My 3.9 had issues starting this morning, about a mile down the road, it **** off and revved back with cruise control on. Then it started backfiring, half a mile later, dead. After a few minutes, tried to start it again. It ran for about a minute, then shut off. Battery was dead, so had it jumped off.

While jumped off, the GEN light came on and it would back fire and die as soon as jumped cables were removed. My initial thought is the alternator has failed, no CEL, the only thing different I've done was put an Edlebrock Cold Air Intake on it.

I just want to make sure I'm not just throwing money at it.

 

Hard to say for sure, but you can yank the alternator and take it to any Advance/AutoZone and they will check it for you for free.

 

Also, CHECK ALL YOUR BATTERY CABLES BOTH ENDS!!!!

But yes, if the battery is failing, it'll run like crap, really bad crap, and even worse ...

RwP

 

Alternator was failing, changed it, but now it'll backfire with sparks out of the tail pipe and die when put in drive.

I've never had one shoot sparks out of the tail pipe..

 

What year 3.9? Backfires are timing related, but without knowing the year of your truck, a lot of differences in the ignition systems.

 

Forgot to include that, my apologies. It's a 96.

 

If it was running fine before the alternator died, I'd be looking at anything you might have touched changing the alternator. Make sure as Ralph said, the battery cables are good and tight, the grounds are good and tight, etc.

Beyond that, to diagnose the ignition timing, you should check plug wires all seated well and you didn't swap any when replacing the alternator. Then check the distributor, make sure it's tight. Check the Cam sensor in the distributor, the Crank sensor on the flywheel. Unfortunately, a ton of things can cause backfires, so just start eliminating the more common ones.

Since it's a 96, it is OBDII, so hook a scanner up to it and see if there are any codes...not all codes throw the CEL.

 

Found the problem, got a short in either the pcm or wire harness. Just touching where the main wire harness plugs in, would shut it off. I believe it to be the pcm, not even zip ties will keep it from shutting off.

 

Might be an open instead.

One of the FIRST things to check - with the battery disconnected, remove and replace the connector three or four times.

See if that helps some.

The contacts could be oxidated.

Next, I would tighten up the female sockets somewhat; they can spring and loose tension over the years, causing an intermittent open. There's been a couple here that have had to do that (I think robertmee, but that's from memory, and I could easily be wrong!)

If none of that helps, THEN replace the ECM.

(If removal and reinsertion 3 or 4 times fixes it, then one more time, but put some dielectric grease - "bulb grease" - on all the contacts on one more removal/reinsertion cycle, to help keep it from oxidating.)

RwP

 

This link is to a process teh guy is using to add a pin but I think the process is useful for tightening up the female side of the connection
http://www.merkurboys.com/wiki/index...xtraPinInstall