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Old 07-14-2014, 10:28 PM   #1
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YJ missing mystery

Hey Guys, not new to jeeps but new to the forums. I have been able to figure out most of my many YJ problems just tinkering around. But this time is different.
Overview: 1995 YJ Rio Grande 2.5L
Problem: Engine miss firing.
Mystery: Severe miss fire during high humidity or after a ran. I live in Florida so it gets humid and rains a lot during the summer. The jeep will miss fire STRICTLY upon start up with high humidity or rain/recent rain. The miss fire seems to "clear itself out" after about 3 to four miles. The more gas you give the more heavily it miss fires. It will idle upon start up with continuous but minor miss firing. HELP I can't find another forum with these symptoms and can't figure it out.

Changed the plugs and wires. No oil leaks.

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Old 07-15-2014, 03:23 AM   #2
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sounds like a spark issue...bad wires or cracked coil pack. pop the hood after dark and look for sparks you might have to spray things down with windex bottle of water...for gawds sake don't use a garden hose!!!

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Old 07-15-2014, 03:35 AM   #3
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I have a 4.2 so I had to ck to see what coil you have on a 2.5 it looks like its a single coil....so we are back to wires also be sure plugs are gapped correctly new plugs are not "pre gapped" I learned this the hardway!!!
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Old 07-15-2014, 05:29 AM   #4
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Did you replace the cap and rotor? If not you should as they are equally important to making spark.

Otherwise maybe the coil is found kaput
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Old 07-15-2014, 05:23 PM   #5
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I have a 4.2 so I had to ck to see what coil you have on a 2.5 it looks like its a single coil....so we are back to wires also be sure plugs are gapped correctly new plugs are not "pre gapped" I learned this the hardway!!!
So gapped the plugs, no I havn't. Looks like you're about to teach me the hard way. haha what does that mean?
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Old 07-15-2014, 05:25 PM   #6
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Did you replace the cap and rotor? If not you should as they are equally important to making spark.

Otherwise maybe the coil is found kaput
Havn't replaced the cap and coil but I was hoping maybe I could test a few other things before spending money taking guesses. any suggestions?
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Old 07-16-2014, 04:22 AM   #7
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The cap and rotor are wear items just like the spark plugs. They aren't expensive I would recommend checking them to see how pitted and warn away they are at the very least. Also to see if the cap is cracked at all
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Old 07-16-2014, 05:32 PM   #8
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The cap and rotor are wear items just like the spark plugs. They aren't expensive I would recommend checking them to see how pitted and warn away they are at the very least. Also to see if the cap is cracked at all
I'll check them out, the cap is like 6 dollars online. No budget breaker there. I just don't get the whole "humidity" thing. Why only then??
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Old 07-16-2014, 07:39 PM   #9
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high humidity means moisture in the engine bay until it gets hot

if the cap is cracked or the wires are bad, the excess moisture will allow arcing of the spark voltage to a closer ground or another wire
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Old 07-16-2014, 09:15 PM   #10
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Exactly what timberwolf said. Same with the coil pack if it's cracked or going bad. It's actually not as uncommon as it sounds. Also I assume your wire kit included the one from the coil pack to the center if the distributor?. I can't think if anything else off the top of my head. Other than to check all your vacuum lines. I don't really suspect that but it's a jeep and there's bound to be one bad one haha the one on the fuel pressure regulator likes to go bad a lot
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Old 07-20-2014, 10:32 AM   #11
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Make any progress?
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Old 07-20-2014, 11:13 AM   #12
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I had the exact problem,new distributor cap took care of it.
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Old 07-21-2014, 04:44 PM   #13
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Make any progress?
Okay so over the weekend I gave the rattle trap a tune up. Changed the oil and filter and added gumout oil treatment, changed the distributor cap and the rotor, changed the fuel filter (which God knows when thats been changed) and also filled the tank and added SeaFoam. The jeep was running like a dream for the past two days and then on my way home today from work was idling slightly rough.

Rough Idle is a slightly different symptom than before (it was missing at any RPM).

I did find a vacumm hose on the top of the air box that goes directly to the engine head. It was split and I'm sure that wasn't helping things. I temporarily sealed the split with gorilla tape thanks to no local auto parts store having it instock.

Any ideas?
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Old 07-21-2014, 09:51 PM   #14
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Rough idle after all you changed sounds vacuum related to me. I mean could be fuel delivery but idk. Check the vacuum connections at the fuel pressure regulator, they go bad there commonly. Do you have access to a fuel pressure gauge? Another common issue is that the fuel return line gets pinched off in the tank due to the tank bottom cupping up into the return line. The solution is to pull the pump assy and trim the steel line in the tank. Gives you a chance to mod to 20 gallons and change out your almost certainly dry rotted lines on top of the motor. Have you tried pulling any codes? I'd try that also. Gives you some things to check anyhow, but 4 bangers are really sensitive to vacuum leaks at idle. So give them all a good once over.
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Old 07-22-2014, 05:38 PM   #15
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Rough idle after all you changed sounds vacuum related to me. I mean could be fuel delivery but idk. Check the vacuum connections at the fuel pressure regulator, they go bad there commonly. Do you have access to a fuel pressure gauge? Another common issue is that the fuel return line gets pinched off in the tank due to the tank bottom cupping up into the return line. The solution is to pull the pump assy and trim the steel line in the tank. Gives you a chance to mod to 20 gallons and change out your almost certainly dry rotted lines on top of the motor. Have you tried pulling any codes? I'd try that also. Gives you some things to check anyhow, but 4 bangers are really sensitive to vacuum leaks at idle. So give them all a good once over.
Gave all the hoses a tracing yesterday and also did a lot of forum digging. The PVC hose that is the clean air intake (the one that goes in the top backside of the valve cover) is split in the end that goes into the top of the airbox. I wrapped in gorilla tape but I am sure its still leaking. However, when I pulled that breather hose I found a lot of oil residue inside and a visible amount on my air filter. According to the forums, the PCV pushes fresh air through the breather hose into the valve cover and sucks blow-by oils and emissions through the exit hose i think into an orphis? (something like that). However, my questions:

If the fresh air breather hose is spraying oil into my air filter, the PCV valve must be stuck open? and/or the exit valve is clogged forcing the blow-by back through the fresh air hose.

Thoughts?
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Old 07-22-2014, 05:46 PM   #16
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I would replace the PCV valve, another cheap item that should be replaced regularly. My understanding is the other thing that can cause it is excessive blow-by due to bad piston rings, etc. Hopefully that isn't it.

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Old 07-22-2014, 06:14 PM   #17
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I would replace the PCV valve, another cheap item that should be replaced regularly. My understanding is the other thing that can cause it is excessive blow-by due to bad piston rings, etc. Hopefully that isn't it.

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Yea, I read that about the pistons as well. I'll be buying a 3 dollar for sale sign if thats the real problem. haha
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Old 07-22-2014, 06:26 PM   #18
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So, late model YJ's (like mine, 95 last year) I can not find a PCV valve for it, I found on one forum that it is actually a CCV and that the valve cannot be replaced, only cleaned. Anyone else heard of that?
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Old 08-24-2014, 10:36 AM   #19
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Hey all. I am troubleshooting a similar issue with my 93 right now, it failed emissions for high HC's and failed evaporative system (fuel). I found a busted hose to the charcoal canister and have the same symptoms as above.

here's what I've found on the "PCV" valve and breather line you all are talking about on the 4.0.

PCV is only a Crank Case Vent Elbow on the back of the valve cover with a restriction to reduce the vacuum at the port. Clean it out.
Breather tube is the elbow on the front of the valve cover. What I am gathering is with older engines, more oil is blown back to the intake wetting the air filter which causes the higher emission results. Remedy is to create a catch can in line to the air box. Basically a drip can which collects blown by oil and allows clean air to reach the air box.

I just started working on my jeep so don't have any results yet but will post once I'm done and emission test results are available.

Hope this helps out.
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Old 08-30-2014, 07:09 PM   #20
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Update: My jeep passed emissions.

for the breather tube, I added a breather filter inside the elbow to catch any oil blowback

CCV elbow, cleaned it.

New air filter

New hose from fuel tank to charcoal canister

Went ahead and replaced 02 sensor just in case. Didn't want to fail again.

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