Poor throttle response after replacing return lines

Theaxehand

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1988 F-350 with 7.3. I bought it not running. Determined the IP needed replaced. Bought new IP and installed it and truck fired right up and ran great. Drove it for a few weeks and decided I would replace the return lines. Got them all installed and I took it for a drive. Cruising down the road just fine until I let off the throttle and the truck died. Fired back up but only with foot on the gas pedal. After this happened I’ve had very poor throttle response and the truck dies every time I let off the throttle.

I have 5.5-6psi at the Schrader valve, new fuel filter, and new lift pump. I’ve checked the IP inlet filter screen and it is clean. I’ve tried the ATF in the fuel filter trick and it didn’t help.

Any help is greatly appreciated!
 

Selahdoor

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Take the return line off, at the IP, so that most of the return lines are AFTER where you took it off.

Blow into it with your mouth. Not an air compressor. It should flow freely. If not, that might be your problem.
 

Selahdoor

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There's your problem.

You need to figure out where the blockage is.

Now pull off the line that runs down behind the engine, and try to blow through that one.

Put an extension on it if you have to. Don't be tempted to just blow through it with the compressor.

But wait. Before you do that, switch the tank selector. Blow on the front one again. See if the problem is in the selector, or could be between it and the currently selected tank. (In other words, if you switch tanks and blow again where you did the first time, and now it is clear, then the problem is in the selector, or in the line between the selector and the currently selected tank. If that makes no difference, go to the rear of the engine, and blow in the line that goes down the back of the engine.)
 

chillman88

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I've seen the cheap kits with holes not properly drilled or having casting flash partially blocking them off. The last cheap kit I installed I had to run a small drill bit through a couple of the barbs. I'd suggest pulling them all back off and inspecting them one by one. It's a pain, but it might be easier than messing around one at a time.
 

Theaxehand

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I’ve pulled the return lines and have determined that the blockage seems to be at the selector valve (if that is possible) or the tank. If I blow really hard into the line to the selector valve I can get a little flow.
 

Selahdoor

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Excellent!

Not that you have a problem. But that you found the problem. LOL

I will assume that you then blew into the first hose again, after removing the back line... To find out if there might also be a problem with the fittings and lines before that. Just in case.


Now, to get further down the line, so to speak... I suggest that you go back to the selector valve. Remove the line that goes from the engine to the valve. With both ends loose like that, Now blow through that line with the air compressor. Also, slip a new piece of line onto the selector valve, and blow through that by mouth.

Hopefully this will solve the problem,. The valve itself will be fine, and blowing out that long line between the engine and valve will cure it.

If that doesn't cure it, the next thing I would do is to pull the lines off the BACK of the selector valve again by mouth. If the valve itself is the problem, then blow through the selector valve strongly. Even with the air compressor. But be careful. Then switch the valve, and blow again.

By the time you get through with all this, if the problem persists, then you know it is in the tank, or in the line between the valve and the tank.

I recently replaced the sender unit in my front tank. I noticed there is a sort of filter clipped onto the end of the line, for the return. I briefly considered taking that off. Since, if that got clogged, it would cause exactly the problem you have. I think in your shoes, if I got through the valve and all, and discovered the problem to be the tank, or the line between the valve and the tank... I'd probably just go ahead and blow right through that last line, into the tank. Blow that filter right off of there.

I can't see that a filter at that point makes any difference. The fuel has already gone through your entire fuel system and returned to there. If it isn't filtered by that point.... What's the point?
 

Selahdoor

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And by the way. Even if the lines are good, If I had gone to this much trouble myself, I would replace at least that line coming down from the engine to the valve, with a new line.
 

Theaxehand

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Fortunately the truck is not my daily driver so I will absolutely run through everything and replace as necessary.

Let me ask you this though, if this blockage is indeed the cause of the lack of power and engine stalling, wouldn’t the engine run right with the return line at the rear of the engine unhooked?
 

Selahdoor

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I would think so, yes. But you'd have diesel fuel running down the back of the engine. :)

I take it you have done that? LOL

And have you blown through the first hose, after removing that back line? (To check out the new fittings and lines, separate from the rest of the system.)


If this doesn't fix the overall problem, then at least this will not be contributing to the overall problem...
 

chillman88

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You could very well have a selector valve failure. When mine died it would start right up, then run out of fuel. Then it would start right back up before running out of fuel again. Both the return and the feed go through the selector valve. Food for thought.
 

Theaxehand

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I would think so, yes. But you'd have diesel fuel running down the back of the engine. :)

I take it you have done that? LOL

And have you blown through the first hose, after removing that back line? (To check out the new fittings and lines, separate from the rest of the system.)


If this doesn't fix the overall problem, then at least this will not be contributing to the overall problem...
I attached a hose and ran it into a jug. The engine acted the same way.

With the selector valve on the front tank, I can blow through if I blow really hard. With the selector valve on the rear tank I can’t blow through at all. I guess now would be the time to pull the selector and inspect it.
 

quickster

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It sounds like it is stuck in between. You can take it off the frame and take it apart. A screw type mechanism switches the valve. You can look for pics here on repairing. I've done it before years ago. If it's bad they sell chinesium ones on evilbay. Or you can do a manual bypass.
 

Selahdoor

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Yup.

Here is a good vid to show how to actually fix the valve, instead of replacing it:

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Big Bart

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From my own experience the valve can stick between tanks and become blocked off. Remember the selector switch does both the fuel line and return line. So even if one is open partly open, the other one could be blocked or partially blocked. I flipped the selector switch a couple times and it now works on my rear tank. I need to replace it as it will not switch properly anymore.
 

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