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Miller cycle engine smokes, eventually runs rough…Replacing spark plugs and valve cover gaskets fail

Car smokes. Gets worse over years. Miller cycle engine. Assumption: Oil leaks into cylinders. After many months, starts running very rough. Worst when warm once it starts running badly.

According to mechanic, when it starts running rough, spark plugs have fouled out (due to oil, presumably) and need replacing. So this time it started running rough, we replaced fouled out spark plug and (expensively) replaced all valve cover gaskets. But this time, unlike all other times spark plugs were replaced, it still runs rough.

If mechanic was right, replacing the spark plug and sealing the gasket better should have fixed rough running, even if only briefly. But it made no change whatsoever. (He now says “Engine is tired. Replace the car.” Of course, he also sells cars.)

Because of the unique nature of this engine (it must be in a Mazda Millenia S-type sedan), this is one of the few situations where only a dealership will have significant experience working on it.

Think about it–only one one sub-model of a sparsely-sold, now-discontinued vehicle ever had this type of engine. Most mechanics have never worked on one, and are unlikely to have the specialized service manual needed for it.

You didn’t tell us how many miles are on this engine, nor did you share its maintenance history, but it is possible that the engine is at the end of its lifespan. In order to get a more experienced set of hands and eyes to evaluate the situation, I think you really need to take this one to the Mazda dealer.

My oops. Doing several things at once. 170,000, replace spark plugs every 6-14 months (more frequently as time passes.

Makes sense. I know that a Miller-cycle engine leaves the intake valve open during part of the compression stroke, so that the engine is compressing against the pressure of the supercharger rather than the pressure of the cylinder walls.

The Miller Cycle takes advantage of the blower pressure which is greater than the pressure that would be accumulated in the initial 20* +/- of the compression stroke. You likely have a failing blower.

Thanks, Rod! That seems like a plausibility to explore, for sure.

At 170K miles, the blower and everything else will be getting pretty tired…

Any thoughts on what would be involved replacing a failing blower? Very rough cost estimate range? We replaced the spark plug on #3, but it’s still throwing a P0303 code, which interprets as #3 cylinder misfire.

It’s also throwing a P1540 (air bypass valve problem) but the air bypass valve is hooked up and plugged in.

True enough, Caddyman. Having just replaced all the valve cover gaskets I guess I’m about equally interested in pouring a little more money into it to make use of the money already spent as I am giving up on it :wink:

If the throttle body is removed the blower screws can be inspected. If they are oily the seals are likely bad, likewise the bearings.

Wow…thanks, Rod! Looks like great advice.

Should I also check the compression in #3 where the cylinder misfire code is, or would that also throw a code if that was the problem?

It would be wise to confirm that the compression is good on all cylinders before investing in a new blower or any other significant repair.