I am trying to diagnose the problem with volvo 940 wagon ('93). It has uneven idle when cold (rpm fluctuates for about +-250 and car vibrates). Also, combined fuel consumption averages to unreasonably low 14-15 MPG.

Recent work done includes replacement of spark plugs wires, cleaning crankcase ventilation, checking vacuum hoses, and oxygen sensor. Any suggestions on where to look would be appreciated.

 

Poor mileage can be caused by a failing engine coolant temperature sensor - the one used by the ECU (not the temp gauge). It can be tested using an ohm meter. Check it prior to replacing. Here is a link to a description of how to test it:

https://www.brickboard.com/FAQ/700-9...eratureSensors

Exhaust leaks upstream of the oxygen sensor can also reduce your mileage. These leaks let oxygen into the exhaust stream causing the ECU to think the engine is running lean and enrich the mixture.

As far as the low idle - check/clean your throttle body and idle air controller. Then make sure the throttle position sensor and throttle body is adjusted properly