Hey everyone -
I've searched around looking for a possible answer to this but have not had luck. Here's the story:
Last Summer I purchased a 2003 Frontier XE 4x4 V6 with about 138K or so on it. The previous owners gave me a stack of papers with all the repairs, maintenance, etc. Awesome people and they had most of the work done that I would have needed to get done (waterpump, timing belt, etc.) at this mileage.
It's been running great since I bought it - never had any issues other than me just getting used to driving a truck again and not a giant VW van or a tiny car. I know basic maintenance and whatnot and am able to do about 50% or so of my own mechanical work, usually it's access to tools that keeps me from doing something. Lame excuse, I know.
So I take the ol' girl up North on a road-trip to pick my girlfriend up from her parents in South Bend, IN. I live in Asheville, NC. It's roughly a 600 mile trip. Before I leave I check my oil (let the engine get warm, turn it off, rest 10 min and then check) and everything is good, I know I have about 8-900 miles before I need to change my oil, but figure I'll just do it when I get back home.
Flash forward 8 hours and I'm almost to my destination in Indiana when I hear a loud clicking while downshifting. Sounds like a lifter or valve or something. VERY loud. My engine has always made a slight ticking noise that just sounds like a lifter or valve, but you have to physically open the hood and listen for it. This sounded like that but intense - I could hear it in the cab with the windows up. I stop, fill up with gas, and let the truck rest before checking the oil. Dipstick looks good. I continue on my way and the noise comes and goes. Sometimes it gets quieter, but it generally stays about the same.
Upon arriving at my lady's house I scour the internet, this forum, etc. looking for answers. I have a few ideas but nothing substantial. I figure maybe it's just loud because I need to change my oil soon. I decide to take the risk (probably not the best idea) and drive to Chicago the next day to visit a friend. Upon driving back to South Bend from Chicago the next day, the clicking is so loud that it sounds like a damn diesel engine. I'm freaking out and take it straight to a Nissan dealer to have them look it over. Guy at the dealership starts talking about ripping the top end of the motor off to figure out what the problem might be, etc. I of course, am freaking out because I don't have the funds to have my engine torn apart or put back together.
Flash forward again, and several hours later, I get a phonecall saying that my truck is ready to be picked up. The mechanic slides a bill to me with a whopping total of $0.00 for putting 3.3 quarts of oil into the engine. He tells me that he checked the oil and found that it was 3.3 quarts low on engine oil and that when he put more oil in the noise dampened down to almost nothing. Of course he'd be more than willing to tear the engine apart and see what that noise is, but he knows I want to get back home to Asheville and dont have the funds. He says that he scoured the engine compartment and can't find a leak anywhere.
I drive the 600 miles back to Asheville and have no problems. I buy a ScanGuageII just in case and use it to watch for any codes (big surprise I have the bad knock sensor, haha) I check the oil every time I stop and don't see any difference. One time I swear it looks a little lower, but I think it's my mind looking for something bad. Yesterday I changed my oil and it was pretty black, but it didn't smell burnt and from what I could see there wasn't any metal or anything in there. I'm gonna plop a magnet in there to see, though. Make sure my engine didn't take a beating.
So basically, long story short - my engine magically disappeared 3.3 quarts of oil, practically the ENTIRE amount of engine oil in the oil pan, and I have no idea how or where it went. I've never seen blue-black smoke from the tailpipe, my air filter is free of oil, there are no drips underneath, my engine oil light never came on, nada....
I've heard of the PCV valve possibly causing this, bad valve gaskets, possibly (but often rare) the piston rings, etc. I also know that sometimes engines just burn oil, but it blows me away that my 28 year old Vanagon drove over 600 miles and didn't burn but maybe 1/2 quart. Or that my last car, a 4 cyl Scion xB didn't burn any oil while on a 3000 mile road trip and I've heard 4cyl are notorious oil burners.
So I guess what I'm asking is: what the hell? Does anybody here have an idea or suggestion as to what it might have been?! Something I should look for, etc? I apologize for the long winded description/story, but I figured I should give all details...haha
Additional info: I was using Valvoline 10w40 high mileage oil with a Bosch filter. I just Seafoamed everything and changed it to Mobil 1 high mileage full synthetic 10w40 with Bosch again, yesterday. On the drive North I was averaging 70mph at the standard 3000 rpm (which seems so high) and getting my usual 17-18 mpg. It was pretty cold the entire drive up, averaging around 30-40ºF outside air temp. On the drive back I averaged about the same, the only difference being that it was mildly warmer 40-50ºF.