check engine light is on and the motor shakes when idling

2014 Ford F-150 8 cylinder 5.0 shakes when idling

I recently purchased a 2014 Ford F-150 8 cylinder 5.0

It has high mileage 105k.

The check engine light is on and the motor shakes when idling though it pulls strong at higher rpm.

I had it checked the computer indicated an issue with cylinder #5. When tested further found that the compression on that cylinder was low 75psi the mechanic also put some oil in and tested again. Went up to only 100 psi.

The mechanic is discouraging me from trying to repair and recommending I try to sell quick.

Am looking for advise and a better explanation of what this could mean and cost?

Don't know if I need to park the car until I resolve one way or another ( mechanic said I could still drive).

Could it be so catastrophic I need to walk away?

Get Your question Answered:

The reason for the check engine light and the shaking motor is that a misfire was detected on the #5 cylinder. Now there can be many reasons for a cylinder to misfire, some of which are an easy fix--- like if it were some bad gas or one spark plug was bad. Other reasons could be more serious and expensive. That's where some testing comes into play to help us determine where the cause of the engine misfire is. When your mechanic conducted a compression test, this is basically testing how well the mechanical portion on this problem operates. The engine is just a big mechanical device that sucks in air, compresses it then blows it out the exhaust. The fuel and spark is what keeps it all going.When he performed the compression test, he found that the mechanical part of the engine was not working (compression was low) properly. He then squirted oil into the cylinder (this is what we call a wet compression test) to lubricate the piston rings and reran the test. If the compression test reading went all the way up to normal when we added the oil, then we would know that the piston rings were bad. When they didn't go up with oil sprayed in there, he knew that there was a mechanical fault that will require the engine to be pulled out and taken apart.That's why he recommend that you sell it now, because the next step in this repair process is to take the engine out and tear it down to see what went wrong. 

Is repairing the internals of the engine that difficult and or expensive ?

Yes and yes. Once the engine is out and torn apart, we should be able to see what the cause is. Most likely, unless you hear some loud knocking noise, the problem would be just in the head and valves. He would then send the head out to a machine shop to be checked and reworked which should run about $400 in machine shop invoice and about $1,000 in parts/labor from the repair shop.Now if the piston or block is involved that needs repair, you'll be looking at installing a good quality low mileage engine from a reputable salvage jobber like LKQ. They run about $2,500-3000 with about the same cost in labor to install it.