AC Fuel Flow Summary
As has been noted earlier, the AC has had its fair share of fuel flow problems. Ranging from the large (a rotten fuel tank) to the tiny (a flake of corrosion in the accelerator pump). Here’s a few things that were done to try and cure it.
Firstly, replace the filters that sit on top of the fuel tank drain plugs. They may not seem like much, but internally the fuel intakes come down inside the tank, and slip inside the filters that are on top of the plugs. This fine mesh is the first line of defence against rubbish in the fuel tank. We thought we could get away with old ones that had a slight tear in them. After a few weekends of running, further inspection showed a pile of gunk sitting inside the filter! This would lead directly to the fuel pump…
The second thing to do was to insert an inline filter between the fuel solenoid (since the AC has two fuel intakes), and the fuel pump. This is just a bog standard plastic inline fuel filter after the solenoid, and is there to save the pump from any rubbish that somehow gets past the gauze filters on the drain plugs. We also took the chance to replace all the fuel hoses between the solenoid and the carburettor. The existing hoses were coming up on 30 years old, so we though it wise to avoid any rubber degradation issues and replace the whole lot in one go.
Roughly at the mid-point now, we opted to change the fuel pump. It had been noted that the fuel was flowing back down the line after the car had been left to sit for a day or two. This made starting it particularly difficult, and was probably the precursor to a failed pump. Add to that all the rubbish that had been coming through the fuel system in those weeks of diagnosis, the pump filter had been cleaned out more often then we had had hot meals. A new pump was fitted, and the fuel draining problem immediately vanished.
Annoyingly though, we were still experiencing problems with fuel flow. Between Gordon and I, we must have stripped the about 20 times. Every time it would work great for 20, maybe 50, miles before reverting to its normal way of not letting any fuel flow when under load.
Convinced it was a problem of more rubbish in the fuel system, we decided to fit another filter after the pump and before the carburettor. Initially we went for the cheap plastic inline filters that are readily available, but we didn’t leave it in that configuration for very long for another reason – we fitted a Filter King instead.
The Filter King not only acts as a filter, but it also has a sedimentation bowl which doubles as a small fuel reserve. Any rubbish floating in the fuel can fall to the bottom of the bowl whilst the fuel in the bowl can continue to be drawn upon by the carburettor. We initially tried the Filter King before the fuel pump, but it’s clearly designed to have fuel pushed through it. The pump couldn’t draw air hard enough to draw fuel into the Filter King, and nothing worked. Now, the AC 3000ME range historically has a problem of fuel starvation during a long corner. This is most likely due to the shape of the fuel tank, but appears to be mostly mitigated by the Filter King now.
Sadly, after all this work, the fuel was still not flowing as planned. At higher revs after 30 minutes of running it would start to cut out. Something was clearly wrong, and all things started to point to the carburettor… no matter how many times we stripped it down and rebuilt it!
When you blip the throttle on the AC, it activates the accelerator pump. This pumps a higher than normal amount of fuel into the carburettor in order to rapidly increase the revs of the engine. It also ensures the engine stays running and doesn’t cough and splutter to a halt. Sound familiar? After much poking and prodding, I discovered that the one of the two accelerator pump outlets was partially blocked! These outlets start at about 4mm across, and narrow to about 0.5mm. That’s why it had passed our cursory blowing through the thing to see if it was clear. Using a small length of wire from the fuel hosing braiding, the passageway was cleared, and tiny flake of white aluminium corrosion fell out…
So that was it, all those problems were caused by a tiny bit of corrosion that had worked itself up to the narrowest gap it could wedge itself into. Although it seems like we were on a wild goose chase for a lot of it, I believe that a lot of worthwhile work was still completed. Replacing the drain plug filters was definitely required, the pump was on the way out, and the Filter King cures some edge case starvation problems, as well as providing a handy visual indicator for the state of fuel flow in the system.