THSCC Forums

Recently (about 6 months ago) my 1999 Miata's engine began using oil. It went from not using a drop to 1 qt between oil changes to 1 qt every 1000 miles.

It seems to be getting progressively worse. Most oil gets burned during highway driving, and when I am in higher revs. So a drive to Greenville and back, plus the autocross brought me down 1/2 qt :(

I was speaking with a few guys at the event last weekend, and they said that it is most likely the valve stem seals.

I asked the guys at Sports and Compacts, and they said that they don't do this kind of work but they could get me a rebuilt engine...

So my question is: if this oil problem gets any worse, can this sort of work be done in a driveway over a weekend/3 days? I looked up this valve job manual on miata.net however I am not sure I want to do/need all the work shown there. I'd settle for new seals, since the rest seems to work fine.

There is nothing wrong witht the engine otherwise. It runs well and makes no weird noises. So I would like to keep it if the cost/benefit is there (and also not to take any chances with a rebuilt engine).

Thank you very much for whatever know-how and advice you can share!

Does it smoke more when you start it up? That is usually a sign of valve seals, but not always. Valve seals are relatively easy to change and cheap compared to rings. Compared to a V8, yours are few and easy to get to.
My Camaro had an oil burning problem at high revs and a catch can solved the problem. The engine was sucking oil through the PCV system when I lifted after being at high revs.
A look at your spark plugs could give evidence of the source of the problem.
Some sweet Miata guy has probably had the same problem and can help.

_________________
Rubbin' is Racin'

You can do a compression/leakdown test and see where you're losing oil. Or you can do what I'm doing, ostrich your head in the sand and just add oil every thousand miles.

_________________
Has no responsibility whatsoever.

As Walter alluded to, I'd check your PCV valve if it hasn't ever been replaced. The NB Miatas have been known to pull oil past an old valve right into the intake system causing a mystery oil leak. It sounds like your problems happened fairly fast and considering how bulletproof the Miata engines usually are, I'd go ahead and replace it to see if that is the issue.

It could be a cheap and easy fix...

Jim

I have not yet seen any smoke coming out of the exhaust, and AFAIK, the spark plugs are clean. They have just been changed about 6,000 mi ago, so I am not sure how excessive the problem would have to be to show there.

In the meantime I have scheduled a leak-down test at Apex Vintage next week, they said they should be able to figure out where exactly the problem lies.

The one part I particularly don't like about this is how fast the oil consumption has grown. If the trend continues, I will need a quart a week in about two months

You'll also find on the NB Miatas that if they are burning oil, you won't see much (if any) smoke out the tail pipe. You'll wind up killing your catalytic converter at some point but they seem to be rather adept at stopping oil smoke.

Put one of these in the car and see if it helps before doing anything expensive.

Jim

Could it be leaking somewhere? No smoke with that much comsumption is strange.

_________________
Rubbin' is Racin'

There are no visible leaks anywhere, and I make sure to look at the underside of the car each time it's on the lift for an oil change or whatever. It's all dry. Even the mechanic at Sports and Compacts said that it is unusually clean and dry for a car that age.

I guess changing the PCV valve can't harm, but I'll also have the Apex Vintage guys perform the leak-down test on the engine. It's not a fortune to pay for a peace of mind, and if there really IS something, I'd rather fix it.

I ordered the part and hope to be able to install the new PCV valve on the weekend.

Good luck.
The leak down test will let you know if the problem is valve seals or rings. I'm pulling for the PCV!

_________________
Rubbin' is Racin'

Well, here's the latest update: I did swap out the PCV valve last week and just got back from Apex Vintage where they performed a leak-down test and found my rings in perfect order! Yay! They found all four cylinders below 10% leakage, within 92-98% range. So it's not the rings.

Now I'll have to see whether the PCV valve makes any difference. If it does, I still consider it not a waste of money to know that my Miata's cylinders are sealing well.

Another positive was that I got to goof off work for an hour this morning :)

That's good news. Did they say anything about the valve seals?
Bad rings equal an expensive repair, usually and engine rebuild. Valve seals are cheap and not hard to replace.
I still hope it is just your PCV.

_________________
Rubbin' is Racin'

They said that the valve stem seals and/or valve guides would be their guess for what's causing the oil use. They recommended I run through a couple cans of Techron to see if that makes any difference (in hope the valves would seal better I guess). When I asked them for a quote to fix the valve seals and/or guides, they said that if they were to pull the engine head, they'd also recommend machining the valves etc. and for that price I can buy an awful lot of oil... :)

My thinking here is that with the engine in good enough shape it can be a worthwhile investment to change the seals though... Will need to save up some money for that though :)

If your stem seals/valve guides were leaking that much oil, you'd be a literal smokescreen off throttle.

When you start up the car after it sat overnight, do you get any smoke? If not, then valve guides or stem seals likely aren't it.

BTW, for a lot of cars, you can do the stem seals/guides with the head on for a inexpensive fix. It's usually more work/time consuming, but if it's not your only car, worth trying. - AB

_________________
'14 Toyota Sequoia Platinum 4WD
Super Westerfield Bros - '93 Integra - LeChump Du Jour
STX 93 - Scion FR-S

It *is* my only car, so I am not sure I'd want to attempt this in my condo's parking lot :)

However, a quart burnt over 1000 miles does not seem to be such an awful lot -- it is even within new engine specs (at least some manufacturers say up to a quart per 750 miles is normal). I am concerned because it did not burn any oil before last Fall.

There is no smoke ever coming out the tailpipe, but Jim Feinberg said earlier in this thread that Miata's cats are good at stopping smoke.

I'll have to see if the oil level stabilizes now with the new PCV valve. If it does, then that was it :) If not, I'll have to do something about the valve seals soon-ish.

I think if you were leaking that much oil past the valve seals, you would probably see some smoke on a cold start. The stock Miata cats are good at trapping oil smoke when they are warm/hot but I tend to doubt they would stop that much when cold. You might have somebody else start your car when it is cold and stand behind it to verify there isn't any initial smoke.

From the symptoms you described and the results of the leak down test, it sounds more and more like a bad PCV. You might think about cleaning out your intake manifold as there is bound to be some oil residue in there if the PCV was bad. A gummed-up intake manifold can cause other minor problems on occasion and may wind up eventually throwing a check engine light. I believe there are step by step instructions in the garage section of miata.net.

If you want some real smoke, try running a little SeaFoam through the engine. It will help clean out deposits left by the burned oil and get rid of any pesky mosquitoes in the area.

Jim