[Diy_efi] High fuel pressure

[Phil Hunter]

> On a 4 cylinder BMW with an early Motronic system, it has a failure
> in the fuel regulator. The pressure rises to the maximum that the
> fuel pump puts out. The injectors are 2.2 ohm. It is near impossible
> to start the engine when cold.
> 
> Why? Is it too rich to burn when cold? Is the high pressure holding
> the fuel injector closed? Other?

On the Bosch/GM system I'm somewhat familiar with, full pump pressure
can easily be double the "vacuum line-off" pressure setting of the FPR,
90+ psi vs. ~40psi. In your case I would expect you to have a very rich
mixture for starting, and even richer when running at anything less
than near WOT (assuming N/A).

Basically the FPR maintains a constant pressure _across_ the fuel
injectors from fuel rail to manifold pressure (vacuum). This is a
fundamental concept that you need to understand how it works. Think
"equilibrium".

A plugged return line can cause the problem you describe, the
return is typically a smaller diameter and easier to restrict from
corrosion, often at the connectors. You might run a temporary fuel
line straight from the FPR return line output to a container, as
a test.


HTHs,
philh
(digest)

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