I have a 1992 3.9 multipoint fuel injection. Each morning for about the first .5 of a mile the engine is constantly missing and poping through the exhaust.Then it straightens out and runs fine and won't do it again until the next morning. I have changed all sensors new fuel pump 31 lbs. presure,no check engine codes. Thanks for any help.Johnny

 

What about your o2 sensors? Does it have o2 sensors on the exhaust?

 

The o2 sensor was the last one i changed it only has one.

 

The problem with a cold-run misfire and swapping sensors is, a lot of the sensors aren't used while the motor is in cold-run (before it goes closed loop after warming up enough).

So, O2s won't affect it then (they're not used until it goes closed-loop, which is not until it gets warm enough. Another strike against using a 160* thermostat ...)

Time to start checking things out, but I do notice a few things you haven't mentioned.

1) How old is the distributor cap? The rotor? The spark plug wires? The coil?

I ask about those because if it gets a bit damp inside the cap, it'll misfire. Also, it could be due to general age on the cap, rotor, and coil. The wires, my experience is they're more prone to misfire when HOT than COLD when getting old, but my experience is limited *grins*

2) What does a compression test say about your motor when run cold and when run hot? Might have some rings that gap too much, or a valve that doesn't seat well until warmed up enough.

3) Check for that splice under the battery tray that the Magnum Gen1's seem to have fits with. Also, while checking that, take the ECU connector loose and reattach it about five or six times (all with the battery disconnected!) and make sure it's not damp sneaking into corrosion/oxidation on the contacts!

RwP

 

I HAVE CHANGED THE COIL, WIRES CAP AND ROTOR .I HAVE DONE A COMPRESSION CHECK WHEN COLD ALL ABOUT 130 LBS. I HAVE CHECKED THE SPLICE IT IS FINE. I HAVE CLEANED THE ECM TERMINALS .I DID DO A VALVE JOB ABOUT 5000 MILES BACK. I HAD IT AT A GARAGE AND IT WAS SHOWING A CHECK ENGINE CODE 22 .THEY THOUGHT THEY HAD RESOLVED THE PROBLEM BY REPLACING THE COOLANT TEMP SENSOR. THEY SAID IT WAS ACTING LIKE A CHOKE CAUSING THE ECM TO SEND EXCESS FUEL TO THE CYLINDERS. WHEN YOU FIRST START IT UP IN THE MORNINGS AND MANUALLY GIVE THE THROTTLE CABLE THE RPM JUMPS ALL OVER WON'T RESPOND TO THE THROTTLE ACTS LIKE IT IS NOT GETTING GAS AND BACKFIRING THROUGH THE MUFFLER. BUT AFTER A SHORT TIME AS IT WARMS UP SLIGHTLY IT RUNS PERFECTLY AND WON'T ACT UP AGAIN UNTIL THE FOLLOWING MORNING.I CAN ALMOST LIVE WITH IT .

 

Checked injectors aren't bugged up?

 

I haven't looked at the injectors yet but they seem to work fine after it has run a short while.

 

One more troubleshooting step - disconnect the battery (just the negative is fine), then using a 5/16" (IIRC, maybe 10mm) socket, take the big connector off and on six or seven times.

See how it runs from cold then.

It should kick out a 12 code after that.

Also, check the wiring to the ECT, make sure it's not cracked (I had to replace my connector due to that failing).

RwP

 

I will try that tommorrow ypu are talking about the plug that hooks up to the ecm the computer ,right?

 

Yep. The ECM is also known as a SBEC (Single Board Electronic Controller) in Mopar-ese for your vintage Dakota (mine is a SMEC - Single Module Electronic Controller).

RwP