Hey everyone, I am new to this board, but I did a search and could not find an answer. I don't have a bonneville myself, but my grandfather has a 1995 bonneville, not supercharged with 120,000 miles on the motor and he is experiencing this issue: he has to add almost 2 quarts of oil between oil changes, but the oil is not being burned off and its not leaking. I know its not being burned off because there is no blue or white smoke coming out of the exhaust, just light water vapor on a cold day. I recently changed his spark plugs, and while they were very worn, there was no oil residue, no soot, and no ash of any kind. I know its not leaking because, I would see the oil drip underneath the car, and there is no oil in his driveway. I lifted his car on stands and looked underneath, and there are no traces of oil of any kind. This leads me to believe that the lighter mollecules in the dino oil are evaporating, especially because when he has to add oil, the oil in the crankcase is dark, and seems to be thicker than normal. He changes his oil every 3,000 miles or so. He uses Mobil 10W 30 motor oil. Otherwise, the engine and the car runs great, like its brand new. Does that sound like a possibility, and what would you all recommend to do to fix this problem. Thanks everyone.

Look in the radiator for oil traces, the oil has to go somewhere and the engine would have to run very hot to vaporize the oil?
Have you done a compression test?

The antifreeze is clean with no oil in it. I have not done a compression test, but the car itself runs great so I don't suspect a loss of compression. Also, when I replaced the spark plugs recently, other than being worn, they were in great shape. By that I mean that they were clean with no oil residue or ash of any kind. My first car burned a little oil, and the spark plugs had some oil residue on them. I did not need to add any oil to that car between oil changes however. Someone told me that the pcv valve might need to be replaced. Its never been replaced before. Could it be causing the problem?

I don't know how that could make you lose oil.. but change it man, if it doesn't rattle, that could be the cause. If the pcv valve [old one] rattles then it was still working [replace it anyways, you've got it out], otherwise, lets wait for the smarter BC members to pipe in.


-justin

-also, thinking about the PCV valve, do this quick test: with the engine idling, remove the oil filler cap, checking for possibility of vacuum in the crankcase.

For what it'* worth, I had an engine years ago that started consuming oil like crazy . Just like you, no matter what I tried, I absolutely could not find out where it was going. -that is, until I completely disassembled it. I found that the "oil control rings" were stuck in the piston grooves. -that is, they no could no longer flex, or move in thier grooves.

The compression test showed nothing wrong (-because the top two rings (compression rings) were still just fine). The bottom line was that the engine was certainly burning the oil, and it was burning it so efficiently that it didn't even smoke. (-it was determined that excessively high combustion chamber temperatures (caused by excessively lean fuel/air mixture) was what caused the oil rings to stick.)

Also, I've used synthetic lubricants in everything I've owned for about the past 20 years, and my experience shows that they do not thin out as much at higher temps. -which, of course, should mean somewhat less consumption. (but I can't prove that). However, they certainly do not have as much high temp "boil off" problem as conventional lubes.

If it'* not leaking, then it is burning it. It definately is not evaporating.

My old 88 Royale (same engine) would consume about a quart every 300 miles the last 20k of its life. PLugs were always really clean, exhaust pipe had just a very SLIGHT residue and the motor ran smooth, strong and quiet. If for not having to add a quart every 300 miles, ya never woulda known anything was wrong. Never knew what was wrong, traded it at 201k. My father followed me on a 70 mile highway trip during the last days and said that when I got in it REALLY hard, it would puff a really LIGHT blue smoke. He could tell I was on it because of the rate I would leave him behind LOL. These engines really are incredible.

Yeah, mine goes through a quart / 3000 miles. However mine is leaking.


-jusitn

Well, thanks a lot for your replies everyone. I guess I'll change the pcv valve tomorrow and hope that the problem is not too serious.

I agree that you are probably burning the oil, it'* not enough where it would really be noticeable. A crankcase ventilation problem can make the car burn more, or it might just be going through the usual routes, piston rings and valve guides.

Old oil will burn off quicker than new oil. So just because you added 2 quarts in the last 3k miles does not mean you shouldn't change it. You might want to try a step up in oil quality, like Castrol.