While cruising, engine cuts out and then springs back to life

'59 CJ5 original F head. This has happened a few times now, though it seems to be random and doesn't happen all the time. When travelling at cruising speed (about 45 MPH) the engine will just completely die. I let off the gas and then the engine comes back to life and I keep driving like nothing happened. It sort of feels like a vapor lock, although in my experience with vapor locks, the engine does not spring back to life right away because it needs to cool down first. Any ideas?

Note everything else seems to be fine -- oil pressure, temperature, etc. Though it does seem to be running a little rich because I can smell the fumes but no smoke as far as I can tell.

 

Sudden engine shut-down is always gas or spark.

I'd check that it's not starved for fuel first. Maybe run it from a gas can on the cowl and see if it still cuts out.

 

I'd suspect the fuel pump.

 

Mine would do it because the cheap Omix air hose would collapse.

 

My ignition switch failed in such a way that it would make and break ignition power while I was driving. Might check that if it seems like it happens when your going over bumps possibly.

 

X2. also check for other bad connections on the ignition circuits. Sudden total cut out is likely electrical. Fuel problems are possible (especially if speed/load related) but often more gradual or intermittant.

 

It has to be electrical, if it was fuel related there should some sputtering involved.

 

I had the same problem. Finally figured out my condenser (about 1 month old from oreilley) was bad.

 

it sounds like what happened to my 59 cj5 awhile back my problem I was starving for fuel because I had one of those fancy chrome hot rod fuel filters that you take apart and clean. even clean it was not letting enough flow so I replaced it with a plastic basic filter from Napa and haven't had a problem since

 

That's sort of what I was thinking as well. It doesn't sputter or act like it is running out of fuel. Lots of good advice here and a few things to check out. I will check the ignition switch and fuel system. For the record, I do have two fuel filters -- the basic plastic see through type -- one before the pump and one after. As for the ignition, I am running a Petronix so I'm wondering if it could be intermittently cutting out.

 

I had a similar problem some time ago, but not quite the same (and not a Jeep). Mine would happen when running at full throttle for maybe a minute and it would just fall on it's face and die. Turns out the carb inlet was partially blocked and at full throttle, it would drain the carb bowl faster than the inlet would allow it to fill. Once I got off of it (which I sort of had to, once it died) the bowl would refill and the engine would act completely normal again (until I floored it again). Yours' is happening at only 45 mph, so if it's restricted, I'd say it's a pretty serious restriction. I would however check on your fuel pump. Maybe it's failed in some way that it can provide enough fuel up to the carb at lower speeds but not at the higher ones.

Good luck

Brad

 

45 mph is pretty much full throttle and top speed. However, I did think of something else that might help. It seems to only happen on flat ground when it is at top speed but NOT full throttle. That may add credence to fuel being the suspect.

 

From this quote, I am leaning toward a fuel issue, letting off the throttle lets the bowl re-fill. Weak pump, restricted filter or line, plugged tank vent, air leak on suction side of pump etc.

 

x2.. I've never driven an F-head, but something seems wrong about 45 mph being "full throttle." When you say full throttle, does that mean you're driving it with your foot on the floor and only getting 45mph? If that's the case, it sounds like it's starving for fuel

 

I agree on something wrong at 45mph and full throttle.
Mine will do 55mph and still have some peddle left. Just running higher RPMs than I care for.

However, I say you will be needing a new fuel pump in the near future. Mine acted exactly like that before it quit.

 

Sorry I guess that was misleading. 45 mph but not "full throttle" in the strict sense of the phrase, only as full as I care to go. It is an el cheapo Omix fuel pump so fuel could be the problem.