05-20-2010
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Senior Member
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Join Date: Oct 2006
Posts: 221
Rep Power: 6
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White smoke means either unburned fuel or water. Unburned fuel can come from either a cold engine (thermostat not closing and allowing the engine to warm up properly) or compression in a cylinder that's not high enough to ignite the fuel in that cylinder.
Water can be leaking into a cylinder through a blown head gasket, or worst case, a hole corroded through from the water jacket to the cylinder.
As Dwayne said, black smoke indicates overloading. The fuel injection pump is controlled by a governor. If the engine is operating slower than the throttle setting, the governor increases fuel flow to the injectors. Normally the engine can produce more power from the more fuel, speed up, then the governor backs off the fuel flow.
If more fuel is being injected than can be burned, the semi-burnt fuel comes out as black smoke. Overproping is one cause, but if this is a new symptom, it could be you have one cylinder not carrying its load (for instance, from low compression), and the other has to work harder to make up for it.
From the description this sounds like the most likely problem -- low compression on one cylinder is giving the white smoke, and the other cylinder being overloaded is giving the black smoke.
Low compression can come from a leaking head gasket (not too bad for cost), a burnt valve (more, but also not too bad), or worn out cylinder wall/rings (bore cylinder oversized and install new piston and rings: very expensive). The bad news is Volvo is horrendous for parts prices on these old engines.
Good luck with that,
Tim
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