390SE Fuel Pressure question

[susser]

We've just dropped the fuel rail pressure of BIG WEJ http://i4.photobucket.com/albums/y127
from 36 to 30 psi; it was running very rich wethink, and doing about 6mpg.
Point is;
Was fuel rail pressure the problem ?
Have we made it too lean ?
Why are there two questions ?

Bu66er that's 3

Edited by susser on Sunday 4th March 12:31

[digga951]

The correct fuel pressure should be around 2.5bar / 36psi. I reckon you have something else causing your car to run too rich.

Maybe its a faulty temperature sensor fooling your ecu into thinking its cold, hence making the mixture richer?

It could also be that one or more injectors are remaining open too long, or even permanently. I checked mine by removing the fuel rail and injectors and then operating the fuel pump manually. If they start spraying/dripping, they are faulty

Dan.

Edited by digga951 on Sunday 4th March 12:44

[jchase]

As per digga said,

But also to add the 390SE with NCK engine modifications also includes a modification to the AFM. This modification will require the base fuel pressure to be increased above 250kPa. Your best bet is to get a wideband lambda sensor fitted, and adjust the fuel pressure to get the correct AFR when the throttle is WFO.

Also, IMHO fiddling with the AFM like this will cause the air temperature correction not to work, so don't expect the fueling to be spot on if the weather changes!

-Jim

[susser]

Good stuff.
I need to get focussed on this before Sat.
Maybe the temp tranbitter is the problem. I Took my pressure readings from the cold start injector. Maybe I shoulda left the plug off it. Or does it only inject on cranking ?.
Think we better set the rail back to 36 anyway, rather than hole a piston.

[jchase]

If you don't have the tools to set the AFR mixture correctly (Base pressure is best taken with the engine not running, but the fuel pump running) You can clean the plugs, and with a nice warmed up engine, drive along at a steady 40mph or close to mid range RPM, and switch the engine off and coast down - inspect the plugs' ceramics - they should be a nice goldeny colour - if they are white you are too lean, and black too rich. On my previous 390se, I was getting spot on 14.7 AFR in this range, with 250kPa (36psi), but was very rich at idle, and a bit lean at top end (16 or so). I wouldn't go below 250kPa, and probably not higher than 280kPa, as the injectors become very non-linear in this range. (Also note that fueling is proportional to the root of the injection pressure, so a 25% increase in pressure should give you only 5% increase in fuel flow - or so the Megasquirt manual says anyways!)

Hope this helps a bit. -jim

[susser]

Thanks Jim
Helps a lot.