Originally posted by Lance Kinley:
Does anyone have the Powertrain Control/Emissions Diagnosis Manual? The service manual refers to this for diagnosing the fuel charging and control system.
I'm trying to figure out if this is normal: With my fuel pressure gauge attached, I see a spike to 39psi when the pump engages during key on, but then falls to 35psi when it shuts down after 1-2 seconds.
With the car running, fuel pressure is around 28psi, which I think is WAY too low. A blip of the throttle gives a spike up to around 40psi, but immedately falls back to 28. If I gradually raise rpms, fuel pressure stays at 28psi.
Also, what kind of voltages are to be expected at the fuel pump? I am trying to determine if there is a problem with the pump or if this is all normal. HELP!
-Lance
This all sounds quite normal to me. When you KOEO (Key On, Engine Off), the pump runs for a few seconds to pressurize the line in preparation to start the engine. If you are noticing an immediate pressure drop when the pump stops running, and that pressure drop continues and does not level off very quickly, I would suspect the fuel presure regulator.
The fluctuations you are observing in fuel pressure when "blipping" the throttle vs. slow and steady increase in rpm are normal also. Fuel pressure in your car is inversely vacuum regulated. The less vacuum, the more fuel pressure (greater engine loads cause less vacuum, hence the need for greater fuel pressure under load). The FPR acts as a valve for the return fuel system and is opened/closed by manifold vacuum. When you "blip" the throttle, the engine experiences a "load" as it tries to overcome the inertia of the crank and everything attached to it as the engine quickly increases in RPMs. This causes a large dip in manifold vacuum (hook up a vacuum gauge to a port directly into the upper intake plenum after the throttle body) which in turn causes the FPR to close the return path for the fuel. Bingo! There is your pressure spike, and is quite normal.
Now, when you gradually increase engine RPM, the sudden "load" never occurs, and hence there is no manifold vacuum dip, and hence no pressure spike. If you could introduce a load on the engine while increasing RPM, you would see the fuel pressure increase proportionally. HTH
PS, Fuel pump voltage should be the same as battery voltage. You need to measure the amperage to see if it is excessive.
