Problem with Intermittent Misfire, Worsens at WOT

#21

Jim,

I know the intake is connected via the breather tube, but my understanding was that it did not open into the intake tracts, only into the head, to permit pressure equalization. Which is why i’m wondering how so much smoke is getting in there. Unless it’s leaking at the center intake manifold gasket, of course. Are there any other paths from the intake to the crankcase?

Even if I had a sensitive flow meter or a 0-15/20 gauge, how would this help for a rather coarse system? I haven’t wrapped all the joints in my exhaust with tape, but I’m sure they’re permitting loss of air pressure because they’re slip fits with no clamps. Clearly, I have a gross leak at the vent tube and I’m wondering 1) whether that’s indicative of a problem and 2) if not, how to close it so that a sensitive flow meter and a pressure gauge would help. Unfortunately, removal requires pulling the intake and it’s also very difficult to reach. Even with a new set of seals, apparently it doesn’t seal well at all.

Matt

#22

The top of the head is open to the crank case via the oil return passages and oil return tube. The valve cover breather hose goes directly to the intake manifold (no pcv valve). So if you pump smoke into the intake it has no choice but to also flow into the block via the cylinder head.

The point of the pressure regulator & gauge is to make sure that the smoke pressure within the intake/engine is high enough to find the small leaks. But you don’t want too much smoke pressure as that can create a leak. The gauge lets you know when the desired pressure has been reached and then you make sure that pressure is held for the duration of the test. The flow meter will show evidence of flow going into the engine/intake before the smoke may have worked it’s way to a leak. The flow meter is nice to have, but not a necessity.

On a stock car the exhaust is essentially gas tight unless it has been damaged. On a Spec E30 that won’t be the case and the slip joints will have to be taped up.

As you may have figured out by not, while not rocket science, a properly run smoke test is not just “blowing smoke”.

The oil return tube, aka “bitch tube”, is no problem to seal. The intake does have to come off, but new oil return tube o-rings and intake manifold gaskets will get the job done. Unless you get real lucky, the bore in the block the tube fits into will need to be cleaned up with a brake cylinder hone and the ends of the tube run against a scratch wheel to remove corrosion. The intake manifold side is usually okay. For the spring and o-rings to work, the tube must slide easily in the block and intake.