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Unread 11-07-2012, 05:32 PM   #1
concretenomad
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Idle Problems

Having trouble with 86 cj7 Idle.
When I start it cold everything is normal. Runs at about 1500 rpms for a few minutes, drops to about 800 for a while. Then after reaching running temp, the rpms shoot up to anywhere from 1800 to 2500... 2800ish after driving for a while. I usually only drive about 5 city miles to work so it doesn't have a chance to get up there. It diesels when it does this. I've checked for a vacuum leak and can't find one, and sprayed carb cleaner down the carb. Also ran 2 cans of Seafoam. It only really shoots up there when it's warmed up. I changed the plugs and they looked like it's running lean (blueish white), which makes sense with the sometimes high idle. Wondering what this could be other than a vacuum leak. I've only had this jeep a few months so I'm not sure exactly all the parts etc. The engine was rebuilt 30,000 ago. It has a 258 I6. Electric choke, assuming it has a carter carb but I haven't verified that.
Any ideas what else I could check for? I'm not a dummy but, not really good at diagnosing.
~Thanks~

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Unread 11-07-2012, 06:41 PM   #2
macscal
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Its been a while since I worked on a 258, but your on the right track. Instead of spraying the carb cleaned in the carb, try spraying the base, the vacuum lines, the ports, etc. while its running. If the idle speed changes, it indicates a vacuum leak. I also seem to remember (been 10 years since I had a cj7 with a 258) that there was a pulse air system with a idle speed control solenoid. From recollection, I believe they can also be the cause of what your describing. I hope someone else will jump in here and give you some ideas and correct or affirm what I said. GOOD LUCK!
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Unread 11-07-2012, 10:47 PM   #3
BagusJeep
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the 1986 had a computer for emissions control and is quite complex compared to earlier vehicles.

Once it warms up it goes into Cruise mode and the computer enters a closed loop operation, maintaining the fuel/air ratio as its primary purpose. It achieves this by measuring oxygen levels and then via a stepper motor adjusting the carb. It MAY be that you have a problem with the O2 sensor or the adjustments and this is manifesting itself in this rising idle when it gets warm.

I am not an expert, you need to get the manual and work through the diagnostic tests.
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Unread 11-08-2012, 03:03 AM   #4
86cj74.2L
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I can't think of anything but the solvac gizmo that is used to increase idle in certain conditions. If it even works. You can remove the vacuum line to it and plug it and disconnect the wire going to the solenoid. But that will not cause it to idle that high.

Then if it still high idles it is a vacuum leak.

Look at where intake bolts to head, carb base (as mentioned earlier) on underside of I take just below carb where the manifold heater mounts, and the miles of vacuum lines.

The looking for leaks works know matter which carb you end up having.

It diesels? You are referring to when you shut it off?
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Unread 11-08-2012, 06:03 AM   #5
On_a_string
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I have the exact same problem on an 84 CJ. Please let us know what you find when you find the problem.
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Unread 11-08-2012, 06:08 AM   #6
86cj74.2L
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Quote:
Originally Posted by On_a_string
I have the exact same problem on an 84 CJ. Please let us know what you find when you find the problem.
When yours high idles did you look for vacuum leaks?

If so and none found try pulling back on the throttle shaft bell crank and see if the idle goes down.
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Unread 11-08-2012, 06:55 AM   #7
thompsoj22
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jeep adventures under the hood

it is how i completely reworked my vacume system. you have to be inducing air under the carb somewhere. if the system works great until op temp than i would rule out leaks. instead, once it starts it's idle problem use a pair of pliers and one by one squeeze each line until you find the one that is allowing the air to bypass. than figuer out why. if you use that recommended web page there is nothing you cant fix when it comes to the vacume/emissions on your jeep. ill close this response in saying that im 53 and have been a mech all my life, i have never seen such a rediculous, screwed up horror of engine management as the AMC pulse air system. whoever designed it should be deported to afghanistan to be with other al qaieda insurgents. if they are allready dead than exume them and send their remains instead!
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Unread 11-08-2012, 11:19 AM   #8
concretenomad
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Yeah, I'm thinking it's mechanical rather than a vacuum leak. I used 1/2 can of starting fluid looking for one. Also, like you said thompsoj22, it is only doing it upon warm up, before that it seems like everything's fine. Squeezing the lines sounds like a good plan. Also I'm going to look further into the solvac unit. Another thing is that it seems like the longer I run it, the worse or higher the rpms go up. I wonder if there's a temperature sensor that is responsible for idle control? This morning I drove the 5 miles to work.. it was idling fine. I turned it off.. fine. Started it right back up, the rpms were at 1700 and wouldn't kick down, even after I gave it a minute, then it dieseled a bit when I turned it off. Will do the line squeeze and look into solvac. Thanks.
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Unread 11-08-2012, 01:37 PM   #9
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What carb do you have?

Make sure your fast idle cam is not getting in the way and causing your high idle. It's just a simple carb that the mcu manipulates the air bleeds for idle and main jet circuit to control mixture. That's if it's even operating. Below is a link to what the BBD with stepper should look like.

YouTube (Short URL)
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There is nothing that will raise the rpm's in the idle system that high. The sol-vac is just to pick up idle to say 900 rpm's and such.

If it is dieseling when you shut it off your combustion chambers are full of buildup. A rich mixture and all but zero ignition advance at idle will help load up the engine. Then the carbon in the combustion chamber will light off the rich mixture when you shut it down.

This picture will show you what the items are.

Things expand when hot don't entirely rule out a vacuum leak. When it high idles, get out and look at if their is a air gap between the curb idle screw and the idle stop. If their is is the fast idle screw touching the fast idle cam? If no to both try pulling back on the throttle and see if it returns back to its normal idle speed.

If you have a dead cylinder that picks up after the engine warms up that would help explain the major increase in idle speed when warm as well as the dieseling.
image-2414047518.jpg  
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Unread 11-09-2012, 11:04 AM   #10
concretenomad
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New development: This morning My drive to work was.. spitting, sputtering, bucking, and not firing right. Looked for a vac leak again...nothing. Going to take it to a mechanic on Monday. Meanwhile... I have the weekend to fiddle with it. ugh.
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Unread 11-11-2012, 03:35 PM   #11
concretenomad
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OK... I feel like I might have gotten somewhere.... just want to check with you guys to be sure. This is what happened today:
I started the jeep, let it idle for a while to warm up and be naughty... nothing. idling at 800. I took the air filter off and noticed a whistling sound I hadn't before. You can't hear it with the air cleaner on the carb... which is why I might have missed it before. The whistling would stop if I moved the throddle about a mm, even changed pitch with a smaller movement. So, it sounded like the whistle was coming from inside the carb. Still was idling ok though. Drove it to the parts store.. high idle the whole way (1500-1700 rpms). Got home to check it out (while it was acting dumb). The whistle was gone. I did also notice that the butterfly was not all the way open, I would say probably about 1/2 way. Does it sound like I need to rebuild the carb? Or is there something else I should do first? I'm guessing there's a gasket inside that's leaking. Opinions? oh, and I did determine that it's the carter 2 barrel.
Also the sputtering around the other day could have been related to this, or the fact that some of the plug wires were slightly loose... we'll see.
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Unread 11-11-2012, 04:48 PM   #12
86cj74.2L
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The butterfly your talking about......is that the thing on top?

If So, That's the choke plate. And it should be strait up and down when warmed up if its working. That would also explain your high idle. The choke closing is raising your idle.
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Unread 11-11-2012, 04:58 PM   #13
concretenomad
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Right... so, is the choke malfunctioning because of the whistling... or is that a separate issue? I'm not sure where to go from here, it's an electric choke. Should I unhook it to see if that solves the problem after it's warmed up? then do I just replace the choke if that does it?
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Unread 11-11-2012, 05:12 PM   #14
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Yeah, but if it was the choke plate causing this, then he would be running rich, but the spark plugs say its lean...
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Unread 11-11-2012, 05:15 PM   #15
86cj74.2L
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Quote:
Originally Posted by concretenomad
Right... so, is the choke malfunctioning because of the whistling... or is that a separate issue? I'm not sure where to go from here, it's an electric choke. Should I unhook it to see if that solves the problem after it's warmed up? then do I just replace the choke if that does it?
You need to find out if you have 12v at the electric choke when the engine is running first. A simple test light from the autoparts store will do it. Simple clamp the alligator clip to something metal in the engine and with the engine running touch the wire connection on the plastic choke housing.

If light lights you have a problem related to the choke or linkage.

If it doesn't light you need to figure out why it's not getting power.
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