I was hoping for some feedback as to the best (price&reliability are important) way to diagnose and fix what is wrong w/ my 98 5-speed w/ 115k. There are two parts:

I did an oil change and then drove home and back 700 miles and when I got back, had very little oil, had to add about 2-3 quarts, no signs of leaks though. Since adding those 2-3 quarts, it has not lost a drop of oil, 3000 miles later. The mechanic suggested that it was leaking oil out the exhaust through piston rings...haven't had a chance to take it in again, any ideas? Should I wait until I do the 120k tune up, probably a little early?

2nd
My check engine light came on about 2-3 weeks ago and produced three codes, knock sensor, and front O2 sensors, both sides. Also, it has been 2-3 weeks since I reset the light and it hasn't come back on, does this mean there is still a problem? So could this be related to the first problem? If so, if that problem was fixed, would these parts need to be replaced? If it isn't related, are these codes related to each other? Will replacing one help the other? Sorry there are so many questions, but I don't know where to start and what to do, I would like to avoid replacing anything that may not need it.

Thank you for your recommendations.

 

1st. how long did you wait after you got home until you checked the oil?after a 700 mile trip i would wait about an hour for all the oil to drain back down into the oil pan. ive also heard that out dipsticks are not reliable but i am unsure if this is true. was the car on a level surface? also for reference it is 1qt between the min and max marks meaning that if you were 2-3 qts low you would not see any oil on the dipstick. also if you were that low i think you would see your oil light come on, im sure our cars dont have much oil pressure with 1-2 quarts of oil in there. a quick check for rings, pull the pcv valve and see how much air/blowby is coming out, of course for this you will need another max with a known good engine to compare.

2nd.
if you get a cel for an o2 and reset it and it doesnt return, dont worry about it, also the knock sensor is a "piggy back" code meaning that when you get an o2 code you most likely get a ks code also, dont worry about that, if the light stays on replace the o2(s) and you more that likely wont get the ks code again. if ks is bad you will get code by itself. worn rings can set 02 codes b/c there is oil getting into the combustion chamber and burning=bad emmisions/low oxygen content sensed by the o2. look for blue smoke at higher rpms (or anytime)is also an indication of burning oil either from piston rings or valve seals (which shouldnt be a problem for the vq, ive never seen one on the road burning oil.)

 

oh if your really worried about your rings just take it in for a compression check. should be about 80-100$ for a v6

 

If your car lost oil initially but then stopped for 3000 miles then I wouldn't worry about it unless it starts losing oil again. In order to "burn" three quarts of oil you would have been blowing out lots of black smoke through the exhaust. And it wouldn't burn oil for a while and then just stop....your piston rings are either bad or they aren't and at 115K I would bet they definitely aren't bad. I would guess maybe you had a leak after your oil was last changed ie filter not put on tight or drain plug leaking or they didn't put enough oil in the first place.
On the codes if your knock sensor is bad it won't set off the CEL. You have to check for the code to see if its bad. I guarantee that code is still there if you check it again. Knock sensor failure is common. I had to replace mine at 78k. The o2 sensors set the actual CEL off. I would bet that it will come back on again later because you probably need at least one of those o2 sensors. I would wait until it comes back and then replace the bad o2 sensor(s)and the knock sensor. Theres lots of info. in the FAQ's and HOW TO's on this stuff.

 

I did the oil change about 4 days before the trip and checked to make sure it wasn't leaking. When I checked it after the trip, the dipstick was dry, engine off, level ground. But it hasn't lost any since.
I will check for the knock sensor code then, I had to replace it a year ago on my 95, so I should remember how to do it. Thanks for the help.

 

did you remeber to replace the oil drain washer?