If it's been running less than a half an hour, it's possible that the lifters haven't filled up to pressure yet; it has to idle a while to stabilize. if it's not that, then look at your ignition timing and your firing order.
SOURCE: backfiring and loss of power
'till you do some in depth testing of all systems, I'd place equal weight on every possibility. Make sure you cover the basics like fuel filter etc before getting crazy with it though. Also check cap. rotor, coil and wires. Idle cylinder pressure is different than that encountered when driving. This additional pressure will cause ignition spark to seek a better easier ground,(not the spark plugs where it should be going) causing missing under load.After checking , if you haven't found anything, its time to begin testing fuel pump pressures, modules and controls.
good luck
SOURCE: 99 jeep wrangler 4.0 backfires and hesitates
Fairly common problem. Typical scenario is start vehicle, runs fine
for 15 to 30 seconds and begins to backfire when placed under load as
customer tries to drive vehicle. Flooring usually makes the problem go
away. Usually does not throw a code.
Fix is to replace the upstream 02 sensor.
SOURCE: jeep wrangler bucking
how long has this been happing..one tank of fuel or more, could have been bad gas, replace filter,, put gas dryier in fuel tank..if it continues could possibly be transmission related..the reving down doesn't sound like transmission though if tansmission was slipping it would rev up..good luck
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If We floor it it makes it worse! and its normally only between 1800rpm-2000rpm
does anyone know what the fuel pressure is supposed to be rail on my 1991 Jeep Wrangler 4.0liter
3.5 bars (50 psi, approx.)
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