Can't figure out backfire through throttle body

[Caman96]

Surprised it even ran!

 

[Road Trip]

I have found my issue. I learned to take someone's word with a grain of salt when buying vehicles, the ad had said there was new plugs and wires on the truck but clearly they havn't been touched in years by the looks of it. truck runs awesome now.

Thanks everyone for the pointers!

@Mitch4x4, good on you for getting back to the forum with what you found +
what solved the problem. I particularly like your photo of all 8 plugs arranged as they
were installed -- they do tell a story.

Speaking of which, they are all uniformly showing signs of a rich mixture and
possible oil consumption? In order to get a better handle on what the state
of the engine is in your newly acquired truck, it would be great if you did the
following:

1) Figure out how accurate your odometer is with the current drivetrain & tires.
Comparing the odo against a single mile marker isn't very accurate, but comparing
your odometer/trip meter against 10 miles worth of mile markers on the Interstate will
easily get you within 1-2% accuracy. (quick example: Stock gearing + oversize tires = odo
shows 9.0 miles indicated when passing the 10th mile marker tells you that your displayed
mileage is off by 10%) Use this correction factor for steps #2 & #3 below.

2) Figure out your Miles Per Gallon after 2-3 fill ups. If the new spark plugs continue
to color like the old ones did, AND you are getting 6 MPG, then this (along with CEL codes,
condition of fuel metering system, O2 sensor data/crossing counts/etc) ...then all this info
will guide us as to where we need to focus next.

On the other hand, if you are getting 12-15+ mpg and the plugs are light tan in color,
then these 2 data points correlate and you will know that the gas you are buying
is being used efficiently, not wasted. Relax & enjoy.

3) It would also be very good to know exactly how many miles you travel per quart of
oil added. Obviously, if the PO treated the oil like he did the spark plugs, it's probably
due for a change. After changing the oil, keep track of the oil consumption so that
we can figure out what condition the engine is in mechanically.

By the way, I'm not an 'all or nothing' guy. If the oil consumption is better than 1,000
miles/quart & the plugs are pretty, I'd leave it alone. If the oil consumption is better
than 500 miles/quart, but the new plugs are all starting to foul again, then I'd run a step or two
hotter heat range spark plug and keep an eye on it. (Note: Historically most automakers won't fix
an oil consuming engine under factory warranty unless it's worse than ~750 miles/quart.)

And if you are adding a quart of oil with every gas tank fill up, then for long-term reliability it's
time to figure out what needs to be done to make things less worn under the hood.

****

Again, thanks again for closing the loop on all this with a clear photo of all 8 plugs
and the solution to your problem - no doubt that this will help others who are trying
to sort out similar symptoms.

Good luck. Looking forward to the oil consumption, MPG, and spark plug coloring
after you put a couple thousand miles on it.

PS - Glad I put that disclaimer on my previous post. On an intake sneeze/blowback (unlit)
or actual intake backfire (ignited) a sharp tune up is the easiest way to figure out if
the problem is tuning related {poor combustion > delayed burn exposed when intake opened
near end of exhaust stroke} -or- mechanically related. {blowback related to poor valve sealing/seating
during compression stroke.}

In your case, replacing the plugs (and distributor) resulted in a smooth running engine,
making a follow-on compression test + leakdown test superfluous. Good actionable info.

 

[Schurkey]

I'm surprised that fouled plugs caused intake-manifold backfiring, but not a complaint of poor power or poor fuel economy.

A cracked exhaust manifold can cause false "lean" signal from the O2 sensor, driving the computer into rich-command.