Engine Smoking Troubleshooting

Last updated: April 26, 2000

Blue Smoke

Blue smoke is caused by burning oil. The question is where the oil is coming from, and how does it get into the combustion chamber.

One likely cause is an oil leak in the turbos.

Another possible cause is a bad MOP (Metering Oil Pump). Since the rotary engine injects oil into the combustion chamber as a normal part of operation, it may be possible for it to fail and start pumping too much oil or leaking it into the combustion chamber.


Date: Tue, 25 Apr 2000 11:39:23 -0500
From: "David Canitz" (dcanitz@royalpurple.com)
Subject: RE: (rx7) [All] Question about oil smoke and engine compression

These are some basic things to look for in 'turbocharged engine oil leaks' albeit these were written for 'turbocharged diesel' engines and may not all apply to the third gen.

(not specific to RX7's so stop reading now if you care to)

Common sources of Turbocharger Oil Leaks (As rewritten from an turbocharger rebuilder)

  1. Oil Inside the Turbo

    Engine oil under pressure enters the bearing housing through the oil inlet supply line. The oil lubricates the bearing and journal surfaces where it picks up heat from the exhaust turbine and as a result of oil turbulence internal in the bearing, air tends to get entrained in the oil. The oil then exits the turbine bearing sump through the drain line (in many cases a gravity drain line) back to the oil reservoir (crankcase). If any thing causes a restriction to this drain line, foamy, air entrained oil will backup into the bearing housing until it is higher than the oil seals on the turbine shaft. The oil (oil mist) can leak into the compressor side of the housing and get carried into the engine under boost and get burned with the fuel mixture.

  2. Types of Seals

    The seals at both the compressor and turbine ends of the bearing housing are typically designed to prevent the high pressure gases from entering the bearing housing and then into the engine crankcase. The prevention of oil entering the compressor and turbine housings is a secondary function of the seals.

  3. Oil at Compressor Inlet / Outlet

    A too small capacity air filter (or one restricted by dirt) can allow the air velocity through an oiled filter to carry over the oil into the compressor inlet. This would be evident only on the compressor outlet. Correction is to remove the restriction at the air filter.

  4. Oil at Compressor Outlet

    A dry type filter will have increased restriction as it becomes dirty. This results in a pressure drop across the filter. Under engine loading, a positive pressure exists at the oil seal behind the compressor impeller so the pressure drop is not noticed. However, in an unloaded engine, ie idle or at low rpm, a potential exists where a partial vacuum occurs at the compressor inlet and behind the compressor impeller simultaneously. If this condition is present for any length of time, it can cause oil to be sucked from the bearing housing through the oil seal and into the compressor housing and then into the engine intake. A differential pressure gauge mounted between the air cleaner and the turbocharger is an excellent method to measure too much intake restriction.

  5. Oil at Turbine Outlet

    See 'Oil inside the Turbo' above. To correct, make sure that the drain port is pointed down at a 35 degree angle and that the drain line does not have any loops or uphill type bends. This s-type bend (loop) could cause a buildup of oil in the line preventing oil draining to the crankcase. Make sure that the crankcase is not overfilled with oil. This could result in the drain line trying to gravity drain into the oil level in the crankcase causing an increase in drain back resistance, backing foamy oil into the turbine bearings.

    If the turbo has recently been replaced or the drain line moved, check to make sure that the drain line hose has not become plugged. Due to the tremendous heat generated by turbos, when old hoses are removed and reinstalled, the internals of the hose ID may fail causing blockage.

  6. Engine Crankcase Breathers

    Make sure that nothing is causing an increase in crankcase pressure (ie bad check valve on a PCV system). This would decrease the ability of the gravity drain back line to the engine crankcase.

Black/Brown Smoke

Black or brown smoke is usually indicative of too rich of a mixture, i.e.- too much fuel or not enough air. Some likely causes are: leaky/worn fuel injectors, bad O2 sensor, or a mis-programmed computer (if this was reprogrammed by a tuner).

White Smoke

This is caused by coolant/water in the combustion chamber. This is commonly a sign of O-ring failure. Start saving for an engine rebuild.

Other possible(?) causes might be a cracked turbo housing (since there are coolant lines to the turbos).

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