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Hey guys. I already have a topic about my crazy stalling problem and low rough idle. I've kind of tracked down the cause of the low and rough idle and I THINK its low fuel pressure. I'm almost positive it is because my fuel pressure lowers as the idle gets lower and rougher. Cruiser is of course trying to help me with it as always but I want to know if anyone else has ever had this problem and what caused it. I have changed the fuel pump, regulator, filter, injectors with Volvo's, checked for leaks everywhere and only found that the lockring is leaking when the tanks close to full. The problem is my fuel pressure starts off at 29 when I start the engine. Then as it warms up and runs longer the fuel pressure slowly drops as has gotten as low as 18 PSI. Like I've said, I've changed the entire fuel system... pressure shoots to 95+ when I squeeze the return line. Does anyone have any insight as to what this could be? Getting good voltage to the pump. Double checking grounds soon. Thanks in advance as always guys.

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Ballast resistor still in place?

Have you ever blown the lines out with air?

Filter in the correct way?

Is the tank properly venting? Check the EVAP lines.

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How do I check the evap lines and how can I tell they're okay? And yes filter is in correct. Ballast resistor is bypassed because the aftermarket one I bought was to many ohms and my fuel pump ran line crap with the higher resistance. Haven't blown air in it, yet.

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I know this because I went through this same problem, but for a different reason (bad gas and varnish/crap everywhere). Do you remember if you tightened both hose clamps that clamp on that little hose going from the pump itself and the pump/sender cage? That bit me in the butt last week, tightened those, and it runs as well as it can until I replace the injectors. Almost sounds like the new regulator is bad. Doesn't the vacuum go down as the engine warms up? If it does, that might account for the loss in pressure as it warms up, the regulator lets more fuel bypass the injectors then it should.

 

Jerry

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Highest vacuum reading will be at idle, and it goes down as rpms increase with the lowest reading being at WOT IIRC.

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How do I check the evap lines and how can I tell they're okay?

It's just 3/16" steel fuel line from tank to EVAP cannister, with rubber fuel line at the connections, check condition of the rubber, and blow air through the line. My thought was if the steel line is corroded/collapsed/blocked it may be creating a vapor lock as the tank won't vent properly (although usually they will pressurize, but it's cheap and simple to check).

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The tank doesnt have any pressure in it because of a lockring leak and a gas cap not being sealed. Do you think this could affect things? my regulators been changed twice. I'm pretty sure I tightened down both hose clamps good but I'll double check when I seal up the lockring. Are the EVAP lines the ones that go to the charcoal cannister and all that? I know they're pretty crusty...

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Yeah they're not in great condition but I don't believe they're leaking or anything. I will check though. Can leaks in those lines cause low pressure somehow?

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Is the gauge you are checking the pressure with a liquid filled gauge?  I know those things lose accuracy as they get warmer, and usually read lower.  This may be giving you a false reading that the fuel pressure is lower than it actually is as it gets warmer.

As far as the low pressure.  What is your rail pressure when cold, with the regular disconnected from vacuum?

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Yes its a liquid filled Jegs gauge which is permanently attached to my fuel rail. The same one Hornbrod used in his write up. When I just run the fuel pump without the engine running it is 34. When the engine first starts it is 29/30. With the engine running and vacuum disconnected it is about 36-38.

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Upon inspecting all of the Evap system that I could from the hood (rained today, can't get under the truck because of puddles) I already have found two problems... on the smaller line from the charcoal cannister (the one that is rubber for a long ways instead of becoming a hard plastic line) I found this crammed inside it. I have no idea what it is OR how the hell it got there and then I blew into the line and heard my air coming from behind the valve cover... weird. But, then I found problem number two. This it seems the heat from the engine burnt a good sized hole into the rubber line. Now, what type of engine problems can these two things cause?

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Its the one that connects to the metal line and goes under the truck. A pretty big vacuum leak seeing the size of that hole... what type of rubber line can I use for this and what size? Is fuel line required? Sounds like a pricey kind of line lol.

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I am guessing that I don't have any leaks in fuel lines. Or the injectors and I'm sure the regulator is working right. Because the pressure stayed at 34 with the engine off for about an hour if not more before it started lowering down.

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Pretty sure this is p/o the vapor line from the fuel tank to the charcoal cannister. If so, it's a 3/8" ID vacuum hose. Cut a piece out and bring it the the Zone or similar and replace the soft hose portion.

 

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Or, remove the EVAP system entirely, plug the end of the vent tube from the tank, and put a vented gas cap on your system.   I would only do this if you don't have emissions inspections and testing in your area.  Otherwise, do as suggested above.  That hose is a bit of a pain to get off of the hard line, Mine was also cracked and was a source of a vacuum leak when I first got my truck.  Made a big difference in how the truck ran.

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I want to keep the evap system for a little bit. I don't have emissions thankfully but doesn't the evap system help with gas mileage? I'll get this fixed up asap. Hoping it fixes it...

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Fixed the line. The trucks running better. Aint fixed, but runs a lot better. I still have little backfires when it gets warmed up but they aint as bad as before. I'm thinking the low fuel pressure is the gauge reading bad because of heat. The idles halfway smooth though.

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I wonder if you don't have a "flapper" in a rubber line. That's wear the line separates internally and flaps the opening closed when the liquid travels through it. 

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This may not be yours, however about a year a go mine lost lots of pressure, turns out the hose in the gas tank had a crack in it. easy fix just pull pump and float stuff out and replaced the line.

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I've got to get around to pulling the pump and all back out of the tank. Got a brand new pump and line in there but I need to check the line to be sure. And I need to check distributor indexing (for idle problems) and rent a fuel pressure gauge to see if my gauge on the rail is even reading correctly

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