Cylinder 5 no compression - JeepForum.com
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post #1 of 14 Old 10-05-2019, 04:35 PM Thread Starter
FrankenCub
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Angry Cylinder 5 no compression

Our WJ has been using 3 quarts of oil every 2500 miles or so, the only noticeable indication has been smoking on restarts. No oil reaching the ground and doesn't smoke driving down the road. Has about 228k miles, wife drives Lyft so she racks them up quick. I finally had time to run a compression test today, she had a hip replacement so time has been tight. Other than gauge readings, there's no indication that #5 has no compression. Runs smooth down the road and idles smooth. What's the chance I'll get lucky and it spit a rocker arm? Not sure when I'm going to have time to pull the valve cover.
Compression readings are as follows.

180# cly 7 180# cyl 8
0# cyl 5 200# cyl 6
185# cyl 3 208# cyl 4
150# cyl 1 200# cyl 2

The plugs are shot, I don't think the PO ever changed them. They are layed out in cylinder sequence.

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post #2 of 14 Old 10-06-2019, 03:34 PM
turbocoupe88
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usually if you spit a rocker off you will have high comp in that cylinder not low compression due to the valve doesn't open
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post #3 of 14 Old 10-06-2019, 04:27 PM
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no misfire codes, claims it idles smooth, and runs good -MIght want to do test over, maybe a valve was open during test.
A compression leak down test will help ID where the compression is leaking If test confirms low or zero pressure. Often a loaner or rental tool.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #4 of 14 Old 10-06-2019, 08:39 PM Thread Starter
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no misfire codes, claims it idles smooth, and runs good -MIght want to do test over, maybe a valve was open during test.
A compression leak down test will help ID where the compression is leaking If test confirms low or zero pressure. Often a loaner or rental tool.
I tested that cylinder multiple times, even unthreading the hose thinking maybe it wasn't sealing. Checked the connections on the gauge to be sure it was sealing up, all fine. There's a noticeable difference in sound when cranking the engine over. Every cylinder sounds like it's building compression when the gauge is in that hole. Number 5 does nothing, just as if all 8 plugs are out or as if it's completely missing that rod and piston.
Honestly, it doesn't give me a warm fuzzy feeling. Starting to wonder if I'd see a hole in the piston if I had a bore scope.

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post #5 of 14 Old 10-06-2019, 08:42 PM Thread Starter
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I can't edit my post so I'll add here. Only code shown are for cat efficiency and large evap leak. No misfire codes.

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post #6 of 14 Old 10-07-2019, 05:51 AM
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I would remove the valve cover - don;t know till we look. and as mentioned a compression leak down test.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #7 of 14 Old 10-07-2019, 12:17 PM
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Intake rocker fell off or broken.
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post #8 of 14 Old 10-07-2019, 08:39 PM Thread Starter
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I have a borescope coming, will run it through #5 plug hole and see if anything is visible then pull valve cover if it looks good. As long as the rain holds off I should know something by this weekend.

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post #9 of 14 Old 10-08-2019, 07:22 AM
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Any chance the check engine light is missing/burnt? Does it light up upon ignition on, then turn off once the engine is running?
Seems pretty odd that a cylinder has no compression, yet no misfire code appears.

The spark plugs being 100% physically undamaged but evenly worn tells me the bore scope image will be clean.

The other thing I'm curious about is if the engine is running and you disconnect the injector to #5, does that change the idle?

3 quarts at 2500 miles is a lot. More than some worn valve stem seals would allow.

Fingers crossed it is just a weird reading on the gauge or a rocker arm spit out. No codes (if the light is operational) is plain mysterious.
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post #10 of 14 Old 10-08-2019, 08:29 AM
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Any chance the check engine light is missing/burnt?
no compression, yet no misfire code appears.

GOOD observation - something missed.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #11 of 14 Old 10-08-2019, 09:25 PM Thread Starter
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Quote:
Originally Posted by Double E View Post
Any chance the check engine light is missing/burnt? Does it light up upon ignition on, then turn off once the engine is running?
Seems pretty odd that a cylinder has no compression, yet no misfire code appears.

The spark plugs being 100% physically undamaged but evenly worn tells me the bore scope image will be clean.

The other thing I'm curious about is if the engine is running and you disconnect the injector to #5, does that change the idle?

3 quarts at 2500 miles is a lot. More than some worn valve stem seals would allow.

Fingers crossed it is just a weird reading on the gauge or a rocker arm spit out. No codes (if the light is operational) is plain mysterious.
CEL works as it should, will come back on after cleared to show my cat and evap codes. I'm hoping the scope shows all good but still worth the look, it a loaner from a friend so no cost to me.
Good idea unhooking the injector. If I get a chance tomorrow I'll try that. Not sure it will do much, I'm not noticing any drivability issues at all. I had to jump on it yesterday on the way back from town to get around a moron in a county highway truck that ran a red light in front of me and pulls like mad. Even with 228k miles on it I'm impressed with how much power it has and how smooth it is.
That's also my thoughts with the oil usage. I'd think it would smoke really bad driving down the road, but it doesn't. Only on restarts, and it's not all the time. Sometimes when it does it smokes more than other times.
Something did come to mind a few minutes ago though. I'm a Chevy guy, always have been. The 4.7 has the same fireing order as a SBC. If I had a plug wire go bad on a SBC I'd notice it. Just wont run as smooth. This engine is butter smooth going down the road. If a spit lifter is possible, I'm wondering if a failed lash adjuster isn't pumping up until the engine is running? I have a manifold leak on that side so I'm hearing the leak.

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post #12 of 14 Old 10-09-2019, 02:55 PM
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The thing with sometimes more oil or less oil smoke on startup can easily be the way the valves end up at shutdown. Closed or open or both shut, whichever valve stem seal is leaking will let oil into that cylinder and then...which one is doing it worse than others.

No plug wires on the 4.7. They are coils on plugs.
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post #13 of 14 Old 10-09-2019, 08:37 PM Thread Starter
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Quote:
Originally Posted by Double E View Post
The thing with sometimes more oil or less oil smoke on startup can easily be the way the valves end up at shutdown. Closed or open or both shut, whichever valve stem seal is leaking will let oil into that cylinder and then...which one is doing it worse than others.

No plug wires on the 4.7. They are coils on plugs.
I don't think I'm going to worry much about the valve seals, I don't believe they would be leaking that much to account for how much oil we've been going through. The plugs also don't show signs of a bad oil burn problem. I'll be looking for other areas now.
I know there isn't plug wires on the 4.7, I was just using that comparatively with a lost cylinder.
Of which I did try unplugging the injector at the trouble cylinder today, it stumbles when unplugged so the cylinder can't truly be dead. Gotta be another reason for the no compression issue.

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post #14 of 14 Old 10-10-2019, 03:32 PM
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Good deal. I wonder if the PCV system is at issue then?
If you pull the TB, does it look like oil has been getting in there?
Might as well clean it up while it is off. The IAC piston and back side of the TB likes to be cleaned every 40K miles or so.

Other than that, I'm suspecting a leak that presents only when running such as the RMS maybe?
Likewise, I'm thinking #5 is good.

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