View Full Version : Odd fuel related issue
Took my truck today, after a couple weeks of sitting, and it started instantly. No issue there. Got out on the interstate and at first it drove just fine. After about 20 minutes, it started to miss or buck like it was not getting enough fuel. I was still able to keep up with traffic and made it to my destination. On the return trip, the same thing. It started instantly (after maybe an hour parked) After it had done it for a while, I pulled over and let the engine idle. Maybe two or three minutes later, I rejoined traffic and all was fine for another five minutes or so.
I had the same problem, although not as bad and I changed the fuel filter, and it seemed to take care of a lot of the issue, but not all. It just seems like the LP is not keeping the fuel bowl full. Can a lift pump 'slow down'? Or should I target the filter again?
master53yoda
08-09-2012, 11:59
the lift pump is powered through an oil pressure switch, The contacts in the switch are really not big enough to carry the pump, as the switch progressively gets bad it lowers the voltage to the pump and reduces its ability to pump sufficient fuel and in time will actual stop working at all. I and many others have used a relay to run the pump with the relay being activated by the oil pressure switch. This normally resolves the problem.
When you open the fuel vent line does it deliver a solid fuel stream, when my switch was failing i did not have a solid fuel stream unless i bypassed the pressure switch.
good luck
Did you splice in near the pump or nearer the OPS?
You need to splice near the OPS so that it will still serve its safety function.
You use the OPS to activate the relay and let the relay carry the heavy current.
Google OPS relay mod for diagrams ect.
This is all provided that your truck is 95 or older-the 96+ trucks have the relay mod built in.
But to answer your original question,yes the LP's will loose pumping ability over time and can even make noise and pulsate but not pump any fuel.
Are you sure the fuel cap is venting properly? Try running with it a little loose.
Truck is a 96, so it would appear that it is already fitted with a relay. I also loosened the cap and had same symptoms. Next step is to replace the pump.
More Power
08-15-2012, 10:53
Truck is a 96, so it would appear that it is already fitted with a relay. I also loosened the cap and had same symptoms. Next step is to replace the pump.
Trying to contact you using your email address, but it appears your email address is no longer valid. Pls send a message to me using the "Contact Us" link at the bottom of this page with your current email address. Scroll down the page to see the hotlink.
Thx,
Jim
To update this
>New lift pump
>New fuel pump relay
>Filler cap off.
I can make the problem occur by basically pushing the fuel usage. After about 15 of towing or heavy acceleration, it begins to starve for fuel. Truck never dies, but misses to the point that speed is limited to about 50mph. If I open the filter vent, fuel will eventually come out, but it takes 10 or 15 seconds for that to happen. Once bowl is full, I have full acceleration and speed, until the same thing starts over. The same fill rate appears whether with engine running, LP activated by key or by jumper wire at the fuse box.
Either the pump is not getting full amperage or there is partial blockage in the line somewhere. The truck never fails to start instantly, and I mean let it glow and hit the key and it fires immediately. It runs fine around town and on errands, but get out on the highway when there is a sustained higher demand, and it chokes.
sctrailrider
11-12-2012, 05:36
From what I understand, there is a screen in the inlet to the IP that sometimes plugs up, checked that yet??
The problem seems to be upstream from that. Once fuel gets to the filter bowl, all is good. The bowl is just not filling fast enough
A LP from a 93 is the best AC Delco option for these trucks,hope that is what you got-aftermarket LP's are not very good quality.
There should be a last ditch screen in the filter manager that can become clogged and limit fuel flow to the filter-it is a stanadyne part #29244,might need replaced-feet face down.
If your fuel tank has any debris the fuel sock on the sending unit my be getting clogged?
Replace the fuel return line from the IP with clear diesel rated line and while the engine idles safely-watch that line for bubbles,bubbles indicate air entry to the fuel system between the fuel level in the tank and the LP.Repairs for the leak are in order.
A LP from a 93 is the best AC Delco option for these trucks,hope that is what you got-aftermarket LP's are not very good quality.
There should be a last ditch screen in the filter manager that can become clogged and limit fuel flow to the filter-it is a stanadyne part #29244,might need replaced-feet face down.
If your fuel tank has any debris the fuel sock on the sending unit my be getting clogged?
Replace the fuel return line from the IP with clear diesel rated line and while the engine idles safely-watch that line for bubbles,bubbles indicate air entry to the fuel system between the fuel level in the tank and the LP.Repairs for the leak are in order.
Thank you for the info on the screen at the filter manager. I also plan on attaching line to actually see if there is proper flow from the tank. I hope this will point to a problem upstream from the LP or downstream, including the pump itself. I have had air leaks in the past, and it is not replicating that problem.
In addition to the "last ditch screen" in the Fuel Filter Manager, there is a wire mesh screen in the Injection Pump. You should check it and clean it.
It is part of the inlet fitting assembly coming into to your Injection Pump. As you are standing on your bumper, looking down at the injection pump, it is the right most fitting coming into to the pump. Remove the fuel line, get a socket or a crows foot (19mm) onto the base fitting. You maybe able to remove it using the upper hex (13mm) I believe. It is a two piece fitting.
Mine came out the first time using the 13mm socket, but the second time I pulled it, I had to use a crows foot as the upper portion of the assembly came loose first, leaving the 19mm portion still on the pump. You need to pull the whole fitting assembly to clean it.
Mine was filthy and plugged and solved my driveability issues. When you are there. Make sure your fuel lines are in decent shape, change the clamp too if needed to a nice stainless steel worm clamp.
A LP from a 93 is the best AC Delco option for these trucks,hope that is what you got-aftermarket LP's are not very good quality.
There should be a last ditch screen in the filter manager that can become clogged and limit fuel flow to the filter-it is a stanadyne part #29244,might need replaced-feet face down.
If your fuel tank has any debris the fuel sock on the sending unit my be getting clogged?
Replace the fuel return line from the IP with clear diesel rated line and while the engine idles safely-watch that line for bubbles,bubbles indicate air entry to the fuel system between the fuel level in the tank and the LP.Repairs for the leak are in order.
That screen looks like it is between the filter and the IP. My problem is that fuel is not filling the bowl fast enough.
If the bowl is filling too slowly then the LP is a poor quality replacement,there is an air leak between the fuel level in the tank and the LP that can't pull fuel with reduced suction from the leak,or the pickup sock in the fuel tank is plugged or a poor quality replacement from an aftermarket sending unit.
Another factor that can influence low fuel flow is a plugged fuel cap vent or the wrong fuel cap-diesel caps are designed to breathe whereas gas caps are built very differently and have found their way onto diesel tanks causing a great many problems.
If the bowl is filling too slowly then the LP is a poor quality replacement,there is an air leak between the fuel level in the tank and the LP that can't pull fuel with reduced suction from the leak,or the pickup sock in the fuel tank is plugged or a poor quality replacement from an aftermarket sending unit.
Another factor that can influence low fuel flow is a plugged fuel cap vent or the wrong fuel cap-diesel caps are designed to breathe whereas gas caps are built very differently and have found their way onto diesel tanks causing a great many problems.
I have had the same symptoms over the last week with three different LP, so I think it is elsewhere, but not 100% certain. I have also run with the filler cap off, with same results. When I can, I will attach a hose to check for air and flow. While I am at it, I will hit the LP with a pure 12v from an outside source to remove truck wiring from the list. At this point, I am leaning towards the pickup sock, which has not been replaced as long as I have had the truck - 10 years! What is the best source for this replacement? Does the entire sender need to be replaced?
Some people will blow low pressure compressed air back from the LP intake line towards the tank with the filler cap OFF to try and dislodge any debris on the sock-and then rod test to see if they are on the right track.Improvement tells you that the tank and sock are dirty.
Also while the line is being fed air it may make any leaks apparent?
Replacement of the sock often involves a new sending unit(the old one is normally rotten on top of the tank),AC Delco only to get a proper sock,unless your sock is salvageable and you get it clean and it fits the draw tube of an aftermarket sending unit?
The aftermarket sending units are being sent out with improper socks that have caused restriction problems:
http://www.thedieselpageforums.com/tdpforum/showthread.php?t=41255
Excellent idea on blowing air back into tank. Thanks
Make sure the cap is off so you don't get a bath when you stop blowing,or damage the tank.
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