Rough idle, misfire, ash deposits on plugs - JeepForum.com

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post #1 of 78 Old 05-08-2016, 12:31 AM Thread Starter
TDodge7
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Rough idle, misfire, ash deposits on plugs

Hi, I've been reading lots of threads on here but up until now never really had a reason to post. Late last fall my GMC Sonoma up and died on me so I decided I was finally going to get a YJ. Every YJ near me was a rusted out mess and it took me months to find what I thought was a decent one. It's a 1990 with the 4.0 from a 1992 YJ swapped in. The PO said the engine came from a wrecked YJ and then he rebuilt it roughly 20k ago. Everything looked good and It made the 80 mile drive home just fine. It made it through the winter blizzard, but as the weather started warming up my YJ started falling apart.

There's lots of things I still need to fix but mainly I have a misfire I'm trying to diagnose. When I first start the Jeep up it idles high for a few seconds and then adjusts the idle down to around 800rpm, the whole time it's shaking slightly and you can hear it missing at the tail pipe, it may be surging ever so slightly at idle too. I don't think it's missing while driving but holding the throttle open under the hook it still feels pretty shakey. After the o2 sensor kicks in it gets a little better but it still pretty rough at idle. Still though it's very smooth driving and it's actually got plenty of power and no hesitation. I can't see the exhaust while driving but I don't see any smoke from the tail pipe, the exhaust does stink and burn my eyes though. No check engine light either.


I mainly work on old motorcycles so fuel injection isn't really my specialty. I've been trying my best to test and diagnose the problem but I'm not 100% on it so I figured I'd post here. First I did the standard tuneup, plugs, wires, cap and rotor, which did absolutely nothing. The plugs that came out were pretty bad, some were very ashy and most of them showed signs of burning oil, most of which was only on one side of the electrode. There was also a fair bit of ash, especially cylinder #6, I've never seen deposits that bad in person before.

(That bent tip on the front plug is from dropping it, it didn't come out that way)



After about 4 miles driving and maybe 20 minutes of idling I pulled the new plugs and wasn't impressed with the results (I couldn't find a pen to write down which plug went to which cylinder so excuse my brothers hand modeling)








From here I went to a compression test, on a cold engine with the throttle wide open I got:

#1 155psi
#2 154psi
#3 160psi
#4 159psi
#5 150psi
#6 152psi

Those numbers sounded good to me but I'll retest the compression warmed up too as soon as it stops raining. So I went on to the leak down test, I wasn't able to find a decent leak down tester anywhere locally so I picked up a garbage one from harbor freight that (surprise surprise) was total garbage. It only tests at 15psi and despite setting it up and adjusting it correctly it gives a different reading every time you use it. I didn't hear any air from the intake or exhaust but did hear an extremely small amount of air from the oil cap on all of the cylinders at tdc but I'm told that's normal. The numbers are meaningless but here they are:


#1 59%, 36%, 19%, 19%, 21%
#2 36%, 28%, 28%, 24%, 30%
#3 29%, 33%, 38%, 26%, 22%
#4 5%, 27%, 5%, 11%, 11%
#5 39%, 6%, 9%, 20%, 10%
#6 53%, 19%, 8%, 9%, 7%

From here I broke out the vacuum gauge. Here I discovered the PO had accidentally switched valve cover breather elbows and had the one with the metered orifice on the wrong hose which was putting the entire crankcase under tons of vacuum (a steady 12hg to be exact) you literally had to fight the oil cap off of it while it was running. I don't know if that had anything to do with the misfire but it's fixed now.

When I hooked the gauge to manifold vacuum though I noticed I get an extremely fast fluctuating needle between 15-18hg, if what I read is true it's suggesting I have bad intake valve guides.


Does it sound like I'm on the right track and it's valve guides/seals or is there something else I should check? This YJ was supposed to be my daily driver and while it's down for the count I'm stuck daily driving my motorcycle rain or shine which is getting pretty old, I really need to fix this thing.

Sorry for the book, I just wanted to cover everything....and at least there's pictures and videos right?

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post #2 of 78 Old 05-08-2016, 11:03 AM Thread Starter
TDodge7
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Also, it has never turned on the check engine light except in the course of diagnosing when I unplugged one of the injectors while it was running, but the next time I came out to start the Jeep the light had turned off so I figured it cleared it's self. I just did the key trick anyway and got 12, 27, 21, 33, 55. The 27 is from unplugging the injector, the 21 however is "Oxygen sensor input voltage stayed above normal operating range". I don't know if that one was triggered because of the injector being unplugged but it's never thrown a check engine light for it. Now that I've repaired the crankcase excessive vacuum problem is that something I'd want to unhook the battery and clear the computer for so it has to relearn everything? I'm not super familiar with obd1 systems.
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post #3 of 78 Old 05-08-2016, 12:03 PM
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Welcome to the forum. Its Mothers Day, so not a lot of guys on here I guess. Yes you should disconnect the battery for at least 10-60 seconds. And then run it and see if that 21 has gone away.

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post #4 of 78 Old 05-08-2016, 02:18 PM
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It's possible your valve guides are bad...id try to order a decent leak down tester off the internet though.
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post #5 of 78 Old 05-08-2016, 04:22 PM
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White ashy plugs usually means a lean condition exists, as you probably already know from motorcycles.
Well, same with the Jeep.
Have you explored the possibility of an intake vacuum leak yet?

If you have some starting fluid laying around, it's certainly worth a looksie.
I would spray it all around the intake manifold and throttle body base.

If it jumps in RPM, you have a leak.
Sometimes intake bolts loosen up, sometimes they break, I even had an intake manifold develop a crack underneath.
Don't know why it cracked, but where it was made it tough to find.

I don't know if it is a vacuum leak, but I would rule it out first before tearing into the valvetrain.

What happens in my Jeep, stays in my Jeep
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post #6 of 78 Old 05-08-2016, 05:03 PM Thread Starter
TDodge7
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I cleared the codes and drove around for a while, so far code 21 hasn't come back. Are the leak down numbers all that important? I haven't returned the harbor freight one yet, I could just hook the quick connect to a regulated 80 or 90psi just to listen for leaks.


I have tested for leaks by spraying carb cleaner, I didn't find any, I should have mentioned that. I also tested with the vacuum gauge, a manifold leak is supposed to show it's self as a steady 5hg reading and mine is doing the fluttery 15-18hg thing.

Last edited by TDodge7; 05-08-2016 at 05:14 PM.
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post #7 of 78 Old 05-08-2016, 05:13 PM
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I like the suggestion for the vacuum leak test. An unlit propane torch is pretty awesome if you don't want to spray carb cleaner everywhere.

How long is it taking to go to closed loop? Is it actually up to temperature when this happens?

Remove the throttle body and clean it up with throttle body cleaner. Make sure to hit the idle air control valve with a toothbrush and cleaner as well.
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post #8 of 78 Old 05-08-2016, 05:33 PM Thread Starter
TDodge7
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It always misses, when you first start it it idles at about 1000rpm, after 30 seconds to a minute it settles down to 700-800 and that's where it stays even after it's warmed up, it's shaky the whole time.

I'm definitely going to clean the tb and iac before do anything like pull the head, though that oil has to be coming from somewhere. For a while I was holding out hope that it was just the excessive crankcase vacuum misting oil into the intake but fixing the vacuum thing didn't help running any, neither did all new tune up parts.
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post #9 of 78 Old 05-08-2016, 06:48 PM
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I don't like testing with carb cleaner, I like ether. It's more of a gas and really revs the motor vs. just causing a stumble.
You can get under the manifold really well.
It may not be vacuum though.

I much prefer ether or smoke or yeah, propane than carb cleaner for vac leaks.

These are things that can cause a miss and erratic vac readings like that.
Weak or broken valve spring, bent push rod, damaged rocker, even a bad head gasket.

The deposits on that plug look suspect. It just doesn't look like oil to me, I don't know.
I'm throwing this out there, are you losing some antifreeze? Could you be burning a little antifreeze in that cylinder?
As in head gasket?

Have you checked your injectors? The fuel pressure?

What happens in my Jeep, stays in my Jeep
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post #10 of 78 Old 05-08-2016, 07:39 PM Thread Starter
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I didn't get any air through the radiator on the 15psi leak down test. I have a leaky freeze plug that I intend to change when I have the intake off for the head work, but that's external. It wouldn't surprise me though if there was a super tiny head gasket leak, one of the first things I did to this YJ was replace the cooling system because the PO ditched the clutch fan in favor of a manually activated pusher fan instead of just replacing the faulty radiator cap it had, I'm sure it's gotten hot before.

Plugs 4, 5, and especially 6 have lots of oil buildup at the base of the electrode and threads (no valve cover leak), it just tapers out to tan and deposits.

I haven't tested fuel psi, I did remove the connectors from 3 of the easier to get to injectors and test them with the multimeter, they all read 15.7 ohm's. I listened to them with a mechanics stethoscope and none of them seemed to be erratic or sounded like they were skipping. I pulled the connector for the #6 injector while it was running and it immediately threw a code 27.
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post #11 of 78 Old 05-09-2016, 05:13 AM
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That is valve lifter or cam.
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post #12 of 78 Old 05-09-2016, 06:36 AM
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It could still burn coolant when hot. You tested the cooling system at 15 psi when cold. There could be one of those chemical goop head gasket bs repairs that leaks when hot and not when cold.

They should put the manufacturers and users of that stuff in prison. There is one reason for that stuff: "SURPRISE! Your used car is a lemon!! Now walk home"

Not to say that is your particular problem in this case though.
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post #13 of 78 Old 05-09-2016, 09:03 AM
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The JF psychic crew is in full force here.


Your an old bike guy. You know you cannot get an accurate plug check with 4 miles of driving and 20 minutes of idling.


The old plugs are different then the new. Same heat range and type but a different brand ?


The ash deposits are common to all of the plugs with #6 being the worse. That is a by product of the fuel and the fuel additives.


Looking at the electrode plus the compression and leakdown numbers it would seem #6 is a bit less efficient then the other 5 cylinders. The extra oil further suggests that fact. If in fact that cylinder was not firing as well as it should that would explain the ash buildup over time.


It would seem you do have a vacuum leak. Standard rules of engagement to find it. Don't forget the vac. line under the dash to control the HVAC door.


Beyond that I`d simply run it after clearing the codes and see what happens as a whole.
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post #14 of 78 Old 05-09-2016, 10:30 AM Thread Starter
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The plugs got switched after the compression test since they were already out, when I had them out again the next day for the leak down test I noticed how bad they already looked and figured it was worth snapping a picture, they weren't out exclusively for plug reading. They are a different brand, all the parts place had was NGK, I was planning on ordering the correct oem Champions.

I had read that the ash deposits can come from fuel additives but the presence of oil and the fact that every other vehicle here runs on the exact same fuel and never has an issue I figured it had to be more mechanical. I'm not saying I don't have some sort of ignition or vacuum problem too but how else would the oil get in there besides leaking past the rings, past the valves, or perhaps misted in with the fuel if the oil somehow made it to the intake?

And what about the vacuum gauge reading off the intake? If the book that came with the gauge is correct there's only two things that should make the needle go crazy like that and that's weak or broken valve springs or worn valve guides.

I cleared the codes but it still runs the same, I drove it around a little and so far no codes have returned except for 12 and 55 which is normal.

I'm going to have my friend bring over his smoke machine and see if we can find any vacuum leaks, replacing a hose would be a lot cheaper then replacing a head. I know some things are hooked up differently from oem, the PO seems to have bypassed the 4wd vacuum switch on the transmission and appears to have run constant vacuum to the switch on the front axle, all other vacuum lines are in their normal spots and connected though.

The only other random thought I have is that my YJ is lifted and the PO put in a transfer case drop so the engine does sit at a little more of an angle than it did stock, I don't know if that could have any factor on it but I figured it was worth mentioning. Though there's millions of jeeps with t/c drops rolling around out there so that probably doesn't have much if anything to do with it.

Last edited by TDodge7; 05-09-2016 at 11:18 AM. Reason: spelling
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post #15 of 78 Old 05-09-2016, 10:32 AM
mike134
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vacuum leak can possibly cause fluttering vacuum gauge needle as well if the leak is along a path that resonates.

Also, check and recheck that two spark plug wires are not crossed.

Loose valve guides is the classic cause of fluttering vacuum gauge needle. But don't start there. Start with the vacuum leak tests.

When you do the test, I recommend pinching off the hose that goes to the 4wd. If it is non stock, you need to seal that off until you are sure it is not the culprit.

The NGK plugs will be as good as the champions as long as they are the proper gap and copper core instead of platinum or irridium.
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