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"Dead" cylinder with compression??


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Hi, hoping someone can shed light on this. I'm being told one cylinder is dead in my 3.7L v6. But ...

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Old 07-24-2018, 03:10 PM   #1 (permalink)
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Default "Dead" cylinder with compression??

Hi, hoping someone can shed light on this.

I'm being told one cylinder is dead in my 3.7L v6. But it has 140 psi of compression. I'm not a mechanic nor engineer. I'm not an idiot either so common sense would lead me to believe dead cylinder = 0 psi. When I asked the mechanic, "how does a dead cylinder give compression?" he said that is what he couldn't figure out.

How is this possible? And what could be causing it? Is it a telling symptom of something specific?

Any thoughts are appreciated.
Thank you!!
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Old 07-24-2018, 07:50 PM   #2 (permalink)
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What do you consider a "dead" cylinder?
Typically, that would be one without ignition (spark). So, is there good spark?
It could have good compression with a stuck valve, are the valves working properly? Broken rockers happen on the V6's.
You might need a better mechanic.
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Old 07-24-2018, 08:53 PM   #3 (permalink)
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Originally Posted by ElCheapo View Post
What do you consider a "dead" cylinder?
Typically, that would be one without ignition (spark). So, is there good spark?
It could have good compression with a stuck valve, are the valves working properly? Broken rockers happen on the V6's.
You might need a better mechanic.
Thank you, ElCheapo.

I consider a "dead" cylinder as non-functional in some critical way. I've no clue what the mechanic considers it as.

Unsure if it has spark. I was told injectors, coils, and plugs plus pcv valve and breather hoses were replaced. "Runs better than it did but no change in the cylinder." Haven't been told anything about valves or rockers. (In reading thru the forums for days now, I understand Jeep plugs should be copper NPK?? Unsure if the mechanic used those or not and if not, if that would make a difference in spark?)

I've also learned here about rockers, lifters, cam, and valve seat drops (altho that one would give 0 or low compression). However the mechanic has not mentioned anything about rockers or valves (besides pcv valve).

Thank you again, and please let me know if the above makes more (or less) sense.
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Old 07-25-2018, 02:46 AM   #4 (permalink)
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Needs copper NGK plugs but that should not cause a completely dead cylinder.

Could be a bad coil pack....swap the bad cylinder coil pack over with a different one.

Could be a bad injector...check the connectors to that injector is on tight.

With battery disconnected re-plug the connectors on the PCM a few times in case you have a dirty connection.

You should get a better mechanic and may have to inspect the valves and lifters on the bad bank.

Do you have any CEL codes posted?
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Old 07-25-2018, 04:26 AM   #5 (permalink)
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Thank you, BillWill. My dad's name was Bill. He had an old CJ he loved. I guess love for my KJ is in my DNA.

Copper NGK plugs. Sorry! I garden and apparently my mind remembered it as NPK (used in fertilizer).

Okay, I was told:

- Coils were replaced (I assume that means coil pack?).

- Injectors were replaced.

- The PCM connection test was mentioned as a hail mary but not tried because of a reason I can't recall. Sorry, the past 2+ weeks of dealing with all this has melted my mind some. But I remember, "that wouldn't work on yours because of" something.

- CEL was "throwing cat codes" but cats were fine upon visual with a scope. They were not removed and inspected. I wasn't given the CEL code numbers. Once I get my KJ towed out I will check via dash/odo scan.

A bit more info that might be helpful:
-The o2 sensors were replaced 4 mos ago. I was told they are now dirty. But CEL o2 sensor codes were not mentioned.

-Oil changes religiously. Small leak in the rear main so I've been checking and adding high-mileage oil regularly between changes for the past 2-3 yrs. It was advised by this mechanic that I do that rather than have the rear main seal repaired. At time of break down it was 2.5 qts low which was very unusual for its regular small leak related consumption.

I've been watching a YouTube video series by Astral Auto Repairs of a partial rebuild on an 03 KJ 3.7L to learn more about and what could be wrong with my engine. The episode I just finished covered examining valves and lifters. So I now have a better idea of what you mean. Still unsure what is wrong with mine. But I am learning in the process and everyone's replies help so much. Any other thoughts you may have are very much appreciated!!

Thank you!
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Old 07-25-2018, 08:27 AM   #6 (permalink)
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Even with an oil leak, it is not likely that only one cylinder would experience oil starvation and prematurely wear out the compression rings... I'd expect that to be nearly all or all cylinders.

140psi is low(er) pressure than typical, but the lowest cylinder should be within 10% of the highest and all would be considered "normal". I don't recall what normal psi is for the 3.7 pentastar. I'd expect upwards of 180.. so something specific to that one weak cylinder (I wouldn't call it dead, I'd call it weak) is going on.

A sticking valve makes sense... allowing some cylinder compression to escape during the combustion cycle.

Did your mech say if he performed the compression test wet, dry, or both? Hint: He should have done it dry first, then wet- meaning adding some oil to each cylinder to see if the reading changed and by how much.

If this is the same mech that gave you misinformation about the A/C compressor, then I third the comment that you need someone better, with more skill.
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Old 07-25-2018, 09:40 AM   #7 (permalink)
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Since the injectors and coils were already replaced, try switching them around, to see if the "dead" issue moves to another cylinder. That would indicate whether new coil or injector is good. It is possible for new parts to be bad.
If bad, get replacements, hopefully problem solved.
If not, then the valve covers need to come off to check valve operation.

If throwing codes, and O2 sensor already dirty, it sounds like you are getting fuel, it is just not burning in the cylinder, which should be spark related.
It could also be wiring for the injector.
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Old 07-25-2018, 11:20 AM   #8 (permalink)
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this may sound far fetched, but I got stumped once and old guy working for me solved it.

car had miss, like dead cylinder, worse at higher RPM.
Scope on ignition all looked good (back when you could use an O scope on cars).
Compression tested good,
Did not have OBD back than.

turned out it had a broken valve spring. It would hold valve well enough at low RPM for compression test, but running valve would bounce causing miss.
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Old 07-25-2018, 03:51 PM   #9 (permalink)
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Guys, THANK YOU SO MUCH!!

JasonJ: Agreed. Again, I'm not a mechanic but common sense = a leak as primary cause would affect all cylinders, not just one. I only mentioned it in relation to knowing the 2.5 qts low was very unusual, because I've been adding oil for past few years and know its related consumption rate.

I appreciate your term of weak cylinder vs dead. And yes, a sticking valve makes sense from what I've learned. I was not told whether compression test was done dry, wet, or both. But I've learned about those here and on jeepforum from my searches.

Yes. This mechanic was the 3rd to give the A/C misinfo. First opinion was by a kid who worked at a Chrysler dealership who was a friend of an acquaintance, in process of getting his ASE, had a lift in his parent's garage, and charged me $100 cash to do the serpentine belt (which I bought separately at autozone) bypass in his driveway that took 20 mins. The 2nd misinfo on A/C was from the shop I was working with prior to this current shop. So 3 mechanics, 2 shops, and 3 summers of sweating until I could get it repaired. This current shop is who did the entire unit replacement.

ElCheapo: Yes, I was told coils and injectors were swapped in the way you described and didn't make a difference. Sorry, I should have mentioned that in my other post.

Okay, that makes sense, it's getting fuel just not sparking. Injector wiring! That wasn't mentioned by this mechanic as a possibility but makes sense as a possibility. I'll keep that in mind and learn more about it. Agreed about valve cover and inspection. I've been learning about that and what to look for. I'm not afraid to turn wrenches and look at or do something myself if I can. I used to hold the drop light or flashlight for my dad when I was little while he worked on his CJ and other family vehicles. And I'd always ask him, "What's that? What does it do?" I used to change my own oil until convenience outweighed time factor.

towpro: Not far fetched at all! Because from what I've learned and understand now, the spring is what closes the valve, right? So if the valve is stuck open, it could be that the spring is non-functional hence keeping it open providing good compression, but in a stuck way. Am I following that correctly?

I saw a post the other day and took notes, so I know better what you mean:
Camshaft lifts lifters
Lifters push pushrod
Pushrod pushes rocker
Rocker pushes valve open
Spring closes valve

I am so very grateful for all of you guys taking the time to help me out. It means so much
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Old 07-25-2018, 05:15 PM   #10 (permalink)
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If the valve is stuck open, it will release compression, and/or hit the piston. If the rocker malfunctions the valve will not open, and you will have compression.

I don't think your problem is the spring, that wouldn't cause a dead cylinder. It seems like it is either spark or, it is in the valve train. Could even be a collapsed lifter.
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