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Unread 07-05-2013, 12:57 AM   #1
bmxboy809
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TJ 4.0 Stumble?

I have a '99 TJ with a 4.0, 140k miles and up until now have had very minimal engine trouble. When I Parked my jeep at the airport two weeks ago everything worked fine, but when I got home I had tons of trouble.

I first noticed a problem when shifting up to fourth gear when all of a sudden the nose dived and the engine sputtered and even with the pedal to the floor nothing happened. By the time I got home the idle was rough and it barely ran. Every time I stepped on it from a stop sign the same thing would happen in every gear. All power is gone and an occasional snap and pop happens until I let off and feather the clutch.

So far I have replaced the distributor cap, rotor, the distributor itself and timing chain. I also checked the coil with an ohm meter and it was fine but The problem still exists. It seems like off a cold start it is fine for a few minutes, but by the time I get around the block the symptoms come back. In addition to changing these parts I tried to run with the upstream o2 sensor unplugged with no luck. No codes or check engine light so far.

Fuel pump may be next on the list of replacement items but that would not explain why it runs fine when cold and bad when warm. Any ideas as to what might be plaguing my TJ?

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Unread 07-05-2013, 01:35 AM   #2
Necko2529
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You left it parked at the airport for ~2 weeks? Could someone have dumped something inside your fuel tank?
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Unread 07-05-2013, 06:13 AM   #3
Jwood621
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Have you replaced the fuel filter?
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Unread 07-05-2013, 07:14 AM   #4
bmxboy809
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I left it at the regional airport in appleton WI. While it is possible someone dumped something in the gas I dnt think its very likely...

I read in my haynes manual that the fuel filter is part of the pump assembly? So if I drop the tank I will do both the filter and the pump.

The wierd part is that everything runs great for the first few minutes on a cold start. So every time I replace something it seems to help but all of a sudden the problem comes back when things warm up.
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Unread 07-05-2013, 08:48 AM   #5
mukluk
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It's doubtful you have a problem with the fuel filter, without some other good reason to replace it or the pump I'd say it'd be a waste of time to jump right into dropping the tank.

To talk things through a bit:
--Jeep runs poorly after sitting for two weeks
--Pushing the throttle to the floor doesn't change things (WOT is open loop, should rule out O2 sensors)
--Starts and runs OK until it warms up a bit
--Disconnecting upstream O2 sensor doesn't change anything (again, should rule out O2 sensor problem)
--Replacing cap, rotor, distributor, and timing chain doesn't fix the problem
--Coil ohms out OK
--Jeep isn't throwing any codes or illuminating the CEL
--Other than the lack of power, a noticable characteristic of the problem is an occasional snap and pop until the engine load is reduced

I'm curious to know, when you say it starts and runs OK when cold, can you immediately flog the hell out of it with absolutely no power issues whatsoever until it warms up? Do you notice the power issue generally starting at a certain point on the temperature gauge or after a certain amount of time? As for the coil, I would ask if this has only been checked when cold (the coil could be functioning when cold but crapping out as it heats up).
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Unread 07-05-2013, 09:19 AM   #6
bmxboy809
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Quote:
Originally Posted by mukluk View Post
It's doubtful you have a problem with the fuel filter, without some other good reason to replace it or the pump I'd say it'd be a waste of time to jump right into dropping the tank.

To talk things through a bit:
--Jeep runs poorly after sitting for two weeks
--Pushing the throttle to the floor doesn't change things (WOT is open loop, should rule out O2 sensors)
--Starts and runs OK until it warms up a bit
--Disconnecting upstream O2 sensor doesn't change anything (again, should rule out O2 sensor problem)
--Replacing cap, rotor, distributor, and timing chain doesn't fix the problem
--Coil ohms out OK
--Jeep isn't throwing any codes or illuminating the CEL
--Other than the lack of power, a noticable characteristic of the problem is an occasional snap and pop until the engine load is reduced

I'm curious to know, when you say it starts and runs OK when cold, can you immediately flog the hell out of it with absolutely no power issues whatsoever until it warms up? Do you notice the power issue generally starting at a certain point on the temperature gauge or after a certain amount of time? As for the coil, I would ask if this has only been checked when cold (the coil could be functioning when cold but crapping out as it heats up).
Your list is all correct. The coil ohmed out great. 1 ohm on the primary and 12.5k ohm on the secondary. It was cold except for sitting In the sun all day.

I can flog the hell out of it on cold start and it runs great. I did not pay close enough attention if it happens at a certain time or temp. I have been driving around the block and just as I get on the home stretch I have the problem. Complete loss of power until I put the clutch in when it goes to a low and rough idle.

I think I will replace the coil since they are pretty cheap and then everything will be new
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Unread 07-05-2013, 12:11 PM   #7
chris142
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Mine did weird things then it threw a code for the TPS. A new TPS fixed it.
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Unread 07-06-2013, 02:22 PM   #8
bmxboy809
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UPDATE: I changed the ignition coil with no change to the situation. The guy at o'reily said he has seen coolant temp sensors go bad without seeing a code and causing issues. Since the sensor was cheap I gave it a try. With the sensor unplugged things were a lot better but not normal. The engine lacked all around power. With the new sensor installed and plugged in things were looking good but the misfires crept back up again. While at autozone having the codes reset the guy there mentioned that his brother had the same symptoms and it ended up being a failing ECU.

I have come to learn the '99 4.0 ecu is specific to that year. Are these known to fail? A friend told me there was a pretty big rain storm while i was gone and there are no covers over the plugs on the front of the ECU mounted on the firewall. I suppose rain could have come in between the hood and wiper cowl.

any thoughts?
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Unread 07-06-2013, 04:18 PM   #9
bmxboy809
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I also just tested my TPS. The signal voltage ranges from .92-4.11 volts. The needle swings smooth between closed and WOT.
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Last edited by bmxboy809; 07-06-2013 at 04:46 PM.. Reason: double checked with my digital meter
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Unread 07-06-2013, 08:19 PM   #10
mukluk
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The PCM is not a common failure item, but it does happen. I doubt the rain storm would have caused any issues with the PCM connectors though if you're concerned you could pull the connectors and check for any corrosion or evidence of arcing.

Couple of things that come to mind -- when you say you're getting misfires and you had codes cleared, does it seem to be the same cylinders misfiring each time? Also, since the temp sensor seems like it may have made a difference, I'm wondering if there is an issue with the wiring for the temp sensor. If you have a FSM for your '99, go to page 14-40 for a check out of the temp sensor and wiring or go to the Fuel System section for the '99 TJ at this link: http://www.jeep4x4center.com/knowledge-base/index.htm
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Unread 07-07-2013, 09:11 AM   #11
bmxboy809
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the only codes I ever got were from unplugging sensors and running that way to try an diagnose. It never gave codes for the exact cylinder misfire. The only code I ever got without unplugging things was related to the upstream O2 sensor, but I think that was just from running crappy. I reset the codes and didn't see that one again.

I was thinking more about why unplugging the coolant temp sensor made a difference. By unplugging that the computer would exit closed loop right? And when the engine is cold it is also not in closed loop. The problem seems to come about when in closed loop. I have not unplugged the downstream o2 sensor yet. Could this one be causing problems?

After searching around quite a bit I have seen several threads where people describe the exact same symptoms but never a solution.

I did pull the plugs on the PCM and everything looked fine, I would be more concerned about water actually getting inside the PCM not just the plugs. I noticed mine is missing the shield that goes over the plugs and PCM. I will have to start looking through the wiring but I am not sure that I will find anything. I did read somewhere about a valve spring problem? What are the symptoms of that issue? This is already a replacement cylinder head but my original valve train was swapped over.
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Unread 07-07-2013, 02:41 PM   #12
mukluk
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Unplugging the temp sensor would have an overall effect due to the fact that it is used as an input for both open and closed loop operation; that it made a somewhat positive change makes me wonder if there may be a wiring problem or poor connection at the sensor. To help rule out the possibility, I'd recommend performing the resistance check outlined in the FSM.

Unplugging the downstream sensor shouldn't have any effect on how the engine runs, its primary purpose is to monitor catalytic converter efficiency (though it does affect the upstream sensor goal voltage as the cat deteriorates).

I wouldn't worry about the PCM wiring shield not being present, I do believe the cover was deleted fairly early on in production (my '00 made in August of '99 that I bought new never had a cover). As for the valve spring problem, the only thing that comes to mind was a TSB I believe detailing misfiring problems -- the cause was traced to carbon buildup on the valves that was due to the engines being run rather conservatively below 3000rpm, doing so didn't generate enough pressure on the valve stem through the lifter to spin the valve -- the fix was to decarbon the valves (you can use a product like Seafoam or you can simply mist water in the intake with a spray bottle while the engine is running).
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Unread 07-07-2013, 03:04 PM   #13
bmxboy809
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I have not gotten a FSM yet, I have gotten by with the Haynes so far. Can anyone post the details of the resistance test? I am assuming the test is for the wiring from the coolant sensor to the PCM? I replaced the sensor already and the problem still persists.

Since the head was gone through last summer I wouldn't think the buildup should be a problem. I havent tried anything like sea foam yet though. Since things run strong before the engine is up to temp, problems with the gas itself can be ruled out right?
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Unread 07-07-2013, 03:20 PM   #14
mukluk
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I believe it should be safe to rule out bad gas or something in the tank. If you look up to post #10 above, you'll see a link I posted that will take you to an online-accessible FSM for your TJ.
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Unread 07-07-2013, 03:27 PM   #15
MrSundays
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Let me say that I had my head in my 00 XJ replaced last year around November. In January I deployed to Japan and was there for 4 months. The jeep was started maybe once every couple weeks. When I got back I didn't have a lick of trouble. Doubt it's because of that.

Also, you say it stops bogging down when you put the clutch in. Theoretically it shouldn't big down then when it's idling if you aren't in gear. Food for thought.

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