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Unread 05-03-2013, 04:52 PM   #1
cavalier
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Cherokee is Misfiring and "hiccuping"

It runs fine until it gets a little warm, then it starts misfiring. I can still drive it, it just "hiccups" every once in a while, especially while going up hills.

I recently changed the plugs, wires, cap, and rotor. I'm not sure what's going on. When I rev my engine and let off the gas, the revs drop too low and it almost starts to choke, but then it bumps back up.

Would a ****ty (airtex) fuel pump cause a misfire? Maybe a clogged injector? Maybe a sensor? HELP!

There's no check engine light. 1992 XJ with 228,000 on the 4.0.

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Unread 05-03-2013, 05:03 PM   #2
tjwalker
It's the crank sensor!
 
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Checking fuel pressure is easy. Fuel pressure gauges can be rented inexpensively from many parts stores. Testing is the only way to confirm a fuel delivery issue.
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Unread 05-03-2013, 06:01 PM   #3
cavalier
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Hmm. Would a bad ignition coil cause a misfire?
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Unread 05-03-2013, 06:54 PM   #4
79chiefs
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Quote:
Originally Posted by cavalier View Post
Hmm. Would a bad ignition coil cause a misfire?
Yes, test the spark.
It should be blue, and snapping spark.
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Unread 05-04-2013, 09:17 AM   #5
cavalier
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So I just did a fuel pressure test. Here's what I got:

Key in "start" position: 36 psi but gradually goes down as I let it sit
Vehicle idling: 32 psi
Giving it gas: goes up for a second, then stays around 32 psi
Disconnect vacuum line to pressure regulator: goes up to about 42

Thoughts?

Now, I do notice that my exhaust is leaking at the manifold gasket. Bubbles of oil are popping in that area (spilled oil, that is). I'm wondering if an exhaust problem would cause a misfire (like a clogged cat). I haven't noticed any loss in power, just a misfire.
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Unread 05-04-2013, 09:47 AM   #6
cavalier
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OH MAN! I just pulled my plugs. THEY ARE FRIED! Check out this pic:

They are only 1 month old!

What's going on?
jeepplug.jpg  
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Unread 05-04-2013, 10:54 AM   #7
tjwalker
It's the crank sensor!
 
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Do all of your plugs look like that?

That looks like ash deposits. Excessive amounts can cause misfires.

*Have you been using any gas or oil additives? Additives can sometimes cause ash deposits.
*What type of oil are you using and how often are you changing it?
*Otherwise, it's an internal problem, possibly head related.
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Unread 05-04-2013, 10:56 AM   #8
cavalier
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Quote:
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Do all of your plugs look like that? How many miles on them?
At least two of them look like this. I'll pull the rest, but I think they do all look like that. About 800 miles on them.
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Unread 05-04-2013, 11:37 AM   #9
cavalier
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So I thought: maybe it's running rich? I pulled the coil and got 1.3 ohms for primary circuit and 12.47 Kohms for secondary. Seems fine to me...
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Unread 05-04-2013, 11:57 AM   #10
79chiefs
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Your fuel pressure is low, and how fast does it drop?
Sounds like bad regulator.
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99 WJ Laredo I6 Auto 2wd-TOTALED still fathers DD till repairs are made
95 E34 540i-DD
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Unread 05-04-2013, 12:01 PM   #11
cavalier
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Quote:
Originally Posted by 79chiefs View Post
Your fuel pressure is low, and how fast does it drop?
Sounds like bad regulator.
It won't drop while I'm running it. At idle, it stays at 32 psi no matter how much gas I give it. Just when I first put the key to "start" and wait a while. It drops about a psi every 20 seconds.
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Unread 05-04-2013, 12:17 PM   #12
cavalier
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Quote:
Originally Posted by tjwalker View Post
Do all of your plugs look like that?

That looks like ash deposits. Excessive amounts can cause misfires.

*Have you been using any gas or oil additives? Additives can sometimes cause ash deposits.
*What type of oil are you using and how often are you changing it?
*Otherwise, it's an internal problem, possibly head related.
No additives. Cheapest 10w 30 I can find. Napa or Supertech. Change it about every 3k.
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Unread 05-04-2013, 02:14 PM   #13
cavalier
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Oh man. I'm starting to think maybe I need a head rebuild. NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO!!!! !!!!!!!!!!!!!!!!!!
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Unread 05-04-2013, 03:15 PM   #14
79chiefs
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You also need a pressure regulator, it should be 49psi +/- 5psi all the time. But should NOT drop that quickly. 1psi/30min i believe is how it should drop when not running.
It does not drop while running because the fuel pump is always running, when it is not running and the key is on it only primes till your start it.
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79 FSJ Cherokee Chief 350SBC-SOLD_88 XJ Laredo I6 Auto 4wd-SOLD_92 XJ Laredo I6 Auto 4wd-SOLD
99 WJ Laredo I6 Auto 2wd-TOTALED still fathers DD till repairs are made
95 E34 540i-DD

Last edited by 79chiefs; 05-04-2013 at 03:17 PM.. Reason: More info
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Unread 05-04-2013, 03:43 PM   #15
cavalier
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Quote:
Originally Posted by 79chiefs View Post
You also need a pressure regulator, it should be 49psi +/- 5psi all the time. But should NOT drop that quickly. 1psi/30min i believe is how it should drop when not running.
It does not drop while running because the fuel pump is always running, when it is not running and the key is on it only primes till your start it.
I'm not sure it needs to be that high. I think 31 psi is the spec.
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