fuel pump timer?

jbugj · #1 11-28-2007, 07:55 PM

My 92 laredo fuel pump runs no more than one second when turning the key to the on position (not starting the vehicle). Is this a programmed time set by the ECU? I have a different make vehicle and that pump runs about 3 seconds. Original fuel pump, the vehicle has to be cranked about 5-7 seconds before starts. I am going to raplace the pump, but was wondering if this "timer" issue is a concern.

citizen@large · #2 11-29-2007, 01:04 PM

The fuel pump only runs long enough at first to pressurize the fuel lines leading to the fuel rail to the injectors for initial cranking/starting purposes.

So, it's not a timer thing per say, but a pressure thing. Mine('98) comes on for a sec or two, and starts almost immediately. My better-half's('92) takes around as long as yours does to turn over.

You would want to turn your key to the ON position in order to pressurize the rail, then check with a fuel-pressure-checker thing from advance/auto zone/ napa... for correct pressure at the rail. (there's a fitting like on a valve-stem on the fuel rail) Haynes manual or = should have the correct #. I forget now what it should be. sry

Check that before you swap in another pump. $

jbugj · #3 12-01-2007, 04:39 PM

Replaced pump, start time reduced tremendously. Checked pressures in the past, and had bleeddown. Probably bad checkvalve in pump, or rubber line in tank could have had a leak. Found that line almost off one time when had driveability issues. Be aware this jeep had two different size pumps. Have to pull assembly to know which yours has, or buy the complete assembly, cost relatively higher. Can get the assembly out without pulling the tank, a little difficult working from below but very do-able, just make sure fuel is low. As far as the "timer", I don't believe the vehicle has a pressure sensor that shuts off the pump when doing the "key-on" thing, so is it the ECU? Kinda moot at this point, but would have helped with a longer pump-run time when I had the starting issue.

citizen@large · #4 12-01-2007, 04:50 PM

Hmm.. That's all very interesting bug...

Fred, Aloha, Dave, etc... Care to chime in here? Is there a timer involved with the fuel pump, or is there a pressure sensor?

Points are not moot I say!

dave92cherokee · #5 12-01-2007, 06:30 PM

I think it's kind of a mixture of both, if you think about it the ECU controls the fuel pump and when it see's lower fuel pressure makes the pump kick up a little to bring pressure back (at least that seems the most common sense thing). So if the pump is bad even when the computer tells it to speed up it can't and takes longer to build the pressure required than a rebuilt or brand new one.

citizen@large · #6 12-01-2007, 07:12 PM

My friend, that is one unique perspective. But Why? Known Truth?

dave92cherokee · #7 12-02-2007, 10:23 AM

Well like I said I'm not 100% sure on that but it does seem the most logical. If you think about it the fuel pump is just a mechanical pump driven by a motor to pump fuel from the tank into the line and to the engine. Once it builds pressure if there was no way of regulating it at startup then it would just continue to run. So either there's a pressure sensor that sends a signal to the computer when the pressure reaches a certain level, or when the key is turned to the on position the ECU sends a signal to the pump for a couple of seconds to build a little pressure in the line for starting. Because if you think about it how many cars do you see that are the ages of our jeeps start when the engine turns over the very first time, jeep's won't even do that brand new it usually takes a couple of revolutions to get it cranking and not needing the starter anymore. That's why I feel sorry for these people that are buying these new Lexus' and Mercedez's with that push button ignition because that's designed to start on first revolution. If I'm wrong I'm sure aloha will chime in and let us know but when I was installing car alarms in the past I remember we had to program them for remote start that we would have to adjust the starting timer depending on vehicle some it was 1 second others it was 3 or more.

AlohaBra · #8 12-02-2007, 11:57 AM

The powertrain control the fuel pump relay. I am only familar with the late models, because it has been a long time since I had an early one (84). I looked at the 2000 MY service manual and found this information.

Anyway, when you turn the key on without cranking, the fuel pump relay energizes for about a second an a half. If you listen carefully, you will hear it shut off. You can repeat this just to make sure the pressure is up. Every time you turn the key on it will energize the fuel pump relay for 2-3 seconds.

The FSM spec says that fuel pressure should be achieved in 1-2 seconds. It is normal for pressure to be zero when sitting overnight.

Tip: I made it a habit to turn the key on and wait for fuel pressure before cranking. It will keep you from loading the battery too much when cranking excessively.

jbugj · #9 12-03-2007, 08:16 AM

My understanding of the fuel system is the pump never shuts off (while vehicle running) to control pressure. I believe the pressure is regulated totally by the fuel pressure regulator on the fuel rail. I agree the remote starter would be an issue with the long crank times like I was having. The "timer" is an interesting issue I don't understand, one other thing I noticed doing the "key-on" thing is once you did it and turned the key off (even waiting a few seconds), the pump would not run the next time the "key-on" was done. I wish I knew whether it is a sensor thing, or built in the ECU, and if the ECU whether this is all normal.

AlohaBra · #10 12-03-2007, 09:25 AM

Are you sure? Mine runs 2-3 seconds every time the key is turned on (1997-2001 series).

ponchonutty · #11 12-03-2007, 02:02 PM

Well, all of the fuel injected cars that I've messed with lately ran on the amount of pressure-not time.

citizen@large · #12 12-03-2007, 10:56 PM

All-Right. Now all you sum-guns are gonna have me paying attention to all of my starts from now on and somethings going to start acting funny.

Thanks! A Lot!!

AlohaBra · #13 12-04-2007, 10:04 AM

Quote:

Originally Posted by ponchonutty

Well, all of the fuel injected cars that I've messed with lately ran on the amount of pressure-not time.

It is not a timer per se. But when you turn the key on, the powertrain control module signals the fuel pump relay. Then it look for a signal from the CPS. If there is no crank, then it times out.

That is so if the fuel line is cut or not connected, the fuel tank won't be emptied.

The fuel filter and the fuel pressure regulator in the Jeep is a combination unit.
It is mechanical and does not signal the control unit. It is located inside the fuel tank.

See the 2000 XJ FSM for more information and wiring for this.

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