Had a great bingo moment today. One of those moments where you've been looking for a fault somewhere for ages and you finally find one.
After doing my fuel flow test last weekend (incorrectly as pointed out above) I repeated it today from the MFI pump return line after the fuel filter. There was no change in the flow, if anything it was a bit higher today at 800ml in 30sec. So closer to spec than before.
After this I fitted my new inline fuel pressure gauge. I bought a decent one as I want a reliable reading and I'm going to leave it in place. Well I put it in thinking my pressure might be low so as it climbed towards 10psi with the pump running I was happy. Then it got to 12, then 15 then all the way round to 18 and off the charts so to speak. This was not so good in one way but in another, the gauge has paid for itself already. So far I had learned that I had decent if not perfect flow, and that my pump can easily put out the PSI required. But clearly there was a problem with the return line somewhere. Blowing down the return pipe I could hear bubbles, it seemed OK. What I didn't know until I read up on it is what you all probably know already in that there is a spring loaded pressure release valve in the return line fitting on the filter housing. I know what I'm going to find before I find it, a stuck shut valve....
Lots of working the valve back and forth, lots of injector cleaner and sprays of every variety wasn't making much progress. It would move back and forth but would stick in whatever position it was pushed to. It needs heat I thought but not blow torch heat as i think its plastic parts inside. A 10min hairdryer blast and another 10 mins of actuating and it is moving freely and spring back shut.
All fitted back together and I now have a nice steady 9psi with the engine not running. So real progress. God knows what the closed shut return fuelling has been doing to my engine all this time. The fuel pressure may have been >20psi the whole time... or give the fact there was no return flow the pump may have just been dead-heading and sending not much fuel to the MFI pump at all...
Now 9psi is down on what is supposedly target pressure which I'm told is 10-12psi with the engine running. But once the engine is running maybe it will come up. It is however possible that the spring has aged in my valve. I hope not though because they are £150 for a new one... has anyone ever had to replace one of these? Having said that I have read that the return valve opens at 9psi and indeed my valve had "9" stamped into, it the indicates to me that it is working as it should, however I fail to see how I could get a 10-12psi reading on the MFI pump return line if the check valve opens at 9psi? Surely this check valve alone dictates the fuel pressure in the whole circuit (assuming all the filters etc are clean and no blockages in the lines etc...)
In any case I have great hope some much improved engine running over the weekend. I might have to re-correlate everything all over again!
Such is life
