I've been slowly trying to add a few more years to an old '96 Splash, 2.3L, 5-speed, 2WD, w/235K miles on it.

Engine starts and idles well. It will slowly settle into 800rpms as it warms. If throttle is advanced, engine responds. But when throttle is released, engine bogs, sounds like it wants to stall, and idles rough while it tries to recover.

Under driving conditions, there's a definite loss of power during accelleration, but it will get up to speed. I'm sure fuel mileage isn't what it should be. No smoke from exhaust. There is an exhaust leak at manifold flange, but not excessive.

Compression is close to 160 psi in all cylinders. Eight Motorcraft AGSF32FM platinum plugs are recent, but not new. They show normal light brown burn on electrodes. Plug wires are recent, but not new. Coil packs are recent, but not new. They display a good hefty spark when wires are lifted one at a time.

No codes. I replaced IAC and vapor canister purge solenoid to eliminate check engine light. Vacuum hoses and connections seem good.

Here's the mystery... While idling, I disconnected the short vacuum line that goes from the intake manifold to the fuel pressure regulator. (See attached photo.) The rough idling immediately smoothed out, and when driven, the engine performance improved remarkably! I am now driving with this line disconnected!

What does this imply? Why would a now open vacuum connection allow this engine to run better than before? Any thoughts? Thanks!



'96 Splash, 2.3L, SOHC, 2WD, 5-speed, 235K miles

 

It most likely means you are sucking fuel right out of the fuel pressure regulator.

Replace the fuel pressure regulator, should be the end to your mystery