90 4.0L backfires and chuggs when accel WTF!

[hockey10588]

so my jeep is all messed up now. when I start my jeep the engine revs really high (~1700 rpm) for a few seconds and then it drops to about 200 rpm and almost stalls. if I give it alot of gas at that point it will stall so I have to ease the pedal so it wont stall but as soon as im off they drop to about 300-400 rpm and thats where it mostly idols at.

when im at a stop sign or whatever and I press the gas the engine really bogs down and the rpm's drop and it kinda chuggs and backfires (sometimes through my tailpipe and sometimes I can almost feel it through the floor where my feet are) but when it hits 3000 rpm my jeep takes off and it feels like someone just hit the NOS button. anyone know whats going on or what I can do? or check?

like I said sometimes (most of the time) it backfires 3 or 4 times and it sounds like its coming from my engine bay or right under my feet. occasionally it backfires at my tailpipe only once. I'm not running a cat so its not clogged but I do have 33's and 3.55 gearing (about to go to 4.88) if that has anything to do with it.

PLZ help!!

[Slo-Sho]

Sounds like a secondary ignition problem...when was the last time it had a tune up?

[hockey10588]

Sounds like a secondary ignition problem...when was the last time it had a tune up?
I would say about a year and a half ago? I changed the plugs, wires, cap, rotor, and fuel filter when I bought it back in september of 2004

[old_man]

Check your MAP sensor.

[XJING]

Check your MAP, but it sounds like a egr problem to me. check the valve under the header make sure it goes in and out, you can move it ( the small rod in the bell shaped valve) with a small screwdriver. Also make sure you have no vacumn leaks to the electrical part of it on the drivers firewall. Good Luck.

[hockey10588]

so I changed my plugs and the 3rd cylinder plug was pretty bad. there was a bunch of ash and carbon buildup so I decided to change all 6 plugs. changed them all and right when it starts it idols a bit smoother but the rpms still drop to around 200. took it out for a test run and for the first 10 min or so it would still do the backfire chugg thing but after awhile it kind of went away but sometimes it still happens.

can anyone tell me more on the EGR valve? I know where it is but what else should I check?

[jeepjay90]

i had something like that with my 90. did a tuneup and ran great for a day.
then i changed my fuel filter, a little better. it wound up being fuel pump. runs like a champ now.

[hockey10588]

sweet I'll try changing the fuel pump. it has about 177000 miles on it and maybe the pump has gone

[Lonestar]

Had problems like this with my 87 4.0L a while back. It started all of the sudden. Changed the fuel filter and pump thinking it was starved for fuel, but that didn't fix anything. Turned out to be the hall-effect trigger sensor in distribuor. Turns out the sensor wheel is bolted to the distributer shaft and the two screws came out cause the sensor wheel to flop around. The two screws where still sitting in the dist. housing so I bolted it back together and haven't had a problem since.

[jeepjay90]

yea mine has 160,000. it seemed the harder i got on the gas the more it was boging down and backfiring, man its amazing all the stupid little things that can go wrong.

[Slip Kid]

I also had a similar problem with a bad fuel pump, except it never made it to 3000RPM. Backfires through intake, can feel it in the gas pedal. funny it did the same thing when I ran it out of gas the other day :sad1:

[hockey10588]

see the hardest part about all of this is people are telling me a million different things to check or replace and I dont even know where to start. is there like a big main problem I should check first and then go down the list from there?

[TRAILREADYXJ]

Check your distributor, make sure it hasn't rotated and screwed up the timing.
The timing isn't adjustable on a 4.0, but if the dist is too far advanced it will cause the exact symptoms. Check for any extra play in the shaft too.
I just swapped another motor in my 90, and had the same problem, I adjusted the dist. and it runs great.

[woodywagon]

You may try removing the idle speed stepper motor from the throtlle body. i had to clean soot from it also replace the o ring