2011 - F70LA, 4 stroke low speed fuel issue.

[jtmac2011]

Yes, lift pump has a mechanical plunger that I assume is driven off a camshaft lobe.

[Tony Harper]

Update, no luck with the 2 suggested fixes. Same issue persists. But the fuel cooler provided another thought for possible repair, could the water pump not provide enough water at low speed? There is definitely a connection with heat...I.E. after start up the fuel problem does not occur until after engine is fully warm, then the 10 minute run time intervals begin.

did they "replace" the low pressure fuel filter hsg or assy or did they just "look at/visually inspect" it ?? the problem cannot be seen it has to be replaced to confirm whether it is faulty or not.
I very much doubt it wil be a water pump issue as the engine requires more heat dissipation at higher speeds than lower speeds & also the overheat alarm would sound, can you confirm that the alarm is operational, to do so, remove the red lanyard from the emergency stop switch & crank engine with it removed & the alarm buzzer should then sound with the clip removed whilst cranking !!

[jtmac2011]

Yes, filter housing was replaced. Engine was run until problem started. The hsg was then changed, problem still present.

And, yes alarm works.

Question: How is the VST vented?

[Tony Harper]

Yes, filter housing was replaced. Engine was run until problem started. The hsg was then changed, problem still present.

And, yes alarm works.

Question: How is the VST vented?

via the p/n 6C5-86120-00 "solenoid valve assy" which is con*****ed by the ECU thru the main engine wiring harness.
another suggestion could be the low pressure inline fuel strainer p/n 6C5-24251-00, this is mounted between the low pressure lift pump & the VST assy, there is also another fuel filter inside the VST p/n of
6C5-13915-00, is connects directly onto the high pressure fuel pump, these are long shots but maybe worth trying !!

[jtmac2011]

ISC failed on last run. Shop has ordered a replacement. Waiting on the results once changed.

[jtmac2011]

via the p/n 6C5-86120-00 "solenoid valve assy" which is con*****ed by the ECU thru the main engine wiring harness.
another suggestion could be the low pressure inline fuel strainer p/n 6C5-24251-00, this is mounted between the low pressure lift pump & the VST assy, there is also another fuel filter inside the VST p/n of
6C5-13915-00, is connects directly onto the high pressure fuel pump, these are long shots but maybe worth trying !!

Another piece of the puzzle. When the problem began on the last run, Tech indicated he pulled the vent tube off the solenoid valve assy and hot gas/fuel (like boiling) spewed out of the tube. Tech believes the heated high pressure from inside the VST is preventing the low pressure fuel from entering the tank. Tech also removed the low pressure fuel line from the VST at that time as well and there was plenty of fuel flow from the disconnected fuel hose while idling. VST cooler was subsequently removed and cleaned again, although he found no obstructions during that cleaning. Water pump impeller was removed for inspection, not entirely bad but a new pump assy ordered. Question: From your comment the ECU operates the 'solenoid valve assy', but what sensor provides the signal to the ECU for the solenoid to operate? I.E. Is it a pressure sensor?, or heat sensor? And, where would I find it?

[jtmac2011]

So, ISC and water pump replaced, same problem. During my test run, I immediately disconnected the vent tube from the solenoid valve assembly as soon as the problem started. Reprimed the VST and ran for another 30 minutes @1000 rpm in gear and cowling removed, no issues found, motor ran perfectly. So, thinking I found the solution I reconnected the VST vent tube and ran an additional 30 minutes, again with cowling removed, but problem didn't return. So now I am confused. My thoughts are the cowling was removed when vent tube was reconnected, so additional heat from under the cowling could be aggravating this problem. Next run I suggested to the Tech that we try one of 2 things: plug the vst vent tube to mimic a faulty solenoid valve; or, install a temporary 'tee' in the vst vent tube and pull it through the cowling so that we have access to its end (this will allow for a quicker disconnect of the tube without removing the cowling).

[jtmac2011]

Finally made progress, the problem was the solenoid valve...I.E. valve was not functioning properly, as a result venting of VST was limited, this caused a back pressure in the VST that resulted in no fuel flow from the low pressure fuel pump.

Fix: The sensors that provide input to the solenoid, by way of the PCU, were out of adjustment and had to be reset using an Analyzer. Ran the engine under all operating conditions since that repair and all is good.