97 4.0 backfires

[redjmp]

My 4.0 started to run a little rough at idle about a year ago but not too bad.
Changed plugs, and filters, the IAC, and cleaned the map sensor with no results
Then it started to stumble occasionally under load a few months ago and progressively has gotten worse to the point of backfiring.

With no load on level ground though it ran smooth.

Changed out the plugs and wires yesterday and now it starts up and runs smooth at high idle until it kicks down and then just backfires constantly.

Any ideas where to start?

No codes are being thrown except and occasional 0442 when I park on a hill.

Checked compression with all cylinders good maybe 1 was a a few lbs lower...

 

[downer]

i would test the coil pack. if you have access to a scanner i'd take a look at the fuel trims. oxygen sensor could cause similar problems.
being a 97, it wouldn't hurt to check for vacuum leaks on the lower intake manifold. seems to be a somewhat common problem also.

 

[RonD]

Yes, reads like vacuum leak.

With engine warmed up and idling unplug the IAC valve.
Idle should drop to about 500rpms or engine may even stall, either is good.

If idle stays up then start checking vacuum lines, unhook a line from intake manifold and put you finger over the open intake port, if idle drops that line has a leak.
Check PCV Valve hose common leak point.

The large air tube from MAF sensor to intake also needs to be air tight, no cracks and good gaskets at both ends.
If all the air coming into the engine doesn't pass thru the MAF sensor then engine will be running lean and have no power.

4.0l's have a common problem with the lower intake bolts getting loose, which creates a vacuum leak, check those.

Lean fuel mix can pre-ignite when entering a hot cylinder causing a back fire.

Low fuel pressure could also be the problem, but usually that wouldn't effect idle