Scoping fuel pump current and RPM
- EricGoodrich
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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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- Andy.MacFadyen
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" 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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- EricGoodrich
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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.
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- EricGoodrich
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- cheryl hartkorn
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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.
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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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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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More information would be necessary before condemning your pump, or wiring, as the cause of intermittent stalling.
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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?
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- EricGoodrich
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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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- EricGoodrich
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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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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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- EricGoodrich
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