First of all quick update - can no longer drive the truck. Permanent backfire no matter how many times I restart it. Currently sitting in my garage.

@moparnick

Yes that's exactly what it sounds like - not sure if its the intake however. The 3:00 minute mark is where you hear it backfire atleast 10 times.

I will try to crawl under tomorrow after work and test it out (I have a multimeter). Do our trucks just unbolt at the cat or do you have enough room to work with?

Never actually had a look there myself.

 

I'm not too familiar with the 3.9 but I do not think it unbolts at the cat. Its probably cats+midpipe as an entire section which the exhaust and tailpipe slide into. Here is what I would do. You need to have the engine running and you also need the O2 sensor in question to remain plugged into the harness.

You will have to cut away the outer sheeting covering the sensor's wires. Once you have all for wires available, gently trim some insulation away until you expose the metal conductor. Try to do it in the way you would peel an apple. Do this in a different section on each of the four wires and only trim maybe half an inch. You will need to use electrical tape to insulate the wires when you finish. A few wraps around each exposed area should do the trick.

Then you need to experiment. Take two of the wires and connect alligator clips to the metal you exposed and connect the other ends of the alligator clips to your multimeter. Then drag your multimeter out from under your truck so you can view it without being underneath. double check that nothing is touching the wires which could cause any shorts.

Now start the truck and let it idle. Look out/get out and check the meter. Let it sit 30s-1min and note the voltage. One combination should give you a volt or less (sensor) and the other combination will probably be several volts (heater).

FYI I have not used this procedure yet but if I had to check this is how I would do it. There is little space and you might have a hard time getting to the wires. You can always unplug the sensors and move the wire to a more comfortable position to strip away a piece of insulation but make sure you reconnect it before starting the engine.

 

Thanks for the detailed reply, Nick.

I plan on testing out the wires tonight so hopefully I will get some results - I'll post them here as soon as I can.

 

Truck died before I could even get to the O2 sensors. Cranks but doesn't turnover - towing it to shop tomorrow.

 

Hmm. It might be something else. Even if you unplugged the O2 sensors it should really still start up. Usually vehicles run open loop until the O2 sensor heats up enough to provide correct data. Maybe the crankshaft position sensor has gone bad. let us know what the garage says.

 

With the problems you are having Im betting on the Computer is toast.

You can get them cheap though not $1K from dealer but $300 or up.

 

I'll keep you posted - and I have already changed the crank sensor.

Really starting to lose hope... Backfire... Just replaced wheel hub... Need to fix front u joint.

Dropped a ton of money into this thing and its turned around and bit me in the ***.

 

4X4 are money pits unless you need 4X4 stay away from any used 4X4 same with any top line model with electric everything they become money pits as they age.

Nice features but more to go wrong with them with age.

 

Truck spent 2 days in the shop - I've been told that it indeed was an o2 sensor causing the backfire. As for the truck not starting - after it sat overnight waiting to be looked at it started fine in the morning for him so all is good... If i had to guess I may have had air in the fuel lines or something - I'm not really sure. I'm picking it up today after work and over the next couple of days I will let you guys know the situation.

 

My computer would do the same thing work great for a week or two then start acting like a crazy drunk the next day.

I though you already changed the O2's