow sensor, then replaced. Sent the engine cpu to flagship one for repair. They sent it back saying no problem found. I don't really know where to look next, looking for suggestions.
A smoke test is fantastic, but it has a massive blind spot that perfectly aligns with why your car behaves normally when cold but stalls once it warms up.
Here is exactly why a smoke test will lie to you, followed by the modified, corrected advice for your
P0174 Lean Bank 2 issue.
Why a Smoke Test Can Give a "False Pass"
1. Pressure vs. Vacuum (The One-Way Valve Effect)
A smoke machine works by gently
pressurizing the
intake manifold with a low-pressure stream (around 0.5 to 1.5 PSI) to push smoke
out of cracks or gaps.
However, when an engine is running, it creates a powerful
vacuum (roughly negative 9 to 10 PSI),
sucking inward. A cracked plastic
intake manifold or a brittle rubber gasket can act like a one-way reed valve. When the smoke machine pushes outward, the gap pinches shut and seals tight—showing zero leaks. The moment you start the engine, the vacuum sucks the gap open, pulling in unmetered air and leaning out Bank 2.
2. The Thermal Expansion Blind Spot
Most shops and DIYers perform smoke tests on a stone-cold engine. Your symptoms specifically state that the car runs great until it is
slightly warmed up.
As the aluminum cylinder heads heat up, they expand at a different rate than the plastic
intake manifold. This thermal expansion pulls the manifold away from the gaskets. A cold smoke test will look completely airtight, but a hot engine will have a massive gap feeding unmetered air directly into Bank 2.
Modified Battle Plan: P0174 Lean Bank 2
Taking into account the battery change (which wiped the computer's memory of how to compensate for the leak) and everything you’ve already replaced, here is your updated diagnostic checklist:
1. Re-Test the Intake Manifold—Directly under Vacuum (The "Propane Test")
Since the smoke test cannot be trusted here, you need to test the
intake manifold while it is actively sucking in air and warmed up to the point of stalling.
- The Method: Get the engine warmed up to where it starts idling poorly. Take a can of regular aerosol carburetor cleaner (or an unlit propane torch with a rubber hose attached) and carefully spray/direct it around the lower intake manifold gaskets specifically on the driver’s side (Bank 2).
- What to watch for: If the engine idle suddenly smooths out or changes speeds when you spray a specific spot, you have just found your leak. The engine is sucking in the chemical/gas as an alternative fuel source, momentarily correcting the lean condition.
2. Isolate the Brake Booster Vacuum Line
The
brake booster utilizes a large volume of engine vacuum and connects right near the back of the intake. An internal tear in the
brake booster diaphragm can leak massive amounts of air
inside the car's cabin behind the brake pedal. A standard engine-bay smoke test will completely miss this.
- The Method: While the engine is warm and idling poorly, use a pair of hose-pinching pliers to completely clamp shut the thick rubber vacuum line leading to the brake booster. If the engine instantly smooths out and stops trying to stall, your brake booster diaphragm is ruptured and acting as a stealth vacuum leak.
3. Verify Live Fuel Trims
If you have access to a basic
OBD-II scan tool that reads live data, look at your
Short Term Fuel Trims (STFT) and
Long Term Fuel Trims (LTFT) at idle versus highway speeds:
- If the fuel trims are highly positive (+15% to +25%) at idle but drop down close to zero when you rev the engine to 2,500 RPM: This is a definitive, undisputed signature of a vacuum/intake leak.
- If the fuel trims stay bad or get worse at high RPM: The intake is fine; you are dealing with a fuel delivery issue (like a weak fuel pump breaking down under heat).
How to Find a Vacuum Leak with Fuel Trims
This video explains exactly how to look past a misleading smoke test by using an
OBD-II scanner to read live fuel trims, which will immediately prove if your
intake manifold is secretly leaking air when hot.
