I have a friend who has had a 2000 Miata since new. She is now widowed and I am helping with some decisions. In their repair records, the car required a main seal at 65,000 miles. She is being told it needs another now at 125,000. In addition, the valve cover gasket has been replaced about five times. Would these be symptoms of something as easy as a PCV valve? I also realize that I need to learn whether the pair is just very picky about any oil leaks whatsoever. Or, would high-mileage oil or another chemical be the first thing to try?
You might check on Miata forums, if it’s a common problem it’ll be discussed there.
Valve cover and rear main seal are pretty common leaks. Gaskets will be dried out on a 20 year old vehicle. So, I’m not surprised that they’re leaking now. It is surprising that those repairs have already been made (numerous times with the valve cover). They may be picky about leaks too, like you said. There’s always a chance the past repairs weren’t done correctly. It wouldn’t hurt to change the pcv valve either way. I would do that, and replace the valve cover gasket. I might or might not do the rear main, depending on how much it’s actually leaking at this point. We owned a Camry with a rear main seal leak and didn’t repair it. Just not worth pulling the motor for a few drips of oil. By the time it hit 200k miles, the leak had gotten to the point that it needed repairing, but we traded the car around 208k anyway. I don’t really put much faith in the high mileage oil or additives. Although I imagine they can slow a leak for a bit by swelling the seals.
How much oil does the car lose per month? Small leaks might be worth monitoring but not fixing at this time. Based on mileage, this recommendation is about as long as the first change.
I’d just monitor the oil level and keep it topped off. Certainly not worth the money to pull the engine on a 22 year old car.
By “main seal”, do you mean the front crankshaft seal?
No Miata experience, but in my diy’er experience crankshaft seal leaks aren’t common as long as the oil and filter are changed frequently enough. Valve cover (camshaft cover) seal leaks are much more common. If the routine maintenance has been done per schedule, correct fluids etc, needing 5 valve cover gaskets and the main seal leak could imply a problem with the pcv system. It could also indicate there’s excessive blow-by in the cylinders due to piston ring problems. That can over-pressure the crankcase and over time cause seals to leak. Replacing the PCV valve makes sense. You’d need a shop to do a complete PCV system test and a test for excessive blow-by. Might start with a simple compression test.
It’s also possible the owners were very sensitive to oil leaks. Most 20+ year old cars are going to leak a little oil here and there.
We are talking about the rear main seal. I need to check more on the owner sensitivity question. This is an unknowledgeable owner with love of the car and high expectations, when it is probably time to sell the car to a car nut.
Blowby: I know what this is, but how is it tested for?
If there’s excessive blow by from engine wear, the engine should be burning oil. As far as checking for blow by, I don’t really know. Compression test, I guess? If the pcv hoses are dripping with oil or the intake manifold is soaked with oil, I think that would indicate excessive blow by.
Miatas are known to leak at the rear of the valve cover gasket. The factory service manual makes it clear that there must be sealant in 4 places along the rear edge, and even then if the mechanic isn’t getting it right, it’s going to leak. Ask me how I know! This is not a reflection of anything wrong with the engine except a design flaw. That leak will also mimic a rear main seal leak as the oil drips from the same places.
Old Miatas hold their value if they are in reasonable shape, and the cars are fundamentally solid, durable little beasts.