missfires and low power while driving 2000 4.0 - JeepForum.com

 
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post #1 of 9 Old 06-19-2012, 02:39 PM Thread Starter
noise_maker
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missfires and low power while driving 2000 4.0

About 3 weeks ago i started getting missfire codes on my 4.0 TJ. Since that time the problem has gotten worse and now has no power at highway speeds. The code thrown is sometimes is cylinder 2, sometimes 3. in the drive way its fine at any RPM and will rap up with out any problem.

Under Load while driving - It has a slight poor throttle response at low RPM but is mostly fine. At higher RPMs (~2600+) it has low power and kinda just chokes out.

Here is what I have done so far
  • I pulled the plugs and checked them they were all fine and were gapped in spec - (autolite double platinums with ~10k on them). I moved them around and the problem stayed the same.
  • I replaced the coil pack with an extra I have from an 2005 and have the same problem.
  • I replaced the fuel injectors with my spare set of freshly cleaned ones and still have the same problem

The vacuum at idle is 18 in/hg. The gauge doesn't shake or jump although every 30 seconds or so it will cycle down to 17 and back up (might be the AC though). with the engine wrapped up in the drive way the vac is rock solid at any given RMP.
from idle if i open the butterfly and release it the vac dives to about 15 then jumps up to 24-25 as i release it.

I'm gonna go pick up a longer hose and check the VAC under load and see if I see anything and maybe check the compression.

Any ideas?

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post #2 of 9 Old 06-19-2012, 04:35 PM
JWebber
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Next would be to check the fuel pressure at the rail. Buy or rent a fuel pressure gauge and do some tests. Test it when ignition on to prime the pump and take note of psi reading. Should be 49-52 psi at prime. Then turn off ignition swith and watch for falling psi readings. If it falls to fast within 10 mins more than 10 psi, faulty regulator valve on top of fuel pump. Then try testing while the engine is at idle and running should see the same reading 49-52 psi, anything lower then this will shows problems. Mine was at 42 at ignition and when I started the engine would go to 37psi.
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post #3 of 9 Old 06-19-2012, 06:36 PM
Knuckelhead
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Clogged catalytic converter?...
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post #4 of 9 Old 06-19-2012, 07:07 PM Thread Starter
noise_maker
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Quote:
Originally Posted by Knuckelhead View Post
Clogged catalytic converter?...
I after some more testing i think you are probably right...

Tapped a vac gauge and fuel pressure gauge to the windshild and went for a drive.

While running regardless of throttle or load fuel pressure is 51 psi.

at wide open throttle vac would initially go to 0 then start to climb back up to about 15 up then at about 3000 RPM the motor started to choke out.
at about that point the motor made more noise then power.

Gonna pull the up stream O2 and see if runs better with it out - if it does then ill break loose the bolts on the cat.


Thanks guys
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post #5 of 9 Old 06-21-2012, 10:15 AM Thread Starter
noise_maker
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Thanks Knuckelhead!.

I pulled the front O2 sensor yesterday and with the exaust leak that created it ran WAY better.

Now to find a higflow cat or an dummy O2 sensor for the after cat one.
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post #6 of 9 Old 06-21-2012, 10:22 AM
JWebber
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Mangaflow has a cat you can get with a bung for 02's after the cat.
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post #7 of 9 Old 06-21-2012, 12:30 PM
Knuckelhead
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Quote:
Originally Posted by noise_maker View Post
Thanks Knuckelhead!.

I pulled the front O2 sensor yesterday and with the exaust leak that created it ran WAY better.

Now to find a higflow cat or an dummy O2 sensor for the after cat one.
Your welcome, keep the operating O2 sensor... IMO
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post #8 of 9 Old 06-21-2012, 01:35 PM Thread Starter
noise_maker
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Yea I plan on initally emptying the cat and spacing out the rear o2 to keep it from throwing a code.

If it still runs crappy ill pop for a high flow cat. Either way I'm gonna keep both o2 sensors.
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post #9 of 9 Old 06-21-2012, 01:54 PM
tjdriver
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ethanol free gas will help you with the clogged convertor problems. At least it will help with the new one you get.
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