Hello all,
My truck has been idling roughly here and there, and has been having trouble getting from gear 2 to 3. For something like that that isn't necessarily consistent, could that be a failing coil pack? The inconsistency part is stumping me. Thanks!

 

What year and what engine?

Manual trans??

You can test coil pack with OHM Meter: youtube.com/watch?v=c1zhgsnyZWw

While this test will tell you if a coil in the pack is bad, passing test doesn't mean it is 100% good, coils get very hot in normal operation, so if a problem only occurs after engine has been running longer than 20mins on a warm day then could still be coil pack

 

1999 Mazda B4000 4.0 OHV automatic.

Thanks, RonD!

 

On the 4.0l engine always clean MAF sensor, every 2 years at least.

Also check for vacuum leak, simple and easy test
After engine is warmed up, like when you get home
Leave it idling and pop the hood
Unplug the two wire connector on the IAC(idle air control) Valve
IAC Valve will close all the way and engine RPMs should drop to about 500, or engine may even stall, either is good, it means no vacuum leaks
If idle stay the same then there is a leak

 

I got another code today, P0171. The truck starts up okay, sometimes it takes an extra second to get the rpms up to ~760. Driving it is okay, there is a barely noticeable loss of power, and shifting in the ~2nd/3rd gear area is a little sketchy.
The only thing I've changed in the last few weeks is the coil pack, I had changed it when I didn't need to, so I figured I could put the old one back in. I'll switch them again, might as well make sure everything is working up to par.

 

P0171 is Lean on Bank 1 passenger side, this can be caused by misfires on that bank

O2 sensors only "see" Oxygen, and if a cylinder misfires then no oxygen was burned in that cylinder so it all gets dumped into exhaust where O2 sees it
And too much oxygen in exhaust is Lean
Too little is Rich

It can also be caused by vacuum leak on lower intake, or dirty injector on that bank, even older O2 sensor on that bank.

Upper intake leaks, MAF problem, or fuel pressure issues would show lean codes for BOTH banks

1999 4.0l will use the 5R55E automatic trans
4R44E/4R55E, and 5R44E/5R55E are exactly the same inside

Read here on 2-3shift issues: Seven Common Problems With The Ford 4R55E Transmission : The Ranger Station

 

Thank you for all of this info. I really wish that CEL codes could pinpoint what to replace, especially with misfires, it's always so open ended.

Kind of sort of unrelated question, would a tiny crack in the cylinder head between intake and exhaust valve seats cause a misfire? And if so, could it block up causing an intermittent misfire?

 

4.0l OHV heads will crack between valves seats if the engine is ever over heated, weak(thin) point in the casting, so if the metal expands too much it will crack, and heated metal expands.

Yes, it would cause a misfire, because that crack would lead directly to a coolant passage so slight loss of compression and coolant can be sucked in, it would also cause air(compression/firing) to be forced into the cooling system.
This air displaces the coolant in the head and increases the pressure in the cooling system forcing Rad Caps 15psi spring to be pushed open and coolant in the radiator is pushed out into the overflow tank, so it gets over filled.

Then engine starts to overheat since air inside doesn't cool as well as coolant(liquid), and much of the coolant is now in or spilling out of the overflow tank in any case.

Heater will usually stop blowing hot as well, as the air from the cylinder goes to high points in the cooling system and effectively blocks coolant flow thru heater core.
And at that time overflow tank would be Bubbling out air as upper rad would only have air in it, (high point as well) and no coolant.

So yes it would cause misfire but that would be a small issue, big problem would be the over heating make vehicle undriveable
No it wouldn't "block up"
lowest compression pressure is 150psi, but once started each cylinder runs at 1,000psi, thats what pushes the pistons down and gives power to crank shaft, so it won't "block up" on its own

There are are products you can add to coolant that contain Liquid Glass type of compounds, these react and become solids at 1,000+ degF which is whats seen inside a cylinder when it fires.
These are sold as Block sealers, and can work to seal cracked head or blown head gaskets temporarily, temporarily is the key word, it is not a fix, replacing head or gasket is the fix
The seal these provide falls out constantly so needs to be replace constantly, and this happens all the time the engine is running, so you don't add it and then change the coolant if it works for you, have to leave it in until you fix the problem

 

I drove the truck today to got get lunch and ironically my new coil pack, and it started, idled, and drove perfectly normally. I'll still replace the coil pack because might as well have a new one in there now that I have one.

When I did my valve job, I noticed something that looked like a teeny tiny crack, but the engine has never over heated (before nor after that repair), which tells me that that is not a problem. I had run out of money so (don't hit me for this), I did not get the heads serviced when they were off, but when I put everything back together, it ran just fine. It just has these little hiccups where it will throw a code, be shaky for a few days, then straighten out again.