Burnt valve?? - JeepForum.com
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post #1 of 40 Old 03-12-2019, 05:17 PM Thread Starter
Georgia1988jeep
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Burnt valve??

So I’m completely stumped on my Jeep. It’s a 1988 wrangler with the 4.2. The exhaust sucks in air while the engine is cold, so I assumed a burnt/sticking valve. But when the engine warms up it usually clears right up like it never even happened. But yet sometimes it’ll start hiccuping and acting up again, sucking air in even when the engine is at full temp, it’s so back and forth I’m unable to really diagnose it, anybody had this problem before? Any help would be much appreciated

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post #2 of 40 Old 03-12-2019, 05:30 PM
bharris68
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Compression test when both cold and warm to narrow down the cylinder in question? Maybe a bore-inspection camera to check for bent or sucked valves once you determine which one is burnt/cracked?
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post #3 of 40 Old 03-12-2019, 05:34 PM
Boojo35
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A burned valve is a burned valve. It should show its symptoms all of the time. Yours sounds more like a sticking valve or broken or weak valve spring.
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If you cannot fix it with a hammer then it has to be an electrical problem.
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post #4 of 40 Old 03-12-2019, 06:02 PM
Jeepsr4me
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Change the intake and exhaust gasket.... before you mess something up.
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post #5 of 40 Old 03-12-2019, 06:24 PM Thread Starter
Georgia1988jeep
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I plan on breaking the engine down this weekend so those gaskets will be replaced
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post #6 of 40 Old 03-12-2019, 06:26 PM Thread Starter
Georgia1988jeep
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I’m waiting on my compression tool to come in the mail, I just recently purchased one, I was leaning more towards a sticky valve, I was planning on pulling the head off this weekend anyway
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post #7 of 40 Old 03-12-2019, 06:28 PM Thread Starter
Georgia1988jeep
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None of the valve springs are broken I checked them recently, but the weak spring definitely sounds like a possibility for sure, all the spark plugs look good even with it sucking in air
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post #8 of 40 Old 03-12-2019, 09:22 PM
blake989
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Did you hold a dollar bill over the tail pipe to get to this diagnosis?

The YJ is no longer my DD. I have sacrificed all future retirement plans for jeep parts. I'm depending on social security!
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post #9 of 40 Old 03-13-2019, 05:23 AM
fishadventure
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Well I’m totally confused. I’ve never seen an exhaust valve suck air. Ever.

Why are you pulling the head?!

I think you’re grasping at cliche-straws, not actually diagnosing. With the very limited info here I’ll wager your compression is fine- and I agree with someone mentioning changing the manifold gasket.

I just wouldn’t be going off half-cocked and pulling the head off without eval of the problem.

Not being mean- just saying don’t buy a new house because of a squeaky hinge.

[size=ď3Ē]Shackles & D-rings are different things.
Cranking IS turning over
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post #10 of 40 Old 03-13-2019, 08:54 AM
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have not seen a burnt valve in so long that I dont think they exist anymore. And those that I did see never self corrected after any running...


Change your intake and exhaust gasket and put the studs and bolts in the right spot,.... and use the right washers.
Betting you got some loose ones and broken ones .. Folks screw the stud placement also. One at the very front and one at the very back. Reason they screw them up is when you take the bolts out sometimes the stud will come at whole since the nut is rust welded on. then you put a bolt in the stud spots and they will not tighten enough as the bolt will bottom out on the head studs..... axk me how I know..
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post #11 of 40 Old 03-13-2019, 09:17 AM
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Quote:
Originally Posted by Georgia1988jeep View Post
...The exhaust sucks in air while the engine is cold,...
I'm curious and interested, but you're going to need to explain this a bit more.

How can you possibly tell that a cylinder is sucking in air, when 5 others are pumping out exhaust? Or are they all sucking in air? Engines don't run at all if they suck in air.

Sorry man, but you're just not making any sense, at least to this old fart.

But as long as we're guessing, I would say your timing is unstable. Might be a terribly loose timing chain, a bad vib damper, a bad distributor.

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post #12 of 40 Old 03-13-2019, 11:12 AM
vadslram
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I question the "sucking air" too. I can't figure how you would know one valve is letting air in once every half second at idle.

Regardless of the diagnosis, if the symptoms go away as it warms it is either a clearance issue or an oil pressure issue. I would lean towards oil first.
How many miles on the engine? is there any audible tapping? does your gauge work?
If your lifter is sticking you should hear a tap and the valves won't work until they gather enough pressure, possibly because the oil passages are cloggged.
If the valve stem is gunked and sticking then it will stay open until the heat expands stuff enough to move.

More info would help. If you've already gone further disregard but if not try some engine flush and an oil change. Can't hurt and only costs about $50 with fresh oil and filter.
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post #13 of 40 Old 03-13-2019, 04:13 PM Thread Starter
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The old dollar bill trick is what I used, the engine will be idling and with a dollar bill up to the exhaust pipe it’ll suck the dollar bill in for just a second then blow it back out, over and over not in a particular order
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post #14 of 40 Old 03-13-2019, 04:15 PM Thread Starter
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Yessir, sucks the bill in at random
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post #15 of 40 Old 03-13-2019, 04:20 PM Thread Starter
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My reasoning behind pulling the head is, if I pull the manifolds off to replace the gaskets I’d just go ahead and pull the cylinder head, but I totally understand why someone would think the way you do, if I could fix the problem with the head on I’m all for it. I’m just lost on the situation because everyone I’ve talked to told me to yank the head and look start from there
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