Hey all, driving back from the salvage yard Friday my car all of a sudden stopped working on the highway. I knew for a fact that it wasn't getting any gas to the engine, but I didn't know why. I looked at the sending arm with the rubber lines going to the engine and they were dry which meant that gas wasn't getting to the engine from the fuel pump in the tank. 20 minutes later, the car started up just fine!!! Like nothing ever happened. My father said that the car did something called "vapor lock" or something like that. It mostly happens when you're in bumper to bumper traffic, which I was for about 30 minutes before that, and the lines heat up to where fuel vapors are in the lines and blocks the gas from getting to the engine. Any ideas for preventing the problem?

 

If your problem was indeed fuel related it probably wasn't vapor lock. Vapor lock was not uncommon in the days of carburetor cars with float bowls and mechanical fuel pumps, but fuel injection has pretty much made that a thing of the ancient past. Since the problem went away after things cooled down, I would suspect something more electrical, like a fuel pump relay.

 

indeed, vapor lock just doesn't happen to pressurized fuel injection systems, which constantly pump fuel from the tank, past the engine at ~44PSI (for LH 2.x), then back to the fuel tank.

I'm with Joseph.... You should be looking into your fuel pumps and the related control circuitry... the 'fuel pump relay' is integrated into the main relay that also powers the injection system (they are two separate relays in the same can), and is in the main relay panel behind the fuses behind the ashtray on a 740/940.

 

I don't think it is the relays, but I'll check them. How are you able to tell if a relay is bad? I know both fuel pumps work, my brother and I both had enough gas in our faces after I checked the sending arm. Anything else that you guys can think of that might be the problem as long as you're posting?

 

I suppose could slip a wire into either side of fuse 11 which is after the fuel relay and hook that up to a 12V test light, other side of the light to ground somewhere. the light would blink on a sec when you turn on the key, and if the engine is running, the light should be on steady.

or probe it with a volt meter....

if you have fuel pumps, and still aren't getting fuel to the injectors, you might check the 'radio suppression relay', this is unique to 740/940's (240's don't have them) and is under the hood, near the power steering reservoir. This feeds the power to the fuel injectors. A hot-when-on signal goes from the main relay (which is the other side of the fuel pump relay) to the RSR's coil, then that relay switches power directly off the battery to the ballast resistor pack directly to the injectors, then the ECU switches the other side of the injectors to ground to fire them. if that radio suppression relay is not working, you get no injector power, and without that, your engine won't start, with dry plugs.