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1999 Ford F-150 burns oil like crazy

I recently purchased a used f150, within 2 days of purchase my oil pressure gauge began bottoming out, and my check engine light came on. I took it to a local mechanic who found that a spark plug was just hanging, no charge from him. Two days later it overheats and so I changed out the thermostat, then the oil pressure sending unit, drained the oil pan, changed the filter and put in fresh oil. Since then when I go any farther than 30 miles away from home the issue starts again, now I find that the radiator is being drained . There’s no visible leaking, and I’m burning oil like crazy.

You visited the mechanic 3 days too late. Always get a pre purchase inspection by your mechanic on a used car.

So what did the mechanic say was wrong when you took it in? Based on the very little information you told us, I’d guess a blown head gasket or a cracked head. I hope you didn’t pay much for this truck because you are about to pay quite a bit to get it fixed.

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The mechanic had the truck three days and never had the same issue. I had hoped it wouldn’t be a blown head gasket but I figured as much. Thanks for your help

Depending, the problem could be a faulty oil filter adapter gasket. Coolant can mix with the motor oil and vice versa. Note in the pic that the 2 larger round holes are where the oil enters and exits the filter. The larger odd shaped opening is where engine coolant flows. The gasket is a fiber block with rubber inserts. Over time the rubber compresses, hardens, and fails to seal. This can be engine destroying.

https://www.autozone.com/collision-body-parts-and-hardware/oil-filter-housing-gasket/fel-pro-oil-filter-housing-gasket-70801/572199_0_11573

What is “crazy”? A quart every 100, 1000 or 5000 miles?

A quart every 100 miles or less

Next step is a dry and wet compression test. If it’s going through a quart per 100 miles or less and the oil is NOT being diluted by engine coolant then it sounds like the end is near. If it’s burning that much oil the catalytic converters and O2 sensors won’t survive at all.

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with that much oil burning the car should be leaving a huge plume of smoke behind while driving it.

There’s no smoke at all, that’s what I find confusing.

Oil burned in the combustion chambers will stick to the substrate in the converter and become soot. This will be discharged out the back as soot; not smoke. This also leads to clogging of the substrate.

Over the years I’ve cut a few converters open on oil burners and the substrate was roughly 30 to 60% clogged. This of course also leads to an engine performance drop along with converter/O2 sensor issues and EGR faults.