Fpr faults

N

nimhbk

New Member
Should the below apply to our cars FPR

The FPR is a small canister mounted on the fuel rail with a vacuum hose attached to the top of it. There is a diaphram in the center of it with fuel pressure on the lower half and engine vacuum on the upper half. If the diaphram ruptures, raw fuel will leak into the intake manifold and cause a rich condition affecting fuel milage and hot engine starts. If ruptured it will act much like an applied choke on a carburated engine (aiding cold starts and hindering hot starts). To check it, simply pull the vacuum hose with the engine running (or key on, engine off) and look for fuel leaking out of the nipple. There should be none.
 
PobodY

PobodY

Moderators
Staff member
Yes that applies, but it's boost rather than vacuum that we need to worry about. - As the boost increases the fuel pressure goes up at the same rate; I think it's 3bar base then a 1:1 rise with boost (so at 1bar there should be 4bar of fuel pressure).

You can see that it's not running rich that's the issue; it's that if the pressure doesn't rise you go lean and melt a piston!
 
G

geoff pine

Well-Known Member
Last edited:
J

Jon Olds

Well-Known Member
Watch out for oscillations, as these 'fast response' ones have a high 'gain' and can be marginally stable unless additional damping is introduced locally
 
PobodY

PobodY

Moderators
Staff member
That's a good tip! So that's usually what goes wrong with the after-market ones then?
 
S

skiddusmarkus

Active Member
Jon Olds said:
Watch out for oscillations, as these 'fast response' ones have a high 'gain' and can be marginally stable unless additional damping is introduced locally
Click to expand...
My pressure on my sard fpr kept going up and up, it turned out to be engine vibrations causing it which stopped when I moved it off a bracket on the block.
 
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