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Unread 11-04-2013, 08:26 AM   #1
bchap05
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1994 YJ Wrangler 
 
Join Date: Apr 2011
Location: Boise, ID
Posts: 104
Backfire hesitation where to start

1994 4.0 with 165k. It started stuttering when I was driving down the highway this weekend. Holding a constant speed it is smooth like normal. But under throttle you can feel a little hesitation that makes the Jeep buck a bit. It got a lot worse yesterday especially in non highway driving when accelerating. Once up to constant speed its smooth again. While accelerating almost sounds like a chopper motorcycle accelerating.

Yesterday I popped hood and looked for anything obvious. It was backfiring and popping at idle mainly out of the exhaust but you can hear it up in the engine compartment as well.

About a year and a half ago the fuel pump was changed as it was causing the Jeep to die while driving. 02 sensor was changed 6 months ago. Fuel filter was changed probably year and a half as well. It's not tripping the CEL. Any idea where to start?

Oh and for a few minutes this morning it was normal. So I don't think its a plugged issue as I imagine that would remain constant. So I'm guessing something electrical.

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Unread 11-04-2013, 12:06 PM   #2
Que89YJ
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1989 YJ Wrangler 
 
Join Date: Apr 2008
Location: Detroit, Michigan
Posts: 13,700
Backfiring is firing at the wrong time. Ifg it is firing the plugs when the intake valves are open it is a timing issue and you will need to check the distributor and the timing chain. Do you hear any chain slap on the timing chain cover?
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Unread 11-04-2013, 05:52 PM   #3
JohnRodriguez
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1989 YJ Wrangler 
 
Join Date: Sep 2004
Location: Silver City, New Mexico
Posts: 321
lean misfire. pull a plug and see if it is bone white. it should have a slight redish color if it is getting enough fuel. bone white if it is lean. filter is probably plugged
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89 Wrangler, 2.5L TBI, D30 Truetrac 4.56 gear front, Super 35 4.56 gears rear, 31" BFG Mud's, 15x9" weld racing pro-stars XP's, 2.5" rancho and 1.25" shackels, superior axles. rear wilwood disc brakes

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Unread 11-05-2013, 08:22 AM   #4
bchap05
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1994 YJ Wrangler 
 
Join Date: Apr 2011
Location: Boise, ID
Posts: 104
Thanks I'll give both of those a shot when I get home. I haven't noticed a chain slap. The backfiring seems to be when it first fires up cold. Then it quits. The stuttering is constant under acceleration. It has not popped a CEL at all but I checked the codes anyways and it was pulling 24 which is the TPS. Replaced the TPS and fixed nothing.
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Unread 11-05-2013, 08:37 AM   #5
Que89YJ
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1989 YJ Wrangler 
 
Join Date: Apr 2008
Location: Detroit, Michigan
Posts: 13,700
You need to disconnect the battery and count to 10 to clear a fault code that is set. If you are getting a tps code it is because the vacuum it is expecting from the map sensor does not agree with the tps position. Double check to see if the code resets. Usually a cold start issue like you are describing is an issue with the engine temp sensor located right next to the tsat housing; cheap $20 part.
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Unread 11-05-2013, 09:44 AM   #6
bchap05
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1994 YJ Wrangler 
 
Join Date: Apr 2011
Location: Boise, ID
Posts: 104
When I changed the TPS last night I pulled the battery to reset and it did clear the code. The only codes now are the normal 12 and 33. The back firing is at cold start. But no matter how warm it is the stuttering is still bad constantly.
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Unread 11-05-2013, 09:52 AM   #7
Que89YJ
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1989 YJ Wrangler 
 
Join Date: Apr 2008
Location: Detroit, Michigan
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If you have a meter you can read the resistance on the sensor. The codes will definitely tell you when something fails out of spec but will not tell you if the sensor range fails in range.
coolantandairtempchart.jpg

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Unread 11-05-2013, 08:52 PM   #8
bchap05
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1994 YJ Wrangler 
 
Join Date: Apr 2011
Location: Boise, ID
Posts: 104
Put new plugs and wires today runs like new. One plug had white residue on it
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Unread 11-05-2013, 09:02 PM   #9
willawolfe
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1994 YJ Wrangler 
 
Join Date: Nov 2012
Posts: 106
I just resolved a similar issue by replacing plugs. My no. 1 plug had a crack in the porcelain. I'm glad your issue was resolved as easily as mine.
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