740 & 760 Fuel System Strangeness

Greetings all. Needing some guidance to those who may have experienced this in the past.

1988 740 Turbo Estate with Auto.

A short while ago it started to not pick up revs ie: no throttle response. then it just died and wouldn't start again. Usual Checks :

  • Fuel Pump Fuses - OK
  • Fuel Relay replaced with new one - Same symptoms
  • Replaced In tank Pump as it was the original one
  • Tested fuel delivery from In Tank to Main in Line external pump-Check
  • Tested Fuel output from the filter-Check
  • Swapped out the Fuel pressure Regulator-same symptoms
  • Then decided to remove the return line from the Fuel pressure regulator - Ah-Ha

So, It started and ran fine(with the line disconnected). Then I cleared the return line with air pressure (disconnected at both ends)

When I applied air pressure to the return line input that runs into the tank , I noticed that the air would simply displace the fuel and return it back out the 'in door' as it were.

So, I am guessing the vent is blocked as it seems to be not allowing the fuel to return to the tank. Or, it may be something in the fuel sender/pump assembly blocked?

Any ideas or prior experience with this problem?

All commentary is appreciated!

Comments

  • The return fuel has to go somewhere, if you were to run the pump constantly with the engine not running, it will have to return back to the tank.

    out of curiosity have you ruled out the air mass meter?

  • edited February 26

    Thanks Ramrod, yes however I think the fuel isn't being returned as there is a blockage within the tank somewhere. ie fuel isn't being returned as the air isn't escaping that needs to be displaced by the fuel that is returning. No I haven't ruled out the MAF. Do you think this could be borked?

  • could it be something you did/changed during the primary fuel pump replacement?

  • Pretty sure if your return line is blocked the fuel pressure will skyrocket and the car will run super rich...

  • The return line isn't blocked. It's blocked st the tank

  • OK, sorry maybe I misunderstood. Based on your theory, if you remove the fuel cap, the return line functions normally? Have you tried that? The evaporative emissions system is designed to not allow fuel vapours to escape from the fuel tank. When functioning properly, when the car is started, vacuum is applied to a valve on the charcoal canister, and fuel vapours get sucked into the inlet manifold. When the car is switched off, IIRC that valve is shut so the tank theoretically can pressurise. Could it be an issue with the vacuum hose routing to the charcoal canister? Test with fuel cap removed...if you’re still getting the “air displaced coming back out the in door” as you state, then I’d suspect hoses are connected incorrectly back at the fuel sender and need to be double-checked.

  • OK, Thank you. Appreciate the insight. I will have another fettle when time allows.

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