Truck is surging, mostly at idle, what's up?

[nelstomlinson]

First the story, then a couple of questions at the bottom.

Friday my truck started surging then died on the way home. I told a little about it in the what did you do with your truck today thread.

This truck is my '89 crewcab. It has a small electric fuel pump on the frame rail between the fuel selector switch and the mechanical fuel pump. It also has a check valve in there, before the mechanical fuel pump. I believe everything else is stock, except that I have deleted the return line which connected to the fuel filter.

When I investigated, I found that the filter on the input of the electric pump was NOT clogged. I found a crack in the hose leading out of the pump, with a drip when the e-pump was running. I changed that bit of hose, and got it restarted, and now it's better, it runs again, but it's still not right.

What it's doing now is it drives OK, with barely noticeable surging at higher RPMs, but will surge noticeably at idle, or when downshifting. There is some hesitation on acceleration, too. I keep getting air out of the schraeder valve when it's not running. I haven't tried bleeding it while running yet.

So far I have changed that bit of fuel line, have checked that the e-pump filter isn't clogged, have changed the main fuel filter and used the e-pump to bleed the air out of it, can't find any more leaks.

Is it possible that the mechanical fuel pump is somehow partially failing, and can't pump enough fuel at low RPMs, and barely enough at high RPMs? Could it be sucking air somehow but not leaking? I have checked the lube oil level, and it hasn't gotten higher. I'm sort of at a loss for what to try next, would appreciate hints or ideas.

 

[riphip]

Is your electric pump pushing fuel TO the mechanical fuel pump? IF SO, you may have ruptured the diaphragm in the mechanical pump causing it to suck air. Check your oil level on the dipstick, wipe on a cloth not a paper towel & see if you can light with a match, etc


Sounds to me that it is sucking air from some point.

 

[nelstomlinson]

Yes, riphip, it must be sucking air, but I can't find where yet.

The e-pump is feeding the mechanical pump, but it's a small one, don't think it's making much pressure. It will fill the filter in a minute or so, but doesn't squirt fuel far out the Schraeder valve - nothing like how the mechanical pump will squirt at idle. I have checked the oil level in the pan, and it hasn't risen any, and no noticeable smell of fuel on the dipstick.

After sleeping on it, I'm going to try snugging down the fuel hoses on each side of the e-pump. They are slipped onto the plastic OEM fuel lines, and a likely trouble spot. I couldn't see any leaks there, but it might be sucking air there anyway. The stores are all closed today, so anything that requires parts will have to wait until after work tomorrow.

 

[The_Josh_Bear]

Yes it sounds like air intrusion rather than lack of fuel flow. Air makes the pump raise RPM for reasons I can't explain, but that's what happens.

Yes air can enter places fuel will not drip out of. It's nearly a staple of our engines but usually up on the return system which you aren't pumping into anymore. The principal stays the same though.

Stay away from the standard worm-drive clamps on your fuel hoses, if at all possible. They distort on small lines and don't heat cycle. I've switched to all spring clamps for everything fuel and coolant under 3/4" in size and never get any more problems. They heat cycle great and maintain pressure cold, hot, warm, whatever.
Considering the extreme cold you get its even more reason to get spring clamps.

You also might consider putting your e-pump on a parallel circuit to the mechanical, to future-proof the ruptured diaphragm situation.

Anywho there must be a place it's sucking air. Could be from the fuel tank all the way up to right before the mechanical pump. I figure if you found one crack there is at least one more you didn't find. But the fuel line slipped over OEM is a good place to start.

 

[IDIBRONCO]

They are slipped onto the plastic OEM fuel lines, and a likely trouble spot.

That would be a good place to start. Why is the electric pump running all the time? If it has to run in order for your engine to run, then your mechanical pump needs to be replaced.

 

[nelstomlinson]

I don't think that e-pump could deliver enough fuel to keep the engine running. The previous owner powered it so it's on when the key is on. I left it that way when I replaced the e-pump, because I was having issues with the return lines - the easy start, die, hard restart problem. Letting the e-pump run while the glow plugs cycle kept that under control. It might make sense to put a switch in that power line and just use the e-pump when I change the main fuel filter, since I believe I have solved the return line problem.

 

[IDIBRONCO]

I don't think that e-pump could deliver enough fuel to keep the engine running. The previous owner powered it so it's on when the key is on. I left it that way when I replaced the e-pump, because I was having issues with the return lines - the easy start, die, hard restart problem. Letting the e-pump run while the glow plugs cycle kept that under control. It might make sense to put a switch in that power line and just use the e-pump when I change the main fuel filter, since I believe I have solved the return line problem.

By saying that the e-pump couldn't keep up, I assume that you mean the mechanical pump couldn't keep up. If that's the case, you're mechanical pump should be replaced. The switch for the e-pump is a good idea. That's how I have mine. Only for priming the filter. That and the one time that the mechanical pump started acting up. It worked as an emergency pump until I replaced the mechanical pump the next day.

 

[nelstomlinson]

No, I mean the e-pump doesn't put out a lot of fuel, and I'm not sure it could keep up. I forget what it was rated for - maybe 30 GPH? I should disconnect it and prove out the mechanical pump, I suppose, just to be sure the little e-pump isn't covering for a weak mechanical pump. I'm not sure that a mechanical pump can fail half way?

 

[IDIBRONCO]

I suppose, just to be sure the little e-pump isn't covering for a weak mechanical pump. I'm not sure that a mechanical pump can fail half way?

That's a good idea. I don't know about failing half way, but one can get weak and not pump enough fuel as it's failing. The one time that I used my electric as an emergency back up, I pulled out from a gas station into heavy traffic. As I shifted to second, the engine acted like it was running out of fuel. AS I kept going down the street, the engine smoothed out and then died. I coasted around a corner onto a side street. I used the e-pump to bleed the air out at the filter and kept it running while I drove home. Then I used it the next morning to get another mechanical pump and get to a place to install the new pump. The mechanical pump was probably still pumping some fuel, but I don't know how much. A weak mechanical pump can also let the engine die if you clutch it from road speed while you're pulling up to a stop light. Sometimes when that happens, you can limp it along by downshifting until you're almost at a stop, but it's only a matter of time. Naturally this only applies to a manual transmission.

 

[nelstomlinson]

Well, I redid the connections on both sides of the e-pump, and it's as bad as ever. I'm going to try to change out the mechanical pump. I have the old Made-in-USA one from the old Red Truck engine (the engine I pulled out of the '94 to make way for the rebuild that I should be working on instead of this).

 

[nelstomlinson]

So I got the old mech pump out, and it's probably dying. It doesn't have a long stroke, and doesn't seem to pump much, compared to the one that's going in.

Is there a trick to getting the new one in? I am pretty sure the arm goes under the cam, but how in the world do I line it up to get the bolts back in? I can't see it from above, can't really see it from below either, and can barely reach it.

 

[IDIBRONCO]

I like to remove the ground cable, anything that's attached to it (or at least move it out of the way), and completely remove the hard line that goes up to the fuel filter. After that, there's quite a bit of room from the bottom.

 

[nelstomlinson]

Got the cables moved, bolts in their holes, can't get the bolts started, threads won't catch.

 

[Philip1]

You probably have the arm sitting on the lobe of the cam, I would recommend spinning the engine over a little bit and try again

 

[nelstomlinson]

You probably have the arm sitting on the lobe of the cam, I would recommend spinning the engine over a little bit and try again

Oh....