Another option is to purchase an OBDII plug in, I have the ScanGuageII. Amazon approx $130.00 on sale. Plug it into your OBDII port, typically found under the dash near the steering column. You can customize it, adding your engine type; liter, fuel tank capacity and throttle position setting just to name a few. Once set up you can monitor the engine with real time fuel economy.
I like the scan tool feature that stores faults along with the perameters when the fault occurred. Sounds like this might help isolate your problem or at least point to a general direction.
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