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Unread 11-28-2014, 07:54 PM   #1
ssgpate
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1997 TJ Wrangler 
 
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Missing Backfire at every startup

Hello, I have a 1997 Jeep Wrangler recently purchased. Every time I start the vehicle about 15 seconds later it starts missing or backfiring and after about 4-6 seconds clears right up and doesn't do it again until the next startup. If you put it to the floor it will go right through themissing but I should not have to do that. It misses hot or cold it does not matter. I have replaced the following parts with no change to the situation.

The Jeep runs great other then that one issue at startup. at this point I need some interference to figure this out as I am running out of parts to replace. I didn't mind buying the parts anyhow just so I will have all new parts but at this point im tired of spending money. Any help is much appreciated.

Rear O2 sensor
New PCU
MAP sensor
IAC Motor
TPS
Plugs
Wires
Rotary button
Distributor cap




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Unread 11-28-2014, 08:06 PM   #2
chevy2169
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Wires plugged in incorrect firing order? Bad distributor timing?
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Unread 11-28-2014, 08:24 PM   #3
SpaceWrangler
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Mine has done that for years. Just let it warm up a minute.
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Unread 11-28-2014, 08:43 PM   #4
ssgpate
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Quote:
Originally Posted by chevy2169 View Post
Wires plugged in incorrect firing order? Bad distributor timing?
Yes they are, other then the few seconds at startup it runs perfect

Quote:
Originally Posted by SpaceWrangler View Post
Mine has done that for years. Just let it warm up a minute.
I know man but that is annoying and there has got to be a reason for this stumble.
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Unread 11-28-2014, 09:33 PM   #5
SpaceWrangler
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The issue might be low fuel pressure at startup, resulting from [partial] fuel rail leakdown overnight, back into the tank.

Cycle the ignition switch a couple times to "prime" the fuel system and see how she likes it.
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Unread 11-29-2014, 04:24 AM   #6
Jonny Jeep
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It's happened to mine before. It's a failed heater in the upstream O2 sensor.

Unplug the sensor, the connector is clipped to the intake manifold near the firewall. If you find the problem doesn't occur with the sensor unplugged, replace the sensor. If it were to still happen with the sensor unplugged the sensor would not be the cause.
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Unread 11-29-2014, 07:47 AM   #7
ssgpate
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Quote:
Originally Posted by Jonny Jeep View Post
It's happened to mine before. It's a failed heater in the upstream O2 sensor.

Unplug the sensor, the connector is clipped to the intake manifold near the firewall. If you find the problem doesn't occur with the sensor unplugged, replace the sensor. If it were to still happen with the sensor unplugged the sensor would not be the cause.
Would that matter if the vehicle is warm? mine does it hot or cold. I think the front o2 is he only sensor I have not replaced.
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Unread 11-29-2014, 05:55 PM   #8
Jonny Jeep
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Depends how bad the heater element has gone and just how cold it gets between starts.
Disconnecting the sensor is a free and easy way of finding out if it's the cause of your problems.

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Unread 12-10-2014, 07:06 PM   #9
ssgpate
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Johnny Jeep thanks you were right it was a failed upstream o2 sensor, the Jeep has not missed since I replaced it. Thanks
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Unread 12-11-2014, 04:46 AM   #10
Jonny Jeep
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Thanks for the update. It helps if threads come to a conclusion when they are found by a search.
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Unread 12-12-2014, 01:28 AM   #11
oakmckinley
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Quote:
Originally Posted by SpaceWrangler
Mine has done that for years. Just let it warm up a minute.
Huh? This mindset doesn't make any sense to me? Why would you instruct someone to just ignore a problem when they clearly asked for help with fixing it?

PO I'm glad you got it fixed!

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Unread 12-13-2014, 11:59 AM   #12
ssgpate
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I'm a tech Advisor on Modded Mustangs, and I would never give advice to a person and say deal with it I have been for years. There is always a solution to every problem you just have to be willing to dig in the weeds.

Although I was optimistic about the resolution I proceeded with the install anyhow. It doesn't make sense why it would buck like that because I thought the PCU was in open loop until it warmed up and bypassed the front o2 sensor.

Either way she is fixed and I appreciate the advice.

Spacewrangler my advice to you would be change your front o2 sensor and report back to this thread with the outcome.
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Unread 12-13-2014, 12:42 PM   #13
Jonny Jeep
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Quote:
Originally Posted by ssgpate View Post
... It doesn't make sense why it would buck like that because I thought the PCU was in open loop until it warmed up and bypassed the front o2 sensor...
I didn't go into the detail of why these symptoms occur in this thread. Having a look at post #9 in the thread linked below should help it make more sense...
http://www.jeepforum.com/forum/f9/scan-tool-2855634/
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Rockauto 5% discount code can be found here... http://www.markviii.org/LOD2/products_services.htm

Online factory service manuals (FSMs) and parts lists can be viewed at http://www.jeep4x4center.com/knowledge-base/index.htm
If the link doesn't work properly, look for the service manual link under Jeep Resources at the bottom of the page.
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