Q – My 86 944 has been acting strangely and now it is dead. For a while, it would run well when cold unless it was damp outside then it would sputter and run rough but after it warmed up for a while, it ran fine. Recently it started running rough even when it was dry outside after it warmed up. Just before it died, I tried an experiment and disconnected the OXS sensor and it ran better. I was thinking the sensor may be the problem but then just before it died, I heard a loud “bang” under the hood and now it will start but dies right away. What is your best guess? David.

A – Unlike the 911 that died in motion, with this one, I don’t need to guess. All the systems can be analyzed, diagnosed, and corrected “Without Guesswork” (Which is the name of the technical guides published by VW since the 60’s).

The three legged stool of internal combustion engines is made up of fuel, air, and spark. When the car was running rough, the challenge becomes which “leg” is it? If you lose any one of the three, the car won’t run. I would start with an under hood inspection and “smoke test” of the intake system. That bang you heard under the hood was probably a backfire and my hunch is that you blew off a vacuum hose or boot. Any time a loud bang occurs under the hood, you have to check connections and hoses for integrity.   A vacuum leak will upset the fuel mixture. A smoke test involves adding pressurized smoke to the intake which can be easily seen with the naked eye.

Before the car failed altogether, unplugging the OXS sensor would richen up the mixture (“limp-in mode”) and hide a vacuum leak…for a while.   While it is possible for a failed OXS sensor to cause the car to stall, it is very unusual. They usually fail “safe” by causing more fuel, rather than less, to be fed to the engine.   Testing an OXS sensor is not difficult and it can be confirmed or eliminated as a problem fairly quickly.

So maybe you can get the car to start and run by correcting vacuum leaks and replacing the sensor. That does not explain the poor running when wet nor the backfire into the intake. For that a “water test” will often bring out the worst in a secondary ignition system. If it results in misfiring or cross firing of the cylinders, you can blow a vacuum hose or boot off the intake manifold. Spark plug wires, cap and rotor can test within factory spec for resistance and still fail a water test miserably.   If the water test does not kill the engine or make it run rough, look inside the cap and rotor for little black squiggly lines from one terminal to another. When spark jumps across a cap or rotor, it often leaves the tell tale evidence called a “carbon trail”. If you see that inside the cap, or if the rotor has cracks or looks burned, replace them. And since you have an 86, these parts may be original. How do I know that?   These cars are incredibly reliable and usually only need spark plug changes, rarely a major tune up.

In the old days, when I started out, electronic ignition was new and most cars used components called “points” and “condensers”. These parts were job security for mechanics (who were renamed “technicians” a little later). After a tune up, the car would run well for a few thousand miles then gradually deteriorate leading to rough running, poor fuel economy. With the advent of Motronic integrated fuel and ignition systems in the eighties, tune ups became a thing of the past. That is until the age and wear of the parts reach critical mass and the car just dies.

The cure for your car is: seal up the intake system, replace worn ignition parts and test the OXS sensor. With that, I bet your 944 will be humming along like a fine watch for many more years. – MC

CategoryMasterCylinder, MC944

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