Intake "smoke test" for vacuum leaks

tails · Postby **tails** » Mon May 16, 2016 12:25 pm

Hi folks,

On the weekend I hooks up a boost leak tester to the intake of my 1992 MX-5 to see if it has any vacuum leaks. The tester was connected to the intake tube directly after the AFM and switched on. A leak was instantly found along that tubing (just after the ribs) and promptly tightened and sealed. After that how ever the tester wouldn't build any pressure...it turns out air was getting into the valve case (when opening the oil cap it would hiss and then a light amount of smoke would come out).

Was I doing something wrong or does this mean the PCV is stuffed?

On the extreme end, we thought it may mean that the engine is extremely tired and air is getting up into the valve cover past the rings. We did a compression test, which came up on the low end (though very even across all cylinders) but I'm not really counting it as the engine was cold and the battery wasn't being jumped. So still inconclusive.

I figured that if the engine was worn to the point where air can escape past the rings that easily, with only the pressure generated by the smoke machine, it would drive terribly, and for the most part, my cars runs pretty well.

Any thoughts?

Postby **NitroDann** » Mon May 16, 2016 12:34 pm

Looks more carefully at your intake tube and where it connects in various places.

tails · Postby **tails** » Mon May 16, 2016 12:51 pm

NitroDann wrote:Looks more carefully at your intake tube and where it connects in various places.

Thanks Dann.

Any idea on the air under the valve cover?

Postby **NitroDann** » Mon May 16, 2016 1:01 pm

Yeah, the pcv system, there are 2 hoses.

hks_kansei · Postby **hks_kansei** » Mon May 16, 2016 1:49 pm

Two PCV hoses.

One on the driver's side, small tube with a check valve that goes into the intake manifold post throttle plate.

Second tube is on passenger side, has no valve, and usually attaches to the intake crosspipe pre-throttle plate. (if the car has an intake fitted this one may have been replaced with just a filter, or piped elsewhere)

The valved one does most of the work of drawing crankcase fumes out, since it's on the intake manifold it's seeing high vaccuum more often than the other side, which only sees vacuum at high throttle positions. (my one just has a hose leading to a filter behind the guard)

tails · Postby **tails** » Mon May 16, 2016 1:58 pm

NitroDann wrote:Yeah, the pcv system, there are 2 hoses.

hks_kansei wrote:Two PCV hoses.

One on the driver's side, small tube with a check valve that goes into the intake manifold post throttle plate.

Second tube is on passenger side, has no valve, and usually attaches to the intake crosspipe pre-throttle plate. (if the car has an intake fitted this one may have been replaced with just a filter, or piped elsewhere)

The valved one does most of the work of drawing crankcase fumes out, since it's on the intake manifold it's seeing high vaccuum more often than the other side, which only sees vacuum at high throttle positions. (my one just has a hose leading to a filter behind the guard)

Thanks guys.

If one were to block off the passenger/exhaust hose, and the valve case was still filling with smoke, I guess it could be isolated to the driver's side with the valve? Can these valves even leak that way? My understanding was that when they leak, they leak into the intake, not the other way around (ie. into the valve cover).

Theoretically, if both of these hoses were found not to be leaking into the valve cover (or blocked off to isolate them during the test), where else could air be getting through from? Am I right in assuming that if it was coming up through the cylinders, then the engine would run terribly if it was that worn?

Postby **NitroDann** » Mon May 16, 2016 2:13 pm

If you want to know youve got to do the tests, in this case you need a proper compression and leakdown test.

tails · Postby **tails** » Tue Sep 06, 2016 10:05 am

Fixed all my issues in the end. Replacing the PCV (and both breather hoses, including the little restrictor on the hot side), fixing a vacuum leak on the intake tube and replacing the AFM seemed to do the trick. One of the previous owners had tampered with the AFM (the bypass bung had been drilled out), so I replaced it with a confirmed untampered unit.

Just thought I'd update the thread with what actually fixed my problem as there's a million of these threads on a few different MX-5 forums and not many of them post the fix.