I'll make the intro short and sweet:

Just past 160,000 miles last week.
Decided to road trip as celebration - destination: Arkansas diamond mine state park (somewhere about 750-790 miles one way)
quick pre-trip at fuel stop in Springfield, Illinois, and noticed oil cap was GONE!! (ohh f---)
Had 5qt mobil-1 10w30 jug at hand for oil change. topped off, cleaned engine bay, sourced an oil cap (with help of a Honda dealer nearby, via phone, at an auto-zone), put rag over oil cap 'hole' and babied 3 miles.

Decided to continue trip.
No ticks, rattles, or clunking from engine. oil checked at next fuel stop (went 306 miles till*)
Oil level barely registering on dip stick.
engine knocking audible from inside car only. normal ticking at idle outside of car.
Shut down for an hour, as i was convinced oil starvation has occurred and engine is on slope to death (baring failure)
Topped off oil again; started car to let idle and "knocking" still audible from inside car only.
was convinced by a few to continue trip (was only an hour and a half away from destination)

After 2 miles of driving: "knocking" went away.

Oil consumption persisted at an increased rate over normal.
From Murfreesboro, AR back to Rockford Illinois, I had to top off at every fuel stop.
Knocking only occurred on on-ramps and off ramps at over suggested limits (always drive over suggested limits on turns...)

Compression test results (cold engine)

cylinder #1~200, #2~205, #3~155, #4~200






I'm certain i'll have to drop the oil pan and inspect and, curiosity has me, open valve cover and inspect.

Not entirely sure what mess i have on my hands just yet.

Possible suggestions?

 

So what do you suppose happened? Was it really low on oil when you found the cap missing? I wouldnt think that just driving with the cap off would cause this.

 

My guess is that i hadn't tightened the cap enough when changing the oil, after bringing it out of storage.

It was quite low on oil when i discovered the cap missing - as most of it splattered all over the hood and engine bay. Not really sure how long i was driving like this.

Being low on oil, as it was, can/will cause damage (oil starvation, really).

 

The knocking you hear could be your "lost" oil cap. Ive read a few threads on here where people have lost their cap only to find it between the firewall and engine after diagnosing rattles..

Every honda ive owned, including my f20c, burned mobil 1 like crazy.

The compression on #3 is not good.. But it could be unrelated. Find that oil cap

 

before tearing it apart, do a leakdown test to see where it's going, out the valves or into the crankcase. If into the crankcase the starvation scuffed the number 3 piston, and that's probably your increased consumption.

been running mobil 1 0w40 in my S. Bought it with 112k on it, started at around a quart every 250 miles (adding before it got a half qt low). After 3 3k changes it's up to a quart every 1200 miles, if I beat on it, consumption increases.

When I got it, the previous owner only ever used 10w30 cheap dino oil. I think the M1 is slowly cleaning out the crud left behind, as the first change, the oil blackened very fast, now it stays cleaner for a longer period.

 

I'd do an oil analysis, to check for metal. Damage from low oil situation.

And a leak-down test on Cyl #3. Low compression maybe due to an unrelated symptom. Like burnt valve(s), from too tight adjustment.

 

Something is going on with those compression #s and that oil consumption, but I like the idea that the knock could be the oil cap. It would explain why it is more audible inside the car and seems random. That cap had to have gone somewhere.

 

I would venture the knocking sound you heard was when the oil cap was removed but that something is wrong with cylinder number three are you saw low compression