Oil smoke from dipstick tube

Hey guys, I’ve got smoke coming out of my oil dipstick tube. 1996 Mercury Mystique 2.0L automatic, 62000 miles. I noticed a burning oil smell after a short (10 mile) drive, and checked the oil when I got home. A few wisps of smoke smelling of burning oil came out of the dipstick tube when I removed the stick. I was just curious how worried I should be about this. As far as I can tell, my cooling system works (radiator hoses are warm, good hot air from the vents, thermostat and coolant recently replaced, hoses inspected, no leaks) and I’m not thinking the burnt oil is caused by overheating. There hasn’t been any noticeable oil consumption, and I’m pretty diligent about checking the oil on this old car. Oil changes every 3000 miles since I’ve owned the car; dealer performed scheduled maintenance for the old owners.

Here’s my thinking: a worn piston ring allowing oil to be burnt and then sucked back through into the crankcase. When I changed my spark plugs, I did notice an oily residue on one of the plugs (cylinder #1), which also leads me towards a piston ring diagnosis (though I was hoping it was a valve seal causing the oily spark plug, or even a spark plug seal). Do you guys think a wet compression test is in order, or should I go ahead and get a leakdown test? I am going to buy a compression tester, but I do not have an air compressor to use with the leakdown tester; I’d have to take it to a shop.

I do need to service the PCV system (and would have done it today, but the parts hadn’t arrived at the store) – would a PCV problem cause the oil to burn, though?

If you guys aren’t thinking piston ring, I would love to hear any other suggestions you may have, and any comments on the urgency of getting it looked at by a real mechanic. Thanks in advance from this novice (but I’m learning a lot from you guys!).


edit: new theory, after doing some more productive homework: My PCV system is messed up and the blowby is normal; it just isn’t getting cleaned out effectively. This allows me to maintain some hope of not having to hire a machine shop, so it’s my preferred theory at the moment. Still going to get that compression tester.

I am fairly confident that replacing the PCV valve PLUS flushing the PCV hose with an appropriate solvent will do the trick. You might even want to go ahead and replace the PCV hose, rather than flushing it with solvent.

And, in answer to one of your questions, a gunked-up PCV valve can definitely cause increased oil consumption, oil leaks, and the odor of oil.

But, in addition to the PCV valve issue itself, I think that there is a bigger issue with your car, namely lax overall maintenance. If your car is really a 1996 model, and if it really has only 62k miles on the odometer, and if you really change the oil every 3,000 miles, then you are going FAR too long between oil changes. By my reckoning, that means you are changing the oil…perhaps only every 10 or 11 months, and that is not often enough with a car that is apparently driven only in local, low-speed driving. With your apparent driving patterns, you need to change the oil at least once every 6 months, and truthfully, every 4 months would be a better idea.

Under the circumstances in which you apparently drive, you need to change oil on the basis of elapsed time, rather than on the basis of odometer mileage, lest you gunk-up the entire engine, not just the PCV system. So, if you have really been changing the oil only every 10-11 months, I would suggest that you pull the valve cover and check for evidence of accumulated sludge.

Unfortunately, I think you will find a lot of sludge, and that does not bode well for your engine.


Good deal Sam. You might get another 100,000 out of that car. Good luck.

@VDCdriver Most of those 60000 miles were put on the car before I got it (two oil changes ago). It was driven by an elderly couple who then bequeathed it to their granddaughter (a college friend), from whom I got the car. And you’re right: I think they did do their maintenance based on mileage, not time. When I got it, I gave it a good tune-up: plugs and wires, changed all the fluids, fuel and air filter, cleaned the throttle and EGR valve. The PCV system was next on the list, a little delayed because it’s a pain to get to.

The car is used to run to the grocery store a few times a week, and also used for 1000-mile marathon driving (I work seasonal jobs in pretty distant areas).

I was planning on replacing the vacuum lines, but just out of curiosity, what would you use to clean them? Good old-fashioned carb cleaner?

The mileage vs. years issue occurred to me too, just as it did to VDC. This is extreme use and requires extreme maintenance.

Re: your theory; the burning oil doesn’t actually have to get sucked into the crankcase. The forces of the combustion pushes it in that direction. A little will normally escape past the rings in a process called “blowby” due to the extreme pressures of combustion combined with the fact that induction system pulling past the PCV valve keeps the crankcase space, which is connected to the valvecover space via the return passages, under slightly lowered pressure.

I suggest following VDC’s advice and letting us know how you make out. But I’m not sure it’ll really make a difference, as I’m not sure you have a real problem. You could always check the compression to determine the state of wear, but if the engine is running well I think you’ve just got an older engine and nothing more. Not a problem.

I commend you on your maintenance and monitoring habits. It’s too bad everyone didn’t keep track of their fluids and operation.

Back in the old days, all cars smoked from the oil dipstick tube, but this was before the days of Positive Crankcase Ventilation (PCV). @VDCdriver is right on with his diagnosis. IF the PCV system is working, there should not be smoke at the oil dipstick tube.
After you have fixed the PCV system, change the oil more frequently and take the car out for an hour drive or so on the highway every couple of weeks or at least once a month.
Also, be certain that the thermostat is working properly to allow the engine to get up to operating temperature.

@the same mountainbike Ah, that is reassuring! Thanks for the explanation about blowby. I hadn’t thought that a noticeable amount of smoke would be produced without another problem causing it, but I still have a lot to learn about cars! I’ll certainly let you guys know if the replacing the PCV valve and hoses fixes it.

@Triedaq You’re right, I should take it out for some long drives. At the moment, it has pretty severe usage: short trips a few times a week, and then cross-country drives every three-six months. The thermostat is only 1000 miles old but it couldn’t hurt to pop it in a pot of hot water (I’ll be draining the coolant next week anyway, to get at the PCV valve – stupid design).

Thanks everybody for reassuring me about this probably-not-a-real-problem!

“I think they did do their maintenance based on mileage, not time”

If I owned a car that probably only had its oil changed ~20 times in 18 years, I would change the oil again right away, drive it in a lot of highway situations for a few weeks, and then change the oil again. I might even do a 3rd oil change in a few more weeks, in the hope of cleaning up the oil pickup screen in the crankcase and cleaning out the smaller oil galleries in the engine. This may not work on 15 year old sludge, but I think that it is worth a try in view of the fairly low cost of the additional oil changes. And then, I would suggest that you change the oil every 4 months in the future.

And, in terms of what to use for cleaning the PCV hose, old-fashioned carb cleaner is probably fine. Just be sure to flush it a few times, or–just replace that hose.

@VDCdriver you’re right, oil is cheap and rebuilds are not. I hadn’t considered the sludge issue when I was doing the tuneup. I’m going to take your advice and change the oil again the next time I’m under the car. What do you think about crankcase additives like Seafoam?

Some Seafoam is also a good idea, but I wouldn’t run the engine very long with it in the crankcase. I would suggest adding some just before you change the oil, and simply drive the car on the highway for…maybe…an hour before draining the oil.

Then, as they say on the shampoo bottle, lather, rinse, repeat…in a few weeks, and perhaps again in a few weeks.

@VDCdriver one oil change down! I had the oil and filter in my trunk already, so I figured I might as well change it today. The oil was pretty brown and worn-looking after just 2200 miles - I think that must be due to the sludge we were discussing. I drove down to the store and picked up some Seafoam, too, and I’ll slosh some in in a few hundred miles.

Thanks for all the advice, guys. Like I said, I’m a novice, but I really like keeping things in good shape (I condition my leather work boots every other weekend, and I keep my knives sharp) and y’all have been super helpful and welcoming and patient with my simple questions. I really do appreciate it.

@Bluegill–That’s what we are here for!

What we are not here for are the folks who seem to be trolling with…rather unbelievable tales…and also the people who have preconceived ideas of what answers they want, and who then viciously reject any responses that don’t conform to their preconceived notions.

The moral of the story is to…keep an open mind…and continue to learn…as the regulars in this forum do, and as you are doing.

Please return whenever you have a question!

You’re doing great. Know how I know? Because you said “I really like keeping things in good shape” and you’re actually “walking the walk”. Caring is the biggest part of the battle. You can learn the rest.

@VDCdriver So, I think I’m having similar issues. Recently I’ve been able to smell burning oil and just yesterday when I turned off my car and pulled the dipstick I saw a little bit of white smoke come out. I drive a 1991 Honda Accord with 256k miles on it and I change my oil about every 2500-3000 miles which is about every 1 1/2 - 2 months for me. I planned to change it again in 2 days. Do you think the issue might be the PCV valve on mine also? And also, I’ve noticed that when I fill up fully on gas I’m able to smell a faint scent of fuel when I start my car. But after about a day after filling up, I don’t smell it anymore. Do you know what could be causing that?

I agree with VDC…While you are cleaning out the PCV system, don’t stop at the hose. Clean out the manifold fitting the hose is connected to so the entire system is restored to normal flow rates…

So do I probably need to do the same thing?

The hint of fuel that you’re smelling is probably some gas that gets blown past tired, worn rings that have lost their “spring” and collects as fumes in the crankcase. That combined with the hot oil fumes and possibly some exhaust fumes that are passing the rings during the power stroke are likely the source of the puff of smoke when you remove the dipstick and the slight gas smell. That would not be abnormal for a well worn, tired old high mileage engine.

But by all means get a new PCV valve. It might help, and it’s dirt cheap. The valve does allow fumes from the crankcase to get drawn into the induction system at a controlled rate, and it might help.

Meanwhile, just keep doing what you’re doing, make sure you monitor the oil between changes, and you should get a lot more useful life from the car.

Ok, thank you. I will replace it and the hose when I change my oil in 2 days and see what happens. I appreciate the help.

deleted by poster
I almost screwed up. :grin: