I'm having intermittent hesitation and drivability problems on my 740 turbo. I've checked stuff like hoses and intakes, various sensors and relays and am pretty sure the problem is a fuel delivery issue. I'm going to check the voltage being delivered to the main fuel pump (which is new). Volvo used pretty small wires to power the main fuel pump, so I'm thinking the pump may just not be getting the oomph it needs to deliver enough fuel under load. The car idles and starts fine.

Anyway, I'm checking stuff, so I guess my questions is just about cars in general. I'm sure there is an engineering reason behind this...anyone know?

 

Im no genius but I am pretty sure one main concern for doing this is because if left on the fuel pump could explode after building up so much pressure, therefore your car would explode. Just remember before tackling an issue like this one, it was done this way for a reason and even if your unsure how well the purpose is before jumping into rigging it up you better do some hard thinking and go about it a good way. Things are always being upgraded on newer cars but correct me if Im wrong there are still fuel pump relays on the new ones. (trust me I am speaking from experience, I have tried to rewire and do things different on things before sometimes it leads to dire consequences) (almost burned down my crx because of a faulty ground and power wire)

 

They use relays for the fuel pump as well as other electrical devices to reduce the current going through the ignition switch. Also, the fuel pump relay is shut off after a preset time with the ignition on and no ignition signal. This is because of in an accident, you don't want the fuel pump running and possibly spraying fuel everywhere.

Russ

 

^^^^What he said^^^^

 

Right answer

 

Thanks guys, that answered my question!

 

I certainly agree with the above, but I would like to add that if your fuel pump relay craps out at an inconvenient place/time you can bypass it to get home. On my '89 245, connecting a jumper wire between the #4 and #6 fuses got me going again. Just remember to disconnect it when you get where you''re going..

 

The fuel pump relay serves a few purposes: It supplies voltage and current to the pumps, voltage to the ignition relay, the injection system and acts as a safety to shut the pumps down if the engine stops - it takes that signal from the coil and shunts the relay to ground to stop the pumps. The wiring is adequate but...

The main issues are: The connectors at the pump get corroded due to exposure and adds to the resistance. This causes the relay to work harder to supply current. In addition, as the pump ages, primarily due to varnish build up, the pump works even harder and draws even more current. As the current demand increases, the relays heat up and get intermittent. They can shunt to ground and cause drivability issues. I just went through this.

Here is my recommendation: Clean the connectors at the pump with electrical contact cleaner then measure the voltage to the pump. Run several (4-5) tanks of fuel system cleaner to clean the pump(s) and then check the amperage draw to make sure the main pump is under 10 amps - the lower the better. If you do this regularly, the pumps will be happy. If you are so inclined, you can wire in a secondary relay to handle the current draw which will be triggered by the main relay. If you need the diagram, I'll post it.

I did precisely this and it completely cleaned up the act in my '81 Turbo. It's fun to drive again.

Hope this helps.

 

That's a good idea. While my main fuel pump is new, it still whines so I'll check the amps.

 

Here is what I use: Marvel Mystery Oil 4-5 ozs at every other fill up or so. My pumps are dead quiet except for their normal whine. They sound healthy. If that appeals to you, consider this: These pumps love the lubrication; but first, run Techron system cleaner (or Gumout with Regane) through several times. It will clean the varnish and deposits from the pump races and the MMO keeps them nice and lubed. The amp readings will drop reflecting their efficiency. Note: You can also use TC-W3 oil at 2-3 ozs which will accomplish the same thing. You're pumps will love you for it. Mine surely do.

In doing this, you don't have to plumb-in a secondary relay. I did it for peace of mind but it is optional. Keeping the pumps healthy keeps the relay from heating up which keeps it healthy much longer. That said, keep a spare if you don't plumb in a secondary current handling relay.

Lastly, use your VOM and check the wires for resistance when you check the amps. It's a good thing to do in practice.