Acceleration Stumble and Rough Idle -
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Unread 01-05-2013, 04:25 PM   #1
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1998 TJ Wrangler 
Join Date: Jun 2012
Location: Pasadena, CA
Posts: 177
Acceleration Stumble and Rough Idle

So the jeep has a rough idle and stumbles when accelerating. Possibly related, I noticed that when I punch the gas in neutral, it revs and then the exhaust pops. Not loudly, just *Vrrooooommmm!... Pop*

I cleaned up the grounds recently, but I noticed that one of the smaller ground wires was damaged. I don't know where it leads to, but it looks like it will need replacing soon. I believe something is off with the ignition timing, I just don't know where to start. HELP!!!

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Unread 01-05-2013, 04:52 PM   #2
It's the crank sensor!
tjwalker's Avatar
1999 XJ Cherokee 
Join Date: Nov 2007
Location: Central Minnesota, MN
Posts: 8,420
Base ignition timing is controlled by the computer.

There are about 89 things that could be causing your problem. Here's where to start...

1. Fix that ground wire! And verify all others. Engine grounds are critical, especially with the RENIX vintage.

2. Tuneup. Do NOT underestimate the importance of having a solid secondary ignition. Fresh Champion copper plugs gapped to .035, quality plug wires, distributor cap and rotor and a new fuel filter.

3. Verify your fuel pressure. Backfiring is usually caused by an "imbalance" of fuel/air ratio. For your 1990, you should have 31 psi of pressure at idle. MUST be verified with a fuel gauge; many parts stores will rent you a gauge very inexpensively; call around. Then disconnect the vacuum line leading to the fuel pressure regulator (be sure that line is pulling good vacuum and is not wet with fuel which would indicate a ruptured regulator). With that line disconnected, pressure should jump to approximately 39 psi.

4. Test your MAP sensor. And verify the vacuum line leading to that sensor as well.

You have to approach this in a "simple to more complex" strategy. Keep us posted!
99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!
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Unread 01-06-2013, 04:56 PM   #3
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1998 TJ Wrangler 
Join Date: Jun 2012
Location: Pasadena, CA
Posts: 177
That sounds like a good plan to me. There are three separate wires grounded at the base of the dipstick. Any clue what they all are? The smallest of them is hanging on by a thread.
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