Re: Can a bad Oil Filter Cover Gasket cause bad gas mileage?
I'd run a compression test just to make sure. The 5.4L engine is very hearty. I'm not familiar with the layout of the oil filter, but it should be connected to the block, if it's been relocated with a kit, there is still no coolant involvement.
You can test your coolant for ph balance, and pressure. The oil you see might be rust, or a mix of things. When coolant starts to turn brown, it's knowingly been neglected and the heater core is generally the first thing to start to rust. Best thing is to back flush the system. If you're certain there is oil in the coolant, then I would strongly advise a compression test to be certain the head gasket is OK. The 5.4L also protects itself from coolant loss/overheating under most circumstances. As said, it's a hearty engine and you don't want to be out somewhere only to find out the hard way that the head gasket is the source of the problem.
Also, keep in mind that the engine oil is under a higher pressure than the coolant. In some cases the head gasket will mix coolant into the oil.
'11 Ram Quad Cab 4x4 SXT 5.7L HEMI (stock); 48,500m
'06 Pontiac G6 GT 3.5L 220hp; 64,000m
'12 Chrysler 200 Limited 3.6L 283hp; 28,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
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