Odd backfire, low power and a bank of wet plugs

[ActuallyaMazda]

Hi everyone. I’ve got another issue on my B4000 that is getting worse as time goes on. I have solved my previous issue with high idle and rough warm starts. Turned out to be the PCM, so I ordered a remanufactured one. It dropped right in and immediately the high idle went away, however I am having a new slew of issues. Hooray!
First off, with a new PCM swap, I installed it with the battery cables disconnected. First startup was a dream. It started up at the RPM it should’ve, and held itself at about 750 when it got warm. That’s fantastic and everything and so I took it out for a drive. I took it at least 10 miles so the PCM could relearn all the values. Got back and parked it and noticed that it had a slight misfire. Drove it to work the next day and the misfire seemed to get a bit worse and now it started to backfire under slight load (anywhere from idle to about 2500rpm). This was all at the start of this week. Fast forward to this weekend and I have replaced spark plugs, wires and ignition coil all with as close to OEM that I could have our parts department order me and it only seems to have made the problem worse. It idles like the IAC is unplugged at about 350rpm and sounds like it is only firing on a cylinder or two. The rest is lopey, sputtering backfire noises out the exhaust.
I don’t have a catalytic converter on this pickup, so I am also clearly able to see the tailpipe spitting out puffs of smoke at idle.
In the process of yanking out my spark plugs this morning, I noticed that on cylinders 4, 5, and 6, the plugs were black and wet while 1, 2, and 3 were dry and looking like normally worn spark plugs.
I have only taken it on a couple drives since the issue has started because it almost seems to get worse every time. What happens is that accelerating from a stop, it’ll backfire and feel like there is barely any power until it gets to about 2500 - 3000rpm and then it decides that it actually wants to work and will run completely normal. Runs normally under WOT as well. Once it gets back down to lower rpms though, the backfire comes right back and so does the power loss. Parking it after a short drive and it’s warmed up as well, it bucks and shakes the whole pickup at about 400rpm and almost will stall it out and sound like it’s trying to suck every last bit of air that it can through the intake. Unplugging MAF makes it run worse in general, unplugging IAC drops the RPM like it should and will sometimes stall it out, unplugging the TPS as well will make it run worse and also make the smoke out the exhaust worse for whatever reason.
I’ve been dumping a lot of money into this lifted POS for a couple months now just trying to get it to start up and run correctly and before you ask, yes I have checked power, grounds and continuity to all electrical components and they all test within range.
Any advice would be greatly appreciated! Thank you!

 

[SenorNoob]

You say 1 2 3 were wet. Oil, gas, other?

 

[ActuallyaMazda]

You say 1 2 3 were wet. Oil, gas, other?

4 5 6 were the wet ones actually. 1 2 3 were dry. It looked to be gas, but it was hard to tell with the plug looking so blackened. I’m leaning more towards it looked like fuel and with the backfire going on that has me thinking it’s that too. It’s also been chewing through fuel like it doesn’t cost 5 bucks a gallon recently.

 

[SenorNoob]

I see now. It's interesting. I know that 94 was a transition year from batch fire to sequential for the 4.0. I wonder if there was an early vs late 94? Would definitely cause problems with fueling. And it sounds like the injectors are staying open.

 

[ActuallyaMazda]

I see now. It's interesting. I know that 94 was a transition year from batch fire to sequential for the 4.0. I wonder if there was an early vs late 94? Would definitely cause problems with fueling. And it sounds like the injectors are staying open.

It definitely looks more and more like injectors might be the culprit. I’m changing my fuel pump out today to see if that makes any kind of difference, but beyond that I’m going to order a set of refurbished injectors.

 

[ActuallyaMazda]

Update: Ran some injector cleaner through the tank to no avail. Took it for a 10 mile drive and at first it seemed like it was working better, but coming to stoplights and idling in drive or park, it will start to bog down. It got really bad about 6 or 7 miles into the drive and it would start backfiring VERY loudly and feeling like it was almost going to shut off until I gave it about half throttle and it would function normally and have full power and no backfiring. Letting off and giving throttle again only brings the issue back until about half throttle. Any ideas on what could be causing this? Is it safe to assume that my injectors are just jam packed full of dirt and whatever else may have run through the fuel tank?

 

[superj]

not if it runs fine at a higher rpm. your issue is something is reading wrong at low rpms.

test your temp sensor/sender
test your idle control valve
test your mass air flow sensor
test your o2 sensor

 

[ActuallyaMazda]

not if it runs fine at a higher rpm. your issue is something is reading wrong at low rpms.

test your temp sensor/sender
test your idle control valve
test your mass air flow sensor
test your o2 sensor

I replaced the O2 sensors last month because before that, the ports for them in the manifold were plugged off by whoever had it previously. I didn’t have much time for actual testing, but unplugging the O2 sensors took away all power loss and backfiring, however now upon restarting it will chop and lope like it’s cammed for a minute or so before leveling out and running great. I did, however, test the IAC and it’s functioning how it should.

 

[superj]

its something in your o2 wiring than. maybe they were removed because the original sensor wires got melted or something?

 

[ActuallyaMazda]

its something in your o2 wiring than. maybe they were removed because the original sensor wires got melted or something?

I’ll take a look later today. At least getting this out of the way, does it matter which connector the O2 sensors plug into? Like say I had the sensor on bank one plugged into the connector that is maybe meant for bank two. Or do they give the same reading to the PCM regardless of where they’re plugged in?

 

[superj]

i have on idea but i kind of think they would have to go to the proper bank

 

[SenorNoob]

If banks were swapped, It'd do ok until an imbalance started. Then the O2 readings would cause computer to adjust each side to extremes because it "sees" the other side.

Running fine on open loop. Goes into closed loop. Bank 1 starts to run rich. Because the computer thinks bank 2 is running rich, it leans bank 2. Then bank 2 running lean causes the computer to think bank 1 is lean. Computer then richens bank 1. Now bank is super rich and bank 2 is super lean. Getting worse every cycle of the engine until it stalls out the computer gives up and throws codes for "at adaptive limits".

Unrelated right now, but; interesting nonetheless, is that something similar can happen even with just 1 O2 sensor. If you reverse the signal wires, it's possible for one line of code in the PCM to see reversed polarity and throw codes for too lean, and another line of code to see just the voltage number without it's sign and throw codes for too rich at the same time.

 

[ActuallyaMazda]

If banks were swapped, It'd do ok until an imbalance started. Then the O2 readings would cause computer to adjust each side to extremes because it "sees" the other side.

Running fine on open loop. Goes into closed loop. Bank 1 starts to run rich. Because the computer thinks bank 2 is running rich, it leans bank 2. Then bank 2 running lean causes the computer to think bank 1 is lean. Computer then richens bank 1. Now bank is super rich and bank 2 is super lean. Getting worse every cycle of the engine until it stalls out the computer gives up and throws codes for "at adaptive limits".

Unrelated right now, but; interesting nonetheless, is that something similar can happen even with just 1 O2 sensor. If you reverse the signal wires, it's possible for one line of code in the PCM to see reversed polarity and throw codes for too lean, and another line of code to see just the voltage number without it's sign and throw codes for too rich at the same time.

Swapped the MAF and that fixed nearly all issues. I dropped a little more cash to get a Delphi one that matched directly up to the part number and that fixed startup and all running conditions. Swapping the O2’s seemingly made a bit of a difference before I got the new MAF, but the MAF was definitely what the majority of the issues were stemming from. Now I just have to fix the steering.