Fresh build backfiring - Page 2 - JeepForum.com

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post #16 of 28 Old 04-30-2013, 05:14 PM
HOSTED67
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Definitely check fuel pressure. Sounds like it may be good enough for idle but running lean under load.

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post #17 of 28 Old 04-30-2013, 08:36 PM Thread Starter
bigb229
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Fuel pressure checked out at 50psi on the rail.....she's at the shop all alone as we speak. First time my bay ever had strange hands on her.
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post #18 of 28 Old 04-30-2013, 09:00 PM
MustangFX93
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Did you make sure the spark plugs are gapped correctly?
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post #19 of 28 Old 04-30-2013, 09:10 PM Thread Starter
bigb229
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Yeah. plugs def. gapped correctly
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post #20 of 28 Old 05-01-2013, 06:34 AM
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New injectors? Old ? New screens and pintles?

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post #21 of 28 Old 05-07-2013, 05:03 AM
HOSTED67
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Fuel pressure 50 psi. Is that while driving or at idle. Needs to be checked while driving and while failing to be an accurate test. Sometimes a fuel pump will not be able to keep up with the needed volume of fuel at speed.
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post #22 of 28 Old 05-07-2013, 10:13 AM Thread Starter
bigb229
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fuel pressure checked out ok. think i have some valve float. gonna pull the head and have it machined for performance springs and drop a comp cam over in it
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post #23 of 28 Old 05-07-2013, 10:17 AM Thread Starter
bigb229
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I will admit that I never ran the thing to check for the fuel pressure as it will only idle and backfire.....but I've also read in the fsm that it needs to maintain 31 psi while over idle
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post #24 of 28 Old 05-07-2013, 10:24 AM
dylantheheeler
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Did you check crank position sensor? May be bad, just throwin out suggestions.


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post #25 of 28 Old 05-07-2013, 10:26 AM
Jeeples
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Quote:
Originally Posted by bigb229 View Post
fuel pressure checked out ok. think i have some valve float. gonna pull the head and have it machined for performance springs and drop a comp cam over in it
How high in the RPM band was the backfiring occurring? Typically valve float is seen at high RPMs near or beyond redline.

Did you not replace the springs when the head was off last?

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It is what Jeeples has *PARTIALLY* stated.
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post #26 of 28 Old 05-07-2013, 11:18 AM Thread Starter
bigb229
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I didn't replace the springs...just cleaned and lapped the valves a little....it idles very smooth...as soon as you put the gas to it it begins to backfire through intake and exhaust. Took it to the shop and they couldn't figure it out either....all of the sensors are new and check out fine. valve train was a bit stiff after the build....sounds like the valves are floating on me.....shoulda replaced the springs. oh well.
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post #27 of 28 Old 05-07-2013, 11:46 AM
ratmonkey
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Fuel pressure should be 49# at all times. 31# was for the 92-95. Be sure you have the right manual for your year.

Test the map sensor and throttle position sensor.

'97 zj 5.2, some stuff, some other suff, and some things that even work sometimes.

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post #28 of 28 Old 05-07-2013, 01:40 PM Thread Starter
bigb229
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So even through cranking it should not waiver below 49#. All sensors are new and checked out
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