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post #1 of (permalink) Old 08-21-2000, 03:30 PM Thread Starter
 
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Cj backfires after shutoff

My heap, I mean CJ, does a funny thing when you drive it for a while and it gets good and hot. Once you shut they key off, the engine shut right off, no run-on, and then 10 seconds later, it backfires. Sometimes the backfire waits 30-40 seconds so I think I'm clear and get out, only to have it sneak one out while I'm standing in the middle of a crowded parking lot. (Can you say duck and run so no one sees you)
It really only does it when its very hot outside, or I've been driving a while.

anyone?

thanks
Sean

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post #2 of (permalink) Old 08-21-2000, 03:53 PM
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Re: Cj backfires after shutoff

I'm not an expert here, but I've picked up a lot from this BBS. It sounds like you have a combination of exhaust leak, burned/chipped exhaust valves, and carbon build-up in the cylinders. What happens is that the carbon retains the engine heat and provides an ignition source. Fresh air is leaking back into the cylinder (along with residual fuel vapor). When it gets enough fuel - BANG! My AMC Hornet used to do the same thing - woke up all the neighbors. You probably don't need an overhaul. Maybe at most a valve job. A good cleaning out of the carbon would do wonders.

If I have explained this wrong, someone set me straight![img]/wwwthreads_images/icons/wink.gif[/img]

post #3 of (permalink) Old 08-22-2000, 04:32 PM Thread Starter
 
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Re: Cj backfires after shutoff

I'm pretty sure that there is a crack in the manifold because when I let off the gas I can hear a crackling noise from the left side of the engine. My friend tells me that this is sparks shooting out. Hopefully not a huge problem. You mentioned cleaning the carbon out. Is there an additive that can do this? Or do things have to come apart?

Thanks

Sean

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post #4 of (permalink) Old 08-22-2000, 05:10 PM
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Re: Cj backfires after shutoff

Boy, where is everyone?
I understand if you bump up the idle and SLOWLY pour water down the throat of the carburator, the water vapor will clean all the carbon out of the cylinder (or most of it). If you do a search on this board for say... water, cylinder, buildup - or something like that you'll find more info.
To find your manifold crack, start the engine and spray WD-40 around the manifold. Watch the fan! When the idle jumps up, you've found where the crack is.

post #5 of (permalink) Old 08-23-2000, 09:45 AM Thread Starter
 
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Re: Cj backfires after shutoff

Is there anything I can do about the crack, or does the whole thing have to be replaced?
I hate to put this pressure on you Attila, but it seems that everyone else is asleep at the keyboard.

thanks

Sean

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