Power loss on hills and above 35 MPH

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edpdx

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gauge.jpg

Over the last week I noticed a lack of power over 35-40 MPH. I can get more speed, but very slowly have to work up to it. If I get on the freeway and try to maintain 60ish, I get vibration and shutter. I've read as many posts as I could find here and have already changed out the spin-on fuel filter, the air cleaner and even the left pump (short strait arm with spring above type). No improvement. I had the transmission rebuilt in July, and the IP rebuilt last year.

I have looked for a vacuum leak and kinks or restrictions in fuel lines. None so far, but I noticed for the first time that fuel pressure- if I am to trust the gauge on top of the spin-on, reads neg 2 at idle. I just put a new fuel pump on ?!?!

Any way to verify if this is a fuel pressure issue besides second-guessing the gauge?
 

dougco1

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Cooperstown NY
Fuel line restriction before filter, that may be the cause of the negative pressure. Most likely a plugged fuel tank sock
 
Last edited:

cucvrus

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When did you change the fuel pump? And what was the last thing you changed or messed with before the symptoms came on? I know you described the replacement pump. But the other difference in fuel pumps is that the correct one has fuel pump pressure immediately on depression of the lever. The incorrect one the pressure is a bit lower in the stroke and the eccentric on the 6.2 Diesel engine does not have enough lobe to compress the lever far enough. I may be a bit off on my description. But I think you get the jest. Good Luck. But by all means no fuel system is called rebuilt until the fuel sock in the tank is removed or replaced. I prefer replacing. Other may have varied opinions. So be it. Have a Great Day. Be Safe.
 

Barrman

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I agree. Negative pressure can only result from the mechanical lift pump not pumping. The IP is doing all the work in other words. Or, as suggested above you have a clogged sock in the tank and the mechanical pump just can't get anything to pump.

It looks like you have the PSI gauge in the filter bleed hole. Remove the pink wire from the IP and then remove the PSI gauge. Crank the engine over. There should be a gush of fuel every other engine revolution. If not, lift pump, kink in a flexible line, crushed metal line or intake sock are your choices.

Far out there choices could be a clogged fuel tank vent system. Remove the fuel cap and see if everything changes to check that. Another rare choice is a broken mechanical lift pump push rod. Pulling the pump is the only way to check that though. If you are going to do that then you might as well order the correct AC Delco lift pump and plan to swap that on at the same time.

I have a mechanical fuel pressure gauge on one of my trucks. It will bounce between 3-7 psi at idle and then steady out around 4-5 going down the road with a steady load on it.
 

edpdx

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cucvrus:
[h=1]Fuel Lift Pump-VIN: J, DIESEL, FI AUTOZONE/DELPHI CHFP906 (THEEbay) is the one I installed. As I recall the spring is on TOP at the rearmost of the lever deep in the housing. Is this the correct one?[/h]
I offer thanks for all responses so far. I want to be sure I have the correct pump before I start dropping the tank. From the steel line coming from the tank -ends in the chassis near the lift pump there is a rubber line, about 5/16" continuing to the lift pump. It's the only hose I think I have not changed out yet. Any particular fuel hose replacement (size/brand) anyone can recommend?

I'll check the fuel from the gauge port on the filter- thanks Barrman.
 

cucvrus

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Off the top of my head the fuel line coming from the tank to feed the lift pump is 3/8". I made a few of the discontinued Z lines at the front frame to pump. I used 3/8" fuel line and 1/2" split loom and it had to be a wee bit longer to avoid a kink but it worked. I am sticking with 3/8" for the feed line fuel hose size. report back. Blow some air back into thetank and that will tell you if you have a clogged fuel sock. Won't last long unclogged but a quick fix none the less. Good Luck.
 

edpdx

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cucvrus, thanks again- from your previous posts on correct lift pumps, I chose as I posted above. I do understand that the lever should be pumping from any pressure applied, but I wasn't able to depress the lever by hand- even a little so I couldn't be sure. The j-code Delphi p/n I gave, does it match the pumps you use? Is there any chance that the pump rod needs to be replaced- I wouldn't think they'd wear out.

Also, there was just a length of rubber hose to the lift pump-no "Z". I'll look in the TM- any pictures of your concoction?
 

cucvrus

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If you had no free play on compression of the fuel pump lever you had the right pump. But I would still open the fuel filter bleeder and crank it over. It should pump fuel. I agree with the clogged kinked line theory. If you have a straights shot steel feed line at the fuel pump someone cut the feed line. Figures. The modifications are never ending. Good Luck.
 

cucvmule

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Any and all "rubber" hoses are suspect. Collapsed hose is also possible.

Any air in the gage and gage will not read correctly, at all. That spot is for air to bleed.

When changing fuel filters, inspection of filter by pouring contents in to clear glass jar, do not close any glass vessel with fuel, it will give an indication of water or how much dirt, debri is in the tank.

Rubber fuel line is easy to use. But use only at areas where needed, not 3 feet. Hard line is the way to go, cuts down on possible places for problems.

Rubber fuel line at bends tend to break down sooner also, eliminate any 90 degree bend with a fitting if you have to use rubber. Vibration also happens and fuel lines need to be secured.

Transmission rubber line works but it breaks down also. Gates makes new composition fuel lines to be compatible with new type fuels.
 

cucvmule

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5 Micron filters are good but 1 micron sure does help cut down on injection pump issues. My 82' civi has a 5 and 1 micron factory filters. The 5 has a water separator on bottom of filter and drain petcock. Every now and then I will check fuel in jar for water. Buying fuel at a good fuel station is almost an illusion as the fuel is handled even in the rain when delivered so there is always a chance for water.
 

cucvmule

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Fuel pump push rods do fail. I have seen one 6.2 diesel broken in 37 years, not likely but possible. But the cam eccentric can be worn down also to the point of no lever action. And the end of the rod can be scrubbed, but also a rare occurrence.

That is another great reason to start all equipment on a regular maintenance schedule.
 

DeadParrot

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oklahoma city, ok
Double check your air filter.

From personal experience, mice will sometimes nibble on that hose from the tank to the frame. Had a fuel starvation issue a block from my house due to a mouse chewing on both lines on a 75 Chevy C10 with dual 20 gallon tanks that were both full.

If your tank to frame line hose is original, it is EOL and could be crumbling away.

It can be easier to remove the bed bolts and a few other things and slide the bed back then to drop the tank(s) if you find you need to replace that tank to frame hose.
 
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