Hi all,

I have been seeing pretty bad fuel economy in my car lately. 1998 V70 AWD (with the AWD prop shaft removed for summer).

Regardless of what my particular case is in terms of mpg and driving style and all that, I was wondering if we could all provide some input as to what engine systems (so forgetting about brakes, tires, weight in trunk, roof racks, etc) affect the fuel mixture so as to produce a list of things to go over when someone experiences poor fuel economy.

For instance:

Evap System
- I know next to nothing about the evap system
- how does this system affect the computer's mixture calculations?
- which parts of this system are suspect? sensors, lines, etc?
- are there any valves/sensors that can be checked?
-- are they electrically operated? or the one-way type?
-- locations?
-- method to check?

Here's the list I can come up with off the top of my head:


Vacuum Lines and Intake Leaks
- any leaks will cause mixture problems, likely rich running
- best checked with smoke test
- alternatively use propane/butane or throttle body cleaner to check for idle fluctuations




Thermostat
- any problems with thermostat will cause engine temp to be wrong ( stuck open = lower temp engine, stuck closed = higher temp engine) which will cause mixture problems
- need to remove to test, might as well just replace it




Engine Temperature Sensor

http://www.brickboard.com/AWD/volvo/1482896/850/quirky_starting_issue.html
Using a multimeter, you can check the resistance at the ECT sensor, the resistance will increase (numbers get smaller as the temp increases):

68F = 2800 Ohms
104F = 1200 Ohms
212F = 150 Ohms

So a cold engine should be around 2800 and a hot engine around 150.

Many on this board suggest swapping out the thermostat and the sensor at the same time, due to need to drain about a quart of antifreeze to do either. I disagree. When the ECT fails, it is easy enough to replace without a lot of spillage - just use a finger to plug the hole. The ECT and thermostat NEED to be OEM, do not get knockoffs.

Running on a 'cold' engine for long periods due to an open thermostat will increase the likelyhood that the plugs are fouled. The ECT reading low will cause the engine to run rich eventhough the cylinder themperature is hot. Replaceing the thermostat should solve your hard start problem.

Klaus

http://www.brickboard.com/AWD/volvo/1456955/850/code_436new_o2_sensor.html
*To test RO2S
-Ignition on.
-Fuel pressure gauge connected.
-Squeeze the hose between the EVAP valve and the intake manifold with hose pliers 115 8957. DTC for the EVAP system can be set.
-Start the engine and let it run at idling speed until the rear HO2S begins operating.
-The rear HO2S begins operating a short while after the rear HO2S preheating starts.
-Increase fuel pressure by squeezing the hose after the fuel pressure regulator with hose pliers 115 8957.
Voltage across the rear HO2S should rise to approx. 0.8 V.
-Remove the hose pliers.
-Undo the hose for the combined instrument at the intake manifold.
-Voltage across the rear HO2S should drop to approx. 0 V.