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Fuel Pressure at high boost or vacuum

1K views 0 replies 1 participant last post by  PMI 
#1 ·
For anyone experiencing rich or lean engine conditions, a few notes about fuel pressure:

The Trionic ECU regulates the amount of fuel injected into the cylinders by controling the legth of time the injectors stay open during each cycle. That
depends on stable fuel pressure, but stable relative to intake manifold pressure.


This requires that the vacuum/boost regulation in the FPR is

a) correct and with no leaks to whatever max boost you run at,

and on modified engines running at more than 15 psi boost,

b) that the Bosch FPR can actually regulate that high, after 10 years or so in the car (!)

The fuel pressure regulation depends on a vacuum line from the TB to the FPR, which is just slipped over a couple metal tubes at the ends. If a leak develops at high boost, or if the FPR diaphram starts to leak, there will be a sudden fuel pressure drop just when you need it the most. The engine will run lean, plugs will run hot and may show excessive burn-off or melting.

If the ECU does not hold the boost back, the engine may misfire.

(Similar thing can happen if the cheap plastic check valve in the PCV line does not hold at high boost, :roll: , or if there is another kind of vacuum leak.)

At idle and steady speed, if the FPR pressure does not follow the manifold pressure down when running on vacuum, the engine will run rich. The car will have poor MPG, the front O2 sensor will foul and eventually fail, "dry" carbon will collect on the plugs, and soot will collect in the exhaust.

If it runs rich beyond the capability of the ECU to regulate, the engine may also misfire.

Saab has a procedure for checking static and dynamic fuel pressure with a fuel line adapter, fuel pressure gauge, and a hand vacuum pump, with engine off. (the fuel pump is turned on and off with a piece of wire and a switch at the fuse panel)

The WIS shows the procedure, complete with the fuel pump connection at the fuse box, the hand pump, adapter for the fuel line, gauge, and a table of values.
 
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