355 Fuel issue

Do you have a Motronic 5.2?

The fuel pump on the 5.2 car will only prime for a few seconds when using the key. After this time, even if you cycle the key, reset the battery or whatever, the pump will not run until the ECU sees engine cranking rpms.

 

Sorry, I forgot.

Did you read my edit?

 

Looking at the parts manual, the inlet is the one on the end of the rail.

https://www.eurospares.co.uk/Ferrari/355/355_(5.2_Motronic)/PartDiagrams/009/Fuel_Pump_and_Pipes

The return is underneath.

 

The location of the feed and return hoses at the fuel block at front center of engine is counter intuitive. The rear hose goes to the front position and the rear hose goes to the front position.

 

I just checked on my car (5.2). My front fuel block hoses (i.e. closest to firewall) go to the end of the rail (which is further forward than the lower connection). The rear hoses (closest to bumper) go to the lower connection on the rail. i.e. the same as the parts manual.

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There should be a shrader valve on top of the fuel filter.

The pump will only prime for a few seconds and only on the first key turn. From there the engine must be cranking. That said you should see pressure come up at the filter valve.

 

Are there any check valves or restrictors in the system (between the fuel filter and the rail). I'm wondering what the pressure difference would be between the fuel filter outlet and the rails. Would the fuel filter outlet pressure be an average of the regulated pressures in theL/R bank rails?

 

No check valve just the regulators on the return.

 

There is a procedure in the work shop manual to verify the fuel pressure and proper regulator function.

The only check valve would be at the fuel pump then it's straight hose to filter then to the splitter block manifold which T's it off to each rail inlet.

I've never understood the dual regulator arrangement on a single pump 5.2 car. Where there is no isolation between sides. Makes total sense on the early cars with dual pumps. All I can think of is it was easier to leave it as is when they went to single plump rather than reengineer the rail setup. Can't say I've seen this arrangement on any other car. You'd expect one rail to loop into the other with a single regulator and return at the end. Nope they came up with that elaborate T manifold thing and a bunch of hoses that chafe and catch on fire.

 

Basically I think so. The pressure regulators are both operating properly and equal all should be the same.

Going back to the posters issue. Step 1 is to just verify pressure is coming up to spec as expected and to do that all you need is a guage. No extra valve modifications necessary. Having a 90 degree fitting on your guage makes it a hell of a lot easier but isn't necessary. Sometimes you may need to loosen the filter bracket to make some room. Once you get a guage on it you'll know in seconds if you're good. And if your system is holding pressure over time per the WSM. Note I've seen a situation where it can work intermittently sometimes giving proper pressure other times not. I've also seen barely finger tight wire connections on the in tank pump