Lift pump starving the engine???

499 Views 5 Replies 2 Participants Last post by OkDually, 2 mo ago

buildwithgowen

Discussion Starter · 2 mo ago

Non-turbo 6.5 diesel has missing and stuttering. While tracing the fuel system for air leaks, I found that if I removed the lift pump and replaced it with a piece of fuel line, the missing and stuttering was gone - engine ran perfectly. Of course I had replaced the old lift pump because it wasn't operating when the ignition activated (oil pressure switch may have been the problem and it was also replaced). I kept the old one and compared with the new one. When I try and blow air through the lift pump in the direction of fuel flow to the engine, the old lift pump allows air to freely flow through, but the when blowing through the new lift pump, the air flow is more constricted. Is this my fuel starving issue? Both pump check valves work. Old lift pump had no part numbers so I got a lift pump for a 6.5 diesel, or did I?? Wrong part? Bad new part? Anyone ever heard of this situation?

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OkDually

· #2 · 2 mo ago

buildwithgowen said:
Non-turbo 6.5 diesel has missing and stuttering. While tracing the fuel system for air leaks, I found that if I removed the lift pump and replaced it with a piece of fuel line, the missing and stuttering was gone - engine ran perfectly. Of course I had replaced the old lift pump because it wasn't operating when the ignition activated (oil pressure switch may have been the problem and it was also replaced). I kept the old one and compared with the new one. When I try and blow air through the lift pump in the direction of fuel flow to the engine, the old lift pump allows air to freely flow through, but the when blowing through the new lift pump, the air flow is more constricted. Is this my fuel starving issue? Both pump check valves work. Old lift pump had no part numbers so I got a lift pump for a 6.5 diesel, or did I?? Wrong part? Bad new part? Anyone ever heard of this situation?
View attachment 657874

IIRC, there are o ring seals on the connector ends of the lift pump that connect to the fuel lines.
If those orings are bad or missing it will allow air to enter the fuel stream

The recommended lift pump is AC/Delco EP158

What are the numbers on the new pump?

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buildwithgowen

Discussion Starter · #3 · 2 mo ago

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OkDually

· #4 · 2 mo ago

The

buildwithgowen said:
View attachment 657876

The cross reference # for that pump is AC/Delco EP309 which will work for your application.
The Ep158 pump produces a little more and is why it is the preferred pump by many.

Does your liftpump have the orings intact?

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buildwithgowen

Discussion Starter · #5 · 2 mo ago

Yes, the O-rings are present, but maybe being compressed for 20 years has compromised them. My simple brain says diesel should leak out if air is entering. Due to my fuel line tests, I have diesel everywhere under the truck. I don't think I had a diesel leak anywhere before changing the lift pump, but who knows. To test the fuel constriction theory, I'll probably install the old pump and see if I get the same engine missing as the new pump may be creating. Thanks for researching the parts issue. Much appreciated.

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OkDually

· #6 · 2 mo ago

buildwithgowen said:
Yes, the O-rings are present, but maybe being compressed for 20 years has compromised them. My simple brain says diesel should leak out if air is entering. Due to my fuel line tests, I have diesel everywhere under the truck. I don't think I had a diesel leak anywhere before changing the lift pump, but who knows. To test the fuel constriction theory, I'll probably install the old pump and see if I get the same engine missing as the new pump may be creating. Thanks for researching the parts issue. Much appreciated.

Fuel does not necessarily have to leak out. if an oring is compromised it can allow air to enter the fuel stream via a vacuum.

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