Jeep running rough

So, on my 98 4.0, A/T I recently replaced the engine computer as my last off-roading adventure thoroughly flooded the Jeep and fried the original computer. I'd been working on the Jeep for the past month repairing the damage, so it hasn't really been driven much in that time besides a couple of miles to test run it after doing some repairs.

Regardless, at first it was running fine. Now though, at idle, it sputters and the tach jumps from idle at ~700 rpm up and down a couple hundred rpm every couple of seconds or so. Even if I rev it up in neutral, it still jumps. When driving it's fine. At first I thought maybe I was just low on gas and Jeep gas gauges are notoriously inaccurate. So I put gas in it and it was fine while driving it right after, although I got the check engine light and the P1494 code for Leak Detection Pump.

Drove it to work today and it's doing the same sputtering thing. I know that when you disconnect the battery, the computers loose their memory and need to relearn driving habits and I've read it can take up to 40 driving trips for the computer to do this and during this learning period things might be rough. I thought it might be this, but it didn't do this the first couple times I had it on the road, so I don't know.

Any Ideas???

If the gauge is off then the sender may be damaged, and the rough running sounds like the whole pump assembly is wasted. Probably bad wires at the assembly, but you might have water in your tank. Check the gas tank wiring, seal, fuel filter, etc.

The water in the gas is definitely something to check. Would that make it run rough just at idle, though, b/c it doesn't sputter at all while driving?

sorry I misread the gauge you were talking about, it doesn't sound like a fuel problem on second read

I'd replace the IAC and see if it fixed the idle problem, then go from there

Where is the IAC located?

It's the cylindrical thing attached to the side of the throttle body next to the valve cover

my 98 is doing something similar except mine is while driveing too and i changed my IAC(idle air controler) with the one that came on it and was working fine before i swaped throttle bodies and nothing changed but theni checked my fuel presure and it was off by 10 pounds so i'm gonna be changeing my fuel pump and assembly becaue my gas gauge is also off also my jeep has had some uderwater advetures too so i think we might be haveing the same problem

Keep me posted on if the fuel pump assembly changes anything!

i deffinaly will

I don't know if it helps, but the Jeep runs smoothly for the first 20-30 seconds after you start it, then starts to sputter.

I checked my distributor cap and rotor, both perfectly clean, no rust at all. I had replaced them only a couple of months ago. I certainly hope it's not the fuel pump, as that's a lot of coin and PITA!

jeepster_259 said:

I don't know if it helps, but the Jeep runs smoothly for the first 20-30 seconds after you start it, then starts to sputter.

I checked my distributor cap and rotor, both perfectly clean, no rust at all. I had replaced them only a couple of months ago. I certainly hope it's not the fuel pump, as that's a lot of coin and PITA!

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if you suspect the fuel pump, get a fuel pressure gauge and test the fuel pressure.

when it sputters, do you get a CEL?

did you replace it with a 98 computer or a different year? did you get it reprogrammed?(dont know if it needs to, just thought id ask).

did the tps get wet?

check that the o2 sensor is not damaged and test.

did you put any dry gas in the gas tank ti eliminate waater/moisture in the fuel system.

when it sputters, does the voltage gauge drop? bad alternator(from getting wet) could cause eratic/stumbling at idle.

I have the CEL light constantly on with the P1494 code for Leak Detection Pump. I've erased the code with the scanner and it just stays on.

I replaced it with a 98 computer, same engine and tranny. It ran perfectly fine at first. It just started doing this recently.

TPS could have gotten wet. Before I had her up and running again, though, I went through and opened up all of my electrical connection and put dielectric grease in them, so I don't think it should be wet still if it was.

How do I test the O2 sensor? That's not a bad idea. What are the symptoms of a bad O2 sensor? Although the only CEL code I get is P1494, afterwards the scanner does say misfire, O2 Sensor, and something else, can't remember. It was saying that when it was running fine, though, so I didn't think anything of that.

Haven't done anything with the gas yet. I'll try that shortly.

No voltage drops when sputtering. Just the tach jumping. And the strange thing, at least to me, is that it only does it while idling, not while driving.

I had the same 1494 code go off. i thought it was 1495, but w/e....anyway....it actually went away after driving for a month...figure it the hose that wraps around back of motor to intake from that system....regardless it went away...
bad 02 could causse rough idle, poor mpg...

I had a CEL and rough idling, no problem while driving. Replaced O2 sensor and problem solved.

Unplug the O2 sensor and see if it helps...

wow, am I have deja vu?

I'm going to try unplugging the O2 sensor tonight. Should I unplug both O2 sensors, or just the upstream or downstream one? Which one was your problem?

upstream controls the air/fuel mixture and the downstream monitors cat convertor operation. i would do the up stream first.

So the Jeep will run without the O2 sensors plugged in? I didn't know that.

i would not ever run my engine with an unpluged upstream o2 because it controls your injector pulse width and spark timieng and the wrong reading could cause detonation

idk if it would or not but i don't want to find out

jeepster_259 said:

So the Jeep will run without the O2 sensors plugged in? I didn't know that.

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i guess it depends on what year, my 89 can run without an o2 sensor. it will just run rich

MTBracer367 said:

i would not ever run my engine with an unpluged upstream o2 because it controls your injector pulse width and spark timieng and the wrong reading could cause detonation

idk if it would or not but i don't want to find out

Click to expand...

what about when the system is in open loop and it doesnt even use the o2 sensor reading, wouldnt that be like running without one?

doesnt the cam sensor control injector pulse and width. maybe not on an obd-2 system? i dont know.

hmm...well, I'll try unplugging one at a time and see what happens.