Jeep Backfires from Air Intake and has a complete lack of power - JeepForum.com

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post #1 of 30 Old 10-10-2007, 09:17 PM Thread Starter
se7vn
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Jeep Backfires from Air Intake and has a complete lack of power

Here's what happens, at 75% throttle, the engine bucks and backfires like there's no gas unless I push the pedal down, the engine surges with full power, anything before 75% throttle, it feels like a bad cat, 5th gear is completely useless because i'm lacking so much power, it even backfires from the intake while idling, but only at high RPM's. Idles at 300 Rpms then once every 2 minutes, the engine steps up and idles at 700 rpms which is where it should idle for about 5 secs then goes back to 300 rpms.......

Here's what I've changed:
Fuel Filter
Fuel Pressure Regulator
Fuel Pump
2 Fuel injectors that were leaking
Full Tune up, Cap, Rotor, Plugs, Plug Wires, even a new ignition coil
Map Sensor
IAC Valve
O2 Sensor
Timing Chain
Coolant Temperature Sensor
Tried Blocking off the EGR to see if that changed it, it didn't change it at all
Tested TPS, voltage goes smooth all through throttle when I go very very slowly
5.0V input, .81 output at idle
Cleaned TB
Checked EGR Solenoid, fine
Catalytic Converter
Harmonic Balancer
Muffler Unhooked and no change in idle<-----------Edit
Crankshaft Position Sensor <--------------------Edit
Tested Compression : I got 1)=150 psi
2)=140 psi
3)=160 psi
4)=155 psi
5)=150 psi
6)=150 psi
did i forget to check anything else...............................??????
Any ideas what it could be?

My guess right now is a vaccum leak I fail to trace down...........


2004 Jeep Liberty Sport 3.7L V6
1989 Jeep Cherokee Laredo 4.0 I6 - 147,520 Miles

Last edited by se7vn; 10-10-2007 at 09:58 PM.
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post #2 of 30 Old 10-10-2007, 09:25 PM
lifted92xj
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did you check the cat or replace it? it could also be the muffler. plus check your oil level i've heard if theres too much oil it can destroy your cat..but thas just what i've heard. it would be loud as hell but i'd try unbolting the cat so its just the manifold and run it like that and see if it still does it (meaning in your driveway) and if it stops you know its the exhaust

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post #3 of 30 Old 10-10-2007, 09:38 PM Thread Starter
se7vn
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if it were clogged, wouldnt it idle and run like crap all the time? if that were it, then why would it idle up to 700 rpms on cue every time?

2004 Jeep Liberty Sport 3.7L V6
1989 Jeep Cherokee Laredo 4.0 I6 - 147,520 Miles
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post #4 of 30 Old 10-10-2007, 09:45 PM
lifted92xj
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sry didnt read that last part there.. the change in idle sounds like a o2 sensor to me, do you have 2 of them? and only changed 1, or just 1

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post #5 of 30 Old 10-10-2007, 09:49 PM Thread Starter
se7vn
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lol, sorry, I know, I wrote a book above, lol, but I'm just getting completely stumped by this, it's been a year now.....I changed the one on the downpipe from the exhaust manifold, I read somewhere that the one further back in the exhaust doesn't really make that much of a difference.......

2004 Jeep Liberty Sport 3.7L V6
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post #6 of 30 Old 10-10-2007, 09:51 PM
cox_xj
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timing is off, very very off

if it backfires though the intake its 98% most likely timing related
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post #7 of 30 Old 10-10-2007, 09:51 PM
lifted92xj
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well it seems like youve done everything short of a motor swap, i'd try that one too, thats the only other thing i can think of

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post #8 of 30 Old 10-10-2007, 09:52 PM
Slowride63
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Quote:
Originally Posted by se7vn
if it were clogged, wouldnt it idle and run like crap all the time? if that were it, then why would it idle up to 700 rpms on cue every time?
I've dealt with several plugged converters over the years, and not one gave any indication of a problem untill the engine was under load. Have you tried disconnecting the exhaust and running it? If not that would be my suggestion, as it will either identify or rule out the muffler and/or cat as the problem.

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post #9 of 30 Old 10-10-2007, 09:57 PM Thread Starter
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I unplugged the muffler two weeks ago and it still ran the same, I didn't try the cat lately though, I'm also thinking it may be ECU related, maybe it's bad? Dunno how, but I do know my timing is off somehow, hence the timing chain replacement, it could also be something stupid like the knock sensor or something, with the renix engine and all these damn sensors it could be a $10 sensor that advances or retards timing......

2004 Jeep Liberty Sport 3.7L V6
1989 Jeep Cherokee Laredo 4.0 I6 - 147,520 Miles
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post #10 of 30 Old 10-10-2007, 10:14 PM Thread Starter
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Anyone else have any ideas?

2004 Jeep Liberty Sport 3.7L V6
1989 Jeep Cherokee Laredo 4.0 I6 - 147,520 Miles
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post #11 of 30 Old 10-10-2007, 10:16 PM
cox_xj
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have you put a timing light on it? to see how off the timing is?
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post #12 of 30 Old 10-10-2007, 10:18 PM Thread Starter
se7vn
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I have one, just have to figure out how to use it LOL, probably do that tomorrow after work....

2004 Jeep Liberty Sport 3.7L V6
1989 Jeep Cherokee Laredo 4.0 I6 - 147,520 Miles
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post #13 of 30 Old 10-10-2007, 10:20 PM
cox_xj
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normally has 2 clips one positive one ground, then a lil clamp that goes on the number 1 plug wire

some have dials to change the timing of the light, some just flash, point at crank pully and see which dash mark its lining up with
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post #14 of 30 Old 10-10-2007, 10:42 PM Thread Starter
se7vn
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should I put some kind of paint mark on the Harmonic Balancer or just point it anywhere? I have the one that just flashes

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1989 Jeep Cherokee Laredo 4.0 I6 - 147,520 Miles
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post #15 of 30 Old 10-10-2007, 10:52 PM
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Put paint on the timing pointer and 0* TDC

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