Running Issue; Vapor Lock?

orange150

orange150

Well-Known Member
Joined
Mar 20, 2005
Location
Fairfax City, VA
My Jeep (4.2 w/ MC2150) has started having a terrible stumbling issue. It's always had a funny little stumble after a hard pull, like up a steep hill. It did it with the Offy intake and 4bbl Holley, it still does it with stock intake and 2150. I just got it back together after sitting for five or six months and now it's having an intermittent stumbling issue that I'm thinking may be vapor lock, it acts like it's out of gas. What makes me think it may not be vapor lock is that I can "work through it".

If I don't stay on the throttle after a hard pull up a hill, or if I down shift to a stop light I will have to stay on the throttle to keep it going or it will cut out. Once I turn it back on though it acts fine, for a little bit.
I replaced the fuel filter just to be sure it wasn't clogged; the one I pulled off wasn't bad at all though.
This last time it did it I was about 10 minutes from the house, got it home (included going up a steep, long hill. Glenwood Ave. for those familiar with Raleigh), got it in the driveway and it was idling fine by then. Revved it a few times in the driveway and couldn't get it to do it, which makes me think it's related to having a load on the engine.

I've not ruled out an ignition issue, I just don't know the best way to test for that. One thing I read said to just crank it up and shake the wiring harness to see if that makes it stumble.
 
Not sure if that has a mechanical or electric fuel pump, but I was having a similar issue in my YJ and it wound up being a weak fuel pump. I replace the pump, and also routed the fuel line away from all the hot stuff in the engine bay and haven't had an issue since. Not sure if that's your case, but sounds like maybe it does not get enough fuel when it's under a load but has enough to run at idle and without having to work hard. Mine was filling the fuel rail with air because it wasnt keeping enough pressure and it was vapor locking. I discovered and repaired this 10 hours from home at Choccolocco ORV Park lol.
 
77GreenMachine said:
Not sure if that has a mechanical or electric fuel pump, but I was having a similar issue in my YJ and it wound up being a weak fuel pump. I replace the pump, and also routed the fuel line away from all the hot stuff in the engine bay and haven't had an issue since. Not sure if that's your case, but sounds like maybe it does not get enough fuel when it's under a load but has enough to run at idle and without having to work hard. Mine was filling the fuel rail with air because it wasnt keeping enough pressure and it was vapor locking. I discovered and repaired this 10 hours from home at Choccolocco ORV Park lol.
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Mech. that I replaced a few years ago. I've got an electric sitting on my bench that I need to install.
 
Loosen the gas cap or even remove it(for testing purposes), it may not be venting properly causing a vacuum on the fuel system that the mechanical ump cannot overcome. it doesn’t take much
 
Not saying this is it, but it could be fuel flash boiling. I went through that with my Fairlane. A return line to the tank fixed it. Try running a tank of non ethanol gas first, see if it goes away. E10 (normal pump gas) has components that begin boiling off at around 180 degrees. Winter gas is even worse so if you're still rocking a tank from before April get refilled first too.

Non ethanol won't start boiling off til after 225 or so.

What was happening on my Fairlane (my only carburetor vehicle) is I could drive all day fine, but if I floored it it would stumble or stall. On restart it was flooded. Finally one day it did it not in traffic, when I popped the hood gas was dripping off air cleaner. I pulled the lid and gas was bubbling out of every vent on the top of the carb, a Holley double pumper.

Basically the fuel would heat soak in the line under pressure between the pump and carb. That raised it's boiling point. When the needle would fly open to refill the bowl after a hard pull, it would drop to atmosphere pressure and instantly flash boil.

Since I wanted to be able to run pump gas I added a return line at the carb, it can be a simple tee fitting with the return leg blocked off with solder and a small .025 or .030 hole drilled.

If you have a mechanical pump it could very well be vapor lock too. An extra electric pump in series, or conversion to full electric pump at the tank will fix it, but I'd consider running a return line too.
 
I'll loosen the gas cap next time to see if that makes a difference.

I just filled it with non-ethanol fuel the other day and will probably continue to as long as it's available to me. Jeep CJs (at least the later ones) all had return lines from the factory. I spoke with @adman02 a few weeks ago who was having vapor lock issues from his return line, if I remember correctly.

I guess the odd thing is that it wasn't doing it, and now it is; and nothing has changed other than sitting for some time.
 
A few things I have experienced, that you might check
Fuel pump. Every CJ that I have got, that had sit a while, including the ones I have had, that also sit a while, usually ended up having fuel pump issues. Big tale tale was getting fuel in my crank case. easy to check, pull the dip stick, note the level, and smell it. If it is over full and smells like gas, then there may the issue.

Fuel boiling(vapor lock.
Did it run fine when it was colder? issue show up more when the temps rose outside?
Might be your issue. Check and make sure lines are not too close to heat area's, re-route them and/or insulate them.
Could be an issue with under the hood heat, not being able to escape.
Still running a clutch fan? It could be bad.
What size breather are you running? a larger one will allow more cooler air into the carb. smaller one, not so much, the heat may be building inside the carb.
A carb spacer might also help.
 
I would check for a clogged screen on pickup tube in gas tank and maybe rust or debris in tank
 
moldman05 said:
I would check for a clogged screen on pickup tube in gas tank and maybe rust or debris in tank
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This. Check the strainer screen in the tank, also check the short piece of rubber hose on the sending unit that connects between strainer and sending unit.

If that rubber hose isn’t rated for ethanol, and has seen some ethanol it will swell up and choke down on fuel supply. It can also squeeze down more with mech pump bc it’s sucking fuel, not having fuel pushed through it.

Also may want to wrap with fuel line with DEI heatshield product to mitigate the boiling. And/or a phenolic carb spacer so less heat transfer from intake up to carb.
 
Had enough time to drop the tank today.
Haven’t opened it up yet, but this surely wasn’t helping. Return line.

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Opened the tank up. The filter sock is completely gone from the 15 gallon tank. The inside of the tank looked brand spanking new.

But I picked up @adman02 old 20 gallon CJ tank and swapped them out. New lines and wiring across the top and the sending unit looked brand new. I’m going to drive it around with the new tank this week to see how it goes.


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Hey. Whatcha gonna do with that 15 gallon tank? Turns out I just picked up a scrambler that needs a new tank and skid plate. : )
 
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