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Fuel pressure questions and troubleshooting

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Old 01-20-2018, 12:12 PM
walmill walmill is offline
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Fuel pressure questions and troubleshooting

94 f150 4.9 efi
Chasing an intermittent hard start. Battery, cables, starter, have been replaced as they were all disgusting. Also did plugs,wires, and coil. I believe dist was replaced at some point by PO.
I believe i've narrowed it down to a fuel issue. I do believe PO had done work on the fuel system but I can't remember exactly what. I do notice that the fuel injectors are not all the same color, I've got black and some creamish ones.
My hard starts happen overnight/after sitting long periods of time. It isn't a temp issue as I'm in Florida and this has been occuring even before it "cooled" down here. If i restart the truck within a few hours of shutting it down it fires right up as soon as my wrist even starts turning the key, but on the first start of the day I might have to hold the key a couple seconds.

I got a gauge from the zone. Hooked it up while on the front tank and was only able to obtain 35-38 psi with engine off. WIth engine on I came up to 46, removing the vacuum to the pressure regulator brought me to 54. There was no gas splashing around from the hole but the hose did have a faint gas smell. At shutdown I held 54 pressure for a couple hours. After about 3 hours I got down to 40, another hour after that 30, about another hour 20, and overnight I dropped to 10. How quickly is pressure supposed to drop?

On the back tank I get a pressure of 52 with the engine off and I maintain around it with engine running. I'm not able to get an accurate reading with the vacuum disconnected because through manipulation a hole developed causing the gauge to wiggle back and forth +/- 5psi.

Cliff notes for those not reading all my BS.

My pressure is only 38 with engine off on the front tank and 52 on rear tank. Does that indicate a problem in the front tank system/lines only?

How quickly does fuel pressure hold without indicating a leak? I maintained pressure for a couple hours, then dropped about 10psi/hr down to 10 and maintained that overnight.

As a side note, I do get a gas smell around the truck when left sitting overnight. No obvious external leak signs.
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Old 01-20-2018, 01:19 PM
88n94 88n94 is online now
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If on the first start of the day you only have to hold the key for a couple seconds, I wouldn't think you have much of a problem. You could try turning the key to run and waiting until the pump quits humming and turn the key back off and repeating this a few times to build up the fuel pressure and see if it starts quicker when cold. Just my opinion.
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Old 01-20-2018, 05:51 PM
walmill walmill is offline
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I also forgot to add that running on the rear tank fills up the front tank. Am I correct in understanding that replacing the front FDM is the usual fix for that?

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Originally Posted by 88n94 View Post
If on the first start of the day you only have to hold the key for a couple seconds, I wouldn't think you have much of a problem. You could try turning the key to run and waiting until the pump quits humming and turn the key back off and repeating this a few times to build up the fuel pressure and see if it starts quicker when cold. Just my opinion.
true enough. But I would like things running smoothly
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Old 01-21-2018, 12:29 AM
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I also forgot to add that running on the rear tank fills up the front tank. Am I correct in understanding that replacing the front FDM is the usual fix for that?
I haven't gotten around to doing it for my own truck but yes, there is an old recall for these fuel pump modules. Replace them both with updated designs, you can still get the pumps from a Ford dealer of aftermarket, either option has corrected the flaw. If/when you order make sure you specify capacity, front/rear tank, steel/plastic, etc.
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Old 01-21-2018, 12:49 PM
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I would like things running smoothly
A few seconds of cranking is still running smoothly.

I think you are on the right track by investigating fuel pressure, but anything near what you have measured will be sufficient to start the engine quickly. Differences between tanks and running vs. idling are pretty much irrelevant as is the issue of one tank filling to the other. There are fixes for that, but you need to tackle one problem at a time.

It sounds as if you truck is holding fuel rail pressure for at least a few hours. I believe you will find that is within spec. What you need to do is to measure your fuel rail pressure before the morning start and also watch the pressure on initial cranking to see how long it takes to build pressure. My experience tells me that if you have approximately 20 lb or more lb fuel rail pressure at start up, the engine will fire quickly.

So, if your fuel rail pressure is not holding overnight, it must be leaking down and this can happen at 3 places. Fuel injectors, fuel pressure regulator, or fuel pump. Since you are going to try to remedy the cross tank filling issue, I would start with the FDM in the tank. Another clue to a faulty pump would be fuel rail pressure that is slow to build from zero. That could indicate a weak pump that also may be close to total failure anyways. You need to check the front and rear tanks separately. Release the pressure on the fuel rail before the test. Most fuel pressure testers have a manual relief valve and drain hose for this purpose.
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Old 02-15-2018, 06:26 PM
walmill walmill is offline
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Finished this up, I hope.
Originally ordered a bump but after further consideration went ahead and got the tank/pump/sender assembly.
Getting the tank down was surprisingly not difficult. Disconnecting the lines wasn't too bad either once I actually knew what the hell I was doing and even with the plastic disconnect tool that looked like something out of a Tonka Truck toy set. Finding/accessing and disconnecting the electrical plug to the pump was probably one of the single most difficult parts of this phase.
Changed the filter, replaced the fuel pressure regulator while I was at it. Used some plastic gloves and zip ties to cover up the exposed fuel lines and catch drips and keep out moisture.

Reassembly is where I ran into trouble.
First off, when removing the glove off one of the fuel lines about a whole cup of gas fell directly into my eyes. That sucker burned but MILK really put it out quick.
Splicing the wires went easily. fuel lines snapped in no problem. I struggled to maneuver the filler neck and get it to seal on the tank and reach up to bolt it to the fender. Had to bring the tank back down and start over. Wasnt able to form the new gas straps to shape and reach where the holes were so I had to use the old ones over and they seemed in good enough shape.
With everything hooked up I clicked the key a couple times to build up the pressure and for once saw 50psi on the front tank.
Fired up right away. I let it sit overnight twice and both times it fired up right away on first run. No leaks, no issues so far. Could be coincidence but it also seems that a weird shifting issue has also smoothed itself out.
Tried running on the rear tank a bit to see if the front one overfilled. So far so good. I hope that issue is also resolved and won't have to replace the back pump or tank.
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