I have a 1998 V70 with about 60K miles. Car starts and runs just fine, perfect, in fact. However, fuel mixture is extremely rich and fuel economy
is only about 13 mpg (vs 20-25mpg previousl). Black smoke and smell of fuel
in exhaust (when present this is in both cold and hot engine conditions) No apparent fuel in oil. No Check Engine Codes. Tried testing ODBC by disconnecting Air Mass Sensor - immediately got check engine which
was reset. Rear 02 sensor was replaced last year in response to check engine error.

Something is telling the engine to run rich without adversely affecting
performance. I have seen a number of references to electronic throttle control problems with the V70, but I think mine predates the ETC.

Car appears to have cable throttle (not electronic) but I may
not be looking at this correctly. On suspicion that some computer control is malfuntioning, have disconnected battery several times to attempt a possible system reset. This seems to give a temporary improvement in the smoking problem (i.e. black smoke goes away for a while; hard to tell if fuel economy improves), but only temporary.

Without any error codes, only symptoms are thus the black smoke and the lousy fuel economy.

I hestitate to take this problem to Volvo without a clear
understanding going in as to the possible causes. Symptoms seem ripe for an
'ooh this is a tough one - open your wallet' approach from a dealer.

Any thoughts as to things to consider would be greatly appreciated.
Thanks in advance for any help!