Stupid fuel economy gauge tricks... (archive)

Let's assume that one of the two relays is your problem (or something else nearby that could be resurrected by a little tapping). If your fuel economy gauge is in working condition, you can use it to help diagnose your problem.
The next time you're driving along and the car stalls, observe what the fuel economy gauge does as the engine slowly drags your car to a halt.
-If it says you're getting abnormally poor fuel economy, even if you let up on the gas pedal (make sure cruise control is off), suspect the main relay. Voltage is not getting to the injectors, and the main relay provides that.
-If it says you're getting abnormally good fuel economy, even if you floor it, the main relay is fine- the ECU is not activating the injectors for some reason. Check the grounds on your engine. (On my car the ground between the valve cover and the firewall was loose.) Also try reseating the ECU connector.
-If it fluctuates as it should (pedal up->good economy, pedal down->poor economy), the main relay is good, and the ECU is actuating the injectors correctly. Could be the fuel pump relay, or an intermittent fuse #1 (corner of the fuse box near the main relay) which is inline with the fuel pumps.

Here is why the economy gauge is helpful here (if you care).
-The fuel consumption rate signal is nothing more than the ECU's injector control signal, run through a resistor. There isn't an actual fuel flow detector.
-The injectors are pulled up to 12V by the main relay, which is always on when the car is running.
-When the ECU fires the injectors, it grounds them (or applies a voltage lower than 12V- I'm not sure), putting a voltage across the injectors.
-This means a low fuel consumption voltage means high fuel consumption, and a high voltage means low consumption.

So:
-When the main relay fails, the injectors are pulled to ground permanently, even when the ECU isn't driving them. This means the fuel consumption signal will be 0 Volts=maximum fuel consumption.
-If your fuel economy gauge reads anything approaching decent economy (and the gauge itself is operational) your main relay is good and is supplying 12 V, as it should.

Hi all,

Just trying to figure out which relay is which around the fuse box. In particular, looking for the fuel pump relay.

Despite driving out an old tank of gas (with added dry gas, and cleaner), refilling, and some more driving, my car seems to cut off totally (exactly as though it ran out of gas), somewhat randomly 'when warm' and in particular, when trying to start the engine warm.

Has happened at a stoplight or two, and once at highway speed. Has not happened when car is started first time for the day or after a few hours of shutoff. Have verified good spark when the warm 'no start' occurred, and no 'vaccuum' in the gas tank.

When running, seems to not miss a beat otherwise (kinda indicating that I'm getting decent fuel pressure when I get it).

tapping (all) of the relays has resulted in the car starting immediately (once I'm back in the car of course). done that 4 times now, car starts immediately.

thanks again for any info!
andy
p.s. what's a good recommended manual for the car. Seems I'm getting to know a lot more about it lately:-)

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