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Old 12-14-2013, 12:21 PM   #1
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In need of diagnostic help..

Basic Jeep History:
Year: 1998
Engine: 4.0L 6 Cylinder
Transmission: 3 Speed auto
Mileage: 83,000
with A/C
Tj Sport
(Just cause everyone asks these)

My jeep is throwing a few codes... P0113, P0123, P0304, and P0306.

P0304 and P0306 are generic misfire codes, P0306 came a week or two before P0304. Obviously one is a cylinder 4 misfire the other is cylinder 6.
They both happened when running it fairly hard.
My best guess is replace the spark plugs and possibly wires or anything ignition wise.
I used seafoam just in-case it was clogged injectors.
I do need a new fuel pressure regulator as well.
Any input would be great.
The misfires aren't the worst of my worries. The other codes have me puzzled.


P0113 is IAT Sensor one High input, P0123 is TPS sensor high input.
After some research I understand that these normally occur if the sensor reads over 5 volts. It can either be a sensor on the same circuit or a ground. I could not find any bad grounds. I tryed replacing the TPS itself and it did not work.
Those were the only high input codes but after using a voltmeter most all my sensors read 5.13v. Surprisingly enough my 02 sensor read 4.18v, but I do not know if that is normal. The only other 2 sensors I checked read 12v, they were the one on the A/C compressor and the one on the valve cover side of the throttle body. I've included labeled pictures with the voltages to better clarify.

Sorry for all the detail but I want the right answer not a million questions.

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Old 12-14-2013, 05:11 PM   #2
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'97-98 4.0L engines were notorious for developing intermittent misfires, my then nearly new '97 had one that drove me crazy. After replacing all the usual stuff & pleading for help in the various Jeep forums, Jeep engineering contacted me about the problem & they were puzzled by it too. They ended up sending me a free care package with a new PCM, fuel injectors, fuel injector wiring harness, clockspring, etc. That was after I had replaced the plugs, distributor cap, rotor, and ignition wiring. Nothing helped, and my Jeep engineer contact stayed with me off & on for the next year.

Finally Jeep figured out the problem that was causing the misfire for my engine, it was from a weak/bad batch of valve springs that made its way into many of the 97-98 4.0L engines.

The fix for that particular problem, which may or may not be what is causing your misfires, is to simply replace the valve springs. The same part number is used. That and clean out the combustion chambers, they can get loaded up with carbon from the misfires.

Here's a Google search on the subject but essentially, it just boils down to replace the valve springs. And sometimes, if the valves got burned from the weak valve springs, it may also require a valve job to completely eliminate the misfires. Replacing my valve springs cured about 90-95% of my misfires. If that Jeep hadn't eventually been stolen I would have had a valve job done to it to fix the intermitten misfire problem completely.

https://www.google.com/#q='97+TJ+4.0...+valve+springs

There is a TSB associated with it but it doesn't really add anything besides to do what was described here. Note it is a TSB (Technical Service Bulletin) and not a Recall so the dealer won't do the TSB for free like they will a recall.

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Old 12-14-2013, 06:07 PM   #3
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Quote:
Originally Posted by Jerry Bransford View Post
'97-98 4.0L engines were notorious for developing intermittent misfires, my then nearly new '97 had one that drove me crazy. After replacing all the usual stuff & pleading for help in the various Jeep forums, Jeep engineering contacted me about the problem & they were puzzled by it too. They ended up sending me a free care package with a new PCM, fuel injectors, fuel injector wiring harness, clockspring, etc. That was after I had replaced the plugs, distributor cap, rotor, and ignition wiring. Nothing helped, and my Jeep engineer contact stayed with me off & on for the next year.

Finally Jeep figured out the problem that was causing the misfire for my engine, it was from a weak/bad batch of valve springs that made its way into many of the 97-98 4.0L engines.

The fix for that particular problem, which may or may not be what is causing your misfires, is to simply replace the valve springs. The same part number is used. That and clean out the combustion chambers, they can get loaded up with carbon from the misfires.

Here's a Google search on the subject but essentially, it just boils down to replace the valve springs. And sometimes, if the valves got burned from the weak valve springs, it may also require a valve job to completely eliminate the misfires. Replacing my valve springs cured about 90-95% of my misfires. If that Jeep hadn't eventually been stolen I would have had a valve job done to it to fix the intermitten misfire problem completely.

https://www.google.com/#q='97+TJ+4.0...+valve+springs

There is a TSB associated with it but it doesn't really add anything besides to do what was described here. Note it is a TSB (Technical Service Bulletin) and not a Recall so the dealer won't do the TSB for free like they will a recall.
I was hoping you'd comment Jerry haha. That's interesting, I did not know that about the valve springs. Parts and labor how much do you think it'd cost to replace them in a shop? I have a few people more knowledgeable than me always willing to help but i'm curious. Also you wouldn't happen to know anything about my sensors reading high input would you?
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Old 12-14-2013, 06:48 PM   #4
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I don't remember exactly what I paid but valve springs aren't expensive and replacing them isn't a major job. The springs can be replaced without removing the head so I'd think maybe two hours of labor. I think I spent less than $250 including parts & labor on mine but the mechanic I use for things like that is really cheap.
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Old 12-14-2013, 08:27 PM   #5
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Alright thank you! Anybody know anything about high voltage to sensors?
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Old 12-14-2013, 09:31 PM   #6
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I like what Jerry said, and I too had a missfire when I bought my 97 Sport 3 years ago with 121K on it. I use Amsoil powerfoam intake cleaner about once a year to clean the intake and valves and combustion chambers. Use as directed on the bottle. I also do once a year is dribble a glass of water into the throttle body while the Jeep is hot and running. About 12 ounces. It cleans carbon out of the combustion chamber too. I have not had a missfire code in 3 years. She will flip 149K real soon too. Good luck.
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Old 12-15-2013, 07:00 AM   #7
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No one knows anything about my sensor problem?
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Old 12-15-2013, 09:45 AM   #8
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Quote:
Originally Posted by CountryBoyJeep View Post
No one knows anything about my sensor problem?
Do you know what voltages the sensors run? Some might run at 12 volts. If anything, put in a new TPS. I did, it was like $30
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Old 12-15-2013, 10:41 AM   #9
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True sensors including the IAT, MAP, & 02 sensors run off the PCM's 5 volt logic circuit voltage. Items you measured the 12 volts at are not really sensors which include the IAC (Idle Air Controller) attached to the throttle body and the A/C pressure switch will show battery voltage.

The voltages at the IAT, MAP, and 02 sensors will be at or close to the PCM's normal 5 volt logic circuitry voltage. They look close to what I would expect but I've never had to worry about them so I don't have any experience with troubleshooting their circuits.

I don't know of those 5.13 volt measurements you're seeing are static/unchanging but I suspect they do vary. But the 4.18v you measured at the 02 sensor has me scratching my head. There are four wires on the TJ's 02 sensors... two have 12v on them for the 02 sensors's internal heater, the other two wires are the 02 sensor's output which varies between 0 and 1 volt. The 02 sensor generates that 0-1 voltage inversely to the amount of oxygen present. Could any of the 02 sensor wiring be damaged so the 02 sensor's heater voltage is being seen by the 02 sensor itself? If not, maybe the 02 is shorted internally so its internal heater circuit is touching the 02 sensor element.

I'd also make sure all three PCM connectors are making good connection, I'd remove all three and reseat them 2-3 times which can help clean a dirty connection. Make sure the screws that holds the PCM to the firewall are screwed in to bare metal as that is part of its ground system.

I never did find anything that would give reasons for both the MAP and IAT signals to read high which is why I'd at least make sure the PCM's connections are good to start with.

This doesn't help much, I know, but it's hard to troubleshoot without actually being there.
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Old 12-15-2013, 12:13 PM   #10
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Plowboy.. I did change mine and it didnt fix the problem.

Quote:
Originally Posted by Jerry Bransford View Post
True sensors including the IAT, MAP, & 02 sensors run off the PCM's 5 volt logic circuit voltage. Items you measured the 12 volts at are not really sensors which include the IAC (Idle Air Controller) attached to the throttle body and the A/C pressure switch will show battery voltage.

The voltages at the IAT, MAP, and 02 sensors will be at or close to the PCM's normal 5 volt logic circuitry voltage. They look close to what I would expect but I've never had to worry about them so I don't have any experience with troubleshooting their circuits.

I don't know of those 5.13 volt measurements you're seeing are static/unchanging but I suspect they do vary. But the 4.18v you measured at the 02 sensor has me scratching my head. There are four wires on the TJ's 02 sensors... two have 12v on them for the 02 sensors's internal heater, the other two wires are the 02 sensor's output which varies between 0 and 1 volt. The 02 sensor generates that 0-1 voltage inversely to the amount of oxygen present. Could any of the 02 sensor wiring be damaged so the 02 sensor's heater voltage is being seen by the 02 sensor itself? If not, maybe the 02 is shorted internally so its internal heater circuit is touching the 02 sensor element.

I'd also make sure all three PCM connectors are making good connection, I'd remove all three and reseat them 2-3 times which can help clean a dirty connection. Make sure the screws that holds the PCM to the firewall are screwed in to bare metal as that is part of its ground system.

I never did find anything that would give reasons for both the MAP and IAT signals to read high which is why I'd at least make sure the PCM's connections are good to start with.

This doesn't help much, I know, but it's hard to troubleshoot without actually being there.
I did reconnect each connector a few times to see if any connector was corroded enough to mess up the circuit. Also all the voltages were read from the connector before the actual sensor. I was hoping throwing out all the voltage would pinpoint inbetween what 2 connections is my problem. From what I've read the cam or crank sensor is on the same circuit and can cause this, where are they? Also where are all the grounds? The main one I saw was off my firewall.
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Old 12-15-2013, 12:24 PM   #11
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Oh, ok then sure those voltages would all appear correct with the sensors unplugged... those were all open circuit voltages you measured. The way to measure those voltages is with the sensors plugged in.

The cam position sensor is located under the round plastic shield under the rotor under the distributor cap, and the crankshaft position sensor is located up high on the driver's side where the transmission bolts to the engine, next to the flywheel.
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Old 12-16-2013, 10:32 AM   #12
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I pretty much just said screw it and took the jeep up to a friends shop, sometimes these things arent worth the frustration. I did tell him that the valve springs may of caused my misfires though so thank you jerry. As for the sensors thats upto them to figure out now. When I get a call back I'll post what the solutions were so anyone with these problems in the future can use this thread as a source of repair.
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Old 12-18-2013, 02:38 PM   #13
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So I took it to my friends dads shop. He says nothing was wrong but it runs 10x better and theres no more codes coming back. I'm pretty sure he fixed my jeep free of charge. Im not complaining though.

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