Engine runs/idles rough, doesn't burn fuel completely

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CherokeeCook

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Hello,
I just got this truck, a 1985 D150 318(5.2L) 2WD. Pretty vanilla truck but it's marvelous.
Long story short, it was running fine until one morning I got in it and it was idling roughly (loaded and unloaded) and it isn't burning all the fuel, puking (definitely) gas out the exhaust. It would want to die when put in gear, but I could keep it going with gas. It had rained the night before (one theory is that moisture got into something electrical). Fast forward to a sunny day, it's still doing. Distributor and button have no obvious defects.
I'm new to this so I humbly come to the great hivemind of the internet for guidance.

Thanks!
 

Joshwaa

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I am not familiar with what that year had. Carb? FI, TBI? Give me some more info and I will try to help you. Do you have a multi-meter. Start first by checking the coil. Should be between .4 and 2 ohms. If no multi then pull a plug wire and see how good of a spark you are getting.
 
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CherokeeCook

CherokeeCook

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It's carburated. I do have a multimeter & was going to check the wires also but I couldn't find a number for what resistance they should be in the Hayne's book. Haven't checked the coil yet because it got dark yo
 

Joshwaa

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Carb huh. Sunk float possibly would be my first guess. Then bad coil. I would not think that all the plug wires went bad at once. When you take off the air cleaner can you see the vent for your float bowl. Check if gas is comming out of it when running.
 

muddy12

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Not too familiar with what carbs dodge used back then(I was a Chevy guy at that time), but does your carb have an automatic choke? If so, it could be sticking.


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CherokeeCook

CherokeeCook

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@muddy12 It does have an automatic choke that has been sticking and making it rev high on start till it finally opens when warm. It's been doing that a long time, it was the next thing I was going to fix before this.

@Joshwaa The book says voltage on the positive side of the coil should be +/- 1 volts of battery voltage. It was pretty much zero. That was with the ignition off, if that makes a difference.
 

Joshwaa

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No I meant put an ohms meter on the + and - side of the coil and see what you get. If it is between .4 and 2 ohms then the coil should be good. Take the wires off of the coil when you test it.
 

crazzywolfie

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@muddy12 It does have an automatic choke that has been sticking and making it rev high on start till it finally opens when warm. It's been doing that a long time, it was the next thing I was going to fix before this.
i doubt it is sticking. the thermo chokes can take forever to fully open especially if the exhaust cross over is clogged. this is why electric chokes were preferred.

the voltage to the coil should not be the same as the battery. should be about 2-3volts less than battery voltage because of the ballast resister.

does it have a ignition module and ballast resister on the firewall by the drivers side hinge or is there a computer on the air cleaner or behind the battery?
 
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CherokeeCook

CherokeeCook

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@crazzywolfie It's got a 4 pin ECU on the firewall. Initially, I thought the coil checked bad but after going back and doing more reading and checking it again (properly), I'm pretty sure it's not bad. So I'm still in the dark about the rough running. I noticed the distributor cap had some corrosion on the points, and the rotor contact was kind of burnt. I'm going to replace those and give it a go.

Regarding the choke, it looks like it is mechanical. It has the bimetal coil spring set into the engine (attached to a vertical rod that impinges on the carb), but it also has a wire coming out of that same assembly going into a small ballast resistor with a couple of other connections. That thing is located between the choke and the coil, and the wire I spoke of is definitely shot. The insulation is toast and the wire is frayed. Do you know what those components might be for? I wonder if that is contributing to the high idle.
 

crazzywolfie

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it is probably themo electric choke. still not the greatest but worst case scenario if it stops working is you just have to wait for the intake to heat it up enough to fully open. you can take the air cleaner off and see if the choke is open all the way or you can try holding it open and giving it a bit of gas. if you hold it open and give it some gas it should start running on the idle circuit.
 
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CherokeeCook

CherokeeCook

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Before it started running rough, I would have to wait about 10mins for the choke to fully open. That whole 10mins till it gets warm enough it revs really high. So Whether or not it's related to this running issue I think I'm just going to replace every choke component, because the previous owner tried on numerous occasions to fix the choke issue with no luck.
 

crazzywolfie

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idling high when a carb is first started for about 10 minutes is normal. i don't know what the temperature is generally like where you live but if it stays above freeing you could try tying the choke open. i wouldn't go replacing any parts unless you are 100% sure it is the issue. some people will just get a manual choke kit and make it work.
 
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