Fuel pressure varies during cranking, no start.

fordman54935

Registered User
Hey guys,

I've posted on here before about my cranks/no start situation with my 94 SC. Found something new to try and figure out. When I crank the car with a fuel pressure guage on, the fuel pressure is doing strange things. On key-on, pressure is about 30psi, which is under spec. When you start cranking, the guage varies from about 35-25 psi and does that approx. every 4th revolution while the engine cranks. This seems very strange and I would think the guage should indicated about 37-40 psi and stay quite stable.

Any ideas what may cause this? The rail holds pressure for a long time once you turn the key off, and there was no fuel smell in the vacuum reference line for the fuel pressure regulator. I was hoping it would bleed down really fast to indicate a stuck injector or bad regulator.

The car still doesn't start and this condition is causing the engine to gas foul the spark plugs.
 
Your fuel pressure regulator is vacuum referenced. With the vac line disconnected you should see 39psi if you jumper the fuel pump on. When cranking, the engine develops vacuum and with the vac line connected you will loose 1 psi of fuel pressure for every psi of negative vacuum. Vacuum on the gauge is measured in inches of mercury, so I'm not sure how that translates into psi.

Point is, it's not unexpected to see lower fuel pressure during cranking. To test this you can pull the vac line off the FPR and you should see consistent 39psi during cranking.

That your engine is fueling but it just floods and doesn't start indicates it thinks it's sparking. If you get spark, and fuel but no start, replace your spark plugs. Flood your engine a couple times and the spark plugs will be junk.
 
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