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Unread 04-09-2007, 11:59 PM   #1
jo8243
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Starting issues & low fuel rail pressure --> help

I have a 2002 WJ 4.7L V8 2WD with 74000 miles. Recently, when it has been sitting for more than 15 minutes, it is hard to restart. This happens whether it has been 15 minutes or 15 hours since it was last turned off. When I try to start it, it takes about 10 seconds of cranking before it fires up. It turns over normally (no problems with starter or electrical system). Once it is cranked it runs perfectly. No hesitation or bucking anywhere from idle to full throttle. Battery voltage is fine, gas is of known good quality. Still has original fuel filter. Spark plugs were changed 10,000 miles ago. Dumped trouble codes and found none.

I did a little troubleshooting to try to narrow down the problem:

I hooked a fuel pressure gauge to the rail via the schrader valve which is located over the passenger side valve cover. When I did this the jeep had been sitting for about 2 hours. The gauge indicated ZERO fuel pressure. I turned the ignition switch to ON but did not start the engine. Fuel pressure remained at zero. I cranked the engine. Took about 10 seconds to start. During cranking, fuel pressure remained zero for about 3-4 seconds, then started building up gradually. When it reached about 20 psi, the engine caught and started running. Fuel pressure went rapidly to 52 psi and stayed there. Engine ran fine.

I then shut down the engine. Fuel pressure rapidly dropped to 10 psi (like in about 3 seconds). From there, over the next 10 minutes it slowly dwindled to zero.

Attempting an engine restarted yielded the same results as above. This time, I stopped the engine and as soon as the fuel pressure dropped to 10 psi, I attempted a re-start. The motor cranked perfectly. I shut down the engine.

The fuel pressure was at 10 psi at this point. I turned the ignition to ON but did not crank the motor. The fuel pressure jumped to 52 psi and then immediately fell back to 10 psi. I then turned the ignition to off. I then repeated this several times (again, not cranking the motor). Same deal... pressure jumps when switch is ON but drops immediately to 10 psi.

This time I waited 10 minutes until the pressure went to zero. I turned the ignition to ON but did not crank the motor. The pressure stayed at zero. I turned the ignition OFF and back ON again. Still zero. Again I went OFF-ON. This time the pressure jumped to about 5 psi and stayed there. Another OFF-ON cycle got it up to 10 psi. I then cranked the motor and it started fine.

Ok, I know that was long, but obviously I have a pressure bleed-down problem somewhere. I checked around the motor and under the jeep and on the tank and didn't see any obvious leaks. I assume that means the leak is either internal to the tank or a fuel injector leaking down. How can I rule out one or the other or move forward in the diagnosis of this problem? I don't have factory service manuals right now but I ordered some...

Also, I saw this TSB on wjjeeps.com, but it is listed as only applying to 99-2001 jeeps... although it sounds awful similar to my problem:

http://www.wjjeeps.com/tsb_list.htm#1400201


Thanks

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Unread 04-10-2007, 12:08 AM   #2
ForwardControl
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After reading that, I would bet hard earned money that you have a bad fuel pump or a poor seal ring like the TSB has indicated. The pump is in the tank itself, as is the seal ring. Check the fuse first, though. Have someone else turn the vehicle to ON while you put your ear up to the tank and listen for your fuel pump to spin up. If you don't hear it, or it is intermittent, then you have a bad pump. If it spins up every time, you probably have a bad seal.

Either way, you get to learn about dropping your fuel tank and pulling the pump module.

It also wouldn't hurt to change your filters. There is one inside the pump module in the tank and one inline along the way up to the engine.
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Unread 04-10-2007, 12:30 AM   #3
jo8243
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Quote:
Originally Posted by ForwardControl
After reading that, I would bet hard earned money that you have a bad fuel pump or a poor seal ring like the TSB has indicated. The pump is in the tank itself, as is the seal ring. Check the fuse first, though. Have someone else turn the vehicle to ON while you put your ear up to the tank and listen for your fuel pump to spin up. If you don't hear it, or it is intermittent, then you have a bad pump. If it spins up every time, you probably have a bad seal.

Either way, you get to learn about dropping your fuel tank and pulling the pump module.

It also wouldn't hurt to change your filters. There is one inside the pump module in the tank and one inline along the way up to the engine.
How can I be sure it's not an injector leaking down?
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Unread 04-10-2007, 12:44 AM   #4
ForwardControl
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Quote:
Originally Posted by jo8243
How can I be sure it's not an injector leaking down?
Even with a bleeding injector, you should be seeing fuel rail pressure immediatley when you turn the ignotion to ON because the pump is supposed to start and the seal is supposed to seal and the fuel is supposed to be at pressure all the way up to the injector inlets. A bleeding injector should not be a large enough leak to keep the running pump from creating pressue. If it was, your engine would not be starting 10 or 20 cranks later beacause it would be very, very flooded.

I think because you are getting clean starts ONCE the fuel rail has enough pressure, the problem is before the fuel rail itself. If it was a slowly bleeding injector you would still get the other cylinders firing with immediate fuel rail pressure. Also, with an injector leaking down I would assume that its cylinder and/or manifold section would be flooded and I THINK you'd be able to tell that...

Since you are not seeing/smelling gasoline, and since it seems the problem is before the fuel rail, I suspect that the issue is in the pump module/housing, and it follows, then, that it is either a bad pump or a bad seal.

The fact that the engine runs once fuel makes it up to the rail tells me that it IS NOT a broken line inside the pump module, or a blocked filter. Although, a slow filter could make any issue worse, hence my recommendation to change the secondary filter, and also the primary filter if you find your way into your pump module.

Disclaimer: I'm NOT a mechanic and I'm speculating here...
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2000 WJ Laredo (Champaign), 4.0L, 242 SelecTrac. [I]Added:[/I] Black front hooks. K&N FIPKII Intake. RB1 Navigation, Mopar iPod Integration. 31" BFG All Terrain T/A KO. Alpine audio upgrade (see link below). [I]Next:[/I] IRO 3" Lift, new shocks.

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Last edited by ForwardControl; 04-10-2007 at 01:15 AM..
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Unread 04-10-2007, 07:34 AM   #5
jo8243
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That sounds reasonable to me. I was thinking along the same lines. I'm not 100% sure there aren't multiple problems here, though.
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Unread 04-10-2007, 09:26 AM   #6
jo8243
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Isn't there a known issue with electrolytic caps on some PCMs going bad and causing the PCM to be unable to turn on the fuel pump? I seem to remember reading something about that before...? I guess I need to break out the multi-meter and make sure the signal to turn on the pump is actually there...

Given the fact that fuel pressure is fine when it gets going, it seems like a startup issue either with the pump motor or PCM or relay.

Comments?

Last edited by jo8243; 04-10-2007 at 10:31 AM..
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Unread 04-10-2007, 09:42 AM   #7
Vestjam
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I think you answered your own questions... Your troubleshooting was right on. Now go do something with the fuel pump...
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Unread 04-10-2007, 04:47 PM   #8
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Every time you cycle the ignition key from Off to Run or Start, the fuel pump should hummm instantly and the pressure requirement is also instantly met (31 to 42psi it depends on the model year). Don’t start the engine for this test as the engine noise masks the fuel pump. Just sit in the jeep with the doors closed and turn the key to Run and listen. If you hear nothing then wait a minute or two at the most. If she kicks in seconds later … place the PCM on a concrete surface, then select a nice Gallagher type sledge hammer and smash that thing into a coma. I think the PCM is blood/chip related to HAL 9000; the computer on Space Odyssey. Actually the capacitors can be replaced, from Radio Shack, if the PCM delays power to the fuel pump.

Some other things to remember: once the fuel line is at pressure, then cycling the key may not produce a sound loud enough to hear from the pump as it becomes more quite was it sees back-pressure. Also, if the PCM does not see the crankshaft or cam shaft position sensor signal, then the fuel pump power if be timed-out by the PCM. This is the case when you do not start the engine. So this is why you see the pressure fall.

Yours also falls/decreases in pressure too fast. But that is not really a problem as it gets to pressure instantly.

Measuring if the fuel pump gets power right away is the right thing to do as you have indicated.
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Last edited by greasefingers; 04-10-2007 at 09:11 PM..
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Unread 04-10-2007, 08:04 PM   #9
jo8243
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I verified that the fuel pump comes on every time I turn the ignition to ON (without cranking the motor). It comes on for about 3 seconds and then turns off. So the fuel pump is doing its job. The question is.... where is it leaking down? Remember from my first post that even though the pump is coming on the pressure does not go up or if it does it does not stay up.

I looked under the car. Above the rear axle on the driver's side there is something that looks like a fuel filter (about 1/2 the size of a 12oz coke can). It has 3 lines running to it. One from the engine and two from the tank. Is this some sort of pressure regulator by chance? If so, could it be allowing fuel to flow back into the tank instead of staying pressurized?

Last edited by jo8243; 04-10-2007 at 10:26 PM..
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Unread 04-10-2007, 09:27 PM   #10
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I do not have that set-up, but it really does not matter how the fuel line de-pressurizes and is very hard to figure out.

There is a check valve on the discharge side of the fuel pump. I confirmed this by disassembling the fuel pump into parts and blowing into the discharge side. Obviously yours is not working or the regulator is leaking back (my regulator is on the fuel rail but yours is not and I am not sure where it is) or it is leaking thru the injectors (highly doubtful)

But it does not really matter. If you turn the ignition key two positions forward (not sure what this is called) then the pressure is immediately maintained then count two seconds and if the engine does not fire, because of no pressure, than the fuel pump may be bad since you have eliminated the PCM power supply.

Since I do not understand your type of fuel filter; replacing it would be a smart thing to do.
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Unread 04-19-2007, 10:17 AM   #11
jo8243
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Would you believe that the day I was getting ready to drop the tank, the problem went away... just like that. It has been almost a week now and it has been fine.

I think I'm still gonna replace the fuel filter/pressure regulator (the one outside the tank).
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Unread 07-30-2012, 08:36 PM   #12
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Quote:
Originally Posted by jo8243 View Post
Would you believe that the day I was getting ready to drop the tank, the problem went away... just like that. It has been almost a week now and it has been fine.

I think I'm still gonna replace the fuel filter/pressure regulator (the one outside the tank).
any problems sence ya changed the fuel filter ?
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