1987 with 2 fuel pumps ?

[raggedyann]

what are the symptoms of a weak in tank pump ? I replaced the frame hi pressure. will run good one day but has low rpm struggle. can hear it backfire on accelation thru the breather every so often. I have pulled each plug wire with it running. all firing good

 

[RonD]

Weak fuel flow usually shows up first when at higher load and speeds, i.e. struggling to maintain 60mph, if in tank pump can't maintain flow then at higher demand fuel pressure will start to drop and engine will start to lose power.

Low RPM is not high demand, most of the fuel flow is diverted back to the fuel tank by the Fuel Pressure Regulator(FPR).

While back fire can indicate lean fuel mix it would just be on 1 cylinder which would make me think injector more than fuel pressure.
Have you tried SeaFoam in the fuel tank or other injector cleaner?

Have you tested fuel pressure?
1987 should have 30-40psi, with FPRs vacuum line connected and disconnected at idle, and then connected but raise RPMs to 2,500 and hold it there while watching fuel pressure, pressure should stay stable, if it is steadily dropping the longer you hold engine at 2,500rpm then that could indicate in tank pump issue, but again you should also notice that as a lack of power at higher speeds.

 

[raggedyann]

thanks Ron, yes pressure chked ok but not done the 2500 rpm test. yes I have run several cans of seafoam thru it.
I use this truck mostly to hunt in and on steep hills im usually at 1000 - 1500rpm and its very aggreviating as this is the speed I need on rough hills.
it will smooth out around 2000rpm , im thinking plugged injector and over the yrs, seafoam or anything else don't help once clogged

 

[RonD]

I would also check distributor shaft for wobble, if possible crank engine while watching rotor to see if it spins true.

Throttle position sensor(TPS) can get worn in certain area causing wrong mix for air flow, '87 will use MAP sensor so not as good as MAF sensor.
Also check the MAP sensors Vacuum hose, small cracks can also cause fuel mix being off.
TPS should have 3 wires, top wire is 5 volts from the computer, lower wire a Ground.
Center wire is what tells computer how much you are pushing down on the gas pedal.
Use a sewing pin to pierce the wire, Volt Meter set to DC volts
With key on, test top wire for 5volts, 4.8-5.2v is OK
With throttle closed Center wire should have under 1volt, .6 to .9 is spec
As you slowly open the throttle the voltage should steadily climb, no jumping up or down, nice and steady, when you get to Wide Open Throttle(WOT) voltage should be above 4.5volts

 

[trail B2]

I had the tank pump quit on my 86.It would cut out at low rpm's but it did not clean out at high rpm's.As I would leave going up a long dirt hill and warmed up and cut out badly about 5 miles out of town.The clue that helped me figure it out was when it cut out I released the valve stem on the injector rail and it released air.What was happening is when the in line pump creates a vacuum behind it it boils the gas in the line creating an air pocket.That's difficult to clear.

 

[raggedyann]

ok, fuel pressure chked ok. I have 30 psi at idle, with the fpr hose removed it has 40psi. at 2500 rpm , it hold steady at 30psi
Now the tps check. it has 5.1 v on the top black wire
the middle wire with throttle closed has 3.97 and at wot it has .57
this don't add up to Ron's specs, if im doin it correct. With key on, I pierced the bottom ground wire and am holding the ground from voltmeter to bottom wire and holding the hot from volt meter to middle wire. I have the volt meter on dc volt (20) scale

 

[RonD]

Use the engine or battery negative as the ground and retest center wire

 

[raggedyann]

ok, I used the battery for ground and it checks according to specs.
wot I have 4.5v
I can test the bottom wire by touching the red probe to the hot battery terminal and ground probe to bottom wire. I am getting 7 volts. Is this correct voltage ? thought I read somewhere I should have 11-12 volts