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If the exhaust is lean, engine control responds by enriching the fuel to air ratio. QA vacuum leak no at the intake manifold can cause this, like a hole in the large accordion style hose at the AMM downstream to the throttle body. Or the PVC piping has failed and leaks vacuum, or the AMM has gone wonky and engine control applies the ultra fuel rich 'limp home' mode.
Also, is the silver accordion preheater hose between the exhaust manifold heat shield connected the air filter box? If you have not replaced the air filter box air temp thermostat, only heated air passes through the air filter box and AMM / MAF and can ruin the AMM. However the this failure appear to cause a lean running condition until engine control can't tolerate the fault and goes ultra rich limp home mode and sets a check engine light.
https://www.fcpeuro.com/products/volvo-engine-air-box-thermostat-1266826
Air intake manifold vacuum fuel pressure directly effects the intake manifold vacuum controlled fuel pressure regulator. With your foot off the gas pedal, fuel passes more easily from the injectors, with 'high' vacuum air intake manifold. Flooring it reduces vacuum to closer to ambient pressure and causes the fuel pressure regulator to restrict fuel passing to the return back to the tank.
The fuel pressure regulator fails in a few ways, usually unresponsive to the air intake manifold pressure, whether the FPR leaks or not, whether you floor it (high fuel injector pressure) or coast or are stopped at a light (high pressure, leaner fuel ratio)
- Restricts fuel return so fuel pressure is higher than should
- Allows too much fuel to pass
- May leak through the vacuum line with or without these other two problems
You checked fuel pressure?
No brake drag?
The oxygen sensor you originally testes was old.
And you are certain the odometer is accurate? Or do you use GPS to verify mileage? Old odometers may be unreliable narrators over the miles.
Hope that helps.
Quiet Veggie Black Bean Soup Boyeeeeeeeeeeeeeeeeeeee
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Beh.
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