Hi - I'm new here, great site. First let me say that I have utilized the 'search' function here extensively and have read/learned a lot about he issues i'm dealing with. Also, it sounds like lots of others have had same/similar dual fuel issues.

Scenario: 87 4wd 6cyl - dual fuel tanks. Truck will suddenly run rough - eventually die. Unclear as to what effect switching between tanks has (i.e. each tank will sometimes run - sometimes not). It has historically been run mainly on the front tank, but historically didn't used to have a problem running on rear when switched either. Have checked/tested the fuel pump relay and believe it to be fine. Have run with a fuel pressure guage and it will reach a normal/proper psi, but then you can watch guage as it starts to drop off (sometimes slowly, sometimes quickly) then motor kills. FYI, you can hear the high pressure pump kick on when you turn key to on.

Impression based on research to date: first I thought that this was fuel selector switch going bad. However, i have read every post that I can find and now I realize that I do not fully understand the role of the in-tank, low pressure pump as it relates to the tank selector switch doing its job (i.e. the possibility for it to either allow fuel to be pumped from one tank to the other and/or pull fuel from front tank while having gauge read from the rear).

In hinesight, I do recall that after I filled up with gas (and then start driving again) that occaisionally I would smell gas upon getting home (or park somewhere).

Can someone please give me some advice on troubleshooting / eliminating pieces in the equation. My impression WAS that it was the selector switch/valve, but as stated above, now I'm not sure if it could be one of the low pressure pumps in one of the tanks.

PLEASE HELP. If i've done a crappy job describing problem and/or if someone needs more info in order to help - please just say so.

 

The in-tank Fuel pumps put out 4 to 5 psi and are used to prime the system and change the Selector Valve/Reservoir, lock it in that position and prime the hi-pressure pump if you run the tank dry or very low. Other than that you do not need the in-tank pumps and the hi-pressure pump will draw from the tank it is set up with (in the selector switch valve) and supply the fuel pressure for the injectors no matter what tank you have the dash switch on.
The check valve is in the High Pressure Pump on the Frame and the High Pressure Pump supplies the pressure for the injectors and the Fuel Pressure Regulator returns the Fuel to the Selector valve for routing back to the Tanks.


Here is a diagram of your system:



The switch on the dash just changes whitch in-tank pumps run.

Here is a diagram of that system:



Here is a digram of the Fuel Selector Valve:

 

"Have run with a fuel pressure gauge and it will reach a normal/proper psi, but then you can watch gauge as it starts to drop off (sometimes slowly, sometimes quickly) then motor kills."

It sounds like the High-Pressure pump may be getting hot and quitting.
Other than the Hi-Pressure pump you may have a plugged up fuel filter, socks in the tanks or a Fuel Pressure Regulator going bad. Fuel dropping while running on a tank does not sound like an in-tank pump (boost pump) or a selector valve.

"I do recall that after I filled up with gas (and then start driving again) that occaisionally I would smell gas upon getting home (or park somewhere)."


This could be an "O" ring sticking on its shaft (of the supply and return cutoff valves) in the fuel selector valve and sending fuel to the front tank while running on the Rear Tank. This is the most common problem with Fuel Selector Valve. The smell comes from the front tank over flowing. The way the valve is made it can not fail and send fuel to the rear tank while on the front.

 

Is getting hot and quitting a sign of it going bad? In otherwords, do they become more sensitive to heat/overheating as they get old and start to fail? I should have mentioned in my first post (but was worried about it getting too long) that the problem does seem to rear itself after it has run for a while, but once it comes out it will repeatedly fail pretty rapidly from that point on. As an example, after driving the truck successfully for about 15 minutes on Saturday it failed, then failed repeatedly as we tried to trouble shoot. A friend tried to start it on Sunday morning and he said it started, ran fine (with good fuel psi), then after about 30 minutes it (the motor and fuel psi) just died suddenly. Then wouldn't start.

Also, although it could just be failing now - never had any problems with it over the recent winter/colder months.




Although I shouldn't "assume" anything, I doubt it is the filter as that has been replaced somewhat recently.

What are "socks in the tanks" and how could I test/fix this (i'm afraid you are going to say - drop the tanks?

I know I can re-search the forum and find a trouble shoot test for the FPR, but if you have any specific tips you'd like to add to this thread that would be great (but I can dig it our of archieves.


If you assume the o-ring is sticking, and if you always run the front tank down so its capacity always excceds the total fuel in the rear (before switching to the rear tank) - can't you avoid this problem and therefore get away without replacing the switch?

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SUBRORD, THANKS FOR YOUR HELP.

 

It has an electric motor on it and they do this sometimes on any application.

They are a kind of filter for the in-tank pumps and yes you drop the tanks or pull the bed (easier) to service them.

This would be an odd way for the FPR to fail but it is a possibility.
If your Fuel Pressure is low (below 33 psi) and you squeeze the return line and if the pressure goes up the FPR is bad. If it does not go up then you have a bad Hi-Pressure Fuel Pump, Injector or clogged sock.
Also if you pull the vacuum line off of the FPR and fuel comes out it is bad.

Yes

 

Do not forget if the ECM does not see the pulse train from the Ignition System it will turn off the Fuel Pumps. A lot of times the Ignition Module or the pick-up coil in the Distributor will cause this.

Also to test the fuel pump pressure you need to ground pin #6 of the test connector with the key on.
The pumps will only run for two seconds after you turn on the key if you do not ground pin#6.

The test Connector:

 

Thanks, this gives me some good basis to go do more testing. Really appreciate your help.

 

OKAY!!!!

First off, my trucks issue was solved by replacing the fuel selector valve.

Some have you may have seen my other thread (RE: picking a selector valve for an 1987 6cyl with dual tanks). Thankfully, I heeded Sub's advice (THANKS SUBFORD!!!) and checked out the selector valve from the Ford dealer (i.e. the canister/mechanical/resevoir style). Although I can't say for sure that none of the one's in the URL link I provided in my other thread (i.e. the ones that 'checker auto' website said were compatible) wouldn't have worked...... they don't look at all like the Ford part. I Know I wouldn't have wanted the "powered" selector valve.

I have to admit something here....... in the end I decided to hire a mechanic friend to change it out. I know, kind of a cop out.... but was a bit short on time, a lot short on patience and wasn't looking forward to crawling under this rust bucket without a lift etc.

I want to thank all of you that responded to my questions along the way. This site is a great resource (site wouldn't be the resource it is without the people posting up all the info). Although the rig is once again running like a top...... i have a feeling i'll be back with more questions. T H A N K YOU!