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Discussion Starter #1
So I have a bone stock 99'. New VP, lift pump (airtex style) and sending unit. Everytime my fuel level gets below 3/8 of a tank the truck hard starts. Today I didn't think I was ever going to get it to start back up. FP is 16 at idle. I have a mechanical fuel gauge ran into the cab with no isolator. What I have noticed is when the truck gets to the 3/8 fuel level the FP holds pretty steady after turning the truck off. It will gradually go down to 1-2 psi after 10 mins or so. Until that level, the FP falls pretty rapidly to zero when the truck is turned off. Once I fill the truck up the hard starts immediately goe away and the FP gauge acts normal again. What in the heck could this be? The only thing I can think of is a cracked suction line above that particular fuel level sucking in air. Leaving the fuel cap loose didnt help matters.
 

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Welcome. Not sure why you are expecting fuel pressure when the truck is off, the should be none. Also that fact is not the cause of your problem. So long as there is any fuel in your VP it should start. Usually you problem is related to an air leak sucking back into the system and VP Often it is from a lose banjo on back of the head or the T fitting below it.

Does it fire at all when trying to start it?
Does the tach move to about 2000 when cranking?
Does the wait to start light come on when the key goes to ON?
With that 3/8 of a tank try pointing the nose down steep hill when parking. This keeps fuel in all the lines to stop/confirm an air leak.
Being the issue is on 3/8 tank only a leak may be lower than the back of thee head.
Check the fuel filter when running for a fuel leak around the plug for the wires to the fuel heater in top of the housing.
 

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Discussion Starter #3
I am not expecting to see any pressure when I shut the truck down, it only does this when I am at 3/8 of a tank. This happended the first time I ran the truck down after the new sending unit. It hard started on me so I went to go fill it up and there was pressure released when I opened the fuel cap. Ran it down a little further this time and it no started on me. Once I got it running I filled it up and problem has gone way. Hot or cold I bump the key and it fires right up. I do have a delayed wait to start....It comes and goes just like most other 99's. Tack doesnt jump when cranking and this last time it didnt even try to fire. There was a large amount of air in the lines as lopped at idle until I could add full to it.
 

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Here's a few things to know:

There should be zero fuel pressure when the engine is not running. Any pressure remaining when the engine isnt running means there's a blockage in the fuel system.

The fuel tank should have zero vacuum or pressure in it when you open the fuel cap. If there is pressure then one or more of the injectors is bleeding pressure into the return fuel system and the fuel tank vent is plugged or capped. If there is a vacuum then the fuel tank vent is also plugged or capped. Know whether or not what you're hearing is pressure escaping or air entering.

The tank module has a flexible plastic line that moves with the tank weight from fuel. If its cracked then you'll get an air leak when the fuel level gets to that point.

It kinda sounds like you have a few things going on here...
 

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Discussion Starter #5
When I drove the truck to the 3/8 fuel level the 2nd time it was with the fuel cap loose. I figure if there was any air coming back thru the return it would go out the cap along with having a plugged or clogged vent. The only thing I can think of is that there is a crack in the supply side that is bringing in air along with the fuel when it gets to that level. I assume it is the system loaded up with air that is causing the fuel pressure holding for up to 15 mins before it goes back to zero.
 

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When it does not start what do you do get it to start, other than add fuel? Are you sure the pressure in the tank is blowing out and not sucking in? How much does it take to fill when at 3/8 tank?
Chances are the starting problem is your BAD ECM. Does it start once you get the WTS light?
Anytime your WTS light does not come on immediately with the key to ON(delayed or not at all) your ECM is going, going, gone.
It will not start until it comes on. It has not booted up until it does. It does not have to stay on(Warmed up or over 60 degrees), it just has to come on.
Why did you replace the sending unit? Just the sender or whole module?
 

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Discussion Starter #7
To get it to start when the tank is at the 3/8 level I just crank on it until it does. Sometimes it takes one 4-5 second try other times it takes 2 or 3 tries. It has done it with the WTS coming on immediately after the key on and when it is delayed. This truck will go long periods of time like this for the last 6 years. Been driving it more often lately but only put 5-6k per year on it. I dont try and start it until I see the light come up and pulses my fuel pressure gauge. The whole module was replaced as my fuel gauge hadnt worked in a while and I got tired of filling up after 400 miles to be safe and not run out of fuel. Funny thing is that its about where I am having to fill it up now. 20-22 gallons. The module was from Orielly's...$100 vs $300 for Geno's Mopar unit. At this point I am just driving around running errands close by to run the tank down. Probably 5-6 starts today and it took just a bump of the key hot or cold. Also batteries have been checked I have a brand new Denso starter on it. New VP, lift pump, hydroboost, starter, radiator, tank module and tires.....It has been an expensive 2019...lol.
 

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If you are waiting until the WTS light comes on, even eventually, It smut be either sucking air like you thought(not common), sucking air somewhere else, an air leak sucking back in sitting. If it stated right after installing module, either something is broke, damaged, wrong module, or installation error, lines or module.
Does it die gong down the road? Does it matter how long it sits between starts? Did the module have the lift pump in it? external, both? fuel line and return line crossed?
If it takes 22 ga to fill at 3/8 tank, the sending unit and gauge are correct(as it gets). Does anything make it start other than adding fuel?
 
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