Hot engine won't idle smooth

[dtrojcak]

I've had my '78 CJ7 with AMC 258 i6 / 4.2l for a little over a year now.
This problem has just started a few weeks ago.
When cranked up cold, it will idle smooth, no problems.
After driving it at highway speeds, even for short distances, it will not idle smooth.
It will sometimes actually die if left alone and the throttle is not pumped.
It runs smooth if throttle is increased.
I replaced the fuel pump but the problem is still happening with the new pump.

Thanks in advance for any help/advice.

 

[dtrojcak]

Video of problem.
https://vimeo.com/126931704

https://vimeo.com/126931875

 

[caveman]

It could be any number of things. When was the last time it had good tune up? It could be plugs or rotor and cap. Make sure that you have the right firing order. Check to see if the choke is opening up when it gets warmed up. Also check for vacuum leaks or unplugged lines. It is definitely skipping and any one of these things can cause it.

 

[Posi]

I hear a vacuum leak

 

[Lumpy Grits]

I hear a vacuum leak

Me too-
Ck ALL of your manifold bolts at the engine, to be sure they are tight.
What carb is that?
Ck the bolts at the base of your carb.
When was the carb last cleaned and rebuilt?
When was the last time you changed that fuel filter?
Confirm that the top outlet on that fuel filter is your return line. And the center outlet 'feeds' the carb.
BTW-That location for the f'filter, is the source of many engine fires--
What happens when you leave the gas cap off the tank and run it??

LG

 

[Hedgehog]

From those videos it never runs very well. LEt me see if I have this right, when you crank it up it keeps on cranking, but after driving around a little, I assume its cranking while you are driving, it wont continue to crank when it should be cranking away at idle. I'm wondering, if you immediately start cranking the engine will it start or crank on it's own?

I agree with the others, it looks like you have a vacuum leak somewhere. When the choke is still on there is enough fuel to keep it running, but when the engine is warm and the choke is off or open the engine is overly lean and it can't keep running without adding some pedal/gas/fuel. Vacuum leaks can be sneaky things, I worked on my CJ and put fresh caps/plugs on the vacuum ports. Within 10 months they cracked and created vacuum leaks. Easy to fix, but still frustrating.

 

[Woolyworm]

I'm going with vac leak. Tighten up all your bolts, check your hoses, try all the easy stuff. Spray starting fluid around the base of the carb and the linkage where it goes through at idle and see if your rpm goes up - don't catch yourself on fire and have an extinguisher on hand.


Wooly

 

[dtrojcak]

OK, a few answers to the questions, which will probably lead to more questions, lol.

I bought the Jeep in March 2014.
When I got it, I replaced the spark plugs, fuel filter, entire exhaust system(manifold to tailpipe), air filter, and oil/filter.
I've put about 1100 miles on the Jeep since all of these repairs/maintenance were completed.

The engine has run fine since I bought it until about a month ago.
I noticed the "whooshing" sound from the beginning. I'm guessing that's what ya'll are saying is a vacuum leak. I didn't think anything of it, since the engine used to idle fine. Being that this is the first non-computer-controlled, no emissions, carburetor engine, I don't have any experience with what noises are normal and which are not, but I'm learning, lol.

There is a hose coming out of the distributor that goes nowhere, just hanging there. It was like that when I bought it though, and it just started this idling problem.

I'll try to check the manifold and carb bolts tomorrow depending on what time I get home.

The outputs of the fuel filter are going to the correct places.

Fuel filter was in that location when I bought the jeep, so I installed the new filter in the same place.
Where would the optimum/safer place be?

Next time it idles rough, I'll remove the gas cap and see if there's a difference.

The fuel pump was allowing the fuel to drain back into the tank, forcing the engine to be cranked over for about a minute or so, before it would start.
At first, I thought this problem was related to the fuel pump getting worse, so I finally replaced the pump, but the problem is still there.

One thing I forgot to mention before, was reminded when someone mentioned the fuel filter, this all seemed to start after the fuel tank was run dry.
If something was picked up from the bottom of the tank, and is partially clogging the fuel filter, could that cause the problem?
I changed the fuel filter last year after I bought the jeep, just because.
I kept the old filter as a emergency spare. I'll try putting it back on and see what happens.

Thanks to everyone for the tips/advice.

I'll report back once I have a chance to check the things ya'll mentioned.

 

[Lumpy Grits]

Put a new fuel filter in and see.
Buy and keep spare fuel filters on hand.
You never ans my question about the hose routing on you fuel filter.
The smaller return line MUST be postioned at 12 o'clock.
Move the filter to the top of the valve cover to reduce fire hazard and boiling the fuel(vapor lock)from the exhaust manifold.
You really need to get a copy of the real factory service manual.
LG

 

[Torxhead]

Interesting about that hose that comes out of the distributor and goes to nowhere, as it operates the vacuum advance at the distributor and should be connected to a manifold vacuum source, like the carb and there should be no open vacuum ports. If the fuel delivery is still in question, a fuel pressure gauge should reveal around 4 p.s.i. to determine the system is operating correctly. When the engine is warmed up, put your hand on the ignition control module located under the coolant recocery bottle and it should not be hot to the touch. Common issues can be caused from cracked fuel inlet lines and a weak carb gasket to the intake manifold.

 

[Hedgehog]

I'm betting on a vacuum leak due o the way the engine is running, not from any sounds that the camera picked up. Torx is mucho grande correct, that hose you don't seem to mind that goes no where plays a huge part in the way your engine will run. First it should be attached to a port in the carb. If it isn't attached you have a BIG vacuum leak. Also if it isn't attached you will not have any vacuum advance in your distributor. IE reduced power at low engine speed. There is a mechanical advance that will take over, but that takes some RMP's to work.

If I were you, when trouble shooting the engines problems I wouldn't discount the work you did previously. Maybe pay special attention to the work you did, I know the first place I look for trouble is in the things I did recently.

Lumpy - In his video you can see his fuel filter, it's not perfect, but it looks good enough to me. I moved my V8 filter by purchasing a filter designed to work in a vertical position and moved it down to just above the fuel pump. No troubles so far.

 

[Lumpy Grits]

I personally know of 3 Jeep CJ engines fires(AMC 258 i6 / 4.2l /6cyl), that were traced back to the fuel filter leaking.
This issue was known back in the day. NTSB even opened an investigation about it.
X2 on what was done last!
The OP now sez, this all started when he ran the gas tank dry. Tells me, crud from the tank may well have plug'd the filter or lines.
LG

 

[Woolyworm]

Don't over tighten your carb to manifold bolts.

X3 on the advance hose.

Depending on your carb you can run the engine with the cover off (mc2100) and you'll be able to see if gas is in the float bowl ruling fuel supply out without needlessly replacing parts from the tank to the carb. Again - don't catch yourself on fire!

I still think that acts like vac leak.


Wooly

 

[Lumpy Grits]

Tho I don't think this is the case here.
Make sure that BOTH(IF you have 2 of them)idle mix screws are set EXACTLY the same......
LG

 

[Woolyworm]

[emoji13] Yes - idle mixture should be checked. Needs air, fuel, spark. Next, need to confirm air/fuel in correct mix. Leaks and or idle mixture can cause imbalances at idle. However-this was a change in behavior indicating something likely broke/wore out/clogged up. A wear item that can cause a vac leak is the throttle plate shaft. Filter and pump have been replaced so fuel is making it to the carb inlet - Make sure the bowl is being filled.

Check idle mixture-
Turn the idle screws all the way in and gently close them, then turn them out 2 turns, start it, then adjust your throttle idle to keep it running-keep it as low as you can while keeping the engine going. Then start turning the screws out a half turn at a time, one then the other, until you get the highest idle you can and then turn them back in s 1/4 turn to 1/2 turn, then readjust your throttle idle down. You can do this more precisely with a vac gauge but this way will get you close.

There is a small hole in each throat of the carb below the throttle plates that gas gets sucked through regardless of the throttle being applied - that is your idle circuit. When you turn the mixture screws in or out you are adjusting the amount of air that gets pulled in to mix with that dribble of gas. It doesn take much of an air leak to screw up that balance and depending on where it is happening different things happen. When you open the throttle it over rides all of that.

After you do that - you should still look for a vac leak. Wiggly the throttle shaft around, any appreciable side to side movement/idle change? Do the spray around the base, and idle change? Work the throttle with your hand slowly from idle to just where it starts to run smoothly - video of that if you can.

Assuming your idle circuit is working properly, the next thing I would do is start checking ignition items.

Good luck! This can be tedious but once you've got it you'll feel like you've accomplished something for sure!


Wooly

 

[Woolyworm]

Um-am I missing something here - where are the 2 mounting studs/nuts on the front of the carb in this photo? Is that a webber?


Wooly

 

[Lumpy Grits]

Good catch-I sure miss'd it.
WW-Do you know what carb that is??
LG

 

[Torxhead]

Nice catch there Woolyworm, pictures really help.

 

[Hedgehog]

Did our OP disappear? Log in problems? Have' heard from him in a little while.

 

[dtrojcak]

Did our OP disappear? Log in problems? Have' heard from him in a little while.

Haven't posted anything 'cause work and life got in the way and I haven't had time to look at it since my original post, so I don't have any updates.

Thanks again for all the responses so far.
I really did not expect this much help this quick. I really appreciate it.

At this point, I'm really hoping it's just a partially clogged fuel filter.
If not, it looks like I have a bunch of other stuff to check.

Someone said something about a couple of bolts missing from carb. I have no idea about any of that.
When I bought the jeep, I changed the fuel filter, engine oil & filter, spark plugs, air filter, and the entire exhaust system. Everything else related/connected to engine was like that when I bought it and has ran the same until a few weeks ago.

I work till Wed-Sat 8a-7p, so it'll probably be Sunday before I get a chance to work on it again.
Please be patient with me and I will update once I do.
Thanks again for all the advice so far.