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Scoping fuel pump current and RPM

  • EricGoodrich
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01 Feb 2018 20:24 #17466 by EricGoodrich
Scoping fuel pump current and RPM was created by EricGoodrich
I posted this on a Facebook group but wanted to share her for the benefit of everyone.

2003 Nissan Maxima. Vehicle came in with the complaint that it won't start. The problem was verified. Although, sometimes it would start but run very poorly and stall almost immediately. Gut instinct says the car isn't getting fuel.

But, the only time I listen to my gut is when we're talking about food.

As you can see the car isn't out of gas. The gauge shows about a third of a tank. Plenty of gas.



Fuel pressure isn't easy to check on this car.
Let's see what's going on with the pump. I removed the fuel pump fuse and installed a loop to scope current. What I found was about a 2.7 amp draw and the pump running at about 7,000rpm. My limited experience has shown about a 5-7 amp draw and 5,000rpm on most of the pumps I've encountered.



What could cause high pump speed but low current draw? I'm suspecting the gauge is wrong and the car is out of gas. I could have just put gas in it and see if it started but the pump is accessed in about five minutes so what the heck, and I just had to look. Plus, the unit needs to be replaced anyway as it ended up being a defective level sensor causing the incorrect fuel reading.

Sure enough. The gas tank is empty.



I've done some testing of my own before using a fuel pump in various conditions, inlet plugged, outlet blocked, held in air, etc. Just to see what effects these scenarios have on current draw and pump speed because some pumps are really hard to access and in the Northeast you don't want to take anything apart because everything is rusted.

POST REPAIR: The fuel sending unit was replaced to fix the fuel gauge reading incorrectly. Once fuel was put in the vehicle I performed the same amperage test and found fuel pump current 5A and pump speed 5,600rpm.

Hopefully this helps someone. Thanks!
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02 Feb 2018 15:48 #17489 by Andy.MacFadyen
Replied by Andy.MacFadyen on topic Scoping fuel pump current and RPM
Another trick added to the box, should also be good for in-tank filter that fall to bits.

" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)



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  • EricGoodrich
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02 Feb 2018 16:00 #17490 by EricGoodrich
Replied by EricGoodrich on topic Scoping fuel pump current and RPM
Definitely.

An update photograph showing the same test with fuel in the tank.

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03 Feb 2018 08:54 #17507 by bruce.oliver
Replied by bruce.oliver on topic Scoping fuel pump current and RPM
We had one yesterday, 1989 Chevy truck, the current was low at 1.9 amps. It only had 3.3v on the power feed to the pump. It would idle but barely
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03 Feb 2018 09:07 #17508 by EricGoodrich
Replied by EricGoodrich on topic Scoping fuel pump current and RPM
I think with low amperage and low RPM together I would want to confirm battery voltage at the pump. The problem would likely be electrical resistance, either in the pump or elsewhere in the circuit.

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  • Tyler
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03 Feb 2018 15:35 - 03 Feb 2018 15:38 #17519 by Tyler
Replied by Tyler on topic Scoping fuel pump current and RPM
For what it's worth, I've also had luck listening to the tank when in a similar situation. Current waveform shows low draw, but the gauge says there's fuel... The pump spinning in an empty tank has a discernible difference in sound to one that's moving fuel. That's when I add gas and recheck the current waveform and pressure.

Can work as a low tech alternative for those that don't have scopes. ;) Or something to consider if you're about to beat on the bottom of a tank, hoping to jar the pump back into life.
Last edit: 03 Feb 2018 15:38 by Tyler.
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03 Feb 2018 15:38 #17520 by EricGoodrich
Replied by EricGoodrich on topic Scoping fuel pump current and RPM
Yep. This one made some noises like it was having mechanical problems. Sucking sounds like it was empty.

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03 Feb 2018 17:07 #17524 by cheryl hartkorn
Replied by cheryl hartkorn on topic Scoping fuel pump current and RPM
i think i read somewhere for every 10 psi of preesure should be an amp of current. can anyone verify how true this is

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04 Feb 2018 10:15 #17541 by Tyler
Replied by Tyler on topic Scoping fuel pump current and RPM

cheryl hartkorn wrote: i think i read somewhere for every 10 psi of preesure should be an amp of current. can anyone verify how true this is


I've heard this figure, too. I don't think it would correlate exactly? But I'd be cool to see some comparisons. :cheer:

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08 Jan 2020 13:30 #36424 by XterraVJ
Replied by XterraVJ on topic Scoping fuel pump current and RPM
Hi Eric,

Thanks for sharing this waveform.

I have a Nissan Xterra(2004) and it stalls intermittently. I have about 3.5 AMPS current at 7K RPM after scoping. Is this low current as per you?

You mentioned taking a few readings and my suspect is a bad connector in the tank(more resistance) leading to a drop in current?

Please advise and thanks in advance.

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08 Jan 2020 13:32 #36425 by XterraVJ
Replied by XterraVJ on topic Scoping fuel pump current and RPM
Maybe wrong in this case. The current above with the new pump is 5AMPS so 50 PSI as per the rule you mentioned.

However the idle pressure reading for Maxima should be 35 PSI as per the manual.

So wondering.

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  • EricGoodrich
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08 Jan 2020 13:43 #36427 by EricGoodrich
Replied by EricGoodrich on topic Scoping fuel pump current and RPM
If low current were the result of a voltage drop in the wiring, the pump rpm would be low as well. In your case, the rpm is high and the current is low indicating, possible low mechanical resistance in the pump, ie it's not working very hard.

More information would be necessary before condemning your pump, or wiring, as the cause of intermittent stalling.

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08 Jan 2020 15:49 - 08 Jan 2020 15:52 #36428 by XterraVJ
Replied by XterraVJ on topic Scoping fuel pump current and RPM
Thanks and seems correct. I took some time and was about to reply..

So low v/g on pump due to voltage drop will reduce current (increase resistance due to drop) but this should reduce the RPM and its not.

What are your suspects?
Last edit: 08 Jan 2020 15:52 by XterraVJ. Reason: Didn't think

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  • EricGoodrich
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08 Jan 2020 15:56 #36429 by EricGoodrich
Replied by EricGoodrich on topic Scoping fuel pump current and RPM
If fuel pressure is okay with your readings, then I wouldn't suspect anything. All of the fuel pumps I've scoped have been about 5 amps and 5krpm, more or less. No gas in tank has been about half the amperage and about 7k rpm or so.

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08 Jan 2020 21:36 #36445 by XterraVJ
Replied by XterraVJ on topic Scoping fuel pump current and RPM
Thanks. So the next thing to check is the fuel pressure. I haven't done that as yet.

If appropriate, I need to stop looking I guess.

I have a P0181 code and this is leading me to suspect the fuel sending unit which also houses the connector for fuel pressure and temperature sensors and fuel level sensor.

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08 Jan 2020 21:46 #36447 by EricGoodrich
Replied by EricGoodrich on topic Scoping fuel pump current and RPM
Well, it's no substitute for a proper diagnosis, but if I were holding the sending unit in my hand, a new one is going back in, even if I can't prove there is something wrong with it. I don't believe in throwing parts at cars but if I've gotten to that point, the time I spent won't be wasted. Haha!

The only time I'll consider "throwing parts at cars" instead of diagnosing something is if I simply cannot get a symptom to happen. I'll give an example. Lady brought a car in for a check engine light. Had a crank sensor code. I checked everything, within reason. Code would not come back for me, sensor circuit checks good no matter what I do. Well, I have two options. I can either have her keep coming back, over and over, wasting her and my time and never fix the car, or we can "try a sensor". I put a new sensor in that car and it never coded for it since. So, there are times when it makes sense to try "something", but I'll exhaust every option I can before considering it.
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08 Jan 2020 22:00 #36448 by XterraVJ
Replied by XterraVJ on topic Scoping fuel pump current and RPM
Thanks Eric. That's not me holding the unit :-).

I'm trying stuff out before I go remove it and that's why these questions.

I know Analysis Paralysis isn't good and can stop an initiative.

I'll check fuel pressure first. I don't have a valve and have to use this around the fuel filter - Is this a good kit?

www.canadiantire.ca/en/pdp/equus-3640-fu...er-kit-0251020p.html

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