I was driving on the highway today, and my truck just died for about 10 seconds or so, then refired, and felt like it was running kind of ragged. I got a code in the computer for secondary fuel pump circuit failure a couple days ago as well. I'm thinking my high pressure pump is on its last legs. What I was wondering, is do the low pressure in-tank pumps provide enough pressure to limp the truck home? The truck has done this on two separate occasions, and then a minute or less later, it catches and keeps going. It's an '87 F150 with the 5 liter, low pressure in-tank pumps, and one high pressure on the frame rail.

 

I had another thought. Are the fuel pumps wired in series? If the in-line pump cuts out, that cuts off the in-tank pump too, is that right?

 

They are not wired in series, the in-tank pump only runs when you have that tank in use. The pump the tank not being used is off.
You could have a worn out fuel pump relay, I would replace it first. ( they are usually green ).
Jimmy

 

I know about the two in-tank pumps. I'm thinking that the high pressure in-line is wired in series with the low pressure in-tank pump. So if the high pressure frame rail pump shuts down, it takes the in tank pump with it. I was curious if this was how they were wired because the truck has stalled while running on both tanks. I'm going to replace the fuel filter this weekend and see if that fixes it. I will double check the relay too, but I don't understand why it would just randomly shut off while driving down the highway and kick right back on10-15 seconds later. Thanks.

 

If the contacts in the relay are worn and corroded, they may vibrate to a bad spot and lose contact. Then when you turn it off, the relay releases, When you start it back up, the contacts hit a good spot and the pump runs again.

They may be wired in series, thru the main pump, then thru the tank switch (front/rear) so only one tank pump runs at a time.There is only 1 relay, now that I think about it.
One quiting, shouldn't cause the other to quit. I haven't seen a pump quit while running. They usually fail to come back on when turned off.
Jimmy
Jimmy

 

Pumps in series would be just bad electrical design. These pumps are not electrically in series! The in tank pump is just a helper and does not produce much pressure and won't run an engine. The high pressure pump is a gear pump which looses the ability to pump if a vapor bubble gets in the line. These pumps have little suction capability. The in tank pump insures that the high pressure pump always sees liquid. A pump failure seems age/mileage appropriate. This is often the result of brushes sticking when they get shorter. Since you don't have to drop the tank, I wouldn't think too long about this. Change the pump and the relay. Then, I hate getting stuck somewhere.

 

I still think the in-tank pumps are wired off of the high pressure pump. I know the low pressure ones won't run the engine properly, but I guess they won't even produce enough to limp it home either. When I unplug the high pressure pump to bleed the pressure off the system, I don't hear the in-tank pump running. That's what I meant about series. I will probably see if I can't get a new one in the next couple days. It's really uncomfortable to have the truck die while cruisin down the highway in the fast lane. The truck has 130000 miles, and the rear in-tank pump has already been replaced, about 40000 miles ago. If the brushes in the motor for the high pressure in line pump stuck, would the pump randomly turn back on a few seconds later? That seems like it is possible to me. Thanks for your help, and I hate getting stuck too.

 

Keep a rubber mallet in the truck. A little hit will do it. A different definition of series. The high pressure and in tank both run on 12V. I can believe there is a connector for the high pressure pump which jumpers the 12V to another connector for the in tank pump. This keeps a common wiring harness between single and two tank trucks. That connector (in a two tank system) connects to a wire harness with the tank switch that selects the two tanks. My Chiltons doesn't give any further detail than both pumps run from 12v.

 

Would the pump cut off if the pressure in the lines got too high? Say if there was a blockage in the line, then when the pressure dropped, it would come back on?

 

no, there is no pressure switch in the system. I just wonder if the ignition module or pick-up coil in the distributor is failing. The EEC uses input from these to detect engine rotation thus run the fuel pumps. I have seen a few of the pick-up coils start to fail and the truck will act like it is out of fuel. then it will restart after it cools off for a short while.
Jimmy

 

That thought crossed my mind as well. However, if the TFI crapped out, would the tach still show rpms?

 

While that senerio could be true, that caused me to look at your first post again. The only way you can get a fuel pump error code is if the relay or the associated wiring close to the relay is bad (connectors on relay or fuse). The computer monitorsthe voltage switched by the relay. This is the only monitoring it does. I would buy a new relay since you got that code!

These early systems are high bypass. 80% of the fuel goes back to the tank from the pressure regulator. Some pumps have their own bypass in case the filter gets clogged. They just run and don't shut off.

 

Would the relay just click off and back on while driving down the road? That's the thing that bothers me. Relays aren't too expensive, I'll get one and see if that fixes it. I'm just nervous about getting myself stranded somewhere. Thanks for your help.

 

the tach would probably still work when the tfi is screwed since the tach is hooked in with the coil

 

But the coil gets its signal from the TFI. If it had something, it would still run. The truck hasn't cut off on me recently, and it's running the same as it has since the MAF swap - not too good. All the times it has cut out on my have been after a high load situation - punching the throttle hard or driving down the highway at a high speed. I was thinking there may be some kind of blockage in the fuel system, but I don't know for sure. I get a code 96, secondary fuel pump circuit failure, pcm to ground. Any ideas where the faulty ground could be?