Elantra Fuel pump relay circuit
 

Electrical problem
2003 Hyundai Elantra 4 cyl Two Wheel Drive Manual 108010 miles

Our car has a peculiar problem. It hesitates briefly as if it is not getting enough fuel- this happens randomly as you are driving or when the car is at idle. There does not seem to be a specific condition which would cause this. At first I thought it was water in the gas, so I drained the tank and replaced the fuel filter. I did find some phase separated fuel at the bottom of the tank, about a cup worth. I tried to freeze this but it would not so It is not water. I thought that was the answer but the problem still exists. Today I had the car and did some investigating and found that the fuel pump relay is hot to the touch- not overly hot just quite warm and audibly clicks as if it is being switched off at the same time the engine stutters. I swapped out the relay with one just like it in the same box and that did not solve the problem. The questions I have are - Do I need a new fuel pump or maybe the fuel injectors are clogged? Is the pump working harder then it should, if so what would be the typical amperage of the fuel pump? Would the ECM be defective? what color is the wire that switches the relay on and off and does this happen through the ECM or the ignition key? I ran fuel cleaner/water absorber in the tank and changed out the spark plugs to no avail. Also the check engine light comes on when it stutters then goes out when it's running smooth but I had the ECM scanned at auto zone and no error codes came up???

-Thanks!

You need a fuel pressure check under the hood to determine if you're getting fuel to the injectors to rule out a fuel problem. You need a special fuel pressure gauge that taps into the system but still allows the car to run. You can clean the injectors by pulling them out, and cleaning them manually.

Correct me if I'm wrong- my understanding is that the relay is being switched off by some other component in the system which is causing the fuel pump to shut off. This would cause the fuel pressure to drop which is causing the engine to stumble for a second then the relay is switched back on and the engine runs fine. I would like to test the fuel pressure but I don't have the equipment and would hate to cut a fuel line when I'm almost positive that the pressure would dip when the relay clicks. I removed the relay completely and the engine died, I assume that this relay turns the pump on and off, so I'm sure whatever is switching this relay on and off is faulty, or a loose wire between the two? I just need to know what controls this relay.
Thanks for the help.

It depends on if there is a check valve in the fuel pump like it's supposed to, so when it shuts off pressure is retained. The relay is controlled by the ecu, so it's real possible that the pump goes on and off. In fact, when you turn the key to the "on" position w/o starting, the pump is supposed to run a few seconds, then shut off.

That's why I'm saying the only way to know for sure is to check fuel pressure under the hood. The repair manual says jumper the system under the hood with the key off, then squeeze the fuel hose to check for pressure.

Thanks so much! I will try this and get back.

I checked this out and it seems I have plenty of fuel pressure, also I can hear the pump power on for a couple of seconds and then power off when I turn the key to the on position without starting. I did not use a fuel pressure gauge but It feels like the hose is hard. This problem is frustrating because it only happens once in a while. We drove the car yesterday without any problems, the car was as smooth as could be. but today the car was bucking violently with the engine power decreasing suddenly then almost at the same time powering back up. Very alarming while driving. Then it will go away as suddenly as it started. Can't quite grasp what this could be. I noticed that there is an electronic valve with two wires coming out of it on what I assume to be the return fuel line (smaller fuel line) close to the engine. Is this valve also controlled by the fuel pump relay or is it something else?
I don't know what this valve does or how to check it for a malfunction. It is just something else I thought of as a possibility.

You may have a crankshaft position sensor issue here. Whenever this happens do you lose the tach? Is you SES light on? A bad Crankshaft sensor will cause a hesitating engine and may happen intermittently but usually will code a P0335.

I do not have a code P0335 saved as far as I know. I went to auto zone for a scan and they told me the ecu was clear. The tach does go out, also for some reason the A/C cuts out when this happens. I don't know what an SES light is, but there are no idiot lights lit. When the engine hesitates the check engine light blinks on for an instant and then turns off when the engine runs smooth, It happens for like a second, as if someone is switching a light switch on and off rapidly. I checked voltage coming into the relay box and it is around 13.5-14.0 volts while this was happening in the driveway. is this crankshaft sensor easy to get to? We drive on very salty roads here in the winter so I'm hoping this is a simple connection issue. I just don't know where to look.
Thanks so much, this forum is a real help.

Edit- Found out that SES= Service engine Soon, same as Check engine.

You'll find pictures attached with the Crank Shaft Position Sensor location. If there not clear enough let me know I can try to make it more clear with my car.
[IMG]file:///C:/DOCUME%7E1/ERIC&A%7E1/LOCALS%7E1/Temp/moz-screenshot-4.jpg[/IMG]
You may want to start by testing your harness and sensor. Facing the connector pin #1 will be at your right. With key in "ON" position you should have 12v on pin #1 and 0v on #2. With engine "OFF" look at the resistance between pin #3 and the ground terminal of your battery: Should be 0 ohm. Pin #2 resistance should be infinite vs ground. Inspection of the sensor itself would require an oscilloscope but can try this easy test: Check the resistance between each pins. The resistance should be anything but 0 (short circuit). I had this happening to me once and was getting ready to change the sensor. It was a Sunday (no part store opened) so I had to put it back in place. I had removed the sensor so I cleaned it with brake cleaner, torqued the 3 pins on the sensor and put everything back in place with dielectric grease. The problem never came back! I may have been lucky or maybe one of my 3 pins was loose in the connector...
If you plan to remove the sensor be very gentle. Theses sensors sticks to the motor body and can crack easily. If it breaks the end of the sensor may end up in your oil pan so be careful.
As for the check engine light it will only come on and code if the problem happens 2 driving cycles in a row.
This sensor is vital. The ECU uses this sensor to determine the RPM, injection duration and ignition timing.

Great Info!!
The pictures are very clear.
I will check this out ASAP and let you know what I find.

Thank you so much!