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Discussion Starter · #1 ·
My jeep has dieseled all summer on shutdown, now that it is getting colder out the problem had gone away.

It is a 4.2l with stock carb. What are the possible causes of this: carb or carb adjustment, cooling system issues or something else.

One more thing, it is starting much easier in the cold versus when it was warm out.
 

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Dieseling is caused by excess combustables in the combustion chamber that want to keep going after the spark is removed.

For instance, running too rich can cause it, as all the fuel isn't burned off through initial combustion. More common a problem is excess carbon built up in or on the cylinder or head, this carbon then absorbs gas and keeps it from igniting properly, then leads to hot spots in the engine that try to selfignite under compression, like a diesel engine. This is how valves burn. It can also be caused by bad plugs. Anything that can compramise the fuels ability to burn cleanly can cause it.

Run a "sensor safe top engine cleaner" through your intake, then once the carbon has been burnt through the system, do a tune up or at least plug replacement. If that didnt correct it, it is likely in the carb. Wih it being temp related it may just be the carb or a little of each.

In the carb it can either be a rich mix or it could not be shutting off immediately. Any black smoke at startup is an indication you are leaking fuel into the head.
 

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Discussion Starter · #3 ·
I ran seafoam through it earlier in the summer, through the vac line and I also ran some through the fuel system.

After the seafoam I did a full tune up (dist cap, rotor, plugs, plug wires, pcv valve, oxygen sensor and air filter.)

Still had dieseling issue.
 

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I think so. I had that question when I moved from Kentucky to Alabama. a member on here told me that jeeps like colder weather.
 

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When you start it warm/hot you have to hold the pedal at least 1/2 way down. That's also as per the manual on a carb'd 4.2. They like starting a bit better cold sometimes due to the auto choke setting. The primary issue with these engines dieseling is that the idle is set too high. It should be no more than 800-850 rpm's hot in park/neutral. The best setting is usually 700-750 rpm.
 

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Discussion Starter · #9 ·
My idle us usually around 1100, but sometimes at red lights it drops to about 800 until I pull out. I am afraid if I drop the idle back to 850 it may stall when the idle drops at the red lights occasionally. I checked my carb for clogged idle tubes what would cause it to idle at 1100 most of the time but drop to 800 occasionally. Maybe I need to figure that out and get the idle speed in the correct range.
 

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My idle us usually around 1100, but sometimes at red lights it drops to about 800 until I pull out. I am afraid if I drop the idle back to 850 it may stall when the idle drops at the red lights occasionally. I checked my carb for clogged idle tubes what would cause it to idle at 1100 most of the time but drop to 800 occasionally. Maybe I need to figure that out and get the idle speed in the correct range.
Your rpm's being that high at idle is why it's dieseling. Also check the adjustment for the carb. solenoid. If it's set too low that's one reason why you would get a sudden drop in rpm's. It's normal for the rpm's to drop a bit like you describe. I would set the idle to 800, bump up the solenoid adjustment and go for a drive that won't cause a problem if you happen to stall. You shouldn't stall out if the carb is able to maintain at least 5-600 rpm. However, your carb can be tired and needs a rebuild.
 

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when my 86' chevy van diesaled on shutdown, my uncle told me there is a check valve on the side of the old carbs that shuts the gas off when the key is shut off. Not sure if this could be a culprate?
 

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Your rpm's being that high at idle is why it's dieseling.

x2.


Make sure your ignition timing is set correctly, eliminate any vacuum leaks...then set your idle RPM to aroud 650-700.

Should eliminate any run on.

FYI ~ the choke setting is adjustable. :thumb:
 
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