Dead Cylinder Below About 1500 rpm's

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hwd2

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Back in the fall of 2012, I had a shake from idle to about 1500. Plugs and wires were fine. I did a compression check and it showed that no.6 had barely 90psi - while the others were from 120 to 130. Valve lash was okay, and the head was rebuilt just prior to me getting the car 8 years earlier. I'd only put on 6 thou during this time.
I then did a 'leakdown' test and there was lots of air flowing into crankcase. A couple others also leaked that way but not as much. There was no 'hissing' from any of the valves.
That pretty much indicated there was a 'ring-sealing' issue. I removed the head and then orientated the pistons so all were below the deck. If I poured some light solvent (such as Zylene) into each bore, it would quickly drain down from the No.6 piston and slower in 5 and 4 and not at all in 1,2, and 3.
Long story - short, I found broken compression rings on 4 of 6 pistons. No.6 had both broken in several pieces and the other 3 only the top ring. I checked all the pieces and they measured out to a 'width' of about 0.060" (compared to new ones at 0.078"), so the rings would 'flutter' and were doomed to fail. Yes, there was a 'ridge' in the stock bores- but not excessive. I elected to hone the cyls. and re-ring (plus bearings, gaskets and seals.
That was 5 years ago and car has run fine, during accumulated 6 thousand miles - - until a couple days ago. Seemingly out of nowhere, I noticed a miss below 1500, and yes, it is again in No.6. The plug and wire are good so I did more tests.
Compression this time showed ALL cylinders at 130 and leakdowns between 20 and 30 percent - no.6 at 20%.
Now the question - - - Do I have the same problem as before or is there something else to examine first? I had intended to do a proper rebuild in a few years, primarily a re-bore, with new pistons - but hoped for (and expected) the engine to run fine throughout.
Have any of you encountered this situation before? Thanks.
 
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Pull the valve cover and look for a broken valve spring on #6.
 
How do you know it is in #6? 130 psig in all cylinders sounds great. Strange that someone assembled the engine with the wrong size piston rings. Any idea how that happened? The outer cylinders tend to run leaner, so would be first to suffer "lean mis-fire". Could also be the spark wire or something in the distributor cap. People have found many caps w/ the tower circle off center. Slant Six Dan has posted the best PN to buy.
 
How do you know it is in #6? 130 psig in all cylinders sounds great. Strange that someone assembled the engine with the wrong size piston rings. Any idea how that happened? The outer cylinders tend to run leaner, so would be first to suffer "lean mis-fire". Could also be the spark wire or something in the distributor cap. People have found many caps w/ the tower circle off center. Slant Six Dan has posted the best PN to buy.

I know it is number 6 because when I remove the H-tension lead from the plug end, while idling, the engine does NOT react in any way. But, when each of the other wires are removed (one by one of course), the engine noticeably stumbles. But, if I set the fast idle to over (approx.) 1500, then remove No.6 lead, the engine reacts. Also, when removing each lead from corresponding plug, they all give healthy sparks - when held close to cyl. head, and that includes No.6, so that kind of tells me the cap, rotor and H.T. leads are okay. I tried several plugs in No.6 and no effect and also swapped with one of the other plugs, and, swapped No. 6 lead with No.1 - and still no power from No.6.
I then removed the valve cover - - and everything looks good. I remembered installing good 318 springs 5 years ago and, the lashes all in spec.
A 'broken' spring would tend to allow cyl. to make power at idle - but lose power when revving higher (acting as a weak spring). At least this is what happened with a friend's LT1 years ago. Bigger concern (with broken spring) would be the risk of 'dropping' a valve into cylinder. I should also add that I am using the Mopar electronic setup (with a HONDA module) and it's been flawless for last 14 years. I checked the pickup to reluctor gaps and all between .008 and .010" - and a reminder again that healthy spark at 800 and at 1500 - yet NO 'combustion' at 800.
Car was parked from Dec.01 thru April 10th. I installed battery, filled carb, tapped pedal twice and it started in two seconds. I did this from passenger side so never worked the accelerator until few minutes on fast idle. It then idled smooth and I insured it on the 11th. and drove off and on for about 30 miles.
It was 3 days later when I parked it in garage to service oil, filter and tranny. I also checked plugs, wires, cap and rotor (and, for reasons unknown) when I started the car again - after the servicing, it immediately did not sound right. That's why I am here now.
 
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I go along with Roy. Check for a vacuum leak at the number 6 intake runner. If you have power brakes, it could be a bad booster, or the hose. There is a port on the number six runner, that supplies vacuum to everything. Check anything connected to that.
 
It also could be a tight rusted exhaust valve/ guide from condensation and setting , have you run it long enough to lube it good?
 
Thank you all for interest and advice. While it could be a vacuum leak or a stuck valve - it is likely neither. As I mentioned before, the engine 'shakes' at slower idle. No.6 is not making power. If I speed it up - to over (about) 1500, it smooths out. When I remove the (No.6) lead and 'ground' it, there is healthy and consistent spark. That should confirm the lead, cap and rotor are fine. Cap has one terminal for each lead/plug but rotor affects all.
If a (manifold) vacuum leak, then noticeable at all RPM's, but it is smooooth at fast idle - and above.
Although I haven't replaced the dist. cap, the inside terminals are all fine, and, there's that good, healthy spark - each and every time. So, with fuel, compression, spark and timing, logic has it that there should be ignition and power. There is not much else I can do - - outside the engine. I know of the 'science' that occurs when compressed air/fuel is ignited and how the resulting gasses act upon the compression rings. The one thing that bothers me the most is that, unlike the first time I discovered the broken rings, this time the (cranking) compression is good - too good. Something just doesn't make sense. If the rings are 'intact' - plus everything else I mentioned above - the engine should fire on all six - all of the time.
I will try to find my original points distributor and give that a try - before making any decisions about the 'internals'.
 
Vacuum leak(gasket) or rings/valves? I would think you have to determin if it is mechanical or not, or if it is intermittent. Could the rings be broke again a ring land problem? Did you hear air leaking during leakdown test? Do you have access to a borescope, you could look around at the valves and seats through the plug hole. I would think a bad valve seat would show low compression.
 
Not to be an ***, but why are you here, arguing with people trying to help you? You could have whipped that valve cover off ten times by now and confirmed or denied a broken valve spring. That would really narrow it down, too. BAM the valve cover is off. It's that simple.

Lastly, you should listen more. Roy and Charlie have close to 150 years experience between them. I have also seen a slant skip on #6 because the power brake hose had a hole in it. The skip only showed up at idle.
 
Not to be an ***, but why are you here, arguing with people trying to help you? You could have whipped that valve cover off ten times by now and confirmed or denied a broken valve spring. That would really narrow it down, too. BAM the valve cover is off. It's that simple.

Lastly, you should listen more. Roy and Charlie have close to 150 years experience between them. I have also seen a slant skip on #6 because the power brake hose had a hole in it. The skip only showed up at idle.


Ummm - well I am not arguing with anyone - just explaining my situation and why all the info given to me was investigated and eliminated. I was honestly hoping to find someone (anyone) who has the knowledge to help - but haven't been able to at this point. Any techie can brag about decades of experience, but, if he (or she) has not encountered a specific problem, then (he or she) is of little help to the person posting the problem.
I did remove the valve cover and reported on it last last night - mainly to appease you. I knew very well that the springs would be fine. The smoothness of the rocker arm operation and smoothness of the engine operation (at 2000 RPM's) confirmed this days ago.
I can also confirm the car has no vacuum leaks and has manual brakes. The only thing I haven't confirmed (yet) is the integrity of the compression rings. The car is running exactly as it did in the fall of 2012. We drove to a steam show about 25 miles away and as soon as I got there and was crawling thru a parking lot, I felt the car 'shake' at the idle speed. I knew right away it was running on 5 cyls. When I revved it beyond 1500, it smoothed out - so the return trip was 'normal'.
As mentioned initially, the cranking compression turned out to be less than 90, and solvent would drain down rapidly into sump.
Only difference today is that I apparently have higher compression in faulty cylinder.
I will leave it at that, thank everyone for their various (and appreciated) ideas and let you get in the last word.
Now back to Allpar.
 
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This isn't about appeasing anyone, it's about helping you. Pay close attention to the intake manifold on the #6 runner. That's where the power brake port is. I've seen them crack right around the where the fitting screws in. I missed where you reported back about the valve spring. You asked for help. We're trying to give it to you. It just seems like you are shooting people down before you even try their suggestions. Lots of great minds here and lots of great experience.


Ummm - well I am not arguing with anyone - just explaining my situation and why all the info given to me was investigated and eliminated.
I did remove the valve cover and reported on it last night - mainly to appease you. I knew very well that the springs would be fine. The smoothness of the rocker arm operation and smoothness of the engine operation (at 2000 RPM's) confirmed this days ago.
I can also confirm the car has no vacuum leaks and has manual brakes. The only thing I haven't confirmed (yet) is the integrity of the compression rings. The car is running exactly as it did in the fall of 2012. We drove to a steam show about 25 miles away and as soon as I got there and was crawling thru a parking lot, I felt the car 'shake' at the idle speed. I knew right away it was running on 5 cyls. When I revved it beyond 1500, it smoothed out.
As mentioned initially, the cranking compression turned out to be less than 90, and solvent would drain down rapidly into sump.
Only difference today is that I apparently have higher compression in faulty cylinder.
I will leave it at that, thank everyone for their various (and appreciated) ideas and let you get in the last word.
Now back to Allpar.
 
I did remove the valve cover and reported on it last last night - mainly to appease you. I knew very well that the springs would be fine.

I was about to jump in and help, but that's, uh, quite an attitude y'got there, so…I know naah-theenk! ¯\_(ツ)_/¯
 
did you try opening up the spark plug gap a little / and go up a heat range #1 and 6 had fuel issues in the manifold / tighten base gasket on carb/ see if carb screws have loosen up and need to be tight
 
It sure sounds like a bad O/F ratio on cyl 6 (probably lean). You are incorrect that a vacuum leak would cause an issue at all rpm's. Exactly the opposite. It would be a fixed added air flow amount (at a given manifold vacuum), so less a percentage as the total air flow (and fuel) increases. Also, at higher power, intake vacuum drops so the leaked-in flow becomes even less. It would be real easy to cap off the vacuum port on the #6 cyl and see the effect, but not sure you are listening to all these great suggestions.
 
I know I'm late to the party, and everything has already been talked about, and I'll throw in with everyone that mentioned vacuum leak,
somewhere on the #6 leg,
I would include lobes going bad, but not all the signs are there for that, And it would be a little strange how that could just mysteriously appear right after a service and ignition inspection.
But I just gotta include the valve lash, if you included that with the "service".. If the valve isn't closing properly when the engine heats up, it will act like a vacuum leak, as the piston squeezes the fuel charge back up into the intake. You can easily see this on a vacuum gauge.
Or if the exhaust is not closing at the right time, then the piston will pull exhaust into the cylinder on the initial part of the intake stroke. You can sometimes feel this at the tailpipe at very low rpm as your hand gets "sucked" onto the pipe every 6th shot. It's really easy to diagnose on a motorcycle with individual pipes for each cylinder.
You can easily catch this by doing a LeakDown test on a fully warmed up engine. It should have shown up on the compression test, again, on a fully warmed up engine.
I have seen this when some contaminant got stuck on the exhaust seat. What I would do was, with the LD tester hooked up, and the cylinder pressurized; I tapped the exhaust valve with the wooden end of a hammer. A few taps later, the valve sealed up and the pressure normalized. I obviously didn't see the contaminants so can't say what it was, but I suspect it was a piece of carbon from the EGR system. If you continue to run it like this, the valve will overheat and be torched, requiring the head to come off again.
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I used to see a lot of this in old lawnmower engines. I would occasionally buy up a non-running single,cheap, clean up the chambers and re-sell it for beer money. The tell-tale was always the same; it "sucked" my hand onto the pipe as I pulled the rope. Sometimes I could get it started by covering the pipe, cuz then the piston would have to "pull" from the carb.
 
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