[cass]

Here's a question that my factory service manual can't answer. The cannister-type fuel filter on my 3.8L 2002 T&C minivan has three fuel lines: one supplying fuel from the tank to the filter, one directing fuel back to the tank from the filter, and one supplying fuel to the engine from the filter. As I understand it, the pressure regulator in the pump module controls pressure to around 50 psi. If the excess fuel is kept in the tank by being discarded by the regulator, why is there a return line from the fuel filter back to the fuel tank? Does the regulator control pressure through the fuel returning from the filter? If so...why?

 

[Bob Lincoln]

Could be that in case of FPR failure at the tank, there is a pressure relief mechanism inside the filter so that pressure won't build up and either blow the filter material apart or cause a leak.

 

[ImperialCrown]

The fuel pressure regulator is in the fuel pump assembly. I believe that the top line return to the tank is an air bubble return to make sure that only liquid fuel is delivered up to the front.
Cut your old one open to see the air/liquid separator baffle and filter element.

 

[dana44]

The purpose of the return fuel from the filter to the tank is to keep the fuel from remaining warm. Cooler fuel does not expand as rapidly thus burns better, prevents vapor lock issues. The system is just doing the same as the recirculating system returning the fuel from the fuel rail back to the tank, just figure it is a safety issue to do it from the filter to the tank instead.

 

[cass]

The purpose of the return fuel from the filter to the tank is to keep the fuel from remaining warm. Cooler fuel does not expand as rapidly thus burns better, prevents vapor lock issues. The system is just doing the same as the recirculating system returning the fuel from the fuel rail back to the tank, just figure it is a safety issue to do it from the filter to the tank instead.

Thanks for your response. Does the fuel heat up that much going from the fuel pump to the fuel filter mounted right on the tank? As a matter of pure conjecture, if the fuel is being returned to the tank from the fuel filter mounted on top of the tank, there's no mechanism to cool the fuel between the fuel filter and the engine. Wouldn't the fuel have plenty of time to heat up once it's in the fuel rail? That is, since there's no fuel return from the fuel rail on the engine (as was not the case with my 1990 Spirit), it seems that the fuel in the engine compartment would be at a relatively high temperature regardless of the fuel return on the filter. Wouldn't the fuel returning to the tank from the filter (located on top of the tank) have very little effect on the temperature of the fuel being suppied to the engine? Am I missing something?

 

[Bob Lincoln]

Vapor lock is virtually impossible with fuel injection, since the pressure keeps it in liquid form, so cooling is very unlikely to be a factor. In fact, you want the fuel hotter at the injector so that it will expand rapidly as it exits the injector and vaporize more readily, burn more completely and not condense into droplets. Controlling the pressure is the function of feedback lines on fuel injection.

 

[dana44]

I was assuming the fuel filter was up somewhere near the engine underneath, didn't realize it was on top of the fuel pump itself.

With that, OK, no need to be returning excess fuel back to the tank, so what about the excess being used to cool the fuel pump itself? If the pumps require extra cooling, using the pump itself to spray the pump to cool it would be by far the best way to prevent overheating.

 

[cass]

The fuel pressure regulator is in the fuel pump assembly. I believe that the top line return to the tank is an air bubble return to make sure that only liquid fuel is delivered up to the front. Cut your old one open to see the air/liquid separator baffle and filter element.

Thanks for the advice. I will definitely do that. I installed a new fuel pump (and filter) last week, so the old filter's still lying on the bench in my basement. I dropped the tank to install the new pump and filter - not hard at all. The tank is plenty big and cumbersome, but made of plastic so it's relatively light and easy to handle. I bought a Denso pump, and found that it was exactly the same as the OEM unit. What was more surprising is that both pumps had "Walbro" imprinted on the cannister body. Seems that Denso is marketing Walbro. The old (OEM) fuel filter is plastic, and the new one (Wix brand) is metal. Interesting. By the way, the fuel return line (from the filter to the tank) is the lowest line coming out of the filter.