i figured it's possible that I'm having a voltage drop to the pump that could cause issue or have caused failure.

I'm a plant maintenance guy, i understand electrical really well, we deal with all kinds and lots of low voltage 24 ac and dc.

 

excelent.
I probably managed to offend you by tellign you about DC motors then hahha
My work here is done
/ swipes hands together haha

 

oh no, you've been very helpful. I wouldn't be here asking for help if i wasn't willing to try all the suggestions people give. We have all been there, can't figure something out after hours of work, then you finally ask for help and the guy flips a switch and its fixed, leaving you looking like a fool.

 

are u saying uve fixed it??

 

not yet. I didn't get to check it this morning, had a job in the am and then had my part for my hvac come in so i installed that before work. I plan to do my voltage check tomorrow morning and fashion up a pressure gauge to see if my pump is healthy. I should have an answer by tomorrow morning.

i probably also need to get s mod to change the title of this thread to low rpm missfire or something, it has some good info in it that could help others.

 

And what number would that be?

There is no spec on the "stall pressure" of the fuel pump, but since the rail pressure is 33-44psi (depending on vacuum), then we can assume the stall pressure would be, oh, 100psi? If you block the fuel return line with your pressure gauge, you'd raise the rail pressure to the pump's stall pressure. Is that a good thing for the FPR? For the injectors? I don't know. And since that test would tell you (basically) nothing, and would only save you a few bucks over the correct test, I sure wouldn't do it.

If you get flow out of the return line at idle, then you know the pump is putting out at least 38psi (if the FPR is holding correctly). Beyond that, you can directly measure the fuel pressure with this rig: https://www.summitracing.com/parts/anm-cp7838 (which has the correct banjo-schraeder adaptor).

No, that's not how an FPR works. The whole point of an FPR is to assure that the pressure ACROSS the injectors is constant. That way, the ECM only needs to adjust the open-time of the injectors, and the liters/min during that open time will be constant. The pressure on the intake side ("manifold pressure") of the injectors goes up and down with throttle position. So the manifold pressure/engine vacuum is fed back into the FPR to keep the rail a constant pressure above the intake. "No" vacuum in the intake is WOT, so the FPR sets its highest pressure (44psi or so) to squirt out fuel at the same rate. But the FPR HAS to restrict the flow to get the pressure up there; so "No" vacuum has the LEAST flow from the FPR to the return line.

 

When voltage is restricted, the current goes down, not up. It's not just a good idea, it's the law. (Ohm's law.) (There are exceptions for boost converters, but this truck is about 20 years too soon for something like that.)

It IS true that an intermittently failing EFI relay could contribute to early failure of the fuel pump. As the fuel pump loses power and coasts down, then gets power and has to spool up, it draws more current during startup (because the back-emf is speed dependent). Forcing the fuel pump to regularly re-start isn't easy on it.

But you'd never get this situation; as soon as the EFI relay opens the whole truck quits.

Failing components in one place can contribute to failure of components elsewhere, but not by violating Ohm's law.

 

i was told by my best friend, who is an electronic engineer that the 12v dc motor will get hotter when its power feed is restricted. i may have got the terminology backwards, but this i am almost certain of. i was also at lunch with another colleague who is an electrician and we were talking about another colleagues fuel pump which had an early failure, and she reitterated the same thing my electronic engineer mate said. so what did they say...
haha

 

yeh nah im right
a predominantly inductive-load (eg, motor) decreasing the source voltage will result in an increase of load-current if the driven-load is constant.
i think somehow the motor draws more current.

 

I know that 2 and 3 phase motors when they lose voltage on one side, or across the phases will draw more current than when they are at their rates voltages, it is definitely true when using a VFD to drive then (until it faults out on over current or under voltage).

for a single winding, 12vdc, with no load, if the voltage is decreased then the current draw will also decrease. If the motor is under load, then the motor will try to stall. Under those conditions i could see the current draw increase with the loss of voltage.

 

indeed a stalled motor draws its max current, that is to say. As motor speed increases from zero current is reduced.

​​​​​​

 

This isn't that hard to understand.

Hop on your electric bike. You can "restrict the voltage" to the motor with the throttle, which in the simplest bikes is just a potentiometer. Do you think you can ride longer (minutes, not miles) at full throttle, because the motor "pulls" more current at half-throttle?

Chuck a paint stirrer in your cordless drill, and dip it into a bucket of water. Will the battery last longer with the trigger pulled to full speed? Do you expect the motor to run hotter at half-speed?

Note that in both of these cases the load (torque) is speed dependent. If you ride your bike at a constant mph, going uphill you'll have to open the throttle to keep it at the same speed because the load has increased. Open throttle - more voltage to motor - more current draw.

Perhaps this IS just a case of getting the terminology backwards. But a fuel pump will not run hotter if you reduce the voltage. It will just run slower. (Well, for that matter, it's immersed in cool gasoline, and gasoline flows through it as a coolant, so it is will nigh impossible to shift the temperature of the fuel pump more than a few degrees just with voltage.)

 

but current isnt power. power is voltage multipled by current. now im not 100% on this stuff so im not going to make any serious hit home points, but i can assure you the people that have told me this are paid to use this information and I assure you I wouldnt go blurting out something that I wasnt confident in.
Now using my own uneducated thoughts on the matter, perhaps it happens becausr the motor slows down and more current is drawn because it cannot draw more volts. Perhaps its something like that.

 

It depends on the nature of the load. If the load is truly constant power regardless of motor speed, current will go up as voltage goes down - but there are very few loads like that. For most loads, the power decreases either directly as speed decreases (constant torque loads), or decreases faster than speed decreases (if torque decreases as speed decreases as well.) The latter case probably describes the fuel pump, since if it turns more slowly the fuel is not being moved as fast through the lines, thus has less flow resistance, thus takes less torque from the pump. So, in this case output power decreases at a greater rate than speed decreases.

But, the other question is, does the motor slow down when the voltage goes down? If it keeps turning the same speed, then output power stays constant as voltage decreases, and the only way that can happen is for the current to increase. So, let's explore that a little.
- In the case of AC induction or synchronous motors, speed is largely set by the power line frequency. Thus, these types of motors tend to increase current and overheat at low power line voltages. They can't slow down without stalling because they are locked to the power line frequency, so all they can do is draw more current trying to provide the same amount of output power. That's why brownouts are so tough on air conditioner compressor motors.
- Speed regulation is different with DC motors. Speed is essentially controlled by back emf*, which is a product of speed and current draw for any given motor. The motor speeds up until back emf minus voltage drop in the windings (due to resistance) equals the driving voltage. So, if you decrease the driving voltage, the motor has to generate less back emf, which means it has to either slow down, or draw less current, or both. In the case of the fuel pump, where output power decreases at a faster rate than speed decreases (as postulated above) the current must also drop to satisfy the power equation.
- If you had a constant torque load on the DC motor, the current would remain roughly constant with decreasing voltage, but the motor would slow down.
- The only way for the current to increase as voltage decreases is if the load actually required more torque as speed decreased. I suppose such loads exist, e.g. pumping some sort of non-Newtonian fluid, but I'm pretty certain the fuel pump isn't such a load.

*Back emf (electromotive force) is the industry term for the reverse voltage that the motor generates as it runs. It effectively generates an opposing voltage which "pushes back" against the driving voltage. This back emf increases as speed increases, and eventually reaches a point where it just balances the driving voltage minus the amount of voltage required to drive current through the windings to provide power. At this point the motor is in equilibrium, and will run at a constant speed until the load changes or the driving voltage changes. If the load is very light, the motor will speed up until the back emf very nearly equals the driving voltage, limiting the current to a very small value and thus taking very little power. As the load increases, the motor slows down, back emf decreases, and current increases to provide more power.

 

I think rjr hit it with that post.

I'll also say this thread has derailed slightly. I love a friendly debate and share of information.

to put it back on track. ..

I'm going to dig into it further tomorrow. It snowed about 6 inches today, tested out the 10 tundra, did very well. And with snow on the ground, propane heater in the garage, I'll stay in there working. Ill update with the findings tomorrow.

 

yehh. welll..... my kid is cuter that urs haha
That is a much more in depth explanation than my mate gave, and perhaps he is missinformed, but i dont know what ur experience or knowledge level is., and u may also be missinformed.
Truth is i cant find anything on the net to back up what i was told...
Nonetheless,, after the discussion at work on the topic, and after we both got the multimeter onto the pump and did a voltage drop test, we found a 3 volt drop on the supply wire when the pump motor was running with engine at idle, replaced the wire, relay, and the pump all at once and there was no more voltage drop when running. After that he never had a problem again. Perhaps he was just unlucky that the first replacement pump was just no good. maybe.
His original symptom was the engine cutting out after about 10 minutes, and then it was ok for about 2 months with the new pump then started doing it again. That experience really got me on that train of thought that faulty wires and relays can break fuel pumps.

 

i think perhaps the load on the pump does vary based on the fuel pressure regulator. if its pumping slowly, the fuel flow is slow and regulator or the in pump regulator (relief valve) responds accordingly closing and trying to increase pressure.

 

To update this, i jumped b and fp in the diagnostic connector. Battery voltage 12.05, voltage to b 11.87. With the fuel pump running i have 10 volts to the pump.

that's pretty low in my opinion.

truck running 14.48 volts at battery and 13.14 at pump

 

ok u need to test if that is loss in the wire or just std voltage drop from running a circuit.
u need to put the red positive tester lead on battery positive and black negative lead on the + close to the pump.
to do this make an aligator extension for the battery and make sure it contacts good.
the drop MUST be less than 1.5 volts.(ideally leas than 0.8 volts).
if not, consider this an issue.

 

and i can absolutely confirm what i said to be true. My mate explained it to me. The fluid doea in fa t act like a newtonian fluid. because of the constant pressure regulated by the fuel system. It does burn out the motor if u have a voltage problem. End of discussion on that.