Engine Fuel Flow Stumper!

[N9VIW]

Guys, I gotta take a break from beating my head against this Jeep, I got a headache and an AMC logo stamped across my forehead...

1982 CJ7 , swapped-in '93 242 with the Gronk MC2100 carb conversion and HEI, T-176 , Dana 300 , 2.73 in stock axles. 20-gal poly tank.

It was my daily driver for the last 6 years, first with the stock AMC 258 i6 / 4.2l and then with this donor 242. I've been driving on the 242 since April, and not a lick of trouble until a month or so ago when it started acting like it was running out of gas occasionally. Now it seems I get less and less further from the house every time I test-drive it; I'm waiting for the day when I won't even be able to get out the driveway.

So far I've replaced the fuel pickup sock, fuel pickup, all rubber lines to the tank including the breathers, all steel lines to and from the tank. Because the 242 has no mechanical pump provision, I have a Carter vane-style electric pump mounted to the frame near the engine. New pump. New filters, both the inline filter I put before the pump (when I was diagnosing a similar issue that wound up being rusty steel lines) as well as the stock dual-outlet filter at the carb. All rubber lines to the filters, the pump, the carb, and the return line to the steel return to the tank. Rebuilt the carb, new needle and seat, and the float floats and does not sink. New air filter with clean prefilter.

It started one day when I got almost to work and it started giving me guff. After work I made it to the local station and filled up, and no problems for weeks.

Next time I was on my way to work and got maybe halfway there. Pulled off at a station and filled up, no problems to work or home again.

Next DAY I made it to the next town over, and it started again, with a full tank! Coaxed it home, and it died on the way. I pulled over and looked it over, couldn't see anything wrong, but the steel line I had from the pump to the carb (at the time) seemed hot. Vapor lock? Got it started again and got it home and took the wife's Tracker to work.

I replaced the steel line from pump to carb with rubber, and took it to work the next day. I made it maybe 5 miles when it started running out! Raced home, coasting and goosing it all the way, took the Tracker again.

I drained the tank, dropped it, cleaned it, blew the lines out, replaced the pickup sock, replaced the inline filters again, rebuilt the carb, and put it all back together with fresh gas.

Took it to my neighbor's house a mile away for a test run, and it died in his driveway. And again a half mile from there, and repeatedly thereafter, every few hundred feet. On one stop I opened the carb with my Leatherman, and the bowl was indeed low, darn near empty. No leaks.

I was able to make it home by pinching off the return hose to force all fuel to the carb, but several times I almost flooded it out and had to rev it up to clear the carb.

When I got home, with the engine off, I ran the fuel pump and noted that, with the return line pinched, the pump would not overwhelm the float needle- it didn't overflow, it just stopped flowing. When I removed the zip tie I'd used to clamp the return line, it started flowing again.

I want to get back in the saddle again, not only because I LOVE our Jeep and I HATE my wife's Tracker, but so she'll have something to drive in my absence.

I'm at an absolute loss- can anyone help?!

Nick

 

[TSB8C]

Fuel pump? No gas in carb bowl after cleaned out tank, new filters, rebuilt carb, insufficient pressure when return line pinched off - seems pump isn't providing sufficient fuel/pressure.

 

[N9VIW]

The thing is, I've been running on the same electric fuel pump since april, and that one was almost 7 years old. I replaced it with the new one, thinking the old one was the problem, and it did not make a difference. I suppose it's possible I got a bad one out of the box, but I would be hard-pressed to prove it since, when it is running open and unobstructed, it flows like crazy.

Does anyone have a suggestion for a good inline fuel pump to use?

 

[N9VIW]

Following a friend's suggestion, I tied the ground lead from the pump directly back to the battery negative post. I replaced the 1/4" rubber lines between the frame line, the inline fuel filter, and the pump with 3/8. (That is a little oversized, but they didn't have 5/16".) I also replaced the 1/4" return rubber line with 3/16", hoping that it would provide more resistance and ensure I wasn't flowing too much back to the tank. Oh, and the filter is definitely oriented with the return port at 12:00.

I took it for a test run and made it about 2 miles. When it died, the inline filter was full. I got it restarted, I backed into a spot off the side of the road and thought, "I've never replaced the HEI module in this distributor since I bought it used 6-1/2 years ago." I had a spare in the console, so I swapped that out. Incidentally, the distributor base was hot to the touch; who thought *that* was a good place to put a heat-generating electrical component?!

Back together, I got it started and continued on. It died a half mile or so later. I checked and still saw fuel in the inline filter. I switched on the ignition and made sure I gave it enough time in case it had to refill the carb, then restarted the Jeep and busted a quick u-turn to head home.

I babied it and idled it all the way home; it died a mile down the road back, then again about a mile later. Even letting the fuel pump run with the engine off, it took a long time to get it restarted. I got it another 100 yards and it died again.

Now I was on the frontage in front of my farm, so I popped the hood and took the top off the carb. The inline filter is still full, but that carb bowl is STONE COLD EMPTY. I switched on the ignition and watched the fuel flow; it dribbled in with no great force, gradually but not with any enthusiasm. At one point, with the float bowl still half-empty, flow STOPPED, but the pump was still running!

I pinched off the return line, and the flow started with a vengeance. It filled the bowl, the float stopped the flow... then it started seeping around the needle. I pressed the float down, and the gas positively JETTED into the bowl. When I let go, it stopped briefly, then started seeping around the needle again. I let go of the return line, and it stopped pushing as it resumed flow back to the tank. I put the works back together and got it the last 100 yards or so home.

So now, the fuel pump is running with decent flow, but most of the flow is going back to the tank rather than into the carb. And it does generate enough pressure to overwhelm the float needle, so I can't just run a single-outlet inline filter, but it's sending more fuel back to the tank rather than pushing it through the filter.

The local APS has an Edelbrock inline electric pump that does 38GPM at 4-7psi max (this one supposedly does 25GPM at 5psi max). Should I replace the pump, or try to find some way to restrict the flow back to the tank without shutting it off altogether? And why did this all happen now, with no other changes?

Nick

 

[N9VIW]

Well, it seems @TSB8C 's suggestion was right. Even though the new pump was, well, new, it was the issue- decent pressure, but insufficient flow.

I suppose I could have tested it by putting a restrictor into the return line so that SOME fuel would go back to the tank, but it would be a period of trial and error that I simply shouldn't have to do if the system were to work properly as designed.

I went to town and got the Edelbrock pump. The instructions said to mount it within a foot of the pump, at a level at or below the bottom of the tank. With a 20-gallon tank that's not going to happen, so I bolted it to the frame at the front spring mount of the rear spring.

I ran both power and ground wires back to the pump rather than grounding it at the pump, so I could tie it to the bonded lead that went back to the negative battery post. Because the new pump had an inline filter on the intake side, I eliminated the inline filter that used to be just before the pump I had mounted to the frame up by the engine, which is good and bad- I like those clear inline filters so I can see if I'm picking debris up from the tank.

After starting the pump and doing a leak check, I took it for another test drive. I went to a gas station about 10 miles away, and it didn't give me a lick of trouble- no hesitation, no stalling, no bucking, just power. I topped off the tank, took a pic at a new billboard by the livestock auction yard, then came home. Again, not a bit of trouble.

Thanks, @TSB8C, you win! And so do I!

 

[N9VIW]

Spoke too soon. I've been driving her since the Edelbrock pump, no troubles until this past Sunday night when I was leaving for work. I got a mile and a half from the house and it gave a lag that put copper in my mouth... it did it again about 50 yards later and I thought, I do NOT need to be stranded on the side of the road at 10:30 at night. Turned around and drove/coasted home, and (urp) drove the tracker again.

Monday afternoon after I slept, I looked under the hood. Nothing seemed out of the ordinary, apart from the fuel filter resting directly on the intake manifold. I rerouted the fuel line and zip tied it up to keep the filter pointed correctly and level, and took it for a test drive. No problems at all.

I hate this, I can't trust my old girl! Any more suggestions?

 

[N9VIW]

Reading back over this, I decided to follow a thought and restrict the return line to ensure the lion's share of fuel went to the carb instead of back to the tank. I cut a section of 3/16" brake line and left the ends slightly crushed by the blade of the tubing cutter. The resultant holes are maybe 1/8".

I cut the return line from the filter at an accessible point, inserted the brake line into the return line, and secured it with hose clamps. I'm hoping that, faced with the option of carb or tank, the majority of pressure and flow goes to the carb where it belongs, and only goes to the tank if it absolutely has to.

I don't know what this will do to the pressure at the float, but that will be borne out by how it behaves from here on out. If it is too restrictive, I can either open up the ends of the brake line stub, or if it's a complete failure I can replace it with a 1/4" barbed coupler.

 

[Project86]

Hope isn't much of a repair strategy. If you want to try to "regulate" pressure in either direction, you'll need some type of needle valve. Ball valve will not work. I'd also find a way to verify the fuel pressure at the carb. IIRC, on my daughters jeep the fuel is regulated to 6psi just before the carb. It's been a while. Just how much fuel pressure does your pump put out?

 

[TSB8C]

Sorry to hear it has messed up again. Seems you're on the right track with controlling pressure/flow to the carb. I have to agree with "project86" on a better or more secure way to do that such as a valve or regulator. But you're interim solution may at least bear out whether this is the needed fix or not.

When I had my '77 CJ5 with the AMC 304 V8, I initially didn't have a return line to the tank and just used the mechanical pump on the engine with a clear inline filter between the pump and carb with no issues. When I later installed EFI on the same engine, I ran a return line and used an external frame-mounted electric pump, also with no issues.

Seems to me if the pump has the proper pressure and flow rate, you're problem should be solved. With your current electric pump, you should see about 4-7 psi with around 30 gph flow rate at the carb, such as an Edelbrock 17301 for example. (I'm assuming your engine isn't hopped up too much requiring more flow) Then a good Spectre adjustable regulator, like the 2519 (5-9 psi range) to tweak it and provide a spot for a gauge as well to complete the package. Of course I've got EFI on my current rig, but use an in-tank electric Walbro pump and an adjustable pressure regulator that is also vacuum controlled. With EFI you need less pressure at higher RPM's due to engine vacuum. But for a carb setup, fixed pressure (low) to feed past the float valve and keep the bowl full is all you need. Enough to feed engine needs and not too much to flood the carb. So a pump that is a bit too much with an adjustable regulator to tweak for your engine needs is the perfect setup. And obviously clean filters and no restricted lines.

After typing this, another thought came to mind on that MC2100 carb - Have you checked/replaced the power valve on the carb. It’s function is to enrich the mixture as manifold vacuum drops (signaling load increase). It is closed at idle, and opens to add more fuel as the manifold vacuum drops. It is located on the bottom side (front) and is contained under a cap with 4 screws. There is also an accelerator pump on the front, also under a cover with 4 screws, but it is identifiable by the accelerator pump linkage. Some people report good results using an OEM 2 stage power valve. This valve is longer than the single stage and is original equipment. It is 39mm long from top to bottom and is labeled "#16", meaning that it will not enrich the mixture until manifold vacuum drops to 16 inches HG.

 

[Project86]

^^^ He's pre-med, I'd listen to him.

 

[Rick1956]

Have you tried driving it without the gas cap? Stranger things have happened with fuel vents...

 

[N9VIW]

@Rick1956 , I have not driven with the gas cap off, but did not have this problem while driving the Jeep with its previous engine (stock AMC 258 i6 / 4.2l with Carter BBD). The problem has only been with this engine and carb.

@Project86 , Haha, good one, haven't seen that one in a hot minute, I should watch it again.

I did put in a pressure regulator with gauge. With that little bit of brake line in the return and the regulator wide open, it registered 4psi. I don't have a barbed fitting with which to replace the brake line stub, so I don't yet know what it was before I put it in. For now, I turned the pressure up to 6psi and will try it for a while.

@TSB8C , I did get a rebuild kit for the 2100, which came with a new single-stage power valve, but it's oddly not stamped anywhere with its open value. I don't recall what the currently installed one is, but even if it didn't work, that would only become evident under partial throttle and heavy load like driving uphill, not bumbling down a forest road at 15mph.

I'm going to give it another go this week, I'll let you all know how it goes.

 

[NP435]

Reading back over this, I decided to follow a thought and restrict the return line to ensure the lion's share of fuel went to the carb instead of back to the tank. I cut a section of 3/16" brake line and left the ends slightly crushed by the blade of the tubing cutter. The resultant holes are maybe 1/8".

I cut the return line from the filter at an accessible point, inserted the brake line into the return line, and secured it with hose clamps. I'm hoping that, faced with the option of carb or tank, the majority of pressure and flow goes to the carb where it belongs, and only goes to the tank if it absolutely has to.

I don't know what this will do to the pressure at the float, but that will be borne out by how it behaves from here on out. If it is too restrictive, I can either open up the ends of the brake line stub, or if it's a complete failure I can replace it with a 1/4" barbed coupler.

My only thought would be build a in line fuel reservoir holding say a pint of gas then flow to the carb. seen this with a 4bt Cummins swap in a f150 is the in tank sock clean? I'm some what baffled as to why you cant flow fuel . I run a 2150 motorcraft on the stock pump with zero issues.

 

[N9VIW]

Going back through my old posts, thought I'd revisit this one with a long-term update:

Since installing the regulator, I have had no issues with the old stalling out. Not sure why the MC2100 in this configuration needed it; I ran this very carb on a different engine ('89 4.0 XJ) with no regulator and it never gave me a lick of trouble.

Only issue now is probably related to the power valve, as when under heavy throttle it tends to bog, then when I get off it, it stumbles. I tried backing off the accelerator pump setting, but it didn't make a difference since the trouble is cruise load, not passing load. I haven't installed the remote vac gauge to see where I'm at when it happens, but that will give me a better idea what level valve I should look for.

 

[Grumpa]

I don't come here a lot so I'm sorry I'm late for the party.
Electric fuel pumps need to either be IN the tank or very near the tank. The reason a fuel regulator worked for you is that it cut down on the draw from the tank. Your fuel pump was collapsing the fuel line somewhere between the tank and the pump. The reason it got worse over time is because the fuel line weakened and it was easier to collapse as time went on. Ask me how I know this.

 

[N9VIW]

@Grumpa, I appreciate the input, but it evades a couple issues:

1) Why did the Carter vane pump work on the frame rail next to the engine with the AMC 258 i6 / 4.2l and not with the 4.0?
2) How does a pressure regulator in the return prevent collapse of the feed line? Suction is suction, and the pump speed never changes.
3) How am I getting fuel line collapse on brand new fuel line? (read up at the first post, I replaced everything from stem to stern, steel AND rubber)
4) Why did I get the same issue with the Edelbrock pump mounted back on the frame just ahead of the tank (as you said, in or very near the tank), but not after I installed the regulator?

The short of it, as far as I can tell, is that the pump was sending most of the fuel back to the tank, which starved the carb. Once I restricted the return the carb had plenty of fuel available, at regulated pressure, with the true excess being returned.

Shortly after my comment back in November, I replaced that brake line stub restrictor with an open brass barb, which dropped my pressure down to 4 psi again. I turned the regulator back up to 6, which is where it's been since. The Edelbrock pump died on me a few weeks back (motor windings opened up), so I replaced it and all is well. Not sure if I got a bum unit or if they're only good for a couple months of daily driving before it's time to replace it, but I got much longer life from the Carter pump I ran before. I may go back to it, if I can figure out how to shoehorn it in above the axle without worrying about it getting hit by anything; it's at least 3x the size of the Edelbrock. Another consideration: Mounted on the frame rail in back, it's much more prone to getting wet from puddle splashes than the Carter did being in the relatively protected confines of the engine compartment. I may build a splash guard for it, maybe that'll make the difference.

 

[Grumpa]

@Grumpa, I appreciate the input, but it evades a couple issues:

1) Why did the Carter vane pump work on the frame rail next to the engine with the AMC 258 i6 / 4.2l and not with the 4.0?
2) How does a pressure regulator in the return prevent collapse of the feed line? Suction is suction, and the pump speed never changes.
3) How am I getting fuel line collapse on brand new fuel line? (read up at the first post, I replaced everything from stem to stern, steel AND rubber)
4) Why did I get the same issue with the Edelbrock pump mounted back on the frame just ahead of the tank (as you said, in or very near the tank), but not after I installed the regulator?

The short of it, as far as I can tell, is that the pump was sending most of the fuel back to the tank, which starved the carb. Once I restricted the return the carb had plenty of fuel available, at regulated pressure, with the true excess being returned.

Shortly after my comment back in November, I replaced that brake line stub restrictor with an open brass barb, which dropped my pressure down to 4 psi again. I turned the regulator back up to 6, which is where it's been since. The Edelbrock pump died on me a few weeks back (motor windings opened up), so I replaced it and all is well. Not sure if I got a bum unit or if they're only good for a couple months of daily driving before it's time to replace it, but I got much longer life from the Carter pump I ran before. I may go back to it, if I can figure out how to shoehorn it in above the axle without worrying about it getting hit by anything; it's at least 3x the size of the Edelbrock. Another consideration: Mounted on the frame rail in back, it's much more prone to getting wet from puddle splashes than the Carter did being in the relatively protected confines of the engine compartment. I may build a splash guard for it, maybe that'll make the difference.

You got me. It was the best I could do.