1996 4.0 auto. Rough idle

Hello guys. I just traded my 1991 F250 for a 1996 4.0, auto with 145k on it. I picked it up from the shop the guy was having it worked on. The story behind this is : #1 Ecu died about 2 weeks back. They replaced it with matching numbers ECU. Jeep ran fine til just about a week ago it died again. #2 Crank position sensor gave up. The shop installed new Cps. #3 engine now has rough idle. New plugs, wires, cap and rotor were installed. Still rough idle. Replaced CTS. No help. Cleaned IAC. no help. Can of heat and injector cleaner in 6 gals of premium. no help. Scanned cleared old codes. no help. Disconnected battery for 30mins. No help. scanned again no codes. The Spark advance moves from 7 to 20. When up to 20 starts to misfire. Drove it home. no lack for power by any means. Ive been in alot of jeeps. When driving you get random misfires. could be three mins between or 3 seconds between misfires. Im running out of ideas. Any thoughts? Oh yeah and the throttle body is clean clean clean.

check fuel pressure, do a compression check, look at the plugs and even replace them. check and test all the ignition system, wires, cap, rotor, even the coil. while you have the cap off check the distributor for excessive play. (wobble in the shaft). if you have the means, see if you can check the timing chain/gears.

got any buddies with similar year jeeps? could try swapping out MAP sensor...that will make it run rich. Or perhaps even a coil.

And you're sure you have the correct firing order when you installed the new wires? And all the wires are fully connected to the plugs (sorry, don't mean to sound condescending...but I've done it myself before)?

I set the firing order directly off the intake numbering. Buddies jeep just got here. Will start swapping parts in a few minutes. Any clues specifically on why the spark advance would move so much but not throw any codes?

ok i cleaned the throttle body and the rough idle went away. still have the radom misfire.

ok i also have a exhaust leak on the number one cylinder. looks like the gasket is busted there

Is there a free/cheap fix for my exhaust gasket being blown out on the #1 cylinder? Does anyone know why the spark advance curve is reaching 20 degrees sporadically? Anyone?

ok, using my digital multimeter I set it to volts AC. With the jeep running I touched the battery terminals. The meter was set to 200 vAC. It registered 30.0. Can anyone help me with this? Is the rectifier bad?

themax01 said:

Is the rectifier bad?

Click to expand...

Possibly, and that's why you're frying the ECU. Test or swap out the alternator, have it tested out of the car. Are you aware that the '96s were recalled to have the ECU re-flashed? Ask a dealer what was involved in the re-flash. The place that did mine hasn't got a clue, and I swear it hasn't run as good since.

Also, not to start a debate on spark plugs, but as said many times on this forum, use only Champion plugs. My son once came home with another brand when he tuned up his '95. Did everything correctly, ran like crap. I told him to get the Champs. Of course, dad's don't know a thing. So he reluctantly got the Champs, and it ran like buttah. Score one for the old man! I've also used Autolite platinums, worked just fine, but now we just use the Champs.

themax01 said:

ok, using my digital multimeter I set it to volts AC. With the jeep running I touched the battery terminals. The meter was set to 200 vAC. It registered 30.0. Can anyone help me with this? Is the rectifier bad?

Click to expand...

Im sorry just wanted to clairify something.... Did you set it to VAC or VDC? Check battery voltage set on meter at DC volts.

I changed out the alternator with a good one from the bone yard. no change. I didnt know about the recall. I didnt check the plugs as the misfire is intermittent. (Will do so tomorrow) @Frontman. I Chose vac to see if the alternator was leaking ac volts into the system due to the rectifier possibly being bad. No such luck. Also the alternator was a freebie from my local yard. It was worth a shot. Tomorrow I will also run a can of seafoam thru it.

Seafoam in the intake. done. Seafoam in the crank case. done. Plugs are champion and the gap is correct. No such luck on the mis going away. I did get it to back fire thru the manifold once. Not healthy at all. Plenty of fuel pouring out the valve. No problem starting up either. Any other suggestions?

Is it possible that when the crankshaft position sensor died that it shorted the ecu?

temp sending unit possibly. I had trouble with mine running rich a while back and that was the cure. Not to bash champion plugs because I have never ran them but I have always run the o'reilly 2 dollar autolites with no problems. if the manifold is cracked try welding it up I have had many issues with that on a different xj. I am in Omaha also, let me know if you want to wheel sometime.

New update. tracked down a 96 ECU swapped it in and magically the door locks and and the tachometer now work. Also my check engine and idiot lights now work. So The misfire at this point is still bad. Then the check engine popped on. I took it to Napa and I scanned the code. Came up with 1391. That is the crank sensor. I have replaced that one, and that code could have been stored in new/used ecu. I immediately cleared the code. Started the jeep and it roared to life. The misfire was very minor but still there. The idle was smooth even. Still looking for the answer to the misfire I tore down the column and found 2 burnt ends going into the Key starter switch. I repaired the burnt ends and reconnected the battery. Now the heep is running hideously. Check engine light pops back on. ARGHHHGHGHG!!! So I take it back to Napa, scan, scode came back. Cleared it. I started the heep back up and now back to running like a champ. WHAT???? This makes no sense. I burn the tires out of the parking lot. The misfire is again barely there, but only noticaable at low cruising speed. Take it home and park it. Shut it off and started it right back up runs fine with light mis. Shut it off for a couple hours and back to running rough and popping and light pops back on. Went to the bone yard and grabbed another ignition. added it in and still no help. ANYONE PLEASE HELP ME!!!!!

On my 96 there have been many causes of random misfires over its life..
1- Shaft wore out in distributor,,rotor was hitting terminals in cap,,could hear and feel a ticking sound when touching the cap.....replaced distributor.
2- Bad plug wire
3- bad fuel injector......way over 200 K
4- Bad connection between injector and injector circuit, this connection has been loose/corroded or just bad. Clean,bend and add some di electric grease. I had to add wire ties to keep the connections tight.

No problems since keeping the connections tight.

I have pulled each of the injector plugs individually. They make the engine run alittle rough but not horrible. I am starting to think my valve train needs to be replaced. Sad part is I only have 145k on the motor. I am taking it in to the shop whom replaced the crank position sensor. We are going to swap that out tomorrow to make sure its not bad. Then it will be on to bigger and better things. Like inspecting my valves and checking the springs.Im not going to give up on this one. It needs some TLC and it will be a great wheeler.

I guess i should shoot you guys a update. My jeep was stolen last Friday night. So I obviously havent fixed it. Sad day.

The jeep is back. Missing the catalytic converter, battery, spare tire, and some misc engine bay parts. Cost me 93.00 to get it out of the impound.

It cost you money. That's great! What a wonderful world we live it.