tomnflora

Could someone help?
got a 1990 jeep cherokee 4.0. it starts up and idles fine, but backfires
under acceleration, whether in drive or neutral/park.
the backfire is not from the end of the tailpipe, but somewhere closer to the engine, before the catalytic converter.
also when idling, it sounds as if there is a loud muffle sound coming from somewhere between the engine and catalytic converter. kinda like the sound you here from a hole in your muffler.
today i replaced: plugs, wires, distributor cap, and EGR valve.
it's still backfiring when, and only when you step on the pedal.
Does anyone have any ideas, or possibly a similar problem from the past?
Thanks.

tampaviking

Sounds like something to do with your fuel. If they don't get enough they will stumble and backfire.

ledangel

I woulda said EGR but since ya replaced that it could be a bad valve in the head. You may want to have the compression tested. Could also be a bad plug. A free way to diagnose a plug or electrical would be to remove the wire to each plug one at a time. Remove one plug start it up give it a little gas (just a little and dont let it run for long), if you still get the back fire re-attach the plug and move to the next one until you either get no back fire or eliminate all the plugs being bad.
If you find a bad plug replace them all of course and hope it doesnt happen again because if it does that would point to a valve or possibly bad rings.
If you do find a bad plug look at it, if it seems caked with oil then your rings may be bad.

tomnflora

since it was doing the exact same thing before i changed the plugs, i'm gonna assume it's not the plugs/wires. i could be wrong and will pull each one individually to check.
i'm thinking maybe the fuel filter? if gas is restricted wouldn't that create a lean mix? filters are pretty cheap, i may replace it.
i think a compression check would be a good idea, unfortunately i don't have the tools for that, so i'm gonna have to take it to somebody. with my luck it would be a bad valve.
when i replaced the spark plugs, they looked pretty good, not caked up, but the electrodes were certainly melted down. the wires were in bad shape, i could see where there were arcs.
but now with new plugs,wires and cap it's still doing the same thing. only now it's got a little more get up and go along with the backfire.

ledangel

I missed that you changed the changed the plugs and stuff...I would still check them individually just to be sure its not cylinder specific.
In some distributors there is a little electronic thing (cant remember the part name) but i believe it acts to regulate the voltage coming into the distributor.
It could also be timing, mark with a grease pen where the distributor is sitting and then loosen the bolt and turn The Distributor slightly (you can do this while it is running) after you turn it each time rev it a bit and see if it reduces or eliminates the back fire.
I always try to diagnose problems the free way before doing the costly act of hit an miss with parts...lol.

Sorry if I am being over descriptive, not sure of your mechanical knowledge.

tampaviking

If you change your fuel filter, It would be a good idea to pull out your pump and check the connectors and the rubber hose that goes on the inside of the tank. To check the pumpo just wait till you have less than half a tank of fuel. And all that holds the pump in is a ring that you spin around on the tank. Mine would backfire for weeks and it was a bad hose from the fuel pump to the tank bracket.

BigCountry96

x2 check to make sure your fuel filter isnt clogged, it should be in the back somewhere near the tank i believe