Hello, I have a 92 dakota that has a 3.9l swapped into it. This thing used to run great. Lots of power and no issues what so ever. Now sometimes its a pain to start, will start then stall, and every now n then it backfires, and if I let it run for a lil while it will devlop a rough idle. I changed the cap,rotor,wires,plugs,air filter,fuel filter, cleaned the throttle body, cleaned and checked the egr system, gone over the wiring with a multi meter and a visual inspections, and replaced a couple vacuum lines that looked shady and triple checked the routing. I also then regapped the plugs I just put in to make sure I didnt screw that up. But it still does it.

Any Ideas?

The only thing I can think of are that stupid hall effect pickup or the fuel pump is crapping out. But maybe someone else knows before I go buy parts.

Thanks

 

First things first:
You made sure the spark plugs have a .035 inch (.89mm) gap, I hope, if you verified and/or changed that as needed before putting them in, there's no need to pull 'em out to check.
However, DO double check to make sure the plug wires are routed properly (refer to the good ol' paint drawing picture given) (I made this mistake, I swear I double checked to make sure each wire was going to the right plug, but, I was wrong) (while your at it, also double check the rotor and cap)
Also maybe check for error codes.
Other than that, I could only suggest vaccum lines (which you said you were careful when you replaced, and double checked) or bad fuel (I don't know, maybe?)
Of course, I am not that experianced with cars.
*As far as the fuel pump crapping out, you can find a fuel pressure gauge to hook up, and watch that*

[IMG]local://upfiles/948/AAC90A30E3CA4D1A9E1360011D2213E9.gif[/IMG]
*hope your cylinders don't look like they do in the pic, lol*

 

Well today I figured it out. It was the hall effect sensor under the dist. cap. Nice. But its all better now.

 

Ok well I thought it was fixed. When I first start it when its cold, it fires right up and runs perfect for a little while then it starts to stumble and backfire out the exhaust. If I try to give it more air to keep it running it wants to shut off and start backfiring out the trottle body. Lack of fuel? Maybe my fuel pump is crapping out? Any suggestions? I am at a loss. It will restart after it starts doing this but it will still backfire and run like total crap. If I try to move it, it just dies. No check engine light has come on so far and I have tried to pull codes and all I get is a bettery has been dissconnected recently and a 55 which I believe is end of codes.

 

Have you checked the timing?????That could be the problem.

 

Timing is dead on, I thought the same thing. Thanks for the input tho. I pulled the plugs again. They were covered in gas. I'm wondering about the vacuum lines. Anyone with a diagram handy? The one under the hood is for a 2.5l not the 3.9l thats in it.

 

Hello. I am still looking for that diagram if someone has it handy. I have a 94 3.9l v6 in my 92 dakota. Ive got so many people completely baffled with this thing. But at least I have it the point now where it runs for a lil bit then it just shuts off like someone turned the key off or...it spits sputters and stumbles itself to death. Nice.

So far I have changed plugs..wires..cap..rotor...crank sensor...map sensor..tps...iac....coolant sensor.... hall effect sensor(the one under the dist. cap). Fuel pressure is fine according to Mr. Mechanic and I have ripped the wiring completely apart and checked for shorts...bad connections etc.... had it hooked up to a diagnostic monitor and got alot of crazy erratic readings but no codes.......set the #1 cylinder at tdc on compression stroke and checked to make sure rotor lined up with it to fire and it does. I checked and rechecked all the plug gaps. The only thing that we got out of the diagnostic reader that my mechanic had was that the auto shutdown is killing it and he told me to just start replacing stuff to see what fixes it.....nice advice. So I got a second opinion...same thing. excellent!

So my guess is still with the vacuum lines or buy a computer. I had checked the vacuum lines before only to realize that the diagram under my hood is for the 2.5l engine that came in the truck...not the 3.9 thats in it now. So now they are pretty messed up. Some are still hooked up..others arent, I have been trying to trial and error this but its not working. Anyone have any other ideas it would be cool to let me know and it would be hellacool to get my eyes on a vacuum diagram. help would greatly be appreciated.

Thanks
Eric

 

The following vaccum diagram is from a 1996 Dakota with the 3.9L.

[IMG]local://upfiles/948/85572CFDA8704A339C525A3B7AC3B523.jpg[/IMG]

 

Thank you....that is awesome. It looks about the same except the egr stuff is missing which is fine since the egr is gone on my truck and I have a set of exhaust manifolds that came off a 99 without the egr port on the passenger side milled out. But this is perfect. Thanks again.

 

I,m not sure about the 3.9 but on the 5.2 this sounds like it could be the plenum gasket.I had 2 318's that did this and that was what it was.