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Discussion Starter · #22 ·
Thanks Everybody, helpful data!

I don’t think low compression causes misfires. At least not by worn out rings. Also, worn out ring cause oil consumption and usually blue smoke, but at the very least the spark plugs would be oily, but you said they looked fine.
My understanding is, that low compression CAN cause misfires, even if just at idle - which is what (mostly) seems to be going on with mine (runs (mostly) fine at higher RPM's).
My logic on rings was, would there be smoke if only one of the compression rings was bad, the other one and the oil rings working fine? (I thought it could prevent excessive oil burn, but I don't know 100%.) But yeah, plugs fine and NO smoke, even at start (checked that for valve stem seals).

However, what can cause both low compression reading and misfires is a problem with the valves, valve seats. That’s where I would start looking.
That would certainly be (slightly) more "user friendly" (at least I'd not have to pull the bottom apart...)

I would most certainly be running a borescope into this before pulling the engine apart. Carbon build up on valves can definitely cause low compression or quick leak down. It's like 30$ for a scope camera on Amazon. Also, check the timing marks line up and the timing chain/belt is tight.

Essentially, check everything else it could be before hap hazardly taking the engine apart.
Great ideas! Will get a borescope. Now would the difference between dry and wet compression tests also show if it was valve build-up as opposed to rings? (I guess it could do some sealing there too...)
I'd not think timing has to do with this, given 4 cylinders seem to be just fine, and Bank 1 seems just fine altogether.

A cylinder leakdown test is the easy way to figure out what’s leaking in a cylinder. You don’t have to guess.
Thanks, great idea! I guess this is when I'll just have to get a compressor too (long desired, never quite "needed" so far...)

Negative fuel trims mean too much air, not too much fuel. (Fuel trim is how much fuel to remove. Removing a negative is a positive gain.) Bank 2 is either running lean or is cutting fuel to your misfiring cylinder causing the negative trim readings.
A negative fuel trim value shows it’s removing fuel because it was too rich. A misfire dumps lots of oxygen in the exhaust, which would cause fuel trim to be positive if it were reacting to a current misfire. I’d guess it wasn’t missing when those fuel trims were recorded. And I don’t think I’d worry too much about the fuel trim until everything else is right.
What I understood for negative fuel trim was, that because there is too much fuel trying to go in there, the PCM is trying to reduce it somehow and trims it. My reasoning was, that due to misfires, there was a build up/excess (unburnt) gas that when it would finally ignite, would make it "rich" and the PCM would then try to counteract this with the leaning of the mix. But mostly, I was just using the fuel trims to try and isolate a pattern, which I DID, when I saw it was Bank 2 that seemed "out of whack" somewhat, so I started to narrow it to there, then I came upon Mode 6 misfire counts and confirmed and refined my findings.

Low compression can definitely cause a misfire. I agree with Truckster that the problem is more likely in the top end.
Anyway, still seems like a top end issue.
And so I'm starting to have some slight hope that "lighter surgery" could suffice :D

Pentastars are known for valve seat issues, especially the exhaust ones and especially on bank 2. The machine shop I use will not do a Pentastar head without doing valve seats.

Valve issues would also explain the fuel trims as it pushes the compression back into the intake or allows it to pull air in from the exhaust.

The crusty plug like you described is often from an exhaust restriction. If a valve isn't opening as much as it could because of a dropped, but still holding in the bore, exhaust valve seat; that can cause the exhaust particles to collect in the cylinder and on the plug.

Have you opened the valve cover to check for rocker issues? Rockers that are missing or severely worn could cause some of the same issues as issues with the valves themselves.
Good data, thanks! Haven't pulled the valve cover but am about to do it.

Any particular things I'd want to check while in there? I guess cranking over the engine could reveal if say, a valve doesn't fully "spring back" after being actuated, and also a worn cam lobe or anything bent/broken up there...
(I had an '07 Grand Caravan that was giving me **** with misfires and backfires in one cylinder until I found that one cam lobe, I believe intake, was worn down to nothing due to a stuck roller in the lifter - hope this is not similar here...)

Pulling the heads on these is a ton of work. I'd try to figure out as much as possible before going that far.
Amen to that! You won't have to say that twice - I didn't/don't really have the "urge" to get in there if I can help it :D

This is complicated stuff. :) And I agree — much more likely to be things other than rings leaking.
I notice it is complicated :D I hope I can take this back to (some level of) simplicity, which I am a big believer of :)

Well, off to the valve cover I go, and I'll let you guys know what "treasures" I found there awaiting :D
 

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Great ideas! Will get a borescope. Now would the difference between dry and wet compression tests also show if it was valve build-up as opposed to rings? (I guess it could do some sealing there too...)
I'd not think timing has to do with this, given 4 cylinders seem to be just fine, and Bank 1 seems just fine altogether.
Dry vs wet shouldn't matter to the valves. It just gets some sealing liquid on the piston rings to prevent blow by, so should give you slightly higher cylinder pressures. If a valve is not sealing, then you're losing combustion pressure out the exhaust or intake, which will make your pressure test show low on that cylinder. This is where the borescope cam can come in handy, set the piston at a point in the stroke where the valves should be closed, then look with the camera and see if any of the valves are off the seat at all. If you find one that is, it's either carboned up, or bent. With the intake manifold off, you'd be able to see if the tops of any valves are heavily carboned, and also have access to clean them. If something is bent, then you're pulling the head to replace some parts at a minimum.
 

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^^ I have a pretty rough idle when stopped in D but it's almost perfectly smooth in N with a slight shake in P. I just changed tranny fluid and filter and now it's also really good in P but still rough in D so I'm assuming it's the torque converter or something not releasing fully in the transmission.
 

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Inside the valve cover all you are looking for is something out of place with your misfiring cylinders. Rockers, lifters, cam, spring, and the valves themselves. Turn the engine over and make sure they all move the same as the others, no valve sits too far down when fully released, and there's no damage from parts slapping around in ways they shouldn't. Check rockers for play when the valves are fully released. If it moves by hand, it's bad. To cause issues like yours, the problem should be pretty obvious if it's inside the cover.
 
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I don’t think the boroscope will show much, especially on a closed valve. A leak down test will be much more useful.

OP got 90psi for the low cylinders, you will never see any gap or anything not looking normal at the valve with that kind of pressure still being made. I think it’s a waste of time and money to get a boroscope for that. If OP had one already, it doesn’t hurt to take a look, but I would not get one specifically for this. He will not see anything there that will pin point the problem.
 

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Discussion Starter · #28 ·
Dry vs wet shouldn't matter to the valves. It just gets some sealing liquid on the piston rings to prevent blow by, so should give you slightly higher cylinder pressures. If a valve is not sealing, then you're losing combustion pressure out the exhaust or intake, which will make your pressure test show low on that cylinder. This is where the borescope cam can come in handy, set the piston at a point in the stroke where the valves should be closed, then look with the camera and see if any of the valves are off the seat at all. If you find one that is, it's either carboned up, or bent. With the intake manifold off, you'd be able to see if the tops of any valves are heavily carboned, and also have access to clean them. If something is bent, then you're pulling the head to replace some parts at a minimum.
Thanks!

So...

I finally got a compressor and a borescope, and went to try them.

By the way, I DID pull thee valve cover a few days ago, and did not find/see anything out of the ordinary - no ground down cam lobes, no loose or chewed up rockers (at least from what I could see).

On putting pressure on cylinder #2 at TDC I get air coming out of the exhaust. If I turn the crank to the next TDC (i.e. after compression stroke vs after exhaust stroke), I get air coming out of the exhaust... Same for cylinder #6. So that would be exhaust valves then I guess.

Funny thing is, on cylinder #4, I seemed to find the opposite, i.e. neither TDC would yield any air leak - which I thought was strange, given my idea is that exhaust valve normally stays open a little past TDC after exhaust stroke, unless the Pentastar is not designed that way.

With the borescope I haven't had much luck yet, although I had only a short time to play with it ans still learning how to get it right *lookinng sideways with a mirror attachment to look at valves hangin or not). I'll try again and report back.

As for looking at the valves through intake port (if I got that straight), how would I do that? With the borescope? And how do I clean the exhaust valve if that indeed is just a buildup on it? (By the way, the amount of air leaking to the exhaust was moderate, i.e. not a really strong rush, unless that was due to the resistance of the exhaust system itself.)
 

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Thanks!

So...

I finally got a compressor and a borescope, and went to try them.

By the way, I DID pull thee valve cover a few days ago, and did not find/see anything out of the ordinary - no ground down cam lobes, no loose or chewed up rockers (at least from what I could see).

On putting pressure on cylinder #2 at TDC I get air coming out of the exhaust. If I turn the crank to the next TDC (i.e. after compression stroke vs after exhaust stroke), I get air coming out of the exhaust... Same for cylinder #6. So that would be exhaust valves then I guess.

Funny thing is, on cylinder #4, I seemed to find the opposite, i.e. neither TDC would yield any air leak - which I thought was strange, given my idea is that exhaust valve normally stays open a little past TDC after exhaust stroke, unless the Pentastar is not designed that way.

With the borescope I haven't had much luck yet, although I had only a short time to play with it ans still learning how to get it right *lookinng sideways with a mirror attachment to look at valves hangin or not). I'll try again and report back.

As for looking at the valves through intake port (if I got that straight), how would I do that? With the borescope? And how do I clean the exhaust valve if that indeed is just a buildup on it? (By the way, the amount of air leaking to the exhaust was moderate, i.e. not a really strong rush, unless that was due to the resistance of the exhaust system itself.)
With the IM removed, the ports on the head are exposed, you can just look into them and you should be looking at intake valves, these are the ones that usually get carboned up. If it is exhaust valve specific, then you'd have to go through the exhaust ports which means removing exhaust manifolds.

Valves can be cleaned by soaking them in carb cleaner and agitating with a hard wire brush. There's walnut blasting. Different ways to do it. A quick youtube search would present you plenty of videos on the subject.
 

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Discussion Starter · #30 ·
With the IM removed, the ports on the head are exposed, you can just look into them and you should be looking at intake valves, these are the ones that usually get carboned up. If it is exhaust valve specific, then you'd have to go through the exhaust ports which means removing exhaust manifolds.

Valves can be cleaned by soaking them in carb cleaner and agitating with a hard wire brush. There's walnut blasting. Different ways to do it. A quick youtube search would present you plenty of videos on the subject.
Thanks!

I will try and see if I can make the borescope reveal some more now.
 

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When you do a leakdown test you have to have the piston at exactly top dead center on the compression stroke. If it’s not it’ll force the piston back down, and there’s no way you can keep that from happening. It’s by far the hardest part of doing a leakdown test, and if you didn’t struggle with that you almost for sure didn’t have the crank and cams in the right place.
 

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Discussion Starter · #33 ·
When you do a leakdown test you have to have the piston at exactly top dead center on the compression stroke. If it’s not it’ll force the piston back down, and there’s no way you can keep that from happening. It’s by far the hardest part of doing a leakdown test, and if you didn’t struggle with that you almost for sure didn’t have the crank and cams in the right place.
I did check it with both TDC's, ie valves open and shut, and I did monitor the position with my breaker bar on the crank pulley. Also, I didn't use too much pressure (at 90 psi it did knock the piston down once so I went to about 60 psi or so. )
 

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With the IM removed, the ports on the head are exposed, you can just look into them and you should be looking at intake valves, these are the ones that usually get carboned up. If it is exhaust valve specific, then you'd have to go through the exhaust ports which means removing exhaust manifolds.

Valves can be cleaned by soaking them in carb cleaner and agitating with a hard wire brush. There's walnut blasting. Different ways to do it. A quick youtube search would present you plenty of videos on the subject.
I guess it’s easy to give nonsensical advice when you are not the one affected. 🙄

This is not a DI engine, there will not be any intake valve deposits, and there will not be any exhaust deposits preventing the valve from closing either, they run too hot for any to form.

Why lead people into a deeper rabbit hole than necessary?
 

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On putting pressure on cylinder #2 at TDC I get air coming out of the exhaust. If I turn the crank to the next TDC (i.e. after compression stroke vs after exhaust stroke), I get air coming out of the exhaust... Same for cylinder #6. So that would be exhaust valves then I guess.

Funny thing is, on cylinder #4, I seemed to find the opposite, i.e. neither TDC would yield any air leak - which I thought was strange, given my idea is that exhaust valve normally stays open a little past TDC after exhaust stroke, unless the Pentastar is not designed that way.
At TDC you should not have any air leaking. I would check the other good cylinder for reference. There could be some valve overlap, but I would think it’s only once the VVT is active and that usually happens at higher RPM. When VVT is at rest, there should not be any overlap. But I’m not 100% on this.

Also, part of the leak down test is to check for how long the pressure stays up. A good cylinder should only leak down few PSI after few minutes.
Again, I would check the good cylinders for reference.
 

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Discussion Starter · #36 ·
At TDC you should not have any air leaking. I would check the other good cylinder for reference. There could be some valve overlap, but I would think it’s only once the VVT is active and that usually happens at higher RPM. When VVT is at rest, there should not be any overlap. But I’m not 100% on this.

Also, part of the leak down test is to check for how long the pressure stays up. A good cylinder should only leak down few PSI after few minutes.
Again, I would check the good cylinders for reference.
Thanks for the data.

Since I don't have leak down gauges, I was only able to perform a "crude" leak down test - hook up the cylinder to the compressor and listen/look/feel :)

Both cylinders that have misfired (#2 and #6) were leaking air into the exhaust at TDC (after compression stroke) - I could hear the hissing of air leaving, and I could feel the air coming out of the tail pipe - not a very strong flow, I might add, but noticeable (it wasn't "gushingout" in other words). With #2, I could also feel/hear a very slight amount of air coming out of the oil filler tube with the cap off (I didn't check #6 with the cap off, I forgot).
Cylinder #4 (working fine), in contrast, had no leak to the exhaust or anywhere, at least observably without special gauges.

I then also played around with the borescope.

Here is a photo of one of the exhaust valves on cylinder #2 when open:
Jaw Eyelash Automotive tire Tints and shades Font

The seating area to me looks either pitted or caked in carbon. But I doubt these could pit - probably break first, or am I wrong there? (I hope it's carbon, for simplicity's sakes, anyways.)

Here is the other exhaust valve of #2:
Tints and shades Rim Automotive tire Eyelash Human leg

Same kind of "pitting" or carbon, as far as I am concerned.

Here are the open intake valves on the same cylinder - can't see a whole lot with the shadows, but the seating surfaces seem clean to me:
Jaw Neck Eyelash Flash photography Tints and shades

Eyelash Iris Tints and shades Circle Darkness


This is the piston top for #2 - no clue if this looks "normal" or this much carbon/ deposit is just out of control:
Road surface Circle Soil Metal Asphalt


Two photos of one of the exhaust valves of #2 when closed:
Eye Eyelash Human body Tints and shades Circle

Eye Eyelash Human body Tints and shades Petal

By the shadow, it appears to me there is some slight gap between the bottom edge of the valve and the seat, which I can't think would be normal - I'd think the valve ought to sit "square" on the seat, correct?
And the explanation for the photo would indeed be the carbon deposit holding it slightly apart?
And thus some level of leak to the exhaust, but which also would explain why the wet compression test would raise the pressure - the slight (enough) gap could get sealed by the oil splashing up and create more pressure momentarily?

Am I just jumping to a "happy (because it could be something worse) but short-sighted" conclusion, or do you think this may be an accurate description of what is there?

Thanks!
 

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The valve should sit flat on the seat in an area usually about a quarter from the bottom of the taper on to a quarter from the top. More contact is better, but typically doesn't happen. Rarely will one sit completely flat on the chamber side, but it will be very close.

Parallel Font


Bad valves are usually pretty easy to spot. Here's a Google image search that shows a lot of different examples. Google Image Result for https://ssch.com.au/wp-content/uploads/2021/05/Bent-Valves-Caused-By-A-Broken-Timing-Belt-320x2001-1.jpg

I was half expecting you to find a dislodged seat. It was a common problem with the first few years. Most machine shops will not do 2011-13 heads without doing the exhaust valve seats. Here's some examples. Bottom is a major failure in a Pentastar. I couldn't find an image of a minor failure picture for it, but this is a Jeep 3.7.

Motor vehicle Automotive lighting Automotive exterior Audio equipment Font


Automotive engine gasket Circle Auto part Aircraft Engineering


A partially dropped seat can still be your problem. That might explain why you think the valve doesn't look like it's sitting right. I don't see it in the picture you took, but you could see it from more angles while looking at it and might have seen something more.

Since you found leakage on the exhaust valve, you're at a point where there's really no other choice than to pull the head and take it to the machine shop. To be sure you got it right, do your improvised leak down just before the piston reaches the top and brace the crank by using a breaker bar to prevent the engine from turning backwards (handle facing rear of van on floor, jack stand, or subframe [beware of pinching lines]). Bracing it the other way could just end up loosening the bolt. Then turn the engine 1 full turn and retest the cylinder. If it blows both times, there's definitely a problem with the valve.
 
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I guess it’s easy to give nonsensical advice when you are not the one affected. 🙄

This is not a DI engine, there will not be any intake valve deposits, and there will not be any exhaust deposits preventing the valve from closing either, they run too hot for any to form.

Why lead people into a deeper rabbit hole than necessary?
He has pressure escaping from cylinders. You have no idea what issue exists, except that it is likely something to do with valves. So you follow the logical steps of troubleshooting. But what would my dumb master tech brain know? I'll bow out and leave this one to the youtube warriors............

Good luck OP!
 

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Those pictures are not really good, perhaps it looks better on your boroscope, but the stills are quite useless. As far as color and amount of carbon on them, it’s normal, from what little I can see.


I don’t think you have any other choice but to pull the head off and do a proper inspection.

I would try the trick with the beaker bar @Sienile described, just before the piston reaches TDC. This will ensure all valves are closed.
 

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Discussion Starter · #40 ·
Pardon my disappearing - I've been slowly but surely getting ready (gathering up tools and supplies, etc.) to indeed pull the head off.

I'd just like to say one thing real quick:
I guess it’s easy to give nonsensical advice when you are not the one affected. 🙄

This is not a DI engine, there will not be any intake valve deposits, and there will not be any exhaust deposits preventing the valve from closing either, they run too hot for any to form.

Why lead people into a deeper rabbit hole than necessary?
He has pressure escaping from cylinders. You have no idea what issue exists, except that it is likely something to do with valves. So you follow the logical steps of troubleshooting. But what would my dumb master tech brain know? I'll bow out and leave this one to the youtube warriors............

Good luck OP!
I do honestly believe everyone chiming in here, has been just honestly trying to help.

We are individuals, with our own - and distinctly unique - set of experiences and knowledge to draw on, in attempting to solve situations. While our answers and approaches may be different, that doesn't necessarily make one or the other wrong - based on one individual's experience and knowledge, one approach may be "righter" than the other, but the other person would find it different - from where they are looking and experiencing.

I want you all to know that as a "fledgling" "mechanic" (I put it in quotes so I may not offend anyone that has worked way harder and longer than I to master this skill), I found each of your answers helpful in broadening my knowledge and wrapping my wits around the mystery called "car". As an experienced student (inn various fields) I also know to carefully observe and evaluate each piece of information/knowledge I receive, and see how it may or may not apply to my situation/context, before "proceeding with it - i.e. I try and use the good ol' "common sense" to my endeavors ;)

I also realize how it may be frustrating at times to see or hear something that conflict with our own reality or experience, but again, I'd just like to repeat that I don't think anyone here has been trying anything but help me solve my issue, which is truly appreciated!

I'd just like to ask to please recognize each other's intention here to help, both me and those reading later on in trying to find answers to similar issues - I think this is a wonderful Team and I believe being Team indeed is the Key to solving a lot of tough ones inn Life ;)

Pardon if I got maybe a little too "philosophical", but I'd just like to make sure noone is left here with a bad taste in their mouths, as I don't believe either of you should have a real reason to. With that said, as we can all be lacking data or knowledge, I believe it's all natural to learn from each other, and offering helpful corrective information in a "neutral fashion" should hopefully make all of us a little better and prevent any ill feelings ;) (True there are "trolls" out there, but I sincerely don't think there have been any in this discussion.)
 
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