Lacking Power
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Lacking Power
96 F-250, 5.8 FI, Auto, w/dual fuel tanks (307K miles) Symptoms: During hard acceleration either from a stop or while trying to pass there was a distinct lack of power when the accelerator is depressed. RPMs would go up slightly and peak within a couple hundred of RPMs. Lean mixture fault code resulted. Believing that an ageing fuel pump was the culprit, I replaced the pump in rear tank. Same symptoms. However I noticed that while trying to accelerate (while using the rear tank) and the symptoms appear and I switch tanks (from rear to front tank) the ability to accelerate returns. Before I drop another $100 on a pump for the front pump I want to determine the source of the lack of power. I have also replaced the pressure regulator on the fuel injector tree. Starting to think it may be a tranny issue, but still not convinced. A co-worker is suggesting the fuel distributor between the two tanks not fully switching between the two.
Any troubleshooting suggestions?
Thanks in advance
Any troubleshooting suggestions?
Thanks in advance
Fleet Owner
Joined: Oct 2005
Posts: 20,783
Likes: 1,751
From: Kentucky
Before spending any money rent a fuel pressure gauge to test the fuel pressure at idle and under load on both tanks.
Have you noticed when running on the rear tank if the front tank is mysteriously filling itself? Your symptoms are classic cross-flow that plagues these trucks, but you need to diagnose the root cause first.
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Lacking Power
Thanks for the response.
The answer to your question about the the tanks cross-filling. I have not noticed that at all. The fuel level sending units are also on the blink so I gauge my fuel levels based on my trip odometer. 250 miles to the front tank and 270 on the rear tank.
Autozone has a pressure gauge but it will only work for a stationary vehicle.
Are the check valves you discuss part of the fuel delivery module? If not, where are they located? I do not recall seeing any check valve assemblies when I swapped out the rear FDM and fuel level sending unit. Perhaps my new FDU needs to be replaced?
The answer to your question about the the tanks cross-filling. I have not noticed that at all. The fuel level sending units are also on the blink so I gauge my fuel levels based on my trip odometer. 250 miles to the front tank and 270 on the rear tank.
Autozone has a pressure gauge but it will only work for a stationary vehicle.
Are the check valves you discuss part of the fuel delivery module? If not, where are they located? I do not recall seeing any check valve assemblies when I swapped out the rear FDM and fuel level sending unit. Perhaps my new FDU needs to be replaced?
Fleet Owner
Joined: Oct 2005
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From: Kentucky
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Thanks for the diagram. It really helps. It is on my list of things to get done this weekend.
The Haynes book says to ground out the fuel pump at the diagnostic data link. Where is that located? I can't seem to find it in the schematics?
Thanks for the help on this.
The Haynes book says to ground out the fuel pump at the diagnostic data link. Where is that located? I can't seem to find it in the schematics?
Thanks for the help on this.
FTE Legend
Joined: Jun 2006
Posts: 31,930
Likes: 1,499
From: Ottawa, Ontario
EEC Test port is under the hood by the drivers side hinge on the inner fender.
While you can make the pump run continuously via the test port what you need to do is test fuel pressure at idle and then when the throttle is goosed briefly or vacuum is removed from the FPR. The fuel system should make 32-35psi at idle and pressure should spike to 42-45psi when the throttle is goosed or vacuum removed from the regulator. Pressure should never drop below the idle readings and fuel pressure should hold when the engine is shut off.. slowly bleeding off over several hours is OK but instant loss of pressure suggests a leaking injector or bad check valves in the FDM.
While you can make the pump run continuously via the test port what you need to do is test fuel pressure at idle and then when the throttle is goosed briefly or vacuum is removed from the FPR. The fuel system should make 32-35psi at idle and pressure should spike to 42-45psi when the throttle is goosed or vacuum removed from the regulator. Pressure should never drop below the idle readings and fuel pressure should hold when the engine is shut off.. slowly bleeding off over several hours is OK but instant loss of pressure suggests a leaking injector or bad check valves in the FDM.
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